cppdraft_translate/cppdraft/class/mem.md

[class.mem]

# 11 Classes [[class]](./#class)

## 11.4 Class members [class.mem]

### [11.4.1](#general) General [[class.mem.general]](class.mem.general)

[member-specification:](#nt:member-specification "11.4.1 General [class.mem.general]")  
[*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]")opt  
[*access-specifier*](class.derived.general#nt:access-specifier "11.7.1 General [class.derived.general]") : [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]")opt

[member-declaration:](#nt:member-declaration "11.4.1 General [class.mem.general]")  
[*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]")opt [*member-declarator-list*](#nt:member-declarator-list "11.4.1 General [class.mem.general]")opt ;  
[*function-definition*](dcl.fct.def.general#nt:function-definition "9.6.1 General [dcl.fct.def.general]")  
[*friend-type-declaration*](#nt:friend-type-declaration "11.4.1 General [class.mem.general]")  
[*using-declaration*](namespace.udecl#nt:using-declaration "9.10 The using declaration [namespace.udecl]")  
[*using-enum-declaration*](enum.udecl#nt:using-enum-declaration "9.8.2 The using enum declaration [enum.udecl]")  
[*static_assert-declaration*](dcl.pre#nt:static_assert-declaration "9.1 Preamble [dcl.pre]")  
[*consteval-block-declaration*](dcl.pre#nt:consteval-block-declaration "9.1 Preamble [dcl.pre]")  
[*template-declaration*](temp.pre#nt:template-declaration "13.1 Preamble [temp.pre]")  
[*explicit-specialization*](temp.expl.spec#nt:explicit-specialization "13.9.4 Explicit specialization [temp.expl.spec]")  
[*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]")  
[*alias-declaration*](dcl.pre#nt:alias-declaration "9.1 Preamble [dcl.pre]")  
[*opaque-enum-declaration*](dcl.enum#nt:opaque-enum-declaration "9.8.1 Enumeration declarations [dcl.enum]")  
[*empty-declaration*](dcl.pre#nt:empty-declaration "9.1 Preamble [dcl.pre]")

[member-declarator-list:](#nt:member-declarator-list "11.4.1 General [class.mem.general]")  
[*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]")  
[*member-declarator-list*](#nt:member-declarator-list "11.4.1 General [class.mem.general]") , [*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]")

[member-declarator:](#nt:member-declarator "11.4.1 General [class.mem.general]")  
[*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") [*virt-specifier-seq*](#nt:virt-specifier-seq "11.4.1 General [class.mem.general]")opt [*function-contract-specifier-seq*](dcl.contract.func#nt:function-contract-specifier-seq "9.4.1 General [dcl.contract.func]")opt [*pure-specifier*](#nt:pure-specifier "11.4.1 General [class.mem.general]")opt  
[*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") [*requires-clause*](temp.pre#nt:requires-clause "13.1 Preamble [temp.pre]") [*function-contract-specifier-seq*](dcl.contract.func#nt:function-contract-specifier-seq "9.4.1 General [dcl.contract.func]")opt  
[*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")  
[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")opt [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt : [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")opt

[virt-specifier-seq:](#nt:virt-specifier-seq "11.4.1 General [class.mem.general]")  
[*virt-specifier*](#nt:virt-specifier "11.4.1 General [class.mem.general]") [*virt-specifier-seq*](#nt:virt-specifier-seq "11.4.1 General [class.mem.general]")opt

[virt-specifier:](#nt:virt-specifier "11.4.1 General [class.mem.general]")  
override  
final

[pure-specifier:](#nt:pure-specifier "11.4.1 General [class.mem.general]")  
= 0

[friend-type-declaration:](#nt:friend-type-declaration "11.4.1 General [class.mem.general]")  
friend [*friend-type-specifier-list*](#nt:friend-type-specifier-list "11.4.1 General [class.mem.general]") ;

[friend-type-specifier-list:](#nt:friend-type-specifier-list "11.4.1 General [class.mem.general]")  
[*friend-type-specifier*](#nt:friend-type-specifier "11.4.1 General [class.mem.general]") ...opt  
[*friend-type-specifier-list*](#nt:friend-type-specifier-list "11.4.1 General [class.mem.general]") , [*friend-type-specifier*](#nt:friend-type-specifier "11.4.1 General [class.mem.general]") ...opt

[friend-type-specifier:](#nt:friend-type-specifier "11.4.1 General [class.mem.general]")  
[*simple-type-specifier*](dcl.type.simple#nt:simple-type-specifier "9.2.9.3 Simple type specifiers [dcl.type.simple]")  
[*elaborated-type-specifier*](dcl.type.elab#nt:elaborated-type-specifier "9.2.9.5 Elaborated type specifiers [dcl.type.elab]")  
[*typename-specifier*](temp.res.general#nt:typename-specifier "13.8.1 General [temp.res.general]")

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L616)

In the absence of a [*virt-specifier-seq*](#nt:virt-specifier-seq "11.4.1 General [class.mem.general]"),
the token sequence = 0 is treated as a [*pure-specifier*](#nt:pure-specifier "11.4.1 General [class.mem.general]") if the type of the [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") ([[dcl.meaning.general]](dcl.meaning.general "9.3.4.1 General"))
is a function type, and
is otherwise treated as a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")[.](#general-1.sentence-1)

[*Note [1](#general-note-1)*:

If the member declaration acquires a function type through
template instantiation,
the program is ill-formed; see [[temp.spec.general]](temp.spec.general "13.9.1 General")[.](#general-1.sentence-2)

— *end note*]

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L628)

The optional [*function-contract-specifier-seq*](dcl.contract.func#nt:function-contract-specifier-seq "9.4.1 General [dcl.contract.func]") ([[dcl.contract.func]](dcl.contract.func "9.4.1 General"))
in a [*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]") shall be present only if
the [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") declares a function[.](#general-2.sentence-1)

[3](#general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L634)

The [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") in a class definition declares the
full set of members of the class; no member can be added elsewhere[.](#general-3.sentence-1)

A [*direct member*](#def:member,direct "11.4.1 General [class.mem.general]") of a class X is a member of X that was first declared within the [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of X,
including anonymous union members ([[class.union.anon]](class.union.anon "11.5.2 Anonymous unions")) and direct members thereof[.](#general-3.sentence-2)

Members of a class are data members, member
functions ([[class.mfct]](#class.mfct "11.4.2 Member functions")), nested types, enumerators,
and member templates ([[temp.mem]](temp.mem "13.7.3 Member templates")) and specializations thereof[.](#general-3.sentence-3)

[*Note [2](#general-note-2)*:

A specialization of a static data member template is a static data member[.](#general-3.sentence-4)

A specialization of a member function template is a member function[.](#general-3.sentence-5)

A specialization of a member class template is a nested class[.](#general-3.sentence-6)

— *end note*]

[4](#general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L650)

A [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") does not itself declare new members of the class
if it is

- [(4.1)](#general-4.1)

a friend declaration ([[class.friend]](class.friend "11.8.4 Friends")),

- [(4.2)](#general-4.2)

a [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]") ([[temp.deduct.guide]](temp.deduct.guide "13.7.2.3 Deduction guides")),

- [(4.3)](#general-4.3)

a [*template-declaration*](temp.pre#nt:template-declaration "13.1 Preamble [temp.pre]") whose [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]") is one of the above,

- [(4.4)](#general-4.4)

a [*static_assert-declaration*](dcl.pre#nt:static_assert-declaration "9.1 Preamble [dcl.pre]"),

- [(4.5)](#general-4.5)

a [*consteval-block-declaration*](dcl.pre#nt:consteval-block-declaration "9.1 Preamble [dcl.pre]"),

- [(4.6)](#general-4.6)

a [*using-declaration*](namespace.udecl#nt:using-declaration "9.10 The using declaration [namespace.udecl]") ([[namespace.udecl]](namespace.udecl "9.10 The using declaration")), or

- [(4.7)](#general-4.7)

an [*empty-declaration*](dcl.pre#nt:empty-declaration "9.1 Preamble [dcl.pre]")[.](#general-4.sentence-1)

For any other [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]"),
each declared entity
that is not an [unnamed bit-field](#def:bit-field,unnamed "11.4.10 Bit-fields [class.bit]") is a member of the class,
and each such [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") shall either
declare at least one member name of the class
or declare at least one unnamed bit-field[.](#general-4.sentence-2)

[5](#general-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L671)

A [*data member*](#def:data_member "11.4.1 General [class.mem.general]") is a non-function member introduced by a[*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]")[.](#general-5.sentence-1)

A [*member function*](#def:member_function "11.4.1 General [class.mem.general]") is a member that is a function[.](#general-5.sentence-2)

Nested types are classes ([[class.name]](class.name "11.3 Class names"), [[class.nest]](#class.nest "11.4.12 Nested class declarations")) and
enumerations ([[dcl.enum]](dcl.enum "9.8.1 Enumeration declarations")) declared in the class and arbitrary types
declared as members by use of a typedef declaration ([[dcl.typedef]](dcl.typedef "9.2.4 The typedef specifier"))
or [*alias-declaration*](dcl.pre#nt:alias-declaration "9.1 Preamble [dcl.pre]")[.](#general-5.sentence-3)

The enumerators of an [unscoped enumeration](dcl.enum "9.8.1 Enumeration declarations [dcl.enum]") defined in the class
are members of the class[.](#general-5.sentence-4)

[6](#general-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L682)

A data member or member function
may be declared static in its [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]"),
in which case it is a [*static member*](#def:member,static "11.4.1 General [class.mem.general]") (see [[class.static]](#class.static "11.4.9 Static members"))
(a [*static data member*](#def:data_member,static "11.4.1 General [class.mem.general]") ([[class.static.data]](#class.static.data "11.4.9.3 Static data members")) or[*static member function*](#def:member_function,static "11.4.1 General [class.mem.general]") ([[class.static.mfct]](#class.static.mfct "11.4.9.2 Static member functions")), respectively)
of the class[.](#general-6.sentence-1)

Any other data member or member function is a [*non-static member*](#def:member,non-static "11.4.1 General [class.mem.general]") (a [*non-static data member*](#def:data_member,non-static "11.4.1 General [class.mem.general]") or[*non-static member function*](#def:member_function,non-static "11.4.1 General [class.mem.general]") ([[class.mfct.non.static]](#class.mfct.non.static "11.4.3 Non-static member functions")), respectively)[.](#general-6.sentence-2)

[7](#general-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L693)

Every object of class type has a unique member subobject
corresponding to each of its direct non-static data members[.](#general-7.sentence-1)

If any non-static data member of a class C is of reference type,
then let D be an invented class
that is identical to C except that each non-static member of D corresponding to
a member of C of type “reference to T”
instead has type “pointer to T”[.](#general-7.sentence-2)

Every member subobject of a complete object of type C has the same size, alignment, and offset
as that of the corresponding subobject of a complete object of type D[.](#general-7.sentence-3)

The size and alignment of C are the same as
the size and alignment of D[.](#general-7.sentence-4)

[8](#general-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L708)

A member shall not be declared twice in the[*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]"), except that

- [(8.1)](#general-8.1)

a nested class or member
class template can be declared and then later defined, and

- [(8.2)](#general-8.2)

an
enumeration can be introduced with an [*opaque-enum-declaration*](dcl.enum#nt:opaque-enum-declaration "9.8.1 Enumeration declarations [dcl.enum]") and later
redeclared with an [*enum-specifier*](dcl.enum#nt:enum-specifier "9.8.1 Enumeration declarations [dcl.enum]")[.](#general-8.sentence-1)

[*Note [3](#general-note-3)*:

A single name can denote several member functions provided their types
are sufficiently different ([[basic.scope.scope]](basic.scope.scope "6.4.1 General"))[.](#general-8.sentence-2)

— *end note*]

[9](#general-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L723)

A redeclaration of a class member outside its class definition shall be
a definition,
an explicit specialization, or
an explicit instantiation ([[temp.expl.spec]](temp.expl.spec "13.9.4 Explicit specialization"), [[temp.explicit]](temp.explicit "13.9.3 Explicit instantiation"))[.](#general-9.sentence-1)

The member shall not be a non-static data member[.](#general-9.sentence-2)

[10](#general-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L730)

A [*complete-class context*](#def:complete-class_context "11.4.1 General [class.mem.general]") of a class (template) is a

- [(10.1)](#general-10.1)

function body ([[dcl.fct.def.general]](dcl.fct.def.general "9.6.1 General")),

- [(10.2)](#general-10.2)

default argument ([[dcl.fct.default]](dcl.fct.default "9.3.4.7 Default arguments")),

- [(10.3)](#general-10.3)

default template argument ([[temp.param]](temp.param "13.2 Template parameters")),

- [(10.4)](#general-10.4)

[*noexcept-specifier*](except.spec#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") ([[except.spec]](except.spec "14.5 Exception specifications")),

- [(10.5)](#general-10.5)

[*function-contract-specifier*](dcl.contract.func#nt:function-contract-specifier "9.4.1 General [dcl.contract.func]") ([[dcl.contract.func]](dcl.contract.func "9.4.1 General")), or

- [(10.6)](#general-10.6)

default member initializer

within the [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of the class or class template[.](#general-10.sentence-1)

[*Note [4](#general-note-4)*:

A complete-class context of a nested class is also a complete-class
context of any enclosing class, if the nested class is defined within
the [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of the enclosing class[.](#general-10.sentence-2)

— *end note*]

[11](#general-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L748)

A class C is complete at a program point P if the definition of C is reachable from P ([[module.reach]](module.reach "10.7 Reachability"))
or if P is in a complete-class context of C[.](#general-11.sentence-1)

Otherwise, C is incomplete at P[.](#general-11.sentence-2)

[12](#general-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L754)

If a [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") matches
the syntactic requirements of [*friend-type-declaration*](#nt:friend-type-declaration "11.4.1 General [class.mem.general]"),
it is a [*friend-type-declaration*](#nt:friend-type-declaration "11.4.1 General [class.mem.general]")[.](#general-12.sentence-1)

[13](#general-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L759)

In a [*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]"),
an = immediately following the [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") is interpreted as introducing a [*pure-specifier*](#nt:pure-specifier "11.4.1 General [class.mem.general]") if the [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") has function type,
otherwise it is interpreted as introducing
a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")[.](#general-13.sentence-1)

