NAME¶

std::exchange - std::exchange

Synopsis¶

Defined in header <utility>
template< class T, class U = T > (since C++14)
T exchange( T& obj, U&& new_value ); (until C++20)
template< class T, class U = T > (since C++20)
constexpr T exchange( T& obj, U&& new_value ); (until C++23)
template< class T, class U = T >
constexpr T exchange( T& obj, U&& new_value ) noexcept(/* see below (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> && (since C++23)
std::is_nothrow_assignable_v<T&, U>

)

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¶

The 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_exchange atomically replaces the value of the atomic object with
atomic_exchange_explicit non-atomic argument and returns the old value of the atomic
(C++11) (function template)
(C++11)

Category:¶

* conditionally noexcept