NAME¶

std::atomic_fetch_sub,std::atomic_fetch_sub_explicit - std::atomic_fetch_sub,std::atomic_fetch_sub_explicit

Synopsis¶

Defined in header <atomic>
template< class T >

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

typename std::atomic<T>::difference_type arg ) noexcept;
template< class T >

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

typename std::atomic<T>::difference_type arg ) noexcept;
template< class T > (1)

T atomic_fetch_sub_explicit( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,

std::memory_order order ) noexcept; (2)
template< class T >

T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,

std::memory_order order ) noexcept;

Performs atomic subtraction. Atomically subtracts arg from the value pointed to by
obj and returns the value obj held previously. The operation is performed as if the
following was executed:

1) obj->fetch_sub(arg)
2) obj->fetch_sub(arg, order)

Parameters¶

obj - pointer to the atomic object to modify
arg - the value to subtract from the value stored in the atomic object
order - the memory sycnhronization ordering for this operation: all values are
permitted.

Return value¶

The value immediately preceding the effects of this function in the modification
order of *obj.

Possible implementation¶

template< class T >
T atomic_fetch_sub( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept
{
return obj->fetch_sub(arg);
}

Example¶

Multiple threads may use fetch_sub to concurrently process an indexed container

// Run this code

#include <string>
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
#include <numeric>

const int N = 50;
std::atomic<int> cnt;
std::vector<int> data(N);

void reader(int id)
{
for (;;) {
int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
if (idx >= 0) {
std::cout << "reader " << std::to_string(id) << " processed item "
<< std::to_string(data[idx]) << '\n';
} else {
std::cout << "reader " << std::to_string(id) << " done\n";
break;
}
}
}

int main()
{
std::iota(data.begin(), data.end(), 1);
cnt = data.size() - 1;

std::vector<std::thread> v;
for (int n = 0; n < 5; ++n) {
v.emplace_back(reader, n);
}
for (auto& t : v) {
t.join();
}
}

Output:¶

reader 2 processed item 50
reader 1 processed item 44
reader 4 processed item 46
<....>
reader 0 done
reader 4 done
reader 3 done

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 required because T is T is deduced from the
deduced from multiple arguments atomic argument only

See also¶

atomically subtracts the argument from the value stored in
fetch_sub the atomic object and obtains the value held previously
(public member function of std::atomic<T>)
atomic_fetch_add adds a non-atomic value to an atomic object and obtains
atomic_fetch_add_explicit the previous value of the atomic
(C++11) (function template)
(C++11)