std::exchange(3)

💖 std::exchange 💖

okay bestie, welcome to the C++ Standard Libary, section 3 — this page is literally all about std::exchange. no cap, it's a tiny utility with main-character energy.

🌸 NAME 🌸

std::exchange - std::exchange

📜 Synopsis 📜

it's defined in header <utility>, periodt.

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)

It replaces the value of obj with new_value and returns the old value of obj. literally a glow-up for obj, and you get the old vibe back. 💅

🧸 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. that's the whole gift, bestie 🫶

🔥 Exceptions 🔥

• (none) until C++23.
• noexcept specification since C++23:

noexcept(
    std::is_nothrow_move_constructible_v<T> &&
    std::is_nothrow_assignable_v<T&, U>
)

🛠️ 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
    {
        n = std::exchange(other.n, 0); // Move n, while leaving zero in other.n
                                       // (note: in self-move-assignment, n is unchanged)
        return *this;
    }
};

🧪 Feature-test macro 🧪

• Feature-test macro: __cpp_lib_exchange_function
• Value: 201304L
• Std: (C++14)
• Feature: std::exchange

✨ Example ✨

#include <iostream>
#include <iterator>
#include <utility>
#include <vector>

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_exchange — atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template) (C++11).
• atomic_exchange_explicit — atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template) (C++11).

🏷️ Category 🏷️

• * conditionally noexcept

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shoutout to cppreference.com for the source, and this version is dated 2024.06.10 — page std::exchange(3). mwah 💖🫶