15 Special member functions [special]

15.8 Copying and moving class objects [class.copy]

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

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.
[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
]
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.
[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
]
[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
]
[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 (possibly cv-qualified) 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
]
A declaration of a constructor for a class X is ill-formed if its first parameter is of type (optionally cv-qualified) 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
]
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.
The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.
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&.119
Otherwise, the implicitly-declared copy constructor will have the form
X::X(X&)
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
  • X does not have a user-declared copy constructor,
  • X does not have a user-declared copy assignment operator,
  • X does not have a user-declared move assignment operator, and
  • 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
]
The implicitly-declared move constructor for class X will have the form
X::X(X&&)
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 if X has:
  • a variant member with a non-trivial corresponding constructor and X is a union-like class,
  • a potentially constructed subobject type M (or array thereof) that cannot be copied/moved because overload resolution, as applied to find M's corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
  • any potentially constructed subobject of a type with a destructor that is deleted or inaccessible from the defaulted constructor, or,
  • for the copy constructor, a non-static data member of rvalue reference type.
A defaulted move constructor that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
[Note
:
A deleted move constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead.
end note
]
A copy/move constructor for class X is trivial if it is not user-provided and if:
  • class X has no virtual functions and no virtual base classes, and
  • the constructor selected to copy/move each direct base class subobject is trivial, and
  • 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.
A copy/move constructor that is defaulted and not defined as deleted is implicitly defined if it is odr-used 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 ([class.temporary]).
end note
]
If the implicitly-defined constructor would satisfy the requirements of a constexpr constructor, the implicitly-defined constructor is constexpr.
Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobjects shall have been implicitly defined.
[Note
:
An implicitly-declared copy/move constructor has an implied exception specification.
end note
]
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:
  • if the member is an array, each element is direct-initialized with the corresponding subobject of x;
  • if a member m has rvalue reference type T&&, it is direct-initialized with static_­cast<T&&>(x.m);
  • 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]).
The implicitly-defined copy/move constructor for a union X copies the object representation of X.
This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [diff.special].