[*Example [1](#general-example-1)*: struct S {using T = void();
 T * p = 0; // OK, [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")virtual T f = 0; // OK, [*pure-specifier*](#nt:pure-specifier "11.4.1 General [class.mem.general]")}; — *end example*]

[14](#general-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L776)

In a [*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]") for a bit-field,
the [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") is parsed as
the longest sequence of tokens
that could syntactically form a [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]")[.](#general-14.sentence-1)

[*Example [2](#general-example-2)*: int a;const int b = 0;struct S {int x1 : 8 = 42; // OK, "= 42" is [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")int x2 : 8 { 42 }; // OK, "{ 42 }" is [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")int y1 : true ? 8 : a = 42; // OK, [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") is absentint y2 : true ? 8 : b = 42; // error: cannot assign to const intint y3 : (true ? 8 : b) = 42; // OK, "= 42" is [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")int z : 1 || new int { 0 }; // OK, [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") is absent}; — *end example*]

[15](#general-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L796)

A [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") shall appear only in the
declaration of a data member[.](#general-15.sentence-1)

(For static data members,
see [[class.static.data]](#class.static.data "11.4.9.3 Static data members"); for non-static data members,
see [[class.base.init]](class.base.init "11.9.3 Initializing bases and members") and [[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates"))[.](#general-15.sentence-2)

A [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") for a non-static data memberspecifies a [*default member initializer*](#def:default_member_initializer "11.4.1 General [class.mem.general]") for the member, and
shall not directly or indirectly cause the implicit definition of a
defaulted default constructor for the enclosing class or the
exception specification of that constructor[.](#general-15.sentence-3)

An immediate invocation ([[expr.const]](expr.const "7.7 Constant expressions")) that
is a potentially-evaluated subexpression ([[intro.execution]](intro.execution "6.10.1 Sequential execution"))
of a default member initializer
is neither evaluated nor checked for whether it
is a constant expression at the point where the subexpression appears[.](#general-15.sentence-4)

[16](#general-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L813)

A member shall not be declared with the extern[*storage-class-specifier*](dcl.stc#nt:storage-class-specifier "9.2.2 Storage class specifiers [dcl.stc]")[.](#general-16.sentence-1)

Within a class definition, a member shall not be declared with the thread_local [*storage-class-specifier*](dcl.stc#nt:storage-class-specifier "9.2.2 Storage class specifiers [dcl.stc]") unless also declared static[.](#general-16.sentence-2)

[17](#general-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L817)

The [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]") may be omitted in constructor, destructor,
and conversion function declarations only;
when declaring another kind of member the [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]") shall contain a [*type-specifier*](dcl.type.general#nt:type-specifier "9.2.9.1 General [dcl.type.general]") that is not a [*cv-qualifier*](dcl.decl.general#nt:cv-qualifier "9.3.1 General [dcl.decl.general]")[.](#general-17.sentence-1)

The[*member-declarator-list*](#nt:member-declarator-list "11.4.1 General [class.mem.general]") can be omitted only after a[*class-specifier*](class.pre#nt:class-specifier "11.1 Preamble [class.pre]") or an [*enum-specifier*](dcl.enum#nt:enum-specifier "9.8.1 Enumeration declarations [dcl.enum]") or in a[friend declaration](class.friend "11.8.4 Friends [class.friend]")[.](#general-17.sentence-2)

A[*pure-specifier*](#nt:pure-specifier "11.4.1 General [class.mem.general]") shall be used only in the declaration of a[virtual function](class.virtual "11.7.3 Virtual functions [class.virtual]") that is not a friend declaration[.](#general-17.sentence-3)

[18](#general-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L830)

The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") in a [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") appertains to each of the entities declared by the [*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]")*s*;
it shall not appear if the optional [*member-declarator-list*](#nt:member-declarator-list "11.4.1 General [class.mem.general]") is omitted[.](#general-18.sentence-1)

[19](#general-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L835)

A [*virt-specifier-seq*](#nt:virt-specifier-seq "11.4.1 General [class.mem.general]") shall contain at most one of each[*virt-specifier*](#nt:virt-specifier "11.4.1 General [class.mem.general]")[.](#general-19.sentence-1)

A [*virt-specifier-seq*](#nt:virt-specifier-seq "11.4.1 General [class.mem.general]") shall appear only in the first declaration of a virtual member
function ([[class.virtual]](class.virtual "11.7.3 Virtual functions"))[.](#general-19.sentence-2)

[20](#general-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L842)

The type of a non-static data member shall not be an
incomplete type ([[basic.types.general]](basic.types.general#term.incomplete.type "6.9.1 General")),
an abstract class type ([[class.abstract]](class.abstract "11.7.4 Abstract classes")),
or a (possibly multidimensional) array thereof[.](#general-20.sentence-1)

[*Note [5](#general-note-5)*:

In particular, a class C cannot contain
a non-static member of class C,
but it can contain a pointer or reference to an object of class C[.](#general-20.sentence-2)

— *end note*]

[21](#general-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L854)

[*Note [6](#general-note-6)*:

See [[expr.prim.id]](expr.prim.id "7.5.5 Names") for restrictions on the use of non-static data
members and non-static member functions[.](#general-21.sentence-1)

— *end note*]

[22](#general-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L860)

[*Note [7](#general-note-7)*:

The type of a non-static member function is an ordinary function type,
and the type of a non-static data member is an ordinary object type[.](#general-22.sentence-1)

There are no special member function types or data member types[.](#general-22.sentence-2)

— *end note*]

[23](#general-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L867)

[*Example [3](#general-example-3)*:

A simple example of a class definition isstruct tnode {char tword[20]; int count;
 tnode* left;
 tnode* right;}; which contains an array of twenty characters, an integer, and two
pointers to objects of the same type[.](#general-23.sentence-1)

Once this definition has been
given, the declarationtnode s, *sp; declares s to be a tnode and sp to be a pointer
to a tnode[.](#general-23.sentence-2)

With these declarations, sp->count refers to
the count member of the object to which sp points;s.left refers to the left subtree pointer of the objects; and s.right->tword[0] refers to the initial character
of the tword member of the right subtree of s[.](#general-23.sentence-3)

— *end example*]

[24](#general-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L892)

[*Note [8](#general-note-8)*:

Non-variant non-static data members of
non-zero size ([[intro.object]](intro.object "6.8.2 Object model"))
are allocated so that later
members have higher addresses within a class object ([[expr.rel]](expr.rel "7.6.9 Relational operators"))[.](#general-24.sentence-1)

Implementation alignment requirements can cause two adjacent members
not to be allocated immediately after each other; so can requirements
for space for managing virtual functions ([[class.virtual]](class.virtual "11.7.3 Virtual functions")) and
virtual base classes ([[class.mi]](class.mi "11.7.2 Multiple base classes"))[.](#general-24.sentence-2)

— *end note*]

[25](#general-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L905)

If T is the name of a class, then each of the following shall
have a name different from T:

- [(25.1)](#general-25.1)

every static data member of class T;

- [(25.2)](#general-25.2)

every member function of class T;
  [*Note [9](#general-note-9)*:
  This restriction does not apply to constructors, which do not have
names ([[class.ctor]](#class.ctor "11.4.5 Constructors"))[.](#general-25.2.sentence-1)
 — *end note*]

- [(25.3)](#general-25.3)

every member of class T that is itself a type;

- [(25.4)](#general-25.4)

every member template of class T;

- [(25.5)](#general-25.5)

every enumerator of every member of class T that is an
unscoped enumeration type; and

- [(25.6)](#general-25.6)

every member of every anonymous union that is a member of classT[.](#general-25.sentence-1)

[26](#general-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L928)

In addition, if class T has a user-declared[constructor](#class.ctor "11.4.5 Constructors [class.ctor]"), every non-static data member of classT shall have a name different from T[.](#general-26.sentence-1)

[27](#general-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L933)

The [*common initial sequence*](#def:common_initial_sequence "11.4.1 General [class.mem.general]") of two standard-layout struct ([[class.prop]](class.prop "11.2 Properties of classes"))
types is the longest sequence of non-static data
members and bit-fields in declaration order, starting with the first
such entity in each of the structs, such that

- [(27.1)](#general-27.1)

corresponding entities
have layout-compatible types ([[basic.types]](basic.types "6.9 Types")),

- [(27.2)](#general-27.2)

corresponding entities have the same alignment requirements ([[basic.align]](basic.align "6.8.3 Alignment")),

- [(27.3)](#general-27.3)

if a [*has-attribute-expression*](cpp.cond#nt:has-attribute-expression "15.2 Conditional inclusion [cpp.cond]") ([[cpp.cond]](cpp.cond "15.2 Conditional inclusion"))
is not 0 for the no_unique_address attribute,
then neither entity is declared with
the no_unique_address attribute ([[dcl.attr.nouniqueaddr]](dcl.attr.nouniqueaddr "9.13.11 No unique address attribute")), and

- [(27.4)](#general-27.4)

either both entities are bit-fields with the same width
or neither is a bit-field[.](#general-27.sentence-1)

[*Example [4](#general-example-4)*: struct A { int a; char b; };struct B { const int b1; volatile char b2; };struct C { int c; unsigned : 0; char b; };struct D { int d; char b : 4; };struct E { unsigned int e; char b; };

The common initial sequence of A and B comprises all members
of either class[.](#general-27.sentence-2)

The common initial sequence of A and C and
of A and D comprises the first member in each case[.](#general-27.sentence-3)

The common initial sequence of A and E is empty[.](#general-27.sentence-4)

— *end example*]

[28](#general-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L967)

Two standard-layout struct ([[class.prop]](class.prop "11.2 Properties of classes")) types are[*layout-compatible classes*](#def:layout-compatible,class "11.4.1 General [class.mem.general]") if
their common initial sequence comprises all members and bit-fields of
both classes ([[basic.types]](basic.types "6.9 Types"))[.](#general-28.sentence-1)

[29](#general-29)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L973)

Two standard-layout unions are layout-compatible if they
have the same number of non-static data members and corresponding
non-static data members (in any order) have layout-compatible
types ([[basic.types.general]](basic.types.general#term.layout.compatible.type "6.9.1 General"))[.](#general-29.sentence-1)

[30](#general-30)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L979)

In a standard-layout union with an [active member](class.union#def:active,union_member "11.5 Unions [class.union]") of struct type T1, it is permitted to read a non-static
data member m of another union member of struct type T2 provided m is part of the common initial sequence of T1 and T2;
the behavior is as if the corresponding member of T1 were nominated[.](#general-30.sentence-1)

[*Example [5](#general-example-5)*: struct T1 { int a, b; };struct T2 { int c; double d; };union U { T1 t1; T2 t2; };int f() { U u = { { 1, 2 } }; // active member is t1return u.t2.c; // OK, as if u.t1.a were nominated} — *end example*]

[*Note [10](#general-note-10)*:

Reading a volatile object through a glvalue of non-volatile type has
undefined behavior ([[dcl.type.cv]](dcl.type.cv "9.2.9.2 The cv-qualifiers"))[.](#general-30.sentence-2)

— *end note*]

[31](#general-31)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1001)

If a standard-layout class object has any non-static data members, its address
is the same as the address of its first non-static data member
if that member is not a bit-field[.](#general-31.sentence-1)

Its
address is also the same as the address of each of its base class subobjects[.](#general-31.sentence-2)

[*Note [11](#general-note-11)*:

There can therefore be unnamed padding within a standard-layout struct object
inserted by an implementation, but
not at its beginning, as necessary to achieve appropriate alignment[.](#general-31.sentence-3)

— *end note*]

[*Note [12](#general-note-12)*:

The object and its first subobject are
pointer-interconvertible ([[basic.compound]](basic.compound "6.9.4 Compound types"), [[expr.static.cast]](expr.static.cast "7.6.1.9 Static cast"))[.](#general-31.sentence-4)

— *end note*]

[32](#general-32)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1016)

A [*data member description*](#def:description,data_member "11.4.1 General [class.mem.general]") is
a quintuple (T, N, A, W, NUA)
describing the potential declaration of a non-static data member where

- [(32.1)](#general-32.1)

T is a type,

- [(32.2)](#general-32.2)

N is an [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") or ⊥,

- [(32.3)](#general-32.3)

A is an alignment or ⊥,

- [(32.4)](#general-32.4)

W is a bit-field width or ⊥, and

- [(32.5)](#general-32.5)

NUA is a boolean value[.](#general-32.sentence-1)

Two data member descriptions are equal
if each of their respective components are the same entities,
are the same identifiers, have equal values, or are both ⊥[.](#general-32.sentence-2)

[*Note [13](#general-note-13)*:

The components of a data member description describe a data member such that

- [(32.6)](#general-32.6)

its type is specified using the type given by T,

- [(32.7)](#general-32.7)

it is declared with the name given by N if N is not ⊥ and is otherwise unnamed,

- [(32.8)](#general-32.8)

it is declared with the [*alignment-specifier*](dcl.attr.grammar#nt:alignment-specifier "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") ([[dcl.align]](dcl.align "9.13.2 Alignment specifier"))
given by alignas(A) if A is not ⊥ and
is otherwise declared without an [*alignment-specifier*](dcl.attr.grammar#nt:alignment-specifier "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]"),

- [(32.9)](#general-32.9)

it is a bit-field ([[class.bit]](#class.bit "11.4.10 Bit-fields")) with the width given by W if W is not ⊥ and is otherwise not a bit-field, and

- [(32.10)](#general-32.10)

it is declared with
the attribute [[no_unique_address]] ([[dcl.attr.nouniqueaddr]](dcl.attr.nouniqueaddr "9.13.11 No unique address attribute"))
if NUA is true and is otherwise declared without that attribute[.](#general-32.sentence-3)

Data member descriptions are represented by reflections ([[basic.fundamental]](basic.fundamental "6.9.2 Fundamental types"))
returned by std​::​meta​::​data_member_spec ([[meta.reflection.define.aggregate]](meta.reflection.define.aggregate "21.4.16 Reflection class definition generation")) and
can be reified as data members of a class using std​::​meta​::​define_aggregate[.](#general-32.sentence-4)

— *end note*]

### [11.4.2](#class.mfct) Member functions [[class.mfct]](class.mfct)

[1](#class.mfct-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1059)

