Namespaces
Variants
Actions

std::atomic_exchange, std::atomic_exchange_explicit

From cppreference.com
< cpp‎ | atomic
 
C++
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++17)(C++17)
this_thread namespace
(C++11)
(C++11)
(C++11)
(C++11)
Cooperative cancellation
Mutual exclusion
(C++11)
(C++11)
(C++17)
Generic lock management
(C++11)
(C++11)
(C++17)
(C++11)
(C++14)
(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
Condition variables
(C++11)
(C++11)
(C++11)
(C++11)
Semaphores
(C++20)(C++20)
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Safe reclamation
(C++26)
(C++26)
(C++26)
Hazard pointers
(C++26)
(C++26)
(C++26)
Atomic types
(C++11)
(C++20)
(C++11)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
(C++11)
Memory ordering
(C++11)
(C++11)(deprecated in C++26)
(C++11)
(C++11)
Free functions for atomic operations
(C++11)(C++11)
(C++11)(C++11)
atomic_exchangeatomic_exchange_explicit
(C++11)(C++11)
(C++11)(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++26)(C++26)
(C++26)(C++26)
(C++11)
(C++20)(C++20)
(C++20)
(C++20)
Free functions for atomic flags
(C++11)(C++11)
(C++11)(C++11)
(C++20)(C++20)
(C++20)(C++20)
(C++20)
(C++20)
 
Defined in header <atomic>
template< class T >

T atomic_exchange( std::atomic<T>* obj,

                   typename std::atomic<T>::value_type desired ) noexcept;
 (1) (since C++11)
template< class T >

T atomic_exchange( volatile std::atomic<T>* obj,

                   typename std::atomic<T>::value_type desired ) noexcept;
 (2) (since C++11)
template< class T >

T atomic_exchange_explicit( std::atomic<T>* obj,
                            typename std::atomic<T>::value_type desired,

                            std::memory_order order ) noexcept;
 (3) (since C++11)
template< class T >

T atomic_exchange_explicit( volatile std::atomic<T>* obj,
                            typename std::atomic<T>::value_type desired,

                            std::memory_order order ) noexcept;
 (4) (since C++11)
1,2) Atomically replaces the value pointed to by obj with the value of desired and returns the value obj held previously, as if by obj->exchange(desired).
3,4) Atomically replaces the value pointed to by obj with the value of desired and returns the value obj held previously, as if by obj->exchange(desired, order).

Contents

[edit] Parameters

obj - pointer to the atomic object to modify
desired - the value to store in the atomic object
order - the memory synchronization ordering

[edit] Return value

The value held previously by the atomic object pointed to by obj.

[edit] Example

A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:

Run this code
#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
std::atomic<bool> lock(false); // holds true when locked
                               // holds false when unlocked
 
int new_line{1}; // the access is synchronized via atomic lock variable
 
void f(int n)
{
    for (int cnt = 0; cnt < 100; ++cnt)
    {
        while (std::atomic_exchange_explicit(&lock, true, std::memory_order_acquire))
            ; // spin until acquired
        std::cout << n << (new_line++ % 80 ? "" : "\n");
        std::atomic_store_explicit(&lock, false, std::memory_order_release);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 8; ++n)
        v.emplace_back(f, n);
    for (auto& t : v)
        t.join();
}

Possible output: 

02222222222222222222222002222222222222222222222222222222222222222222222222222222
22222222200022222222202222211111111111110000011111111100000000000000110001111111
00011111000001111110000011111100000111000000001111111111111110000010000001001111
11011111111011111011000000000000111100000000000001111000011133333333333333333333
33333333333333333333333333333333333333333333333333333333333333333333333333333333
44444444444444444444444444444444444444444444444444444444444444444444444444444444
44444444444444444444555555555555555555555555555555555555555555555555555555555555
55555555555555555555555555555555555555556666666666666666666666666666666666666666
66666666666666666666666666666666666666666666666666666666666677777777777777777777
77777777777777777777777777777777777777777777777777777777777777777777777777777777

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0558R1 C++11 exact type match was required because
T was deduced from multiple arguments 		T is only deduced
from obj

[edit] See also

 atomically replaces the value of the atomic object and obtains the value held previously
(public member function of std::atomic<T>) [edit]
(C++11)(C++11)(C++11)(C++11)
 atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not
(function template) [edit]
(deprecated in C++20)(removed in C++26)
 specializes atomic operations for std::shared_ptr
(function template)
C documentation for atomic_exchange, atomic_exchange_explicit
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_exchange&oldid=171254"