NAME

std::atomic::exchange - std::atomic::exchange

Synopsis

T exchange( T desired, std::memory_order order = (1)
std::memory_order_seq_cst ) noexcept; (since C++11)
T exchange( T desired, std::memory_order order = (2)
std::memory_order_seq_cst ) volatile noexcept;

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

The volatile-qualified version (2) is deprecated if (since C++20)
std::atomic<T>::is_always_lock_free is false.

Parameters

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

Return value

The value of the atomic variable before the call.

Example

// Run this code

#include <algorithm>
#include <atomic>
#include <cstddef>
#include <iostream>
#include <syncstream>
#include <thread>
#include <vector>

int main(){
const std::size_t ThreadNumber = 5;
const int Sum = 5;
std::atomic<int> atom{0};
std::atomic<int> counter{0};

// lambda as thread proc
auto lambda = [&](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)
<< '#' << 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 < ThreadNumber; ++i){
v.emplace_back(lambda, i);
}

for (auto& tr : v){
tr.join();
}

std::cout << ThreadNumber << " threads adding 0 to "
<< Sum << " takes total "
<< counter << " times\n";
}

Possible output:

#1 (140552371918592) wrote 0 replacing the old value 0
#2 (140552363525888) wrote 0 replacing the old value 0
#1 (140552371918592) wrote 1 replacing the old value 0
#1 (140552371918592) wrote 2 replacing the old value 1
#2 (140552363525888) wrote 1 replacing the old value 1
#1 (140552371918592) wrote 3 replacing the old value 2
#1 (140552371918592) wrote 4 replacing the old value 2
#2 (140552363525888) wrote 2 replacing the old value 3
#2 (140552363525888) wrote 4 replacing the old value 0
#3 (140552355133184) wrote 0 replacing the old value 4
#0 (140552380311296) wrote 0 replacing the old value 0
#0 (140552380311296) wrote 1 replacing the old value 4
#4 (140552346740480) wrote 0 replacing the old value 1
#4 (140552346740480) wrote 2 replacing the old value 0
#4 (140552346740480) wrote 3 replacing the old value 2
#4 (140552346740480) wrote 4 replacing the old value 3
5 threads adding 0 to 5 takes total 16 times

See also

atomic_exchange atomically replaces the value of the atomic object with
atomic_exchange_explicit non-atomic argument and returns the old value of the atomic
(C++11) (function template)
(C++11)
exchange replaces the argument with a new value and returns its
(C++14) previous value
(function template)