If a member function is attached to the global module and is defined ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions")) in its class definition,
it is inline ([[dcl.inline]](dcl.inline "9.2.8 The inline specifier"))[.](#class.mfct-1.sentence-1)

[*Note [1](#class.mfct-note-1)*:

A member function is also inline if it is declaredinline, constexpr, or consteval[.](#class.mfct-1.sentence-2)

— *end note*]

[2](#class.mfct-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1069)

[*Example [1](#class.mfct-example-1)*: struct X {typedef int T; static T count; void f(T);};void X::f(T t = count) { }

The definition of the member function f of class X inhabits the global
scope; the notation X​::​f indicates that the function f is a member of class X and in the scope of class X[.](#class.mfct-2.sentence-1)

In
the function definition, the parameter type T refers to the
typedef member T declared in class X and the default
argument count refers to the static data member count declared in class X[.](#class.mfct-2.sentence-2)

— *end example*]

[3](#class.mfct-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1090)

Member functions of a local class shall be defined inline in their class
definition, if they are defined at all[.](#class.mfct-3.sentence-1)

[4](#class.mfct-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1095)

[*Note [2](#class.mfct-note-2)*:

A member function can be declared (but not defined) using a typedef for
a function type[.](#class.mfct-4.sentence-1)

The resulting member function has exactly the same type
as it would have if the function declarator were provided explicitly,
see [[dcl.fct]](dcl.fct "9.3.4.6 Functions") and [[temp.arg]](temp.arg "13.4 Template arguments")[.](#class.mfct-4.sentence-2)

[*Example [2](#class.mfct-example-2)*: typedef void fv();typedef void fvc() const;struct S { fv memfunc1; // equivalent to: void memfunc1();void memfunc2();
 fvc memfunc3; // equivalent to: void memfunc3() const;};
fv S::* pmfv1 = &S::memfunc1;
fv S::* pmfv2 = &S::memfunc2;
fvc S::* pmfv3 = &S::memfunc3; — *end example*]

— *end note*]

### [11.4.3](#class.mfct.non.static) Non-static member functions [[class.mfct.non.static]](class.mfct.non.static)

[1](#class.mfct.non.static-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1121)

A non-static member function may be called for an object of
its class type, or for an object of a class derived ([[class.derived]](class.derived "11.7 Derived classes"))
from its class type, using the class member
access syntax ([[expr.ref]](expr.ref "7.6.1.5 Class member access"), [[over.match.call]](over.match.call "12.2.2.2 Function call syntax"))[.](#class.mfct.non.static-1.sentence-1)

A non-static
member function may also be called directly using the function call
syntax ([[expr.call]](expr.call "7.6.1.3 Function call"), [[over.match.call]](over.match.call "12.2.2.2 Function call syntax")) from within
its class or a class derived from its class, or
a member thereof, as described below[.](#class.mfct.non.static-1.sentence-2)

[2](#class.mfct.non.static-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1131)

[*Note [1](#class.mfct.non.static-note-1)*:

An implicit object member function can be declared with[*cv-qualifier*](dcl.decl.general#nt:cv-qualifier "9.3.1 General [dcl.decl.general]")*s*, which affect the type of the this pointer ([[expr.prim.this]](expr.prim.this "7.5.3 This")),
and/or a [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]") ([[dcl.fct]](dcl.fct "9.3.4.6 Functions"));
both affect overload resolution ([[over.match.funcs]](over.match.funcs "12.2.2 Candidate functions and argument lists"))[.](#class.mfct.non.static-2.sentence-1)

— *end note*]

[3](#class.mfct.non.static-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1143)

An implicit object member function may be declared
virtual ([[class.virtual]](class.virtual "11.7.3 Virtual functions")) or pure virtual ([[class.abstract]](class.abstract "11.7.4 Abstract classes"))[.](#class.mfct.non.static-3.sentence-1)

### [11.4.4](#special) Special member functions [[special]](special)

[1](#special-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1154)

Default constructors ([[class.default.ctor]](#class.default.ctor "11.4.5.2 Default constructors")),
copy constructors, move constructors ([[class.copy.ctor]](#class.copy.ctor "11.4.5.3 Copy/move constructors")),
copy assignment operators, move assignment operators ([[class.copy.assign]](#class.copy.assign "11.4.6 Copy/move assignment operator")),
and prospective destructors ([[class.dtor]](#class.dtor "11.4.7 Destructors")) are[*special member functions*](#def:special_member_functions)[.](#special-1.sentence-1)

[*Note [1](#special-note-1)*:

The implementation will implicitly declare these member functions for some class
types when the program does not explicitly declare them[.](#special-1.sentence-2)

The implementation will implicitly define them
as needed ([[dcl.fct.def.default]](dcl.fct.def.default "9.6.2 Explicitly-defaulted functions"))[.](#special-1.sentence-3)

— *end note*]

An implicitly-declared special member function is declared at the closing} of the [*class-specifier*](class.pre#nt:class-specifier "11.1 Preamble [class.pre]")[.](#special-1.sentence-4)

Programs shall not define implicitly-declared special member functions[.](#special-1.sentence-5)

[2](#special-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1175)

Programs may explicitly refer to implicitly-declared special member functions[.](#special-2.sentence-1)

[*Example [1](#special-example-1)*:

A program may explicitly call or form a pointer to member
to an implicitly-declared special member function[.](#special-2.sentence-2)

struct A { }; // implicitly declared A​::​operator=struct B : A { B& operator=(const B &);};
B& B::operator=(const B& s) {this->A::operator=(s); // well-formedreturn *this;} — *end example*]

[3](#special-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1193)

[*Note [2](#special-note-2)*:

The special member functions affect the way objects of class type are created,
copied, moved, and destroyed, and how values can be converted to values of other types[.](#special-3.sentence-1)

Often such special member functions are called implicitly[.](#special-3.sentence-2)

— *end note*]

[4](#special-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1200)

Special member functions obey the usual access rules ([[class.access]](class.access "11.8 Member access control"))[.](#special-4.sentence-1)

[*Example [2](#special-example-2)*:

Declaring a constructor protected
ensures that only derived classes and friends can create objects using it[.](#special-4.sentence-2)

— *end example*]

[5](#special-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1208)

Two special member functions are of the same kind if

- [(5.1)](#special-5.1)

they are both default constructors,

- [(5.2)](#special-5.2)

they are both copy or move constructors
with the same first parameter type, or

- [(5.3)](#special-5.3)

they are both copy or move assignment operators
with the same first parameter type
and the same [*cv-qualifier*](dcl.decl.general#nt:cv-qualifier "9.3.1 General [dcl.decl.general]")*s* and [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]"), if any[.](#special-5.sentence-1)

[6](#special-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1219)

An [*eligible special member function*](#def:special_member_function,eligible "11.4.4 Special member functions [special]") is a special member function for which:

- [(6.1)](#special-6.1)

the function is not deleted,

- [(6.2)](#special-6.2)

the associated constraints ([[temp.constr]](temp.constr "13.5 Template constraints")), if any, are satisfied, and

- [(6.3)](#special-6.3)

no special member function of the same kind
whose associated constraints, if any, are satisfied
is more constrained ([[temp.constr.order]](temp.constr.order "13.5.5 Partial ordering by constraints"))[.](#special-6.sentence-1)

[7](#special-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1229)

For a class, its non-static data members, its non-virtual direct base classes,
and, if the class is not abstract ([[class.abstract]](class.abstract "11.7.4 Abstract classes")), its virtual base
classes are called its [*potentially constructed subobjects*](#def:potentially_constructed_subobjects)[.](#special-7.sentence-1)

### [11.4.5](#class.ctor) Constructors [[class.ctor]](class.ctor)

#### [11.4.5.1](#class.ctor.general) General [[class.ctor.general]](class.ctor.general)

[1](#class.ctor.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1240)

A [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") declares a [*constructor*](#def:constructor "11.4.5.1 General [class.ctor.general]") if it is a
function declarator ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")) of the form

[*ptr-declarator*](dcl.decl.general#nt:ptr-declarator "9.3.1 General [dcl.decl.general]") ( [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") ) [*noexcept-specifier*](except.spec#nt:noexcept-specifier "14.5 Exception specifications [except.spec]")opt [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt

where the [*ptr-declarator*](dcl.decl.general#nt:ptr-declarator "9.3.1 General [dcl.decl.general]") consists solely of an[*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]"), an optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]"),
and optional surrounding parentheses, and the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") has
one of the following forms:

- [(1.1)](#class.ctor.general-1.1)

in a friend declaration ([[class.friend]](class.friend "11.8.4 Friends")),
the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is a [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") that names a constructor ([[class.qual]](class.qual "6.5.5.2 Class members"));

- [(1.2)](#class.ctor.general-1.2)

otherwise, in a [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") that belongs to the[*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of a class or class template,
the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is the
injected-class-name ([[class.pre]](class.pre "11.1 Preamble")) of the immediately-enclosing entity;

- [(1.3)](#class.ctor.general-1.3)

otherwise, the[*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is a [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") whose [*unqualified-id*](expr.prim.id.unqual#nt:unqualified-id "7.5.5.2 Unqualified names [expr.prim.id.unqual]") is
the injected-class-name of its lookup context[.](#class.ctor.general-1.sentence-1)

Constructors do not have names[.](#class.ctor.general-1.sentence-2)

In a constructor declaration, each [*decl-specifier*](dcl.spec.general#nt:decl-specifier "9.2.1 General [dcl.spec.general]") in the optional[*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]") shall befriend,inline,constexpr,consteval, or
an [*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]")[.](#class.ctor.general-1.sentence-3)

[*Example [1](#class.ctor.general-example-1)*: struct S { S(); // declares the constructor};

S::S() { } // defines the constructor — *end example*]

[2](#class.ctor.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1285)

A constructor is used to initialize objects of its class type[.](#class.ctor.general-2.sentence-1)

[*Note [1](#class.ctor.general-note-1)*:

Because constructors do not have names, they are never found during
unqualified name lookup; however an explicit type conversion using the functional
notation ([[expr.type.conv]](expr.type.conv "7.6.1.4 Explicit type conversion (functional notation)")) will cause a constructor to be called to
initialize an object[.](#class.ctor.general-2.sentence-2)

The syntax looks like an explicit call of the constructor[.](#class.ctor.general-2.sentence-3)

— *end note*]

[*Example [2](#class.ctor.general-example-2)*: complex zz = complex(1,2.3);
cprint( complex(7.8,1.2) ); — *end example*]

[*Note [2](#class.ctor.general-note-2)*:

For initialization of objects of class type see [[class.init]](class.init "11.9 Initialization")[.](#class.ctor.general-2.sentence-4)

— *end note*]

[3](#class.ctor.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1305)

An object created in this way is unnamed[.](#class.ctor.general-3.sentence-1)

[*Note [3](#class.ctor.general-note-3)*:

[[class.temporary]](class.temporary "6.8.7 Temporary objects") describes the lifetime of temporary objects[.](#class.ctor.general-3.sentence-2)

— *end note*]

[*Note [4](#class.ctor.general-note-4)*:

Explicit constructor calls do not yield lvalues, see [[basic.lval]](basic.lval "7.2.1 Value category")[.](#class.ctor.general-3.sentence-3)

— *end note*]

[4](#class.ctor.general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1315)

[*Note [5](#class.ctor.general-note-5)*:

Some language constructs have special semantics when used during construction;
see [[class.base.init]](class.base.init "11.9.3 Initializing bases and members") and [[class.cdtor]](class.cdtor "11.9.5 Construction and destruction")[.](#class.ctor.general-4.sentence-1)

— *end note*]

[5](#class.ctor.general-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1322)

A constructor can be invoked for aconst,volatile orconstvolatile object[.](#class.ctor.general-5.sentence-1)

const andvolatile semantics ([[dcl.type.cv]](dcl.type.cv "9.2.9.2 The cv-qualifiers")) are not applied on an object under construction[.](#class.ctor.general-5.sentence-2)

They come into effect when the constructor for the
most derived object ([[intro.object]](intro.object "6.8.2 Object model")) ends[.](#class.ctor.general-5.sentence-3)

[6](#class.ctor.general-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1340)

The address of a constructor shall not be taken[.](#class.ctor.general-6.sentence-1)

[*Note [6](#class.ctor.general-note-6)*:

A return statement in the body of a constructor
cannot specify a return value ([[stmt.return]](stmt.return "8.8.4 The return statement"))[.](#class.ctor.general-6.sentence-2)

— *end note*]

[7](#class.ctor.general-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1349)

A constructor shall not be a coroutine[.](#class.ctor.general-7.sentence-1)

[8](#class.ctor.general-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1352)

A constructor shall not have an explicit object parameter ([[dcl.fct]](dcl.fct "9.3.4.6 Functions"))[.](#class.ctor.general-8.sentence-1)

#### [11.4.5.2](#class.default.ctor) Default constructors [[class.default.ctor]](class.default.ctor)

[1](#class.default.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1357)

A [*default constructor*](#def:constructor,default "11.4.5.2 Default constructors [class.default.ctor]") for a class X is a constructor of class X for which each parameter
that is not a function parameter pack
has a default argument
(including the case of a constructor with no parameters)[.](#class.default.ctor-1.sentence-1)

If there is no user-declared constructor or constructor template for classX,
a non-explicit constructor having no parameters is implicitly declared
as defaulted ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions"))[.](#class.default.ctor-1.sentence-2)

An implicitly-declared default constructor is an
inline public member of its class[.](#class.default.ctor-1.sentence-3)

[2](#class.default.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1374)

A defaulted default constructor for class X is defined as deleted if

- [(2.1)](#class.default.ctor-2.1)

any non-static data member with no default member initializer ([class.mem]) is
of reference type,

- [(2.2)](#class.default.ctor-2.2)

X is a non-union class and
any non-variant non-static data member of const-qualified type
(or possibly multidimensional array thereof)
with no [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") is not const-default-constructible ([[dcl.init]](dcl.init "9.5 Initializers")),

- [(2.3)](#class.default.ctor-2.3)

any non-variant potentially constructed subobject, except for a non-static data member
with a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]"),
has class type M (or possibly multidimensional array thereof)
and overload resolution ([[over.match]](over.match "12.2 Overload resolution"))
as applied to find M's corresponding constructor
does not result in a usable candidate ([[over.match.general]](over.match.general "12.2.1 General")), or

