std::function<R(Args...)>::operator=
From cppreference.com
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) |