20 General utilities library [utilities]

20.2 Utility components [utility]

20.2.3 exchange [utility.exchange]

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template<class T, class U = T> constexpr T exchange(T& obj, U&& new_val);
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Effects: Equivalent to: 
T old_val = std::move(obj);
obj = std::forward<U>(new_val);
return old_val;