11 Classes [class]

11.4 Class members [class.mem]

11.4.4 Constructors [class.ctor]

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

1
#
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
:
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
#
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
:
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
#
[Note
:
All forms of copy/move constructor may be declared for a class.
[Example
: 
struct X {
  X(const X&);
  X(X&);            // OK
  X(X&&);
  X(const X&&);     // OK, but possibly not sensible
};
— end example
]
— end note
]
4
#
[Note
:
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
: 
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
#
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
: 
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
#
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 defined as 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
#
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&.103
Otherwise, the implicitly-declared copy constructor will have the form 
X::X(X&)
8
#
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
:
When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor.
— end note
]
9
#
The implicitly-declared move constructor for class X will have the form 
X::X(X&&)
10
#
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 type M (or array thereof) that cannot be copied/moved because overload resolution ([over.match]), as applied to find M's corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
  • (10.2)
    a variant member whose corresponding constructor as selected by overload resolution is non-trivial,
  • (10.3)
    any potentially constructed subobject of a type with a destructor that is deleted or inaccessible from the defaulted constructor, or,
  • (10.4)
    for the copy constructor, a non-static data member of rvalue reference type.
[Note
:
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
]
11
#
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
#
A copy/move constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.
[Note
:
The copy/move constructor is implicitly defined even if the implementation elided its odr-use ([basic.def.odr], [class.temporary]).
— end note
]
If the implicitly-defined constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined constructor is constexpr.
13
#
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
:
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
:
Default member initializers of non-static data members are ignored.
See also the example in [class.base.init].
— 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
#
The implicitly-defined copy/move constructor for a union X copies the object representation ([basic.types]) 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.
103)
This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [class]: classes'>[diff.class].
⮥