std::exchange (C++14)

In a Nutshell

Assigns a new value to a variable and hands you its old value at the same time, sparing you a hand-written temporary.

Header

#include <utility>

Core API Quick Reference

Operation	Signature	Description
Replace and return the old value	template<class T, class U = T> T exchange(T& obj, U&& new_value);	Replaces obj with new_value and returns the old value of obj

Minimal Example

// Standard: C++14
#include <iostream>
#include <utility>

int main() {
    int a = 10, b = 20;
    // Swap a and b without a temporary variable
    a = std::exchange(b, a);
    std::cout << a << " " << b << "\n"; // Output: 20 10

    // Print the first few Fibonacci numbers
    for (int x{0}, y{1}; x < 50; x = std::exchange(y, x + y))
        std::cout << x << " ";
}

Embedded Applicability: Medium

• A pure inline function, with no extra heap allocation or system-call overhead
• Relies on move semantics; when using it with custom types, verify the actual cost of the move constructor/assignment
• Very concise for implementing move constructors and state machine transitions, a good fit for resource-rich targets
• constexpr since C++20, so it can be used at compile time

Compiler Support

GCC	Clang	MSVC
5.0	3.4	19.0

See Also

• cppreference: std::exchange

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