- [(2.4)](#class.default.ctor-2.4)

any potentially constructed subobject S has
class type M (or possibly multidimensional array thereof),M has
a destructor that is deleted or inaccessible from the defaulted default
constructor, and
either S is non-variant or S has a default member initializer[.](#class.default.ctor-2.sentence-1)

[3](#class.default.ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1401)

A default constructor for a class X is[*trivial*](#def:constructor,default,trivial "11.4.5.2 Default constructors [class.default.ctor]") if it is not user-provided and if

- [(3.1)](#class.default.ctor-3.1)

X has no virtual functions ([[class.virtual]](class.virtual "11.7.3 Virtual functions")) and no virtual base
classes ([[class.mi]](class.mi "11.7.2 Multiple base classes")), and

- [(3.2)](#class.default.ctor-3.2)

no non-static data member of X has
a default member initializer ([class.mem]), and

- [(3.3)](#class.default.ctor-3.3)

all the direct base classes of X have trivial default constructors, and

- [(3.4)](#class.default.ctor-3.4)

either X is a union or
for all the non-variant non-static data members of X that are of class
type (or array thereof), each such class has a trivial default constructor[.](#class.default.ctor-3.sentence-1)

Otherwise, the default constructor is[*non-trivial*](#def:constructor,default,non-trivial "11.4.5.2 Default constructors [class.default.ctor]")[.](#class.default.ctor-3.sentence-2)

[4](#class.default.ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1425)

If a default constructor of a union-like class X is trivial,
then for each union U that is either X or an anonymous union member of X,
if the first variant member, if any, of U has implicit-lifetime type ([[basic.types.general]](basic.types.general "6.9.1 General")),
the default constructor of X begins the lifetime of that member
if it is not the active member of its union[.](#class.default.ctor-4.sentence-1)

[*Note [1](#class.default.ctor-note-1)*:

It is already the active member if U was value-initialized[.](#class.default.ctor-4.sentence-2)

— *end note*]

Otherwise,
an implicitly-defined ([[dcl.fct.def.default]](dcl.fct.def.default "9.6.2 Explicitly-defaulted functions")) default constructor performs the set of
initializations of the class that would be performed by a user-written default
constructor for that class with no[*ctor-initializer*](class.base.init#nt:ctor-initializer "11.9.3 Initializing bases and members [class.base.init]") ([[class.base.init]](class.base.init "11.9.3 Initializing bases and members")) and an empty[*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]")[.](#class.default.ctor-4.sentence-3)

If that user-written default constructor would be ill-formed,
the program is ill-formed[.](#class.default.ctor-4.sentence-4)

If that user-written default constructor would be constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers")),
the implicitly-defined
default constructor is constexpr[.](#class.default.ctor-4.sentence-5)

Before the defaulted default constructor for a class is
implicitly defined,
all the non-user-provided default constructors for its base classes and
its non-static data members are implicitly defined[.](#class.default.ctor-4.sentence-6)

[*Note [2](#class.default.ctor-note-2)*:

An implicitly-declared default constructor has an
exception specification ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#class.default.ctor-4.sentence-7)

An explicitly-defaulted definition might have an
implicit exception specification, see [[dcl.fct.def]](dcl.fct.def "9.6 Function definitions")[.](#class.default.ctor-4.sentence-8)

— *end note*]

[5](#class.default.ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1458)

[*Note [3](#class.default.ctor-note-3)*:

A default constructor is implicitly invoked to initialize
a class object when no initializer is specified ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#class.default.ctor-5.sentence-1)

Such a default constructor needs to be accessible ([[class.access]](class.access "11.8 Member access control"))[.](#class.default.ctor-5.sentence-2)

— *end note*]

[6](#class.default.ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1466)

[*Note [4](#class.default.ctor-note-4)*:

[[class.base.init]](class.base.init "11.9.3 Initializing bases and members") describes the order in which constructors for base
classes and non-static data members are called and
describes how arguments can be specified for the calls to these constructors[.](#class.default.ctor-6.sentence-1)

— *end note*]

#### [11.4.5.3](#class.copy.ctor) Copy/move constructors [[class.copy.ctor]](class.copy.ctor)

[1](#class.copy.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1477)

A non-template constructor for classX is
a
copy
constructor if its first parameter is of typeX&,const X&,volatile X& orconst volatile X&,
and either there are no other parameters
or else all other parameters have default arguments ([[dcl.fct.default]](dcl.fct.default "9.3.4.7 Default arguments"))[.](#class.copy.ctor-1.sentence-1)

[*Example [1](#class.copy.ctor-example-1)*:

X​::​X(const X&) andX​::​X(X&,int=1) are copy constructors[.](#class.copy.ctor-1.sentence-2)

struct X { X(int);
 X(const X&, int = 1);};
X a(1); // calls X(int); X b(a, 0); // calls X(const X&, int); X c = b; // calls X(const X&, int); — *end example*]

[2](#class.copy.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1512)

A non-template constructor for class X is a move constructor if its
first parameter is of type X&&, const X&&,volatile X&&, or const volatile X&&, and either there are
no other parameters or else all other parameters have default
arguments ([[dcl.fct.default]](dcl.fct.default "9.3.4.7 Default arguments"))[.](#class.copy.ctor-2.sentence-1)

[*Example [2](#class.copy.ctor-example-2)*:

Y​::​Y(Y&&) is a move constructor[.](#class.copy.ctor-2.sentence-2)

struct Y { Y(const Y&);
 Y(Y&&);};extern Y f(int);
Y d(f(1)); // calls Y(Y&&) Y e = d; // calls Y(const Y&) — *end example*]

[3](#class.copy.ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1531)

[*Note [1](#class.copy.ctor-note-1)*:

All forms of copy/move constructor can be declared for a class[.](#class.copy.ctor-3.sentence-1)

[*Example [3](#class.copy.ctor-example-3)*: struct X { X(const X&);
 X(X&); // OK X(X&&);
 X(const X&&); // OK, but possibly not sensible}; — *end example*]

— *end note*]

[4](#class.copy.ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1546)

[*Note [2](#class.copy.ctor-note-2)*:

If a classX only has a copy constructor with a parameter of typeX&,
an initializer of typeconstX orvolatileX cannot initialize an object of typecv X[.](#class.copy.ctor-4.sentence-1)

[*Example [4](#class.copy.ctor-example-4)*: struct X { X(); // default constructor X(X&); // copy constructor with a non-const parameter};const X cx;
X x = cx; // error: X​::​X(X&) cannot copy cx into x — *end example*]

— *end note*]

[5](#class.copy.ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1572)

A declaration of a constructor for a classX is ill-formed if its first parameter is of typecv X and either there are no other parameters or else all other parameters have
default arguments[.](#class.copy.ctor-5.sentence-1)

A member function template is never instantiated to
produce such a constructor signature[.](#class.copy.ctor-5.sentence-2)

[*Example [5](#class.copy.ctor-example-5)*: struct S {template<typename T> S(T);
 S();};

S g;

void h() { S a(g); // does not instantiate the member template to produce S​::​S<S>(S);// uses the implicitly declared copy constructor} — *end example*]

[6](#class.copy.ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1597)

If the class definition does not explicitly declare a copy constructor,
a non-explicit one is declared [*implicitly*](#def:constructor,copy,implicitly_declared "11.4.5.3 Copy/move constructors [class.copy.ctor]")[.](#class.copy.ctor-6.sentence-1)

If the class definition declares a move
constructor or move assignment operator, the implicitly declared copy
constructor is defined as deleted; otherwise, it is
defaulted ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions"))[.](#class.copy.ctor-6.sentence-2)

The latter case is deprecated if the class has a user-declared copy assignment
operator or a user-declared destructor ([[depr.impldec]](depr.impldec "D.6 Implicit declaration of copy functions"))[.](#class.copy.ctor-6.sentence-3)

[7](#class.copy.ctor-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1607)

The implicitly-declared copy constructor for a classX will have the formX::X(const X&) if each potentially constructed subobject of a class typeM (or array thereof)
has a copy constructor whose first parameter is of typeconstM& orconstvolatileM&[.](#class.copy.ctor-7.sentence-1)[89](#footnote-89 "This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [diff.class].")

Otherwise, the implicitly-declared copy constructor will have the formX::X(X&)

[8](#class.copy.ctor-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1636)

If the definition of a class X does not explicitly declare
a move constructor, a non-explicit one will be
implicitly declared as defaulted if and only if

- [(8.1)](#class.copy.ctor-8.1)

X does not have a user-declared copy constructor,

- [(8.2)](#class.copy.ctor-8.2)

X does not have a user-declared copy assignment operator,

- [(8.3)](#class.copy.ctor-8.3)

X does not have a user-declared move assignment operator, and

- [(8.4)](#class.copy.ctor-8.4)

X does not have a user-declared destructor[.](#class.copy.ctor-8.sentence-1)

[*Note [3](#class.copy.ctor-note-3)*:

When the move constructor is not implicitly declared or explicitly supplied,
expressions that otherwise would have invoked the move constructor might instead invoke
a copy constructor[.](#class.copy.ctor-8.sentence-2)

— *end note*]

[9](#class.copy.ctor-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1661)

The implicitly-declared move constructor for class X will have the formX::X(X&&)

[10](#class.copy.ctor-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1667)

An implicitly-declared copy/move constructor is an
inline public member of its class[.](#class.copy.ctor-10.sentence-1)

A defaulted copy/​move constructor for a class X is defined as deleted ([[dcl.fct.def.delete]](dcl.fct.def.delete "9.6.3 Deleted definitions")) if X has:

- [(10.1)](#class.copy.ctor-10.1)

a potentially constructed subobject of type M (or possibly multidimensional array thereof) for which
 overload resolution ([[over.match]](over.match "12.2 Overload resolution")), as applied to find M's corresponding constructor,
 either does not result in a usable candidate ([[over.match.general]](over.match.general "12.2.1 General")) or,
 in the case of a variant member, selects a non-trivial function,

- [(10.2)](#class.copy.ctor-10.2)

any potentially constructed subobject of
 class type M (or possibly multidimensional array thereof)
 where M has
 a destructor that is deleted or inaccessible from the defaulted
 constructor, or,

- [(10.3)](#class.copy.ctor-10.3)

for the copy constructor, a non-static data member of rvalue reference type[.](#class.copy.ctor-10.sentence-2)

[*Note [4](#class.copy.ctor-note-4)*:

A defaulted move constructor that is defined as deleted is ignored by overload
resolution ([[over.match]](over.match "12.2 Overload resolution"), [[over.over]](over.over "12.3 Address of an overload set"))[.](#class.copy.ctor-10.sentence-3)

Such a constructor would otherwise interfere with initialization from
an rvalue which can use the copy constructor instead[.](#class.copy.ctor-10.sentence-4)

— *end note*]

[11](#class.copy.ctor-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1696)

A copy/move constructor for classX is
trivial
if it is not user-provided and if

- [(11.1)](#class.copy.ctor-11.1)

classX has no virtual functions ([[class.virtual]](class.virtual "11.7.3 Virtual functions"))
and no virtual base classes ([[class.mi]](class.mi "11.7.2 Multiple base classes")), and

- [(11.2)](#class.copy.ctor-11.2)

the constructor selected to copy/move each direct base class subobject is trivial, and

- [(11.3)](#class.copy.ctor-11.3)

for each non-static data member ofX that is of class type (or array thereof),
the constructor selected to copy/move that member is trivial;

otherwise the copy/move constructor is[*non-trivial*](#def:constructor,copy,nontrivial "11.4.5.3 Copy/move constructors [class.copy.ctor]")[.](#class.copy.ctor-11.sentence-1)

[12](#class.copy.ctor-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1724)

[*Note [5](#class.copy.ctor-note-5)*:

The copy/move constructor is implicitly defined even if the implementation elided
its odr-use ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule"), [[class.temporary]](class.temporary "6.8.7 Temporary objects"))[.](#class.copy.ctor-12.sentence-1)

— *end note*]

If an implicitly-defined ([[dcl.fct.def.default]](dcl.fct.def.default "9.6.2 Explicitly-defaulted functions")) constructor would be constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers")),
the implicitly-defined
constructor is constexpr[.](#class.copy.ctor-12.sentence-2)

[13](#class.copy.ctor-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1733)

Before the defaulted copy/move constructor for a class is
implicitly defined,
all non-user-provided copy/move constructors for its
potentially constructed subobjects
are implicitly defined[.](#class.copy.ctor-13.sentence-1)

[*Note [6](#class.copy.ctor-note-6)*:

An implicitly-declared copy/move constructor has an
implied exception specification ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#class.copy.ctor-13.sentence-2)

— *end note*]

[14](#class.copy.ctor-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1744)

The implicitly-defined copy/move constructor for a non-union classX performs a memberwise copy/move of its bases and members[.](#class.copy.ctor-14.sentence-1)

[*Note [7](#class.copy.ctor-note-7)*:

Default member initializers of non-static data members are ignored[.](#class.copy.ctor-14.sentence-2)

— *end note*]

The order of initialization is the same as the order of initialization of bases
and members in a user-defined constructor (see [[class.base.init]](class.base.init "11.9.3 Initializing bases and members"))[.](#class.copy.ctor-14.sentence-3)

Let x be either the parameter of the constructor or, for the move constructor, an
xvalue referring to the parameter[.](#class.copy.ctor-14.sentence-4)

Each base or non-static data member
is copied/moved in the manner appropriate to its type:

- [(14.1)](#class.copy.ctor-14.1)

if the member is an array, each element is
direct-initialized with the corresponding subobject of x;

- [(14.2)](#class.copy.ctor-14.2)

if a member m has rvalue reference type T&&, it is direct-initialized withstatic_cast<T&&>(x.m);

- [(14.3)](#class.copy.ctor-14.3)

otherwise, the base or member is direct-initialized with the corresponding base or member of x[.](#class.copy.ctor-14.sentence-5)

Virtual base class subobjects shall be initialized only once by
the implicitly-defined copy/move constructor (see [[class.base.init]](class.base.init "11.9.3 Initializing bases and members"))[.](#class.copy.ctor-14.sentence-6)

[15](#class.copy.ctor-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1774)

The implicitly-defined copy/move constructor for a unionX copies the object representation ([[basic.types.general]](basic.types.general#term.object.representation "6.9.1 General")) of X[.](#class.copy.ctor-15.sentence-1)

For each object nested within ([[intro.object]](intro.object "6.8.2 Object model"))
the object that is the source of the copy,
a corresponding object o nested within the destination
is identified (if the object is a subobject) or created (otherwise),
and the lifetime of o begins before the copy is performed[.](#class.copy.ctor-15.sentence-2)

[89)](#footnote-89)[89)](#footnoteref-89)

This implies that the reference parameter of the
implicitly-declared copy constructor
cannot bind to avolatile lvalue; see [[diff.class]](diff.class "C.7.7 [class]: classes")[.](#footnote-89.sentence-1)

### [11.4.6](#class.copy.assign) Copy/move assignment operator [[class.copy.assign]](class.copy.assign)

[1](#class.copy.assign-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1787)

A user-declared [*copy*](#def:copy) assignment operator X​::​operator= is a
non-static non-template member function of class X with exactly one
non-object parameter of type X, X&, const X&,volatile X&, or const volatile X&[.](#class.copy.assign-1.sentence-1)[90](#footnote-90 "Because a template assignment operator or an assignment operator taking an rvalue reference parameter is never a copy assignment operator, the presence of such an assignment operator does not suppress the implicit declaration of a copy assignment operator. Such assignment operators participate in overload resolution with other assignment operators, including copy assignment operators, and, if selected, will be used to assign an object.")

