std::exchange (C++14)

In a Nutshell

Assigns a new value to a variable while retrieving its old value, eliminating the need for a manual temporary variable.

Header

#include <utility>

Core API Quick Reference

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

Minimal Example

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

int main() {
    int a = 10, b = 20;
    // 交换 a 和 b，无需临时变量
    a = std::exchange(b, a);
    std::cout << a << " " << b << "\n"; // 输出: 10 10

    // 打印斐波那契数列前几项
    for (int x{0}, y{1}; x < 50; x = std::exchange(y, x + y))
        std::cout << x << " ";
}

Embedded Applicability: Medium

• It is a pure inline function with no extra heap allocation or system call overhead.
• It relies on move semantics; when used with custom types, we must verify the actual overhead of move construction or assignment.
• It is very concise for implementing move constructors and state machine transitions, making it suitable for resource-rich scenarios.
• Starting with C++20, it supports constexpr and 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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