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)

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:

struct S
{
  int* p;
  int n;
  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;
  }
}