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std::atomic<T>::exchange

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< cpp‎ | atomic‎ | atomic

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(C++11)

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hardware_destructive_interference_sizehardware_constructive_interference_size

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atomic_exchangeatomic_exchange_explicit

(C++11)(C++11)

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(C++11)(C++11)(C++11)(C++11)

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(C++26)(C++26)

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atomic::operator=

atomic::is_lock_free

atomic::operator T

atomic::exchange

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(C++20)

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(C++20)

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(C++17)

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Specialized for integral,
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atomic::operator+=atomic::operator-=

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(C++26)

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(C++26)

atomic::operator++atomic::operator++(int)atomic::operator--atomic::operator--(int)

Specialized for integral types only

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atomic::fetch_or

atomic::fetch_xor

atomic::operator&=atomic::operator|=atomic::operator^=

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

Run this code

#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

atomic_exchangeatomic_exchange_explicit

(C++11)(C++11)

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]

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