157 lines
6.6 KiB
Markdown
157 lines
6.6 KiB
Markdown
[class.default.ctor]
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# 11 Classes [[class]](./#class)
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## 11.4 Class members [[class.mem]](class.mem#class.default.ctor)
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### 11.4.5 Constructors [[class.ctor]](class.ctor#class.default.ctor)
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#### 11.4.5.2 Default constructors [class.default.ctor]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1357)
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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
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that is not a function parameter pack
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has a default argument
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(including the case of a constructor with no parameters)[.](#1.sentence-1)
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If there is no user-declared constructor or constructor template for classX,
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a non-explicit constructor having no parameters is implicitly declared
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as defaulted ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions"))[.](#1.sentence-2)
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An implicitly-declared default constructor is an
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inline public member of its class[.](#1.sentence-3)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1374)
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A defaulted default constructor for class X is defined as deleted if
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- [(2.1)](#2.1)
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any non-static data member with no default member initializer ([[class.mem]](class.mem "11.4 Class members")) is
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of reference type,
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- [(2.2)](#2.2)
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X is a non-union class and
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any non-variant non-static data member of const-qualified type
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(or possibly multidimensional array thereof)
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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")),
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- [(2.3)](#2.3)
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any non-variant potentially constructed subobject, except for a non-static data member
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with a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]"),
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has class type M (or possibly multidimensional array thereof)
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and overload resolution ([[over.match]](over.match "12.2 Overload resolution"))
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as applied to find M's corresponding constructor
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does not result in a usable candidate ([[over.match.general]](over.match.general "12.2.1 General")), or
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- [(2.4)](#2.4)
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any potentially constructed subobject S has
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class type M (or possibly multidimensional array thereof),M has
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a destructor that is deleted or inaccessible from the defaulted default
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constructor, and
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either S is non-variant or S has a default member initializer[.](#2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1401)
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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
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- [(3.1)](#3.1)
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X has no virtual functions ([[class.virtual]](class.virtual "11.7.3 Virtual functions")) and no virtual base
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classes ([[class.mi]](class.mi "11.7.2 Multiple base classes")), and
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- [(3.2)](#3.2)
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no non-static data member of X has
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a default member initializer ([[class.mem]](class.mem "11.4 Class members")), and
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- [(3.3)](#3.3)
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all the direct base classes of X have trivial default constructors, and
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- [(3.4)](#3.4)
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either X is a union or
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for all the non-variant non-static data members of X that are of class
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type (or array thereof), each such class has a trivial default constructor[.](#3.sentence-1)
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Otherwise, the default constructor is[*non-trivial*](#def:constructor,default,non-trivial "11.4.5.2 Default constructors [class.default.ctor]")[.](#3.sentence-2)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1425)
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If a default constructor of a union-like class X is trivial,
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then for each union U that is either X or an anonymous union member of X,
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if the first variant member, if any, of U has implicit-lifetime type ([[basic.types.general]](basic.types.general "6.9.1 General")),
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the default constructor of X begins the lifetime of that member
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if it is not the active member of its union[.](#4.sentence-1)
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[*Note [1](#note-1)*:
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It is already the active member if U was value-initialized[.](#4.sentence-2)
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â *end note*]
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Otherwise,
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an implicitly-defined ([[dcl.fct.def.default]](dcl.fct.def.default "9.6.2 Explicitly-defaulted functions")) default constructor performs the set of
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initializations of the class that would be performed by a user-written default
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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]")[.](#4.sentence-3)
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If that user-written default constructor would be ill-formed,
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the program is ill-formed[.](#4.sentence-4)
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If that user-written default constructor would be constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers")),
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the implicitly-defined
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default constructor is constexpr[.](#4.sentence-5)
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Before the defaulted default constructor for a class is
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implicitly defined,
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all the non-user-provided default constructors for its base classes and
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its non-static data members are implicitly defined[.](#4.sentence-6)
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[*Note [2](#note-2)*:
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An implicitly-declared default constructor has an
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exception specification ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#4.sentence-7)
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An explicitly-defaulted definition might have an
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implicit exception specification, see [[dcl.fct.def]](dcl.fct.def "9.6 Function definitions")[.](#4.sentence-8)
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â *end note*]
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1458)
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[*Note [3](#note-3)*:
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A default constructor is implicitly invoked to initialize
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a class object when no initializer is specified ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#5.sentence-1)
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Such a default constructor needs to be accessible ([[class.access]](class.access "11.8 Member access control"))[.](#5.sentence-2)
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â *end note*]
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1466)
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[*Note [4](#note-4)*:
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[[class.base.init]](class.base.init "11.9.3 Initializing bases and members") describes the order in which constructors for base
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classes and non-static data members are called and
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describes how arguments can be specified for the calls to these constructors[.](#6.sentence-1)
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â *end note*]
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