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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../utility/exchange.html below:

std::exchange - cppreference.com

template< class T, class U = T >
T exchange( T& obj, U&& new_value );

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

The old value of obj.

std::exchange can be used when implementing move constructors and, for the members that don't require special cleanup, move assignment operators:

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 case of self-move-assignment, n is unchanged
        // Also note: if n is an opaque resource handle that requires
        //            special cleanup, the resource is leaked.
        return *this;
    }
};

#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, ...

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