std::atomic_fetch_add,std::atomic_fetch_add_explicit(3)

NAME
std::atomic_fetch_add,std::atomic_fetch_add_explicit - std::atomic_fetch_add,std::atomic_fetch_add_explicit

Synopsis
   Defined in header <atomic>
   template< class T >
   T atomic_fetch_add( std::atomic<T>* obj, (1) (since C++11)
                       typename std::atomic<T>::difference_type arg )
   noexcept;
   template< class T >
   T atomic_fetch_add( volatile std::atomic<T>* obj, (2) (since C++11)
                       typename std::atomic<T>::difference_type arg )
   noexcept;
   template< class T >
   T atomic_fetch_add_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_add_explicit( volatile std::atomic<T>* obj,
                        typename (4) (since C++11)
   std::atomic<T>::difference_type arg,
                        std::memory_order order ) noexcept;
   Performs atomic addition. Atomically adds arg to 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_add(arg)
   3,4) obj->fetch_add(arg, order)
   If std::atomic<T> has no fetch_add 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 add to 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
   Single-writer/multiple-reader lock can be made with std::atomic_fetch_add. Note that
   this simplistic implementation is not lockout-free. // Run this code
 #include <atomic>
 #include <chrono>
 #include <iostream>
 #include <string>
 #include <thread>
 #include <vector>
 using namespace std::chrono_literals;
 // meaning of cnt:
 // 5: readers and writer are in race. There are no active readers or writers.
 // 4...0: there are 1...5 active readers, The writer is blocked.
 // -1: writer won the race and readers are blocked.
 const int N = 5; // four concurrent readers are allowed
 std::atomic<int> cnt(N);
 std::vector<int> data;
 void reader(int id)
 {
     for (;;)
     {
         // lock
         while (std::atomic_fetch_sub(&cnt, 1) <= 0)
             std::atomic_fetch_add(&cnt, 1);
         // read
         if (!data.empty())
             std::cout << ("reader " + std::to_string(id) +
                        " sees " + std::to_string(*data.rbegin()) + '\n');
         if (data.size() == 25)
             break;
         // unlock
         std::atomic_fetch_add(&cnt, 1);
         // pause
         std::this_thread::sleep_for(1ms);
     }
 }
 void writer()
 {
     for (int n = 0; n < 25; ++n)
     {
         // lock
         while (std::atomic_fetch_sub(&cnt, N + 1) != N)
             std::atomic_fetch_add(&cnt, N + 1);
         // write
         data.push_back(n);
         std::cout << "writer pushed back " << n << '\n';
         // unlock
         std::atomic_fetch_add(&cnt, N + 1);
         // pause
         std::this_thread::sleep_for(1ms);
     }
 }
 int main()
 {
     std::vector<std::thread> v;
     for (int n = 0; n < N; ++n)
         v.emplace_back(reader, n);
     v.emplace_back(writer);
     for (auto& t : v)
         t.join();
 }

Output:
 writer pushed back 0
 reader 2 sees 0
 reader 3 sees 0
 reader 1 sees 0
 <...>
 reader 2 sees 24
 reader 4 sees 24
 reader 1 sees 24
   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 adds the argument to the value stored in the
   fetch_add atomic object and obtains the value held previously
                        (public member function of std::atomic<T>)
   atomic_fetch_sub subtracts a non-atomic value from an atomic object and
   atomic_fetch_sub_explicit obtains the previous value of the atomic
   (C++11) (function template)
   (C++11)
   C documentation for
   atomic_fetch_add,
   atomic_fetch_add_explicit