std::atomic_fetch_add, std::atomic_fetch_add_explicit

#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();
}

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