Namespaces
Variants
Views
Actions

std::atomic<T>::exchange

From cppreference.com
< cpp‎ | atomic‎ | atomic
 
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++20)
this_thread namespace
(C++11)
(C++11)
(C++11)
Mutual exclusion
(C++11)
(C++11)  
Generic lock management
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Condition variables
(C++11)
Semaphores
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
Safe Reclamation
(C++26)
(C++26)
Hazard Pointers





Atomic types
(C++11)
(C++20)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
 
 
T exchange( T desired, std::memory_order order =
                           std::memory_order_seq_cst ) noexcept;
(1) (since C++11)
T exchange( T desired, std::memory_order order =
                           std::memory_order_seq_cst ) volatile noexcept;
(2) (since C++11)

Atomically replaces the underlying value with desired (a read-modify-write operation). Memory is affected according to the value of order.

It is deprecated if std::atomic<T>::is_always_lock_free is false and overload (2) participates in overload resolution.

(since C++20)

Contents

[edit] Parameters

desired - value to assign
order - memory order constraints to enforce

[edit] Return value

The value of the atomic variable before the call.

[edit] Example

#include <algorithm>
#include <atomic>
#include <cstddef>
#include <iostream>
#include <syncstream>
#include <thread>
#include <vector>
 
int main()
{
    constexpr int thread_count{5};
    constexpr int sum{5};
 
    std::atomic<int> atom{0};
    std::atomic<int> counter{0};
 
    auto increment_to_sum = [&](const int id)
    {
        for (int next = 0; next < sum;)
        {
            // each thread is writing a value from its own knowledge
            const int current = atom.exchange(next);
            counter++;
            // sync writing to prevent from interrupting by other threads
            std::osyncstream(std::cout)
                << "Thread #" << id << " (id=" << std::this_thread::get_id()
                << ") wrote " << next << " replacing the old value "
                << current << ".\n";
            next = std::max(current, next) + 1;
        }
    };
 
    std::vector<std::thread> v;
    for (std::size_t i = 0; i < thread_count; ++i)
        v.emplace_back(increment_to_sum, i);
 
    for (auto& tr : v)
        tr.join();
 
    std::cout << thread_count << " threads take "
              << counter << " times in total to "
              << "increment 0 to " << sum << ".\n";
}

Possible output:

Thread #1 (id=139722332333824) wrote 0 replacing the old value 0.
Thread #2 (id=139722323941120) wrote 0 replacing the old value 0.
Thread #1 (id=139722332333824) wrote 1 replacing the old value 0.
Thread #1 (id=139722332333824) wrote 2 replacing the old value 1.
Thread #1 (id=139722332333824) wrote 3 replacing the old value 2.
Thread #1 (id=139722332333824) wrote 4 replacing the old value 3.
Thread #0 (id=139722340726528) wrote 0 replacing the old value 0.
Thread #3 (id=139722315548416) wrote 0 replacing the old value 0.
Thread #3 (id=139722315548416) wrote 1 replacing the old value 4.
Thread #0 (id=139722340726528) wrote 1 replacing the old value 1.
Thread #4 (id=139722307155712) wrote 0 replacing the old value 1.
Thread #4 (id=139722307155712) wrote 2 replacing the old value 2.
Thread #4 (id=139722307155712) wrote 3 replacing the old value 2.
Thread #4 (id=139722307155712) wrote 4 replacing the old value 3.
Thread #2 (id=139722323941120) wrote 1 replacing the old value 0.
Thread #0 (id=139722340726528) wrote 2 replacing the old value 1.
Thread #2 (id=139722323941120) wrote 2 replacing the old value 4.
Thread #0 (id=139722340726528) wrote 3 replacing the old value 2.
Thread #0 (id=139722340726528) wrote 4 replacing the old value 3.
5 threads take 19 times in total to increment 0 to 5.

[edit] See also

atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic
(function template) [edit]
(C++14)
replaces the argument with a new value and returns its previous value
(function template) [edit]