11 Classes [class]

11.4 Class members [class.mem]

11.4.5 Constructors [class.ctor]

11.4.5.1 General [class.ctor.general]

1
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A declarator declares a constructor if it is a function declarator ([dcl.fct]) of the form
ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:
Constructors do not have names.
In a constructor declaration, each decl-specifier in the optional decl-specifier-seq shall be friend, inline, constexpr, consteval, or an explicit-specifier.
[Example 1: struct S { S(); // declares the constructor }; S::S() { } // defines the constructor — end example]
2
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A constructor is used to initialize objects of its class type.
[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]) will cause a constructor to be called to initialize an object.
The syntax looks like an explicit call of the constructor.
— end note]
[Example 2: complex zz = complex(1,2.3); cprint( complex(7.8,1.2) ); — end example]
[Note 2: 
For initialization of objects of class type see [class.init].
— end note]
3
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An object created in this way is unnamed.
[Note 3: 
[class.temporary] describes the lifetime of temporary objects.
— end note]
[Note 4: 
Explicit constructor calls do not yield lvalues, see [basic.lval].
— end note]
4
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[Note 5: 
Some language constructs have special semantics when used during construction; see [class.base.init] and [class.cdtor].
— end note]
5
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A constructor can be invoked for a const, volatile or const volatile object.
const and volatile semantics ([dcl.type.cv]) are not applied on an object under construction.
They come into effect when the constructor for the most derived object ([intro.object]) ends.
6
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The address of a constructor shall not be taken.
[Note 6: 
A return statement in the body of a constructor cannot specify a return value ([stmt.return]).
— end note]
7
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A constructor shall not be a coroutine.
8
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A constructor shall not have an explicit object parameter ([dcl.fct]).

11.4.5.2 Default constructors [class.default.ctor]

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A default constructor 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).
If there is no user-declared constructor or constructor template for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).
An implicitly-declared default constructor is an inline public member of its class.
2
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A defaulted default constructor for class X is defined as deleted if
  • (2.1)
    any non-static data member with no default member initializer ([class.mem]) is of reference type,
  • (2.2)
    any non-variant non-static data member of const-qualified type (or possibly multidimensional array thereof) with no brace-or-equal-initializer is not const-default-constructible ([dcl.init]),
  • (2.3)
    X is a union and all of its variant members are of const-qualified type (or possibly multidimensional array thereof),
  • (2.4)
    X is a non-union class and all members of any anonymous union member are of const-qualified type (or possibly multidimensional array thereof),
  • (2.5)
    any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer or a variant member of a union where another non-static data member has a brace-or-equal-initializer, has class type M (or possibly multidimensional array thereof) and overload resolution ([over.match]) as applied to find M's corresponding constructor either does not result in a usable candidate ([over.match.general]) or, in the case of a variant member, selects a non-trivial function, or
  • (2.6)
    any potentially constructed subobject has class type M (or possibly multidimensional array thereof) and M has a destructor that is deleted or inaccessible from the defaulted default constructor.
3
#
A default constructor is trivial if it is not user-provided and if
  • (3.1)
    its class has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • (3.2)
    no non-static data member of its class has a default member initializer ([class.mem]), and
  • (3.3)
    all the direct base classes of its class have trivial default constructors, and
  • (3.4)
    for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.
Otherwise, the default constructor is non-trivial.
4
#
An implicitly-defined ([dcl.fct.def.default]) 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]) and an empty compound-statement.
If that user-written default constructor would be ill-formed, the program is ill-formed.
If that user-written default constructor would be constexpr-suitable ([dcl.constexpr]), the implicitly-defined default constructor is constexpr.
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.
[Note 1: 
An implicitly-declared default constructor has an exception specification ([except.spec]).
An explicitly-defaulted definition might have an implicit exception specification, see [dcl.fct.def].
— end note]
5
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[Note 2: 
A default constructor is implicitly invoked to initialize a class object when no initializer is specified ([dcl.init.general]).
Such a default constructor needs to be accessible ([class.access]).
— end note]
6
#
[Note 3: 
[class.base.init] 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.
— end note]

11.4.5.3 Copy/move constructors [class.copy.ctor]

1
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A non-template constructor for class X is a copy 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]).
[Example 1: 
X​::​X(const X&) and X​::​X(X&,int=1) are copy constructors.
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
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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]).
[Example 2: 
Y​::​Y(Y&&) is a move constructor.
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
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[Note 1: 
All forms of copy/move constructor can be declared for a class.
[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
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[Note 2: 
If a class X only has a copy constructor with a parameter of type X&, an initializer of type const X or volatile X cannot initialize an object of type cv X.
[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
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A declaration of a constructor for a class X is ill-formed if its first parameter is of type cv X and either there are no other parameters or else all other parameters have default arguments.
A member function template is never instantiated to produce such a constructor signature.
[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
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If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly.
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]).
The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor ([depr.impldec]).
7
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The implicitly-declared copy constructor for a class X will have the form X::X(const X&) if each potentially constructed subobject of a class type M (or array thereof) has a copy constructor whose first parameter is of type const M& or const volatile M&.89
Otherwise, the implicitly-declared copy constructor will have the form X::X(X&)
8
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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)
    X does not have a user-declared copy constructor,
  • (8.2)
    X does not have a user-declared copy assignment operator,
  • (8.3)
    X does not have a user-declared move assignment operator, and
  • (8.4)
    X does not have a user-declared destructor.
[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.
— end note]
9
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The implicitly-declared move constructor for class X will have the form X::X(X&&)
10
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An implicitly-declared copy/move constructor is an inline public member of its class.
A defaulted copy/​move constructor for a class X is defined as deleted ([dcl.fct.def.delete]) if X has:
  • (10.1)
    a potentially constructed subobject of type M (or possibly multidimensional array thereof) for which overload resolution ([over.match]), as applied to find M's corresponding constructor, either does not result in a usable candidate ([over.match.general]) or, in the case of a variant member, selects a non-trivial function,
  • (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)
    for the copy constructor, a non-static data member of rvalue reference type.
[Note 4: 
A defaulted move constructor that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
Such a constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead.
— end note]
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A copy/move constructor for class X is trivial if it is not user-provided and if
  • (11.1)
    class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • (11.2)
    the constructor selected to copy/move each direct base class subobject is trivial, and
  • (11.3)
    for each non-static data member of X 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.
12
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[Note 5: 
The copy/move constructor is implicitly defined even if the implementation elided its odr-use ([basic.def.odr], [class.temporary]).
— end note]
If an implicitly-defined ([dcl.fct.def.default]) constructor would be constexpr-suitable ([dcl.constexpr]), the implicitly-defined constructor is constexpr.
13
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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.
[Note 6: 
An implicitly-declared copy/move constructor has an implied exception specification ([except.spec]).
— end note]
14
#
The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members.
[Note 7: 
Default member initializers of non-static data members are ignored.
— 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]).
Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter.
Each base or non-static data member is copied/moved in the manner appropriate to its type:
  • (14.1)
    if the member is an array, each element is direct-initialized with the corresponding subobject of x;
  • (14.2)
    if a member m has rvalue reference type T&&, it is direct-initialized with static_cast<T&&>(x.m);
  • (14.3)
    otherwise, the base or member is direct-initialized with the corresponding base or member of x.
Virtual base class subobjects shall be initialized only once by the implicitly-defined copy/move constructor (see [class.base.init]).
15
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The implicitly-defined copy/move constructor for a union X copies the object representation ([basic.types.general]) of X.
For each object nested within ([intro.object]) 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.
89)89)
This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [diff.class].
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