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
{
if(this != &other)
n = std::exchange(other.n, 0); // move n, while leaving zero in other.n
return *this;
}
};

Feature-test macro: __cpp_lib_exchange_function

Example

// Run this code

#include <iostream>
#include <utility>
#include <vector>
#include <iterator>

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, ...