[*Note [1](#class.copy.assign-note-1)*:

More than one form of copy assignment operator can be declared for a class[.](#class.copy.assign-1.sentence-2)

— *end note*]

[*Note [2](#class.copy.assign-note-2)*:

If a classX only has a copy assignment operator with a non-object parameter of typeX&,
an expression of type constX cannot be assigned to an object of typeX[.](#class.copy.assign-1.sentence-3)

[*Example [1](#class.copy.assign-example-1)*: struct X { X();
 X& operator=(X&);};const X cx;
X x;void f() { x = cx; // error: X​::​operator=(X&) cannot assign cx into x} — *end example*]

— *end note*]

[2](#class.copy.assign-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1835)

If the class definition does not explicitly declare a copy assignment operator,
one is declared [*implicitly*](#def:assignment_operator,copy,implicitly_declared "11.4.6 Copy/move assignment operator [class.copy.assign]")[.](#class.copy.assign-2.sentence-1)

If the class definition declares a move
constructor or move assignment operator, the implicitly declared copy
assignment operator is defined as deleted; otherwise, it is
defaulted ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions"))[.](#class.copy.assign-2.sentence-2)

The latter case is deprecated if the class has a user-declared copy constructor
or a user-declared destructor ([[depr.impldec]](depr.impldec "D.6 Implicit declaration of copy functions"))[.](#class.copy.assign-2.sentence-3)

The implicitly-declared copy assignment operator for a classX will have the formX& X::operator=(const X&) if

- [(2.1)](#class.copy.assign-2.1)

each direct base class B of X has a copy assignment operator whose non-object parameter is of typeconst B&, const volatile B&, or B, and

- [(2.2)](#class.copy.assign-2.2)

for all the non-static data members of X that are of a class type M (or array thereof),
each such class type has a copy assignment operator whose non-object parameter is of typeconst M&, const volatile M&,
or M[.](#class.copy.assign-2.sentence-4)[91](#footnote-91 "This implies that the reference parameter of the implicitly-declared copy assignment operator cannot bind to a volatile lvalue; see [diff.class].")

Otherwise, the implicitly-declared copy assignment operator
will have the formX& X::operator=(X&)

[3](#class.copy.assign-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1875)

A user-declared move assignment operator X​::​operator= is
a non-static non-template member function of class X with exactly
one non-object parameter of type X&&, const X&&, volatile X&&, orconst volatile X&&[.](#class.copy.assign-3.sentence-1)

[*Note [3](#class.copy.assign-note-3)*:

More than one form of move assignment operator can be declared for a class[.](#class.copy.assign-3.sentence-2)

— *end note*]

[4](#class.copy.assign-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1884)

If the definition of a class X does not explicitly declare a
move assignment operator, one
will be implicitly declared as defaulted if and only if

- [(4.1)](#class.copy.assign-4.1)

X does not have a user-declared copy constructor,

- [(4.2)](#class.copy.assign-4.2)

X does not have a user-declared move constructor,

- [(4.3)](#class.copy.assign-4.3)

X does not have a user-declared copy assignment operator, and

- [(4.4)](#class.copy.assign-4.4)

X does not have a user-declared destructor[.](#class.copy.assign-4.sentence-1)

[*Example [2](#class.copy.assign-example-2)*:

The class definitionstruct S {int a;
 S& operator=(const S&) = default;}; will not have a default move assignment operator implicitly declared because the
copy assignment operator has been user-declared[.](#class.copy.assign-4.sentence-2)

The move assignment operator may
be explicitly defaulted[.](#class.copy.assign-4.sentence-3)

struct S {int a;
 S& operator=(const S&) = default;
 S& operator=(S&&) = default;}; — *end example*]

[5](#class.copy.assign-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1924)

The implicitly-declared move assignment operator for a class X will have the formX& X::operator=(X&&)

[6](#class.copy.assign-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1930)

The implicitly-declared copy/move assignment operator for classX has the return typeX&[.](#class.copy.assign-6.sentence-1)

An implicitly-declared copy/move assignment operator is an
inline public member of its class[.](#class.copy.assign-6.sentence-2)

[7](#class.copy.assign-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1938)

A defaulted copy/move assignment operator for
class X is defined as deleted if X has:

- [(7.1)](#class.copy.assign-7.1)

a non-static data member of const non-class
 type (or possibly multidimensional array thereof), or

- [(7.2)](#class.copy.assign-7.2)

a non-static data member of reference type, or

- [(7.3)](#class.copy.assign-7.3)

a direct non-static data member of class type M (or possibly multidimensional array thereof) or
 a direct base class M that cannot be copied/moved because overload resolution ([[over.match]](over.match "12.2 Overload resolution")),
 as applied to find M's corresponding assignment operator,
 either does not result in a usable candidate ([[over.match.general]](over.match.general "12.2.1 General")) or,
 in the case of a variant member, selects a non-trivial function[.](#class.copy.assign-7.sentence-1)

[*Note [4](#class.copy.assign-note-4)*:

A defaulted move assignment operator that is defined as deleted is ignored by
overload resolution ([[over.match]](over.match "12.2 Overload resolution"), [[over.over]](over.over "12.3 Address of an overload set"))[.](#class.copy.assign-7.sentence-2)

— *end note*]

[8](#class.copy.assign-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1961)

Because a copy/move assignment operator is implicitly declared for a class
if not declared by the user,
a base class copy/move assignment operator is always hidden
by the corresponding assignment operator of a derived class ([[over.assign]](over.assign "12.4.3.2 Simple assignment"))[.](#class.copy.assign-8.sentence-1)

[*Note [5](#class.copy.assign-note-5)*:

A [*using-declaration*](namespace.udecl#nt:using-declaration "9.10 The using declaration [namespace.udecl]") in a derived class C that names an assignment operator from a base class
never suppresses the implicit declaration of
an assignment operator of C,
even if the base class assignment operator would be
a copy or move assignment operator
if declared as a member of C[.](#class.copy.assign-8.sentence-2)

— *end note*]

[9](#class.copy.assign-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1978)

A copy/move assignment operator for classX is
trivial
if it is not user-provided and if

- [(9.1)](#class.copy.assign-9.1)

classX has no virtual functions ([[class.virtual]](class.virtual "11.7.3 Virtual functions"))
and no virtual base classes ([[class.mi]](class.mi "11.7.2 Multiple base classes")), and

- [(9.2)](#class.copy.assign-9.2)

the assignment operator selected to copy/move each direct
base class subobject is trivial, and

- [(9.3)](#class.copy.assign-9.3)

for each non-static data member ofX that is of class type (or array thereof),
the assignment operator selected to copy/move that member is trivial;

otherwise the copy/move assignment operator is[*non-trivial*](#def:assignment_operator,copy,non-trivial "11.4.6 Copy/move assignment operator [class.copy.assign]")[.](#class.copy.assign-9.sentence-1)

[10](#class.copy.assign-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2006)

An implicitly-defined ([[dcl.fct.def.default]](dcl.fct.def.default "9.6.2 Explicitly-defaulted functions")) copy/move assignment operator is constexpr[.](#class.copy.assign-10.sentence-1)

[11](#class.copy.assign-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2009)

Before the defaulted copy/move assignment operator for a class is
implicitly defined,
all non-user-provided copy/move assignment operators for
its direct base classes and
its non-static data members are implicitly defined[.](#class.copy.assign-11.sentence-1)

[*Note [6](#class.copy.assign-note-6)*:

An implicitly-declared copy/move assignment operator has an
implied exception specification ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#class.copy.assign-11.sentence-2)

— *end note*]

[12](#class.copy.assign-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2020)

The implicitly-defined copy/move assignment operator for a
non-union class X performs memberwise copy/move assignment of its subobjects[.](#class.copy.assign-12.sentence-1)

The direct
base classes of X are assigned first, in the order of their declaration in the[*base-specifier-list*](class.derived.general#nt:base-specifier-list "11.7.1 General [class.derived.general]"), and then the immediate non-static data members ofX are assigned, in the order in which they were declared in the class
definition[.](#class.copy.assign-12.sentence-2)

Let x be either the parameter of the function or, for the move operator, an
xvalue referring to the parameter[.](#class.copy.assign-12.sentence-3)

Each subobject is assigned in the manner appropriate to its type:

- [(12.1)](#class.copy.assign-12.1)

if the subobject is of class type,
as if by a call to operator= with the subobject as the object expression
and the corresponding subobject of x as a single function argument
(as if by explicit qualification; that is,
ignoring any possible virtual overriding functions in more derived classes);

- [(12.2)](#class.copy.assign-12.2)

if the subobject is an array, each element is assigned,
in the manner appropriate to the element type;

- [(12.3)](#class.copy.assign-12.3)

if the subobject is of scalar type,
the built-in assignment operator is used[.](#class.copy.assign-12.sentence-4)

It is unspecified whether subobjects representing virtual base classes
are assigned more than once by the implicitly-defined copy/move assignment
operator[.](#class.copy.assign-12.sentence-5)

[*Example [3](#class.copy.assign-example-3)*: struct V { };struct A : virtual V { };struct B : virtual V { };struct C : B, A { };

It is unspecified whether the virtual base class subobjectV is assigned twice by the implicitly-defined copy/move assignment operator forC[.](#class.copy.assign-12.sentence-6)

— *end example*]

[13](#class.copy.assign-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2063)

The implicitly-defined copy/move assignment operator for a
union X copies the object representation ([[basic.types.general]](basic.types.general#term.object.representation "6.9.1 General")) of X[.](#class.copy.assign-13.sentence-1)

If the source and destination of the assignment are not the same object, then
for each object nested within ([[intro.object]](intro.object "6.8.2 Object model"))
the object that is the source of the copy,
a corresponding object o nested within the destination is created,
and the lifetime of o begins before the copy is performed[.](#class.copy.assign-13.sentence-2)

[14](#class.copy.assign-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2072)

The implicitly-defined copy/move assignment operator for a class
returns the object for which the assignment operator is invoked,
that is, the object assigned to[.](#class.copy.assign-14.sentence-1)

[90)](#footnote-90)[90)](#footnoteref-90)

Because
a template assignment operator or an assignment operator
taking an rvalue reference parameter is never a copy assignment operator,
the presence of such an assignment operator does not suppress the
implicit declaration of a copy assignment operator[.](#footnote-90.sentence-1)

Such assignment operators
participate in overload resolution with other assignment operators, including
copy assignment operators, and, if selected, will be used to assign an object[.](#footnote-90.sentence-2)

[91)](#footnote-91)[91)](#footnoteref-91)

This implies that the reference parameter of the
implicitly-declared copy assignment operator cannot bind to avolatile lvalue; see [[diff.class]](diff.class "C.7.7 [class]: classes")[.](#footnote-91.sentence-1)

### [11.4.7](#class.dtor) Destructors [[class.dtor]](class.dtor)

[1](#class.dtor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2082)

A declaration whose [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") has an [*unqualified-id*](expr.prim.id.unqual#nt:unqualified-id "7.5.5.2 Unqualified names [expr.prim.id.unqual]") that begins with a ~ declares a [*prospective destructor*](#def:destructor,prospective "11.4.7 Destructors [class.dtor]");
its [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") shall be a function declarator ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")) of the form

[*ptr-declarator*](dcl.decl.general#nt:ptr-declarator "9.3.1 General [dcl.decl.general]") ( [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") ) [*noexcept-specifier*](except.spec#nt:noexcept-specifier "14.5 Exception specifications [except.spec]")opt [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt

where the [*ptr-declarator*](dcl.decl.general#nt:ptr-declarator "9.3.1 General [dcl.decl.general]") consists solely of an[*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]"), an optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]"),
and optional surrounding parentheses, and the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") has
one of the following forms:

- [(1.1)](#class.dtor-1.1)

in a [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") that belongs to the[*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of a class or class template
but is not a friend
declaration ([[class.friend]](class.friend "11.8.4 Friends")), the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is~[*class-name*](class.pre#nt:class-name "11.1 Preamble [class.pre]") and the [*class-name*](class.pre#nt:class-name "11.1 Preamble [class.pre]") is the
injected-class-name ([[class.pre]](class.pre "11.1 Preamble")) of the immediately-enclosing entity or

- [(1.2)](#class.dtor-1.2)

otherwise, the[*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is [*nested-name-specifier*](expr.prim.id.qual#nt:nested-name-specifier "7.5.5.3 Qualified names [expr.prim.id.qual]")~[*class-name*](class.pre#nt:class-name "11.1 Preamble [class.pre]") and the [*class-name*](class.pre#nt:class-name "11.1 Preamble [class.pre]") is the injected-class-name of the
class nominated by the [*nested-name-specifier*](expr.prim.id.qual#nt:nested-name-specifier "7.5.5.3 Qualified names [expr.prim.id.qual]")[.](#class.dtor-1.sentence-1)

