Std::exchange (3) Linux Manual Page

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 );

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)

Possible implementation

  template<class T, class U = T>
  T exchange(T& obj, U&& new_value)
  {
      T old_value = std::move(obj);
      obj = std::forward<U>(new_value);
      return old_value;
  }

Notes

This function can be used when implementing move_assignment_operators and move_constructors:

struct S

{
    int *p;
    int n;
    S(S &&other)
        : p{std::exchange(other.p, nullptr)}, n{std::exchange(other.n, 0)}
    {
    }
    S &operator=(S &&other)
    {
        p = std::exchange(other.p, nullptr); // move p, while leaving nullptr in other.p
        n = std::exchange(other.n, 0);       // move n, while leaving zero in other.n
        return *this;
    }
};

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();
}

Output:

  0
  0
  12

1, 2, 3, 4,

f()

std::exchange (3) Linux Manual Page