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,
   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)