A prospective destructor shall take no arguments ([[dcl.fct]](dcl.fct "9.3.4.6 Functions"))[.](#class.dtor-1.sentence-2)

Each [*decl-specifier*](dcl.spec.general#nt:decl-specifier "9.2.1 General [dcl.spec.general]") of the [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]") of a prospective destructor declaration (if any)
shall befriend,inline,virtual, orconstexpr[.](#class.dtor-1.sentence-3)

[2](#class.dtor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2120)

If a class has no user-declared
prospective destructor,
a prospective destructor is implicitly
declared as defaulted ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions"))[.](#class.dtor-2.sentence-1)

An implicitly-declared prospective destructor is an
inline public member of its class[.](#class.dtor-2.sentence-2)

[3](#class.dtor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2130)

An implicitly-declared prospective destructor for a class X will have the form~X()

[4](#class.dtor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2136)

At the end of the definition of a class,
overload resolution is performed
among the prospective destructors declared in that class
with an empty argument list
to select the [*destructor*](#def:destructor "11.4.7 Destructors [class.dtor]") for the class,
also known as the [*selected destructor*](#def:destructor,selected "11.4.7 Destructors [class.dtor]")[.](#class.dtor-4.sentence-1)

The program is ill-formed if overload resolution fails[.](#class.dtor-4.sentence-2)

Destructor selection does not constitute
a reference to,
or odr-use ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule")) of,
the selected destructor,
and in particular,
the selected destructor may be deleted ([[dcl.fct.def.delete]](dcl.fct.def.delete "9.6.3 Deleted definitions"))[.](#class.dtor-4.sentence-3)

[5](#class.dtor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2151)

The address of a destructor shall not be taken[.](#class.dtor-5.sentence-1)

[*Note [1](#class.dtor-note-1)*:

A return statement in the body of a destructor
cannot specify a return value ([[stmt.return]](stmt.return "8.8.4 The return statement"))[.](#class.dtor-5.sentence-2)

— *end note*]

A destructor can be invoked for aconst,volatile orconstvolatile object[.](#class.dtor-5.sentence-3)

const andvolatile semantics ([[dcl.type.cv]](dcl.type.cv "9.2.9.2 The cv-qualifiers")) are not applied on an object under destruction[.](#class.dtor-5.sentence-4)

They stop being in effect when the destructor for the
most derived object ([[intro.object]](intro.object "6.8.2 Object model")) starts[.](#class.dtor-5.sentence-5)

[6](#class.dtor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2175)

[*Note [2](#class.dtor-note-2)*:

A declaration of a destructor that does not have a [*noexcept-specifier*](except.spec#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") has the same exception specification as if it had been implicitly declared ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#class.dtor-6.sentence-1)

— *end note*]

[7](#class.dtor-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2181)

A defaulted destructor for a class X is defined as deleted if

- [(7.1)](#class.dtor-7.1)

X is a non-union class and
 any non-variant potentially constructed subobject has class type M (or possibly multidimensional array thereof) where M has a destructor that is deleted or
 is inaccessible from the defaulted destructor,

- [(7.2)](#class.dtor-7.2)

X is a union and
  * [(7.2.1)](#class.dtor-7.2.1)

overload resolution to select a constructor to
 default-initialize an object of type X either fails or
 selects a constructor that is either deleted or not trivial, or

  * [(7.2.2)](#class.dtor-7.2.2)

X has a variant member V of
 class type M (or possibly multi-dimensional array thereof)
 where V has a default member initializer and M has a destructor that is non-trivial, or,

- [(7.3)](#class.dtor-7.3)

for a virtual destructor, lookup of the non-array deallocation
 function results in an ambiguity or in a function that is deleted or
 inaccessible from the defaulted destructor[.](#class.dtor-7.sentence-1)

[8](#class.dtor-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2210)

A destructor for a class X is trivial if it is not user-provided and if

- [(8.1)](#class.dtor-8.1)

the destructor is not virtual,

- [(8.2)](#class.dtor-8.2)

all of the direct base classes of X have trivial destructors, and

- [(8.3)](#class.dtor-8.3)

either X is a union or
for all of the non-variant non-static data members of X that are of class
type (or array thereof), each such class has a trivial destructor[.](#class.dtor-8.sentence-1)

Otherwise, the destructor is[*non-trivial*](#def:destructor,non-trivial "11.4.7 Destructors [class.dtor]")[.](#class.dtor-8.sentence-2)

[9](#class.dtor-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2225)

A defaulted destructor is a constexpr destructor
if it is constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers"))[.](#class.dtor-9.sentence-1)

[10](#class.dtor-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2229)

Before a
defaulted destructor for a class is implicitly defined, all the non-user-provided
destructors for its base classes and its non-static data members are
implicitly defined[.](#class.dtor-10.sentence-1)

[11](#class.dtor-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2235)

A prospective destructor can be
declared virtual ([[class.virtual]](class.virtual "11.7.3 Virtual functions"))
and with a [*pure-specifier*](#nt:pure-specifier "11.4.1 General [class.mem.general]") ([[class.abstract]](class.abstract "11.7.4 Abstract classes"))[.](#class.dtor-11.sentence-1)

If the destructor of a class is virtual and
any objects of that class or any derived class are created in the program,
the destructor shall be defined[.](#class.dtor-11.sentence-2)

[12](#class.dtor-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2245)

[*Note [3](#class.dtor-note-3)*:

Some language constructs have special semantics when used during destruction;
see [[class.cdtor]](class.cdtor "11.9.5 Construction and destruction")[.](#class.dtor-12.sentence-1)

— *end note*]

[13](#class.dtor-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2252)

After executing the body of the destructor and destroying
any objects with automatic storage duration allocated within the body, a
destructor for classX calls the destructors forX's
direct non-variant non-static data members other than anonymous unions,
the destructors forX's
non-virtual direct base classes and, ifX is the most derived class ([[class.base.init]](class.base.init "11.9.3 Initializing bases and members")),
its destructor calls the destructors forX's
virtual base classes[.](#class.dtor-13.sentence-1)

All destructors are called as if they were referenced with a qualified name,
that is, ignoring any possible virtual overriding destructors in more
derived classes[.](#class.dtor-13.sentence-2)

Bases and members are destroyed in the reverse order of the completion of
their constructor (see [[class.base.init]](class.base.init "11.9.3 Initializing bases and members"))[.](#class.dtor-13.sentence-3)

[*Note [4](#class.dtor-note-4)*:

Areturn statement ([[stmt.return]](stmt.return "8.8.4 The return statement")) in a destructor might not directly return to the
caller; before transferring control to the caller, the destructors for the
members and bases are called[.](#class.dtor-13.sentence-4)

— *end note*]

Destructors for elements of an array are called in reverse order of their
construction (see [[class.init]](class.init "11.9 Initialization"))[.](#class.dtor-13.sentence-5)

[14](#class.dtor-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2287)

A destructor is invoked implicitly

- [(14.1)](#class.dtor-14.1)

for a constructed object with static storage duration ([[basic.stc.static]](basic.stc.static "6.8.6.2 Static storage duration")) at program termination ([[basic.start.term]](basic.start.term "6.10.3.4 Termination")),

- [(14.2)](#class.dtor-14.2)

for a constructed object with thread storage duration ([[basic.stc.thread]](basic.stc.thread "6.8.6.3 Thread storage duration")) at thread exit,

- [(14.3)](#class.dtor-14.3)

for a constructed object with automatic storage duration ([[basic.stc.auto]](basic.stc.auto "6.8.6.4 Automatic storage duration")) when the block in which an object is created exits ([[stmt.dcl]](stmt.dcl "8.10 Declaration statement")),

- [(14.4)](#class.dtor-14.4)

for a constructed temporary object when its lifetime ends ([[conv.rval]](conv.rval "7.3.5 Temporary materialization conversion"), [[class.temporary]](class.temporary "6.8.7 Temporary objects"))[.](#class.dtor-14.sentence-1)

In each case, the context of the invocation is the context of the construction of
the object[.](#class.dtor-14.sentence-2)

A destructor may also be invoked implicitly through use of a[*delete-expression*](expr.delete#nt:delete-expression "7.6.2.9 Delete [expr.delete]") ([[expr.delete]](expr.delete "7.6.2.9 Delete")) for a constructed object allocated
by a [*new-expression*](expr.new#nt:new-expression "7.6.2.8 New [expr.new]") ([[expr.new]](expr.new "7.6.2.8 New")); the context of the invocation is the[*delete-expression*](expr.delete#nt:delete-expression "7.6.2.9 Delete [expr.delete]")[.](#class.dtor-14.sentence-3)

[*Note [5](#class.dtor-note-5)*:

An array of class type contains several subobjects for each of which
the destructor is invoked[.](#class.dtor-14.sentence-4)

— *end note*]

A destructor can also be invoked explicitly[.](#class.dtor-14.sentence-5)

A destructor is [*potentially invoked*](#def:potentially_invoked "11.4.7 Destructors [class.dtor]") if it is invoked or as specified in [[expr.new]](expr.new "7.6.2.8 New"),[[stmt.return]](stmt.return "8.8.4 The return statement"), [[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates"),[[class.base.init]](class.base.init "11.9.3 Initializing bases and members"), and [[except.throw]](except.throw "14.2 Throwing an exception")[.](#class.dtor-14.sentence-6)

A program is ill-formed if a destructor that is potentially invoked is deleted
or not accessible from the context of the invocation[.](#class.dtor-14.sentence-7)

[15](#class.dtor-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2319)

At the point of definition of a virtual destructor (including an implicit
definition), the non-array deallocation function is
determined as if for the expression delete this appearing in a
non-virtual destructor of the destructor's class (see [[expr.delete]](expr.delete "7.6.2.9 Delete"))[.](#class.dtor-15.sentence-1)

If the lookup fails or if the deallocation function has
a deleted definition ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions")), the program is ill-formed[.](#class.dtor-15.sentence-2)

[*Note [6](#class.dtor-note-6)*:

This assures that a deallocation function corresponding to the dynamic type of an
object is available for the[*delete-expression*](expr.delete#nt:delete-expression "7.6.2.9 Delete [expr.delete]") ([[class.free]](#class.free "11.4.11 Allocation and deallocation functions"))[.](#class.dtor-15.sentence-3)

— *end note*]

[16](#class.dtor-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2332)

In an explicit destructor call, the destructor is specified by a~ followed by a[*type-name*](dcl.type.simple#nt:type-name "9.2.9.3 Simple type specifiers [dcl.type.simple]") or [*computed-type-specifier*](dcl.type.simple#nt:computed-type-specifier "9.2.9.3 Simple type specifiers [dcl.type.simple]") that denotes the destructor's class type[.](#class.dtor-16.sentence-1)

The invocation of a destructor is subject to the usual rules for member
functions ([[class.mfct]](#class.mfct "11.4.2 Member functions"));
that is, if the object is not of the destructor's class type and
not of a class derived from the destructor's class type (including when
the destructor is invoked via a null pointer value), the program has
undefined behavior[.](#class.dtor-16.sentence-2)

[*Note [7](#class.dtor-note-7)*:

Invoking delete on a null pointer does not call the
destructor; see [[expr.delete]](expr.delete "7.6.2.9 Delete")[.](#class.dtor-16.sentence-3)

— *end note*]

[*Example [1](#class.dtor-example-1)*: struct B {virtual ~B() { }};struct D : B {~D() { }};

D D_object;typedef B B_alias;
B* B_ptr = &D_object;

void f() { D_object.B::~B(); // calls B's destructor B_ptr->~B(); // calls D's destructor B_ptr->~B_alias(); // calls D's destructor B_ptr->B_alias::~B(); // calls B's destructor B_ptr->B_alias::~B_alias(); // calls B's destructor} — *end example*]

[*Note [8](#class.dtor-note-8)*:

An explicit destructor call must always be written using
a member access operator ([[expr.ref]](expr.ref "7.6.1.5 Class member access")) or a [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") ([[expr.prim.id.qual]](expr.prim.id.qual "7.5.5.3 Qualified names"));
in particular, the[*unary-expression*](expr.unary.general#nt:unary-expression "7.6.2.1 General [expr.unary.general]")~X() in a member function is not an explicit destructor call ([[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators"))[.](#class.dtor-16.sentence-4)

— *end note*]

[17](#class.dtor-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2380)

[*Note [9](#class.dtor-note-9)*:

Explicit calls of destructors are rarely needed[.](#class.dtor-17.sentence-1)

One use of such calls is for objects placed at specific
addresses using a placement[*new-expression*](expr.new#nt:new-expression "7.6.2.8 New [expr.new]")[.](#class.dtor-17.sentence-2)

Such use of explicit placement and destruction of objects can be necessary
to cope with dedicated hardware resources and for writing memory management
facilities[.](#class.dtor-17.sentence-3)

[*Example [2](#class.dtor-example-2)*: void* operator new(std::size_t, void* p) { return p; }struct X { X(int); ~X();};void f(X* p);

void g() { // rare, specialized use:char* buf = new char[sizeof(X)];
 X* p = new(buf) X(222); // use buf[] and initialize f(p);
 p->X::~X(); // cleanup} — *end example*]

— *end note*]

[18](#class.dtor-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2409)

Once a destructor is invoked for an object, the object's lifetime ends;
the behavior is undefined if the destructor is invoked
for an object whose lifetime has ended ([[basic.life]](basic.life "6.8.4 Lifetime"))[.](#class.dtor-18.sentence-1)

[*Example [3](#class.dtor-example-3)*:

If the destructor for an object with automatic storage duration is explicitly invoked,
and the block is subsequently left in a manner that would ordinarily
invoke implicit destruction of the object, the behavior is undefined[.](#class.dtor-18.sentence-2)

— *end example*]

[19](#class.dtor-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2419)

[*Note [10](#class.dtor-note-10)*:

The notation for explicit call of a destructor can be used for any scalar type
name ([[expr.prim.id.dtor]](expr.prim.id.dtor "7.5.5.5 Destruction"))[.](#class.dtor-19.sentence-1)

Allowing this makes it possible to write code without having to know if a
destructor exists for a given type[.](#class.dtor-19.sentence-2)

For example:typedef int I;
I* p;
p->I::~I();

— *end note*]

[20](#class.dtor-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2434)

A destructor shall not be a coroutine[.](#class.dtor-20.sentence-1)

