cppdraft_translate/cppdraft/class/pre.md

[class.pre]

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

## 11.1 Preamble [class.pre]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L11)

A class is a type[.](#1.sentence-1)

Its name becomes a [*class-name*](#nt:class-name "11.1 Preamble [class.pre]") ([[class.name]](class.name "11.3 Class names")) within its
scope[.](#1.sentence-2)

[class-name:](#nt:class-name "11.1 Preamble [class.pre]")  
[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")  
[*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]")

A [*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") or
an [*elaborated-type-specifier*](dcl.type.elab#nt:elaborated-type-specifier "9.2.9.5 Elaborated type specifiers [dcl.type.elab]") ([[dcl.type.elab]](dcl.type.elab "9.2.9.5 Elaborated type specifiers")) is used to
make a [*class-name*](#nt:class-name "11.1 Preamble [class.pre]")[.](#1.sentence-3)

An object of a class consists of a
(possibly empty) sequence of members and base class objects[.](#1.sentence-4)

[class-specifier:](#nt:class-specifier "11.1 Preamble [class.pre]")  
[*class-head*](#nt:class-head "11.1 Preamble [class.pre]") { [*member-specification*](class.mem.general#nt:member-specification "11.4.1 General [class.mem.general]")opt }

[class-head:](#nt:class-head "11.1 Preamble [class.pre]")  
[*class-key*](#nt:class-key "11.1 Preamble [class.pre]") [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt [*class-head-name*](#nt:class-head-name "11.1 Preamble [class.pre]") [*class-property-specifier-seq*](#nt:class-property-specifier-seq "11.1 Preamble [class.pre]")opt [*base-clause*](class.derived.general#nt:base-clause "11.7.1 General [class.derived.general]")opt  
[*class-key*](#nt:class-key "11.1 Preamble [class.pre]") [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt [*base-clause*](class.derived.general#nt:base-clause "11.7.1 General [class.derived.general]")opt

[class-head-name:](#nt:class-head-name "11.1 Preamble [class.pre]")  
[*nested-name-specifier*](expr.prim.id.qual#nt:nested-name-specifier "7.5.5.3 Qualified names [expr.prim.id.qual]")opt [*class-name*](#nt:class-name "11.1 Preamble [class.pre]")

[class-property-specifier-seq:](#nt:class-property-specifier-seq "11.1 Preamble [class.pre]")  
[*class-property-specifier*](#nt:class-property-specifier "11.1 Preamble [class.pre]") [*class-property-specifier-seq*](#nt:class-property-specifier-seq "11.1 Preamble [class.pre]")opt

[class-property-specifier:](#nt:class-property-specifier "11.1 Preamble [class.pre]")  
final  
trivially_relocatable_if_eligible  
replaceable_if_eligible

[class-key:](#nt:class-key "11.1 Preamble [class.pre]")  
class  
struct  
union

A class declaration where the [*class-name*](#nt:class-name "11.1 Preamble [class.pre]") in the [*class-head-name*](#nt:class-head-name "11.1 Preamble [class.pre]") is a [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]") shall be an explicit specialization ([[temp.expl.spec]](temp.expl.spec "13.9.4 Explicit specialization")) or
a partial specialization ([[temp.spec.partial]](temp.spec.partial "13.7.6 Partial specialization"))[.](#1.sentence-5)

A [*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") whose[*class-head*](#nt:class-head "11.1 Preamble [class.pre]") omits the[*class-head-name*](#nt:class-head-name "11.1 Preamble [class.pre]") defines an [*unnamed class*](#def:class,unnamed "11.1 Preamble [class.pre]")[.](#1.sentence-6)

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

An unnamed class thus can't
be final[.](#1.sentence-7)

— *end note*]

Otherwise, the [*class-name*](#nt:class-name "11.1 Preamble [class.pre]") is an [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]");
it is not looked up, and the [*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") introduces it[.](#1.sentence-8)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L81)

The component name of the[*class-name*](#nt:class-name "11.1 Preamble [class.pre]") is also bound in the scope of the class (template)
itself; this is known as the [*injected-class-name*](#def:injected-class-name "11.1 Preamble [class.pre]")[.](#2.sentence-1)

For purposes of access checking, the injected-class-name is treated as
if it were a public member name[.](#2.sentence-2)

A [*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") is commonly referred to as a [*class
definition*](#def:definition,class "11.1 Preamble [class.pre]")[.](#2.sentence-3)

