std::exchange

 

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Defined in header <utility>
template< class T, class U = T > T exchange( T& obj, U&& new_value );		(since C++14) (constexpr since C++20) (conditionally noexcept since C++23)

Replaces the value of obj with new_value and returns the old value of obj.

Parameters

obj	-	object whose value to replace
new_value	-	the value to assign to obj
Type requirements
- T must meet the requirements of MoveConstructible. Also, it must be possible to move-assign objects of type U to objects of type T.

Return value

The old value of obj.

Exceptions

(none)	(until C++23)
noexcept specification:   noexcept(     std::is_nothrow_move_constructible_v<T> &&     std::is_nothrow_assignable_v<T&, U> )	(since C++23)

Possible implementation

template<class T, class U = T> constexpr // Since C++20 T exchange(T& obj, U&& new_value) noexcept( // Since C++23 std::is_nothrow_move_constructible<T>::value && std::is_nothrow_assignable<T&, U>::value ) { T old_value = std::move(obj); obj = std::forward<U>(new_value); return old_value; }

Notes

std::exchange can be used when implementing move assignment operators and move constructors:

struct S
{
    int n;
 
    S(S&& other) noexcept : n{std::exchange(other.n, 0)} {}
 
    S& operator=(S&& other) noexcept
    {
        n = std::exchange(other.n, 0); // Move n, while leaving zero in other.n
                                       // (note: in self-move-assignment, n is unchanged)
        return *this;
    }
};

Feature-test macro	Value	Std	Feature
__cpp_lib_exchange_function	201304L	(C++14)	std::exchange

Example

Run this code

#include <iostream>
#include <iterator>
#include <utility>
#include <vector>
 
class stream
{
public:
    using flags_type = int;
 
public:
    flags_type flags() const { return flags_; }
 
    // Replaces flags_ by newf, and returns the old value.
    flags_type flags(flags_type newf) { return std::exchange(flags_, newf); }
 
private:
    flags_type flags_ = 0;
};
 
void f() { std::cout << "f()"; }
 
int main()
{
    stream s;
 
    std::cout << s.flags() << '\n';
    std::cout << s.flags(12) << '\n';
    std::cout << s.flags() << "\n\n";
 
    std::vector<int> v;
 
    // Since the second template parameter has a default value, it is possible
    // to use a braced-init-list as second argument. The expression below
    // is equivalent to std::exchange(v, std::vector<int>{1, 2, 3, 4});
 
    std::exchange(v, {1, 2, 3, 4});
 
    std::copy(begin(v), end(v), std::ostream_iterator<int>(std::cout, ", "));
 
    std::cout << "\n\n";
 
    void (*fun)();
 
    // The default value of template parameter also makes possible to use a
    // normal function as second argument. The expression below is equivalent to
    // std::exchange(fun, static_cast<void(*)()>(f))
    std::exchange(fun, f);
    fun();
 
    std::cout << "\n\nFibonacci sequence: ";
    for (int a{0}, b{1}; a < 100; a = std::exchange(b, a + b))
        std::cout << a << ", ";
    std::cout << "...\n";
}

Output:

0
0
12
 
1, 2, 3, 4,
 
f()
 
Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

See also

swap	swaps the values of two objects (function template)
atomic_exchangeatomic_exchange_explicit (C++11)(C++11)	atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template)