A move assignment operator is a non-template
with the name operator= that can be called with an argument of the same class type and copies the content of the argument, possibly mutating the argument.
Syntax
For the formal move assignment operator syntax, see
. The syntax list below only demonstrates a subset of all valid move assignment operator syntaxes.
return-typeoperator=(parameter-list);(1) return-typeoperator=(parameter-list)function-body(2) return-typeoperator=(parameter-list-no-default) = default;(3) return-typeoperator=(parameter-list) = delete;(4) return-typeclass-name::operator=(parameter-list)function-body(5) return-typeclass-name::operator=(parameter-list-no-default) = default;(6) class-name- the class whose move assignment operator is being declared, the class type is given as T in the descriptions below parameter-list- a
of only one parameter, which is of type T&&, constT&&, volatileT&& or constvolatileT&¶meter-list-no-default- a
of only one parameter, which is of type T&&, constT&&, volatileT&& or constvolatileT&& and does not have a default argument function-body- the
of the move assignment operator return-type- any type, but T& is favored in order to be consistent with scala types Explanation
1) Declaration of a move assignment operator inside of class definition.
2-4) Definition of a move assignment operator inside of class definition.
3) The move assignment operator is explicitly-defaulted.
4) The move assignment operator is deleted.
5,6) Definition of a move assignment operator outside of class definition (the class must contain a declaration (1)).
6) The move assignment operator is explicitly-defaulted.
structX{X&operator=(X&&other);// move assignment operator// X operator=(const X other); // Error: incorrect parameter type};unionY{// move assignment operators can have syntaxes not listed above,// as long as they follow the general function declaration syntax// and do not viloate the restrictions listed aboveautooperator=(Y&&other)->Y&;// OK: trailing return typeY&operator=(thisY&&self,Y&other);// OK: explicit object parameter// Y& operator=(Y&&, int num = 1); // Error: has other non-object parameters};The move assignment operator is called whenever it is selected by
, e.g. when an object appears on the left-hand side of an assignment expression, where the right-hand side is an rvalue of the same or implicitly convertible type.
Move assignment operators typically transfer the resources held by the argument (e.g. pointers to dynamically-allocated objects, file descriptors, TCP sockets, thread handles, etc.), rather than make copies of them, and leave the argument in some valid but otherwise indeterminate state. Since move assignment doesn’t change the lifetime of the argument, the destructor will typically be called on the argument at a later point. For example, move-assigning from a
or from a
may result in the argument being left empty. A move assignment is less, not more restrictively defined than ordinary assignment; where ordinary assignment must leave two copies of data at completion, move assignment is required to leave only one.
Implicitly-declared move assignment operator
If no user-defined move assignment operators are provided for a class type, and all of the following is true:
there are no user-declared
;
there are no user-declared
;
there are no user-declared
;
there is no user-declared
,
then the compiler will declare a move assignment operator as an inlinepublic member of its class with the signature T&T::operator=(T&&).
A class can have multiple move assignment operators, e.g. both T&T::operator=(constT&&) and T&T::operator=(T&&). If some user-defined move assignment operators are present, the user may still force the generation of the implicitly declared move assignment operator with the keyword default.
The implicitly-declared move assignment operator has an exception specification as described in
dynamic exception specification
(until C++17)
(since C++17).
Because some assignment operator (move or copy) is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.
Implicitly-defined move assignment operator
If the implicitly-declared move assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if
or
needed for constant evaluation
(since C++14).
For union types, the implicitly-defined move assignment operator copies the object representation (as by
).
For non-union class types, the move assignment operator performs full member-wise move assignment of the object's direct bases and immediate non-static members, in their declaration order, using built-in assignment for the scalars, memberwise move-assignment for arrays, and move assignment operator for class types (called non-virtually).
The implicitly-defined move assignment operator for a class T is
if
T is a
, and
the assignment operator selected to move each direct base class subobject is a constexpr function, and
for each non-static data member of T that is of class type (or array thereof), the assignment operator selected to move that member is a constexpr function.
(since C++14)
(until C++23)The implicitly-defined move assignment operator for a class T is
.
(since C++23)As with copy assignment, it is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined move assignment operator:
structV{V&operator=(V&&other){// this may be called once or twice// if called twice, 'other' is the just-moved-from V subobjectreturn*this;}};structA:virtualV{};// operator= calls V::operator=structB:virtualV{};// operator= calls V::operator=structC:B,A{};// operator= calls B::operator=, then A::operator=// but they may only call V::operator= onceintmain(){Cc1,c2;c2=std::move(c1);}Deleted move assignment operator
The implicitly-declared or defaulted move assignment operator for class T is defined as deleted if any of the following conditions is satisfied:
T has a non-static data member of a const-qualified non-class type (or possibly multi-dimensional array thereof).
