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

From cppreference.com
< cpp‎ | utility‎ | functional‎ | function
 
 
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Old binders and adaptors
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
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(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
 
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);

[edit] 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.

[edit] Return value

*this

[edit] 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).

[edit] 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);
}

[edit] 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

[edit] See also

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

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