shared_mutex.cpp

// https://en.cppreference.com/w/cpp/thread/shared_mutex
// https://godbolt.org/z/eoqErv
#include <iostream>
#include <shared_mutex>
#include <thread>
#include <vector>

int g_value = 0;
std::shared_mutex g_mutex;

void Print(std::thread::id id, bool set, bool start) {
  static std::mutex m;
  std::lock_guard lock{ m };
  std::cout << "[" << (start ? '+' : ' ') << "] " << (set ? "set" : "   ") << " thread: " << id << std::endl;
}

int GetValue() {
  std::shared_lock lock{ g_mutex };
  Print(std::this_thread::get_id(), false, true);
  std::this_thread::sleep_for(std::chrono::seconds{ 2 });
  Print(std::this_thread::get_id(), false, false);
  return g_value;
}

void SetValue(int value) {
  std::unique_lock lock{ g_mutex };
  Print(std::this_thread::get_id(), true, true);
  std::this_thread::sleep_for(std::chrono::seconds{ 3 });
  Print(std::this_thread::get_id(), true, false);
}

int main(int argc, char* argv[]) {
  std::vector<std::thread> threads;
  threads.emplace_back([]() {
    while (true) {
      SetValue(1);
      std::this_thread::sleep_for(std::chrono::milliseconds{ 100 });
    }
  });

  for (auto i = 0; i < 3; i++) {
    threads.emplace_back([]() {
      while (true) {
        GetValue();
        std::this_thread::sleep_for(std::chrono::milliseconds{ 100 });
      }
    });
  }

  for (auto& thread : threads) {
    thread.join();
  }
}