T has a non-static data member of a reference type.
T has a
potentially constructed subobject
of class type M (or possibly multi-dimensional array thereof) such that the overload resolution as applied to find M's move assignment operator
does not result in a usable candidate, or
in the case of the subobject being a
, selects a non-trivial function.
A deleted implicitly-declared move assignment operator is ignored by
.
Trivial move assignment operator
The move assignment operator for class T is trivial if all of the following is true:
It is not user-provided (meaning, it is implicitly-defined or defaulted);
T has no virtual member functions;
T has no virtual base classes;
the move assignment operator selected for every direct base of T is trivial;
the move assignment operator selected for every non-static class type (or array of class type) member of T is trivial.
A trivial move assignment operator performs the same action as the trivial copy assignment operator, that is, makes a copy of the object representation as if by
. All data types compatible with the C language are trivially move-assignable.
Eligible move assignment operator
A move assignment operator is eligible if it is not deleted.
(until C++20)A move assignment operator is eligible if all following conditions are satisfied:
It is not deleted.
Its
(if any) are satisfied.
No move assignment operator whose associated constraints are satisfied is
.
(since C++20)Triviality of eligible move assignment operators determines whether the class is a
.
Notes
If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an
(either a
such as a nameless temporary or an
such as the result of std::move), and selects the copy assignment if the argument is an
(named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.
It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined move assignment operator (same applies to
).
See
assignment operator overloading
for additional detail on the expected behavior of a user-defined move-assignment operator.
Example
Run this code
#include<iostream>#include<string>#include<utility>structA{std::strings;A():s("test"){}A(constA&o):s(o.s){std::cout<<"move failed!\n";}A(A&&o):s(std::move(o.s)){}A&operator=(constA&other){s=other.s;std::cout<<"copy assigned\n";return*this;}A&operator=(A&&other){s=std::move(other.s);std::cout<<"move assigned\n";return*this;}};Af(Aa){returna;}structB:A{std::strings2;intn;// implicit move assignment operator B& B::operator=(B&&)// calls A's move assignment operator// calls s2's move assignment operator// and makes a bitwise copy of n};structC:B{~C(){}// destructor prevents implicit move assignment};structD:B{D(){}~D(){}// destructor would prevent implicit move assignmentD&operator=(D&&)=default;// force a move assignment anyway };intmain(){Aa1,a2;std::cout<<"Trying to move-assign A from rvalue temporary\n";a1=f(A());// move-assignment from rvalue temporarystd::cout<<"Trying to move-assign A from xvalue\n";a2=std::move(a1);// move-assignment from xvaluestd::cout<<"\nTrying to move-assign B\n";Bb1,b2;std::cout<<"Before move, b1.s = \""<<b1.s<<"\"\n";b2=std::move(b1);// calls implicit move assignmentstd::cout<<"After move, b1.s = \""<<b1.s<<"\"\n";std::cout<<"\nTrying to move-assign C\n";Cc1,c2;c2=std::move(c1);// calls the copy assignment operatorstd::cout<<"\nTrying to move-assign D\n";Dd1,d2;d2=std::move(d1);}Output:
Trying to move-assign A from rvalue temporary move assigned Trying to move-assign A from xvalue move assigned Trying to move-assign B Before move, b1.s = "test" move assigned After move, b1.s = "" Trying to move-assign C copy assigned Trying to move-assign D move assigned Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++11 the conditions where defaulted move assignment operators are
defined as deleted did not consider multi-dimensional array types consider these types
C++11 a defaulted move assignment operator that would
call a non-trivial copy assignment operator was
deleted; a defaulted move assignment operator that
is deleted still participated in overload resolution allows call to such
copy assignment
operator; made ignored
in overload resolution
C++11 specification for a defaulted move assignment operator
involving a virtual base class was missing added
C++11 a volatile subobject made of a defaulted
move assignment operator non-trivial (
) triviality not affected
C++11 a defaulted move assignment operator for class T
was not defined as deleted if T is abstract and has
non-move-assignable direct virtual base classes the operator is defined
as deleted in this case
C++20 a move assignment operator was not eligible if there
is another move assignment operator which is more
constrained but does not satisfy its associated constraints it can be eligible in this case
C++11 the implicitly-defined move assignment operator for
union types did not copy the object representation they copy the object
representation See also