Boost's io_service (since then renamed as io_context) is an implementation of a thread-safe event queue. Threads can put events in the queue (timer events, asynchronous read and write requests for networking, but one can also just push a generic lambda).

This example sets up two timers which will regularly push events in the queue, while two threads processes these events. It's from before c++ had lambdas - nowadays it can be written much more tersely.

I'll try to write a short example after when I'm not on my phone - it should be reduceable to 5-6 lines of setup at most

=> very dumb example of processing messages from ten threads

    #include <boost/asio.hpp>
    
    #include <atomic>
    #include <iostream>
    #include <thread>
    
    int main() {
      boost::asio::io_context io;
    
      using work_guard_type = boost::asio::executor_work_guard<boost::asio::io_context::executor_type>;
      work_guard_type work_guard(io.get_executor());
    
      std::vector<std::jthread> t;
      for(int i = 0; i < 10; i++)
        t.emplace_back(std::jthread([&io] { io.run(); }));
      
      std::atomic_int counter{};
      while(counter < 100)
        io.post([&] { 
            std::cerr << std::this_thread::get_id() << '\n'; 
            counter++; 
        });
      
      io.stop();
      return 0;
    }