A class is considered defined after the closing brace of its[*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") has been seen even though its member
functions are in general not yet defined[.](#2.sentence-4)

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 class; the attributes in
the [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") are thereafter considered attributes of the class
whenever it is named[.](#2.sentence-5)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L97)

If a [*class-head-name*](#nt:class-head-name "11.1 Preamble [class.pre]") contains a [*nested-name-specifier*](expr.prim.id.qual#nt:nested-name-specifier "7.5.5.3 Qualified names [expr.prim.id.qual]"),
the [*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") shall not inhabit a class scope[.](#3.sentence-1)

If its [*class-name*](#nt:class-name "11.1 Preamble [class.pre]") is an [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]"),
the [*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") shall correspond to
one or more declarations nominable in
the class, class template, or namespace to which the[*nested-name-specifier*](expr.prim.id.qual#nt:nested-name-specifier "7.5.5.3 Qualified names [expr.prim.id.qual]") refers;
they shall all have the same target scope, and the target scope of the[*class-specifier*](#nt:class-specifier "11.1 Preamble [class.pre]") is that scope[.](#3.sentence-2)

[*Example [1](#example-1)*: namespace N {template<class>struct A {struct B; };}using N::A;template<class T> struct A<T>::B {}; // OKtemplate<> struct A<void> {}; // OK — *end example*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L121)

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

The [*class-key*](#nt:class-key "11.1 Preamble [class.pre]") determines
whether the class is a union ([[class.union]](class.union "11.5 Unions")) and
whether access is public or private by default ([[class.access]](class.access "11.8 Member access control"))[.](#4.sentence-1)

A union holds the value of at most one data member at a time[.](#4.sentence-2)

— *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L129)

Each [*class-property-specifier*](#nt:class-property-specifier "11.1 Preamble [class.pre]") shall appear at most once
within a single [*class-property-specifier-seq*](#nt:class-property-specifier-seq "11.1 Preamble [class.pre]")[.](#5.sentence-1)

Whenever a [*class-key*](#nt:class-key "11.1 Preamble [class.pre]") is followed
by a [*class-head-name*](#nt:class-head-name "11.1 Preamble [class.pre]"),
one of the identifiers final, trivially_relocatable_if_eligible,
or replaceable_if_eligible, and a colon or left brace,
the identifier is interpreted as a [*class-property-specifier*](#nt:class-property-specifier "11.1 Preamble [class.pre]")[.](#5.sentence-2)

[*Example [2](#example-2)*: struct A;struct A final {}; // OK, definition of struct A,// not value-initialization of variable finalstruct X {struct C { constexpr operator int() { return 5; } }; struct B trivially_relocatable_if_eligible : C{}; // OK, definition of nested class B,// not declaration of a bit-field member// trivially_relocatable_if_eligible}; — *end example*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L153)

If a class is marked with the [*class-property-specifier*](#nt:class-property-specifier "11.1 Preamble [class.pre]")final and that class appears as a [*class-or-decltype*](class.derived.general#nt:class-or-decltype "11.7.1 General [class.derived.general]") in a [*base-clause*](class.derived.general#nt:base-clause "11.7.1 General [class.derived.general]") ([[class.derived]](class.derived "11.7 Derived classes")), the program is ill-formed[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L158)

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

Complete objects of class type have nonzero size[.](#7.sentence-1)

Base class subobjects and
members declared with the no_unique_address attribute ([[dcl.attr.nouniqueaddr]](dcl.attr.nouniqueaddr "9.13.11 No unique address attribute"))
are not so constrained[.](#7.sentence-2)

— *end note*]

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L166)

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

Class objects can be assigned ([[over.assign]](over.assign "12.4.3.2 Simple assignment"), [[class.copy.assign]](class.copy.assign "11.4.6 Copy/move assignment operator")),
passed as arguments to functions ([[dcl.init]](dcl.init "9.5 Initializers"), [[class.copy.ctor]](class.copy.ctor "11.4.5.3 Copy/move constructors")), and
returned by functions (except objects of classes for which copying or moving has
been restricted; see [[dcl.fct.def.delete]](dcl.fct.def.delete "9.6.3 Deleted definitions") and [[class.access]](class.access "11.8 Member access control"))[.](#8.sentence-1)

Other plausible operators, such as equality comparison,
can be defined by the user; see [[over.oper]](over.oper "12.4 Overloaded operators")[.](#8.sentence-2)

— *end note*]