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) (until C++20)
template< class T, class U = T > constexpr T exchange( T& obj, U&& new_value );		(since C++20) (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> &&     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

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, ...

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)