NAME¶

std::atomic_exchange,std::atomic_exchange_explicit - std::atomic_exchange,std::atomic_exchange_explicit

Synopsis¶

Defined in header <atomic>
template< class T >

T atomic_exchange( std::atomic<T>* obj, (1) (since C++11)

typename std::atomic<T>::value_type desired )
noexcept;
template< class T >

T atomic_exchange( volatile std::atomic<T>* obj, (2) (since C++11)

typename std::atomic<T>::value_type desired )
noexcept;
template< class T >

T atomic_exchange_explicit( std::atomic<T>* obj,
typename std::atomic<T>::value_type (3) (since C++11)
desired,

std::memory_order order ) noexcept;
template< class T >

T atomic_exchange_explicit( volatile std::atomic<T>* obj,
typename std::atomic<T>::value_type (4) (since C++11)
desired,

std::memory_order order ) noexcept;

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).

Parameters¶

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

Return value¶

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

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

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 is only deduced
T was deduced from multiple arguments from obj

See also¶

atomically replaces the value of the atomic
exchange object and obtains the value held previously
(public member function of std::atomic<T>)

atomic_compare_exchange_weak
atomic_compare_exchange_weak_explicit atomically compares the value of the atomic
atomic_compare_exchange_strong object with non-atomic argument and performs
atomic_compare_exchange_strong_explicit atomic exchange if equal or atomic load if
(C++11) not
(C++11) (function template)
(C++11)
(C++11)

std::atomic_exchange(std::shared_ptr) specializes atomic operations for
std::atomic_exchange_explicit(std::shared_ptr) std::shared_ptr
(deprecated in C++20) (function template)
(removed in C++26)
C documentation for
atomic_exchange,
atomic_exchange_explicit