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)
function& operator=( F&& f );
template< class F >
function& operator=( std::reference_wrapper <F> f ) noexcept;
(5)
(since C++11)
function& operator=( std::reference_wrapper <F> f ) noexcept;
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 |