std:: atomic_fetch_sub, std:: atomic_fetch_sub_explicit

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atomic_fetch_sub atomic_fetch_sub_explicit
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Defined in header <atomic>
template < class T >

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

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

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

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

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

std:: memory_order order ) noexcept ;
(3) (since C++11)
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 ;
(4) (since C++11)

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:

1,2) obj - > fetch_sub ( arg )
3,4) obj - > fetch_sub ( arg, order )

If std::atomic<T> has no fetch_sub member (this member is only provided for integral , floating-point (since C++20) and pointer types except bool ), the program is ill-formed.

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 synchronization ordering

Return value

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

Example

Multiple threads may use std::atomic_fetch_sub to concurrently process an indexed container.

Run this code 
#include <atomic>
#include <iostream>
#include <numeric>
#include <string>
#include <thread>
#include <vector>
 
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 was required because
T was deduced from multiple arguments 		T is only deduced
from obj

See also

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