C++17 std::optional optimization guide as expected in concurrent environment with lambda capture
I'm following best practices but I just started working with I've hit a wall trying to I've been struggling with this for a few days now and could really use some help... I'm working with an scenario when using `std::optional` in a C++17 multi-threaded environment. I have a producer thread that populates an `std::optional<int>` and a consumer thread that reads from it. The optional seems to be empty when accessed from the consumer thread, even though it was set in the producer thread. Here's a simplified version of my code: ```cpp #include <iostream> #include <optional> #include <thread> #include <mutex> #include <chrono> std::optional<int> sharedValue; std::mutex mtx; void producer() { std::this_thread::sleep_for(std::chrono::seconds(1)); // Simulate work std::lock_guard<std::mutex> lock(mtx); sharedValue = 42; std::cout << "Producer set value to 42" << std::endl; } void consumer() { std::this_thread::sleep_for(std::chrono::seconds(2)); // Wait to ensure producer runs first std::lock_guard<std::mutex> lock(mtx); if (sharedValue.has_value()) { std::cout << "Consumer read value: " << *sharedValue << std::endl; } else { std::cout << "Consumer found no value" << std::endl; } } int main() { std::thread t1(producer); std::thread t2(consumer); t1.join(); t2.join(); return 0; } ``` Even after waiting for the producer to run, the consumer often reports "Consumer found no value". I've ensured that access to `sharedValue` is protected by a mutex, but it still seems that the consumer sometimes misses the update. Am I missing something in the threading model or the way I'm using `std::optional`? Any advice on best practices for this scenario would be greatly appreciated. Also, I compiled with `g++ 7.5.0` with `-pthread` flag enabled for threading support. What am I doing wrong? This is part of a larger web app I'm building. I'm on macOS using the latest version of C++. Has anyone dealt with something similar?