C++17 `std::optional` Not Resetting After Assignment in Multi-threaded Environment
I've looked through the documentation and I'm still confused about I'm migrating some code and I'm learning this framework and I need help solving I'm sure I'm missing something obvious here, but I'm working on a multi-threaded application using C++17 and I've run into an issue with `std::optional`... In one thread, I am assigning a value to an `std::optional<int>`, and in another thread, I am trying to reset it. However, it seems like the reset is not taking effect as expected. Here’s a simplified version of what I have: ```cpp #include <iostream> #include <thread> #include <optional> #include <chrono> std::optional<int> sharedValue; void writer() { std::this_thread::sleep_for(std::chrono::milliseconds(100)); // Simulate some work sharedValue = 42; std::cout << "Writer: Assigned value 42" << std::endl; } void reader() { std::this_thread::sleep_for(std::chrono::milliseconds(50)); // Ensure writer runs first if (sharedValue.has_value()) { std::cout << "Reader: Value is " << sharedValue.value() << std::endl; sharedValue.reset(); std::cout << "Reader: Value reset" << std::endl; } else { std::cout << "Reader: No value" << std::endl; } } int main() { std::thread t1(writer); std::thread t2(reader); t1.join(); t2.join(); if (sharedValue.has_value()) { std::cout << "Main: Value is still " << sharedValue.value() << std::endl; } else { std::cout << "Main: Value is reset" << std::endl; } return 0; } ``` When I run this code, I frequently see the output "Main: Value is still 42" even after the reader thread resets the optional. The synchronization concerns me, as I am aware that `std::optional` is not thread-safe. What I’ve tried is wrapping the access to `sharedValue` with `std::mutex`, but it hasn't resolved the issue. Here’s how I attempted to add mutex: ```cpp #include <mutex> std::mutex mtx; void writer() { std::this_thread::sleep_for(std::chrono::milliseconds(100)); std::lock_guard<std::mutex> lock(mtx); sharedValue = 42; std::cout << "Writer: Assigned value 42" << std::endl; } void reader() { std::this_thread::sleep_for(std::chrono::milliseconds(50)); std::lock_guard<std::mutex> lock(mtx); if (sharedValue.has_value()) { std::cout << "Reader: Value is " << sharedValue.value() << std::endl; sharedValue.reset(); std::cout << "Reader: Value reset" << std::endl; } else { std::cout << "Reader: No value" << std::endl; } } ``` This change does prevent data races, but the issue persists. I suspect there's a timing issue or another subtlety I'm missing with the thread interaction. Any insights into why the reset might not take effect or how to properly handle this scenario would be greatly appreciated. Also, are there other thread-safe patterns for managing shared state in C++17 that might be more effective? For context: I'm using Cpp on macOS. Has anyone else encountered this? I'm working on a microservice that needs to handle this. Any ideas what could be causing this? The project is a service built with Cpp. Could someone point me to the right documentation? For context: I'm using Cpp on Linux. What would be the recommended way to handle this? Any pointers in the right direction?