### [11.4.8](#class.conv) Conversions [[class.conv]](class.conv)

#### [11.4.8.1](#class.conv.general) General [[class.conv.general]](class.conv.general)

[1](#class.conv.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2441)

Type conversions of class objects can be specified by constructors and
by conversion functions[.](#class.conv.general-1.sentence-1)

These conversions are called[*user-defined conversions*](#def:conversion,user-defined "11.4.8.1 General [class.conv.general]") and are used for implicit type conversions ([[conv]](conv "7.3 Standard conversions")),
for initialization ([[dcl.init]](dcl.init "9.5 Initializers")),
and for explicit type conversions ([[expr.type.conv]](expr.type.conv "7.6.1.4 Explicit type conversion (functional notation)"), [[expr.cast]](expr.cast "7.6.3 Explicit type conversion (cast notation)"), [[expr.static.cast]](expr.static.cast "7.6.1.9 Static cast"))[.](#class.conv.general-1.sentence-2)

[2](#class.conv.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2455)

User-defined conversions are applied only where they are unambiguous ([[class.member.lookup]](class.member.lookup "6.5.2 Member name lookup"), [[class.conv.fct]](#class.conv.fct "11.4.8.3 Conversion functions"))[.](#class.conv.general-2.sentence-1)

Conversions obey the access control rules ([[class.access]](class.access "11.8 Member access control"))[.](#class.conv.general-2.sentence-2)

Access control is applied after ambiguity resolution ([[basic.lookup]](basic.lookup "6.5 Name lookup"))[.](#class.conv.general-2.sentence-3)

[3](#class.conv.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2460)

[*Note [1](#class.conv.general-note-1)*:

See [[over.match]](over.match "12.2 Overload resolution") for a discussion of the use of conversions in function calls[.](#class.conv.general-3.sentence-1)

— *end note*]

[4](#class.conv.general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2465)

At most one user-defined conversion (constructor or conversion function)
is implicitly applied to a single value[.](#class.conv.general-4.sentence-1)

[*Example [1](#class.conv.general-example-1)*: struct X {operator int();};

struct Y {operator X();};

Y a;int b = a; // error: no viable conversion (a.operator X().operator int() not considered)int c = X(a); // OK, a.operator X().operator int() — *end example*]

#### [11.4.8.2](#class.conv.ctor) Conversion by constructor [[class.conv.ctor]](class.conv.ctor)

[1](#class.conv.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2488)

A constructor that is not explicit ([[dcl.fct.spec]](dcl.fct.spec "9.2.3 Function specifiers"))
specifies a conversion from
the types of its parameters (if any)
to the type of its class[.](#class.conv.ctor-1.sentence-1)

[*Example [1](#class.conv.ctor-example-1)*:

[🔗](#:Jessie)

struct X { X(int);
 X(const char*, int = 0);
 X(int, int);};

void f(X arg) { X a = 1; // a = X(1) X b = "Jessie"; // b = X("Jessie",0) a = 2; // a = X(2) f(3); // f(X(3)) f({1, 2}); // f(X(1,2))} — *end example*]

[2](#class.conv.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2512)

[*Note [1](#class.conv.ctor-note-1)*:

An explicit constructor constructs objects just like non-explicit
constructors, but does so only where the direct-initialization syntax ([[dcl.init]](dcl.init "9.5 Initializers"))
or where casts ([[expr.static.cast]](expr.static.cast "7.6.1.9 Static cast"), [[expr.cast]](expr.cast "7.6.3 Explicit type conversion (cast notation)")) are explicitly
used; see also [[over.match.copy]](over.match.copy "12.2.2.5 Copy-initialization of class by user-defined conversion")[.](#class.conv.ctor-2.sentence-1)

A default constructor can be an explicit constructor; such a constructor
will be used to perform default-initialization
or value-initialization ([[dcl.init]](dcl.init "9.5 Initializers"))[.](#class.conv.ctor-2.sentence-2)

[*Example [2](#class.conv.ctor-example-2)*: struct Z {explicit Z(); explicit Z(int); explicit Z(int, int);};

Z a; // OK, default-initialization performed Z b{}; // OK, direct initialization syntax used Z c = {}; // error: copy-list-initialization Z a1 = 1; // error: no implicit conversion Z a3 = Z(1); // OK, direct initialization syntax used Z a2(1); // OK, direct initialization syntax used Z* p = new Z(1); // OK, direct initialization syntax used Z a4 = (Z)1; // OK, explicit cast used Z a5 = static_cast<Z>(1); // OK, explicit cast used Z a6 = { 3, 4 }; // error: no implicit conversion — *end example*]

— *end note*]

#### [11.4.8.3](#class.conv.fct) Conversion functions [[class.conv.fct]](class.conv.fct)

[conversion-function-id:](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]")  
operator [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]")

[conversion-type-id:](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]")  
[*type-specifier-seq*](dcl.type.general#nt:type-specifier-seq "9.2.9.1 General [dcl.type.general]") [*conversion-declarator*](#nt:conversion-declarator "11.4.8.3 Conversion functions [class.conv.fct]")opt

[conversion-declarator:](#nt:conversion-declarator "11.4.8.3 Conversion functions [class.conv.fct]")  
[*ptr-operator*](dcl.decl.general#nt:ptr-operator "9.3.1 General [dcl.decl.general]") [*conversion-declarator*](#nt:conversion-declarator "11.4.8.3 Conversion functions [class.conv.fct]")opt

[1](#class.conv.fct-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2563)

A declaration
whose [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") has
an [*unqualified-id*](expr.prim.id.unqual#nt:unqualified-id "7.5.5.2 Unqualified names [expr.prim.id.unqual]") that is a [*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]") declares a [*conversion function*](#def:function,conversion "11.4.8.3 Conversion functions [class.conv.fct]");
its [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") shall be
a function declarator ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")) of the form

[*noptr-declarator*](dcl.decl.general#nt:noptr-declarator "9.3.1 General [dcl.decl.general]") [*parameters-and-qualifiers*](dcl.decl.general#nt:parameters-and-qualifiers "9.3.1 General [dcl.decl.general]")

where the [*noptr-declarator*](dcl.decl.general#nt:noptr-declarator "9.3.1 General [dcl.decl.general]") consists solely of
an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]"),
an optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]"), and
optional surrounding parentheses, and
the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") has one of the following forms:

- [(1.1)](#class.conv.fct-1.1)

in a [*member-declaration*](#nt:member-declaration "11.4.1 General [class.mem.general]") that belongs to
the [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of a class or class template
but is not a friend declaration ([[class.friend]](class.friend "11.8.4 Friends")),
the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is a [*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]");

- [(1.2)](#class.conv.fct-1.2)

otherwise, the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is a [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") whose [*unqualified-id*](expr.prim.id.unqual#nt:unqualified-id "7.5.5.2 Unqualified names [expr.prim.id.unqual]") is a [*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]")[.](#class.conv.fct-1.sentence-1)

[2](#class.conv.fct-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2589)

A conversion function shall have no non-object parameters and
shall be a non-static member function of a class or class template X;
its declared return type is the [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") and
it specifies a conversion from X to
the type specified by the [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]"),
interpreted as a [*type-id*](dcl.name#nt:type-id "9.3.2 Type names [dcl.name]") ([[dcl.name]](dcl.name "9.3.2 Type names"))[.](#class.conv.fct-2.sentence-1)

A [*decl-specifier*](dcl.spec.general#nt:decl-specifier "9.2.1 General [dcl.spec.general]") in the [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]") of a conversion function (if any) shall not be
a [*defining-type-specifier*](dcl.type.general#nt:defining-type-specifier "9.2.9.1 General [dcl.type.general]")[.](#class.conv.fct-2.sentence-2)

[3](#class.conv.fct-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2600)

[*Note [1](#class.conv.fct-note-1)*:

A conversion function is never invoked for
implicit or explicit conversions of an object
to the same object type (or a reference to it),
to a base class of that type (or a reference to it),
or to cv void[.](#class.conv.fct-3.sentence-1)

Even though never directly called to perform a conversion,
such conversion functions can be declared and can potentially
be reached through a call to a virtual conversion function in a base class[.](#class.conv.fct-3.sentence-2)

— *end note*]

[*Example [1](#class.conv.fct-example-1)*: struct X {operator int(); operator auto() -> short; // error: trailing return type};

void f(X a) {int i = int(a);
 i = (int)a;
 i = a;}

In all three cases the value assigned will be converted byX​::​operator int()[.](#class.conv.fct-3.sentence-3)

— *end example*]

[4](#class.conv.fct-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2628)

A conversion function may be explicit ([[dcl.fct.spec]](dcl.fct.spec "9.2.3 Function specifiers")), in which case it is only considered as a user-defined conversion for direct-initialization ([[dcl.init]](dcl.init "9.5 Initializers"))[.](#class.conv.fct-4.sentence-1)

Otherwise, user-defined conversions are not restricted to use in assignments and initializations[.](#class.conv.fct-4.sentence-2)

[*Example [2](#class.conv.fct-example-2)*: class Y { };struct Z {explicit operator Y() const;};

void h(Z z) { Y y1(z); // OK, direct-initialization Y y2 = z; // error: no conversion function candidate for copy-initialization Y y3 = (Y)z; // OK, cast notation}void g(X a, X b) {int i = (a) ? 1+a : 0; int j = (a&&b) ? a+b : i; if (a) {}} — *end example*]

[5](#class.conv.fct-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2652)

The[*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") shall not represent a function type nor an array type[.](#class.conv.fct-5.sentence-1)

The[*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") in a[*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]") is the longest sequence of
tokens that could possibly form a [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]")[.](#class.conv.fct-5.sentence-2)

[*Note [2](#class.conv.fct-note-2)*:

This prevents ambiguities between the declarator operator * and its expression
counterparts[.](#class.conv.fct-5.sentence-3)

[*Example [3](#class.conv.fct-example-3)*: &ac.operator int*i; // syntax error:// parsed as: &(ac.operator int *)i// not as: &(ac.operator int)*i

The * is the pointer declarator and not the multiplication operator[.](#class.conv.fct-5.sentence-4)

— *end example*]

This rule also prevents ambiguities for attributes[.](#class.conv.fct-5.sentence-5)

[*Example [4](#class.conv.fct-example-4)*: operator int [[noreturn]] (); // error: noreturn attribute applied to a type — *end example*]

— *end note*]

[6](#class.conv.fct-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2681)

[*Note [3](#class.conv.fct-note-3)*:

A conversion function in a derived class hides only
conversion functions in base classes that convert to the same type[.](#class.conv.fct-6.sentence-1)

A conversion function template with a dependent return type hides only
templates in base classes that correspond to it ([[class.member.lookup]](class.member.lookup "6.5.2 Member name lookup"));
otherwise, it hides and is hidden as a non-template function[.](#class.conv.fct-6.sentence-2)

Function overload resolution ([[over.match.best]](over.match.best "12.2.4 Best viable function")) selects
the best conversion function to perform the conversion[.](#class.conv.fct-6.sentence-3)

[*Example [5](#class.conv.fct-example-5)*: struct X {operator int();};

struct Y : X {operator char();};

void f(Y& a) {if (a) { // error: ambiguous between X​::​operator int() and Y​::​operator char()}} — *end example*]

— *end note*]

[7](#class.conv.fct-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2709)

Conversion functions can be virtual[.](#class.conv.fct-7.sentence-1)

[8](#class.conv.fct-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2713)

A conversion function template shall not have a
deduced return type ([[dcl.spec.auto]](dcl.spec.auto "9.2.9.7 Placeholder type specifiers"))[.](#class.conv.fct-8.sentence-1)

[*Example [6](#class.conv.fct-example-6)*: struct S {operator auto() const { return 10; } // OKtemplate<class T>operator auto() const { return 1.2; } // error: conversion function template}; — *end example*]

### [11.4.9](#class.static) Static members [[class.static]](class.static)

#### [11.4.9.1](#class.static.general) General [[class.static.general]](class.static.general)

[1](#class.static.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2732)

A static member s of class X may be referred to
using the [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") expression X​::​s; it is not
necessary to use the class member access syntax ([[expr.ref]](expr.ref "7.6.1.5 Class member access")) to
refer to a static member[.](#class.static.general-1.sentence-1)

A static member may be
referred to using the class member access syntax, in which case the
object expression is evaluated[.](#class.static.general-1.sentence-2)

[*Example [1](#class.static.general-example-1)*: struct process {static void reschedule();};
process& g();

void f() { process::reschedule(); // OK, no object necessary g().reschedule(); // g() is called} — *end example*]

[2](#class.static.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2753)

Static members obey the usual class member access rules ([[class.access]](class.access "11.8 Member access control"))[.](#class.static.general-2.sentence-1)

When used in the declaration of a class
member, the static specifier shall only be used in the member
declarations that appear within the [*member-specification*](#nt:member-specification "11.4.1 General [class.mem.general]") of
the class definition[.](#class.static.general-2.sentence-2)

[*Note [1](#class.static.general-note-1)*:

It cannot be specified in member declarations that appear in namespace scope[.](#class.static.general-2.sentence-3)

— *end note*]

#### [11.4.9.2](#class.static.mfct) Static member functions [[class.static.mfct]](class.static.mfct)

[1](#class.static.mfct-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2766)

[*Note [1](#class.static.mfct-note-1)*:

The rules described in [[class.mfct]](#class.mfct "11.4.2 Member functions") apply to static member
functions[.](#class.static.mfct-1.sentence-1)

— *end note*]

[2](#class.static.mfct-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2772)

[*Note [2](#class.static.mfct-note-2)*:

A static member function does not have a this pointer ([[expr.prim.this]](expr.prim.this "7.5.3 This"))[.](#class.static.mfct-2.sentence-1)

A static member function cannot be qualified with const,volatile, or virtual ([[dcl.fct]](dcl.fct "9.3.4.6 Functions"))[.](#class.static.mfct-2.sentence-2)

— *end note*]

#### [11.4.9.3](#class.static.data) Static data members [[class.static.data]](class.static.data)

[1](#class.static.data-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2783)

A static data member is not part of the subobjects of a class[.](#class.static.data-1.sentence-1)

If a
static data member is declared thread_local there is one copy of
the member per thread[.](#class.static.data-1.sentence-2)

