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

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
function::function
function::~function
function::operator=
function::swap
function::assign (until C++17)
function::operator bool
function::operator()
function::target_type
function::target
Non-member functions
operator==operator!= (until C++20)
swap(std::function)
Helper classes
uses_allocator<std::function> (until C++17)
Deduction guides(C++17)

operator=	replaces or destroys the target (public member function of `std::move_only_function`)
assign (removed in C++17)	assigns a new target (public member function)