|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Member functions | ||||
atomic::exchange | ||||
(C++20) | ||||
(C++20) | ||||
(C++20) | ||||
| Constants | ||||
(C++17) | ||||
| Specialized member functions | ||||
| Specialized for integral, floating-point(C++20) and pointer types | ||||
| Specialized for integral and pointer types only | ||||
(C++26) | ||||
(C++26) | ||||
| Specialized for integral types only | ||||
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)volatilenoexcept; | (2) | (since C++11) |
Atomically replaces the underlying value withdesired (a read-modify-write operation). Memory is affected according to the value oforder.
It is deprecated ifstd::atomic<T>::is_always_lock_free isfalse and overload(2) participates in overload resolution. | (since C++20) |
Contents |
| desired | - | value to assign |
| order | - | memory order constraints to enforce |
The value of the atomic variable before the call.
#include <algorithm>#include <atomic>#include <cstddef>#include <iostream>#include <syncstream>#include <thread>#include <vector> int main(){constexprint thread_count{5};constexprint sum{5}; std::atomic<int> atom{0};std::atomic<int> counter{0}; auto increment_to_sum=[&](constint id){for(int next=0; next< sum;){// each thread is writing a value from its own knowledgeconstint current= atom.exchange(next); counter++;// sync writing to prevent from interrupting by other threadsstd::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.
(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] |
