std::function<R(Args...)>::operator= - cppreference.com (original) (raw)

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

#include #include #include   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] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | assigns a new target (public member function) [edit] |