std::atomic_fetch_sub,std::atomic_fetch_sub_explicit(3)

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, (1) (since C++11)
                       typename std::atomic<T>::difference_type arg )
   noexcept;
   template< class T >
   T atomic_fetch_sub( volatile std::atomic<T>* obj, (2) (since C++11)
                       typename std::atomic<T>::difference_type arg )
   noexcept;
   template< class T >
   T atomic_fetch_sub_explicit( std::atomic<T>* obj,
                        typename (3) (since C++11)
   std::atomic<T>::difference_type arg,
                        std::memory_order order ) noexcept;
   template< class T >
   T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj,
                        typename (4) (since C++11)
   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:
   1,2) 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 is only deduced
                      T was deduced from multiple arguments from obj

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)
   C documentation for
   atomic_fetch_sub,
   atomic_fetch_sub_explicit