👀 Views: 3 💬 Answers: 1 📅 Created: 2025-06-06

c++ threads lambda cpp

I'm optimizing some code but I'm learning this framework and I'm sure I'm missing something obvious here, but I've been banging my head against this for hours. I'm experiencing unexpected behavior when trying to use a lambda function with captured variables in a C++14 application with `std::thread`. Specifically, I'm attempting to capture a local variable by reference within a lambda, but the output is not as expected. Here's a simplified version of my code: ```cpp #include <iostream> #include <thread> #include <vector> #include <chrono> void process(std::vector<int>& data, int start) { for (int i = start; i < data.size(); ++i) { data[i] *= 2; std::this_thread::sleep_for(std::chrono::milliseconds(100)); } } int main() { std::vector<int> data = {1, 2, 3, 4, 5}; int start = 0; std::thread t1([&data, &start]() { process(data, start); }); start = 2; // Change start after thread creation t1.join(); for (const auto& val : data) { std::cout << val << ' '; } std::cout << std::endl; return 0; } ``` I expected the output to be `1 2 6 8 10`, since I changed the `start` variable to `2` after starting the thread. However, the actual output I get is `1 2 2 4 5`. It seems like the lambda captures the `start` variable by reference, but it doesn't reflect the updated value once the thread starts executing. I've tried switching the capture to `start` by value, but that doesn't help since I need it to be dynamic. Is there an explanation for why this is happening, and how can I properly manage the captured value to get the expected behavior? I've also looked into using `std::atomic` for `start`, but that feels like overkill for such a simple use case. Any suggestions would be appreciated! This is part of a larger service I'm building. Any ideas what could be causing this? My development environment is Linux. How would you solve this? I'm working on a application that needs to handle this. Could someone point me to the right documentation? This issue appeared after updating to Cpp latest. What would be the recommended way to handle this? I'm developing on Ubuntu 22.04 with Cpp. Has anyone else encountered this? I'm coming from a different tech stack and learning Cpp. Thanks in advance!