C++ Atomic Library - exchange

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Description

It atomically replaces the value pointed to by obj with the value of desr and returns the value obj held previously, as if by obj → exchange(desr)

It atomically replaces the value pointed to by obj with the value of desr and returns the value obj held previously, as if by obj → exchange(desr, order)

Declaration

Following is the declaration for std::atomic_exchange.

template< class T >
T atomic_exchange( std::atomic<T>* obj, T desr );

C++11

template< class T >
T atomic_exchange( volatile std::atomic<T>* obj, T desr );

Following is the declaration for std::atomic_exchange_explicit.

template< class T >
T atomic_exchange_explicit( std::atomic<T>* obj, T desr, 
                            std::memory_order order );

C++11

template< class T >
T atomic_exchange_explicit( volatile std::atomic<T>* obj, T desr, 
                            std::memory_order order );

Parameters

• obj − It is used in pointer to the atomic object to modify.

• desr − It is used the value to store in the atomic object.

• order − It is used synchronise the memory order for the value.

Return Value

It returns the value held previously by the atomic object pointed to by obj.

Exceptions

No-noexcept − this member function never throws exceptions.

Example

In below example for std::atomic_exchange and std::atomic_exchange_explicit.

#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
 
std::atomic<bool> lock(false); 
                               
 
void f(int n) {
   for (int cnt = 0; cnt < 100; ++cnt) {
      while(std::atomic_exchange_explicit(&lock, true, std::memory_order_acquire))
         ; 
      std::cout << "Output from thread " << n << '\n';
      std::atomic_store_explicit(&lock, false, std::memory_order_release);
   }
}
int main() {
   std::vector<std::thread> v;
   for (int n = 0; n < 10; ++n) {
      v.emplace_back(f, n);
   }
   for (auto& t : v) {
      t.join();
   }
}

The sample output should be like this −

Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
Output from thread 0
....................