std::function<R(Args...)>:: operator=

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std::function
function & operator = ( const function & other ) ;
(1) (since C++11)
function & operator = ( function && other ) ;
(2) (since C++11)
function & operator = ( std:: nullptr_t ) noexcept ;
(3) (since C++11)
template < class F >
function & operator = ( F && f ) ;
(4) (since C++11)
template < class F >
function & operator = ( std:: reference_wrapper < F > f ) noexcept ;
(5) (since C++11)

Assigns a new target to std::function .

1) Assigns a copy of target of other , as if by executing function ( other ) . swap ( * this ) ;
2) Moves the target of other to * this . other is in a valid state with an unspecified value.
3) Drops the current target . * this is empty after the call.
4) Sets the target of * this to the callable f , as if by executing function ( std:: forward < F > ( f ) ) . swap ( * this ) ; . This operator does not participate in overload resolution unless f is Callable for argument types Args... and return type R .
5) Sets the target of * this to a copy of f , as if by executing function ( f ) . swap ( * this ) ;

Parameters

other - another std::function object to copy the target of
f - a callable to initialize the target with
Type requirements
-
F must meet the requirements of Callable .

Return value

* this

Notes

Even before allocator support was removed from std::function in C++17, these assignment operators use the default allocator rather than the allocator of * this or the allocator of other (see LWG issue 2386 ).

Example

Run this code 
#include <cassert>
#include <functional>
#include <utility>
 
int inc(int n) { return n + 1; }
 
int main()
{
    std::function<int(int)> f1;
    std::function<int(int)> f2(inc);
    assert(f1 == nullptr and f2 != nullptr);
 
    f1 = f2; // overload (1)
    assert(f1 != nullptr and f1(1) == 2);
 
    f1 = std::move(f2); // overload (2)
    assert(f1 != nullptr and f1(1) == 2);
    // f2 is in valid but unspecified state
 
    f1 = nullptr; // overload (3)
    assert(f1 == nullptr);
 
    f1 = inc; // overload (4)
    assert(f1 != nullptr and f1(1) == 2);
 
    f1 = [](int n) { return n + n; }; // overload (4)
    assert(f1 != nullptr and f1(2) == 4);
 
    std::reference_wrapper<int(int)> ref1 = std::ref(inc);
    f1 = ref1; // overload (5)
    assert(f1 != nullptr and f1(1) == 2);
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2132 C++11 the overload ( 4 ) taking a Callable object might be ambiguous constrained
LWG 2401 C++11 assignment operator ( 3 ) from std::nullptr_t not required to be noexcept required

See also

replaces or destroys the target
(public member function of std::move_only_function )
(removed in C++17)
assigns a new target
(public member function)
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