If a static data member is not declaredthread_local there is one copy of the data member that is shared by all
the objects of the class[.](#class.static.data-1.sentence-3)

[2](#class.static.data-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2790)

A static data member shall not be mutable ([[dcl.stc]](dcl.stc "9.2.2 Storage class specifiers"))[.](#class.static.data-2.sentence-1)

A static data member shall not be a direct member ([class.mem])
of an unnamed ([[class.pre]](class.pre "11.1 Preamble")) or local ([[class.local]](class.local "11.6 Local class declarations")) class or
of a (possibly indirectly) nested class ([[class.nest]](#class.nest "11.4.12 Nested class declarations")) thereof[.](#class.static.data-2.sentence-2)

[3](#class.static.data-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2796)

The declaration of a non-inline
static data member in its class definition
is not a definition and may be of an incomplete type other thancv void[.](#class.static.data-3.sentence-1)

[*Note [1](#class.static.data-note-1)*:

The [*initializer*](dcl.init.general#nt:initializer "9.5.1 General [dcl.init.general]") in the definition of a
static data member is in the scope of its
class ([[basic.scope.class]](basic.scope.class "6.4.7 Class scope"))[.](#class.static.data-3.sentence-2)

— *end note*]

[*Example [1](#class.static.data-example-1)*: class process {static process* run_chain; static process* running;};

process* process::running = get_main();
process* process::run_chain = running;

The definition of the static data member run_chain of classprocess inhabits the global scope; the notationprocess​::​run_chain indicates that the member run_chain is a member of class process and in the scope of classprocess[.](#class.static.data-3.sentence-3)

In the static data member definition, the[*initializer*](dcl.init.general#nt:initializer "9.5.1 General [dcl.init.general]") expression refers to the static data
member running of class process[.](#class.static.data-3.sentence-4)

— *end example*]

[*Note [2](#class.static.data-note-2)*:

Once the static data member has been defined, it exists even if
no objects of its class have been created[.](#class.static.data-3.sentence-5)

[*Example [2](#class.static.data-example-2)*:

In the example above, run_chain and running exist even
if no objects of class process are created by the program[.](#class.static.data-3.sentence-6)

— *end example*]

The initialization and destruction of static data members is described in[[basic.start.static]](basic.start.static "6.10.3.2 Static initialization"), [[basic.start.dynamic]](basic.start.dynamic "6.10.3.3 Dynamic initialization of non-block variables"), and [[basic.start.term]](basic.start.term "6.10.3.4 Termination")[.](#class.static.data-3.sentence-7)

— *end note*]

[4](#class.static.data-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2840)

If a non-volatile non-inline const static data member is
of integral or enumeration type,
its declaration in the class definition can specify a[*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") in which every[*initializer-clause*](dcl.init.general#nt:initializer-clause "9.5.1 General [dcl.init.general]") that is an [*assignment-expression*](expr.assign#nt:assignment-expression "7.6.19 Assignment and compound assignment operators [expr.assign]") is a constant expression ([[expr.const]](expr.const "7.7 Constant expressions"))[.](#class.static.data-4.sentence-1)

The member shall still be defined in a namespace scope if
it is odr-used ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule")) in the program and the
namespace scope definition shall not contain an [*initializer*](dcl.init.general#nt:initializer "9.5.1 General [dcl.init.general]")[.](#class.static.data-4.sentence-2)

The declaration of an inline static data member (which is a definition)
may specify a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")[.](#class.static.data-4.sentence-3)

If the
member is declared with the constexpr specifier, it may be
redeclared in namespace scope with no initializer (this usage is
deprecated; see [[depr.static.constexpr]](depr.static.constexpr "D.7 Redeclaration of static constexpr data members"))[.](#class.static.data-4.sentence-4)

Declarations of other
static data members shall not specify a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")[.](#class.static.data-4.sentence-5)

[5](#class.static.data-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2857)

[*Note [3](#class.static.data-note-3)*:

There is exactly one definition of a static data member
that is odr-used ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule")) in a valid program[.](#class.static.data-5.sentence-1)

— *end note*]

[6](#class.static.data-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2863)

[*Note [4](#class.static.data-note-4)*:

Static data members of a class in namespace scope have the linkage of the name of the class ([[basic.link]](basic.link "6.7 Program and linkage"))[.](#class.static.data-6.sentence-1)

— *end note*]

### [11.4.10](#class.bit) Bit-fields [[class.bit]](class.bit)

[1](#class.bit-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2871)

A [*member-declarator*](#nt:member-declarator "11.4.1 General [class.mem.general]") of the form

[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")opt [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt : [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]")opt

specifies a bit-field[.](#class.bit-1.sentence-1)

The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") appertains
to the entity being declared[.](#class.bit-1.sentence-2)

A bit-field shall not be a static member[.](#class.bit-1.sentence-3)

A bit-field shall have integral or (possibly cv-qualified) enumeration type;
the bit-field semantic property is not part of the type of the class member[.](#class.bit-1.sentence-4)

The [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") shall be an integral constant expression
with a value greater than or equal to zero and
is called the [*width*](simd.general#def:width "29.10.1 General [simd.general]") of the bit-field[.](#class.bit-1.sentence-5)

If the width of a bit-field is larger than
the width of the bit-field's type
(or, in case of an enumeration type, of its underlying type),
the extra bits are padding bits ([[basic.types.general]](basic.types.general#term.padding.bits "6.9.1 General"))[.](#class.bit-1.sentence-6)

Allocation of bit-fields within a class object isimplementation-defined[.](#class.bit-1.sentence-7)

Alignment of bit-fields is implementation-defined[.](#class.bit-1.sentence-8)

Bit-fields are packed into some addressable allocation unit[.](#class.bit-1.sentence-9)

[*Note [1](#class.bit-note-1)*:

Bit-fields straddle allocation units on some machines and not on others[.](#class.bit-1.sentence-10)

Bit-fields are assigned right-to-left on some machines, left-to-right on
others[.](#class.bit-1.sentence-11)

— *end note*]

[2](#class.bit-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2905)

A declaration for a bit-field that omits the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") declares an [*unnamed bit-field*](#def:bit-field,unnamed "11.4.10 Bit-fields [class.bit]")[.](#class.bit-2.sentence-1)

Unnamed bit-fields are not
members and cannot be initialized[.](#class.bit-2.sentence-2)

An unnamed bit-field shall not be declared with a cv-qualified type[.](#class.bit-2.sentence-3)

[*Note [2](#class.bit-note-2)*:

An unnamed bit-field is useful for padding to conform to
externally-imposed layouts[.](#class.bit-2.sentence-4)

— *end note*]

As a special case, an unnamed bit-field with a width of zero specifies
alignment of the next bit-field at an allocation unit boundary[.](#class.bit-2.sentence-5)

Only
when declaring an unnamed bit-field may the width be zero[.](#class.bit-2.sentence-6)

[3](#class.bit-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2920)

The address-of operator & shall not be applied to a bit-field,
so there are no pointers to bit-fields[.](#class.bit-3.sentence-1)

A non-const reference shall not bind to a bit-field ([[dcl.init.ref]](dcl.init.ref "9.5.4 References"))[.](#class.bit-3.sentence-2)

[*Note [3](#class.bit-note-3)*:

If the initializer for a reference of type const T& is
an lvalue that refers to a bit-field, the reference is bound to a
temporary initialized to hold the value of the bit-field; the reference
is not bound to the bit-field directly[.](#class.bit-3.sentence-3)

See [[dcl.init.ref]](dcl.init.ref "9.5.4 References")[.](#class.bit-3.sentence-4)

— *end note*]

[4](#class.bit-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2935)

If a value of integral type (other than bool) is stored
into a bit-field of width N and the value would be representable
in a hypothetical signed or unsigned integer type
with width N and the same signedness as the bit-field's type,
the original value and the value of the bit-field compare equal[.](#class.bit-4.sentence-1)

If the value true or false is stored into a bit-field of
type bool of any size (including a one bit bit-field), the
original bool value and the value of the bit-field compare
equal[.](#class.bit-4.sentence-2)

If a value of an enumeration type is stored into a bit-field of the
same type and the width is large
enough to hold all the values of that enumeration type ([[dcl.enum]](dcl.enum "9.8.1 Enumeration declarations")),
the original value and the value of the bit-field compare equal[.](#class.bit-4.sentence-3)

[*Example [1](#class.bit-example-1)*: enum BOOL { FALSE=0, TRUE=1 };struct A { BOOL b:1;};
A a;void f() { a.b = TRUE; if (a.b == TRUE) // yields true{ /* ... */ }} — *end example*]

### [11.4.11](#class.free) Allocation and deallocation functions [[class.free]](class.free)

[1](#class.free-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2966)

Any allocation function for a classT is a static member (even if not explicitly declaredstatic)[.](#class.free-1.sentence-1)

[2](#class.free-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2974)

[*Example [1](#class.free-example-1)*: class Arena;struct B {void* operator new(std::size_t, Arena*);};struct D1 : B {};

Arena* ap;void foo(int i) {new (ap) D1; // calls B​::​operator new(std​::​size_t, Arena*)new D1[i]; // calls ​::​operator new[](std​::​size_t)new D1; // error: ​::​operator new(std​::​size_t) hidden} — *end example*]

[3](#class.free-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2993)

Any deallocation function for a classX is a static member (even if not explicitly declaredstatic)[.](#class.free-3.sentence-1)

[*Example [2](#class.free-example-2)*: class X {void operator delete(void*); void operator delete[](void*, std::size_t);};

class Y {void operator delete(void*, std::size_t); void operator delete[](void*);}; — *end example*]

[4](#class.free-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L3014)

Since member allocation and deallocation functions arestatic they cannot be virtual[.](#class.free-4.sentence-1)

[*Note [1](#class.free-note-1)*:

However, when the[*cast-expression*](expr.cast#nt:cast-expression "7.6.3 Explicit type conversion (cast notation) [expr.cast]") of a[*delete-expression*](expr.delete#nt:delete-expression "7.6.2.9 Delete [expr.delete]") refers to an object of class type with a virtual destructor,
because the deallocation function is chosen by the destructor
of the dynamic type of the object, the effect is the same in that case[.](#class.free-4.sentence-2)

[*Example [3](#class.free-example-3)*: struct B {virtual ~B(); void operator delete(void*, std::size_t);};

struct D : B {void operator delete(void*);};

struct E : B {void log_deletion(); void operator delete(E *p, std::destroying_delete_t) { p->log_deletion();
 p->~E(); ::operator delete(p); }};

void f() { B* bp = new D; delete bp; // 1: uses D​::​operator delete(void*) bp = new E; delete bp; // 2: uses E​::​operator delete(E*, std​::​destroying_delete_t)}

Here, storage for the object of classD is deallocated byD​::​operator delete(),
and
the object of class E is destroyed
and its storage is deallocated
by E​::​operator delete(),
due to the virtual destructor[.](#class.free-4.sentence-3)

— *end example*]

— *end note*]

[*Note [2](#class.free-note-2)*:

Virtual destructors have no effect on the deallocation function actually
called when the[*cast-expression*](expr.cast#nt:cast-expression "7.6.3 Explicit type conversion (cast notation) [expr.cast]") of a[*delete-expression*](expr.delete#nt:delete-expression "7.6.2.9 Delete [expr.delete]") refers to an array of objects of class type[.](#class.free-4.sentence-4)

[*Example [4](#class.free-example-4)*: struct B {virtual ~B(); void operator delete[](void*, std::size_t);};

struct D : B {void operator delete[](void*, std::size_t);};

void f(int i) { D* dp = new D[i]; delete [] dp; // uses D​::​operator delete[](void*, std​::​size_t) B* bp = new D[i]; delete[] bp; // undefined behavior} — *end example*]

— *end note*]

[5](#class.free-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L3092)

Access to the deallocation function is checked statically,
even if a different one is actually executed[.](#class.free-5.sentence-1)

[*Example [5](#class.free-example-5)*:

For the call on line “// 1” above,
ifB​::​operator delete() had been private, the delete expression would have been ill-formed[.](#class.free-5.sentence-2)

— *end example*]

[6](#class.free-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L3102)

[*Note [3](#class.free-note-3)*:

If a deallocation function has no explicit [*noexcept-specifier*](except.spec#nt:noexcept-specifier "14.5 Exception specifications [except.spec]"), it
has a non-throwing exception specification ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#class.free-6.sentence-1)

— *end note*]

### [11.4.12](#class.nest) Nested class declarations [[class.nest]](class.nest)

[1](#class.nest-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L3111)

A class can be declared within another class[.](#class.nest-1.sentence-1)

A class declared within
another is called a [*nested class*](#def:class,nested "11.4.12 Nested class declarations [class.nest]")[.](#class.nest-1.sentence-2)

[*Note [1](#class.nest-note-1)*:

See [[expr.prim.id]](expr.prim.id "7.5.5 Names") for restrictions on the use of non-static data
members and non-static member functions[.](#class.nest-1.sentence-3)

— *end note*]

[*Example [1](#class.nest-example-1)*: int x;int y;

struct enclose {int x; static int s; struct inner {void f(int i) {int a = sizeof(x); // OK, operand of sizeof is an unevaluated operand x = i; // error: assign to enclose​::​x s = i; // OK, assign to enclose​::​s::x = i; // OK, assign to global x y = i; // OK, assign to global y}void g(enclose* p, int i) { p->x = i; // OK, assign to enclose​::​x}};};

inner* p = 0; // error: inner not found — *end example*]

[2](#class.nest-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L3146)

[*Note [2](#class.nest-note-2)*:

Nested classes can be defined
either in the enclosing class or in an enclosing namespace;
member functions and static data members of a nested class can be
defined either in the nested class or in an enclosing namespace scope[.](#class.nest-2.sentence-1)

[*Example [2](#class.nest-example-2)*: struct enclose {struct inner {static int x; void f(int i); };};

int enclose::inner::x = 1;

void enclose::inner::f(int i) { /* ... */ }class E {class I1; // forward declaration of nested classclass I2; class I1 { }; // definition of nested class};class E::I2 { }; // definition of nested class — *end example*]

— *end note*]

[3](#class.nest-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L3175)

A friend function ([[class.friend]](class.friend "11.8.4 Friends")) defined
within a nested class has no special access rights to
members of an enclosing class[.](#class.nest-3.sentence-1)