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

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

#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.

See also