Snippet for C++ multi-thread stuff

safe_mt.hpp

#pragma once

#include <functional>
#include <mutex>

class safe_mt {
private:
  std::mutex m_mutex;
  std::condition_variable m_condvar;

public:
  safe_mt() {}

  void wait(const std::function<bool(void)> &fn) {
    std::unique_lock lock(m_mutex);
    m_condvar.wait(lock, fn);
  }

  void run(const std::function<void(void)> &fn, bool all = true) {
    std::lock_guard lock(m_mutex);
    fn();
    all ? m_condvar.notify_all() : m_condvar.notify_one();
  }
};

test_safe_mt.hpp

#include <iostream>
#include <string>
#include <thread>
#include <vector>

#include "safe_mt.hpp"

auto test_fn = []() {
  int v = 0;
  safe_mt safemt;
  std::vector<std::thread> tpool;

  tpool.emplace_back(std::thread{[&]() {
    safemt.wait([&]() { return v > 0; });
    std::cout << "1" << std::endl;
  }});

  tpool.emplace_back(std::thread{[&]() {
    safemt.wait([&]() { return v > 1; });
    std::cout << "2" << std::endl;
  }});

  auto fn = [&]() { safemt.run([&]() { v++; }, true); };
  tpool.emplace_back(std::thread{fn});
  tpool.emplace_back(std::thread{fn});

  for (auto &th : tpool)
    if (th.joinable())
      th.join();

  return 1;
};

int main() {
  std::ios_base::sync_with_stdio(false);

  safe_mt safemt;

  auto a = 0;
  std::thread t1{[&]() { safemt.run([&]() { a = test_fn(); }); }};

  std::thread t2{[&]() {
    safemt.wait([&]() { return a > 0; });
    std::cout << "-\n3" << std::endl;
  }};

  t2.join();
  t1.join();
}

//  OUTPUT
// ---------
// 1       2
// 2   OR  1
// -       -
// 3       3