C++ 线程：如何使用 lambda function 将主线程中的参数传递给另一个线程

Question

I am trying to pass a bool flag from the main thread to another thread with lambda function, but it does not seem to work. My idea is to create a LED_thread blinks LED every one second while the main thread does something else. Once the main task finishes, the stop_blink_flag will turn on and the LED should stop blinking. Here is my work:

int main() {
    bool stop_blink_flag = false;

    std::thread LED_thread{ [stop_blink_flag] {
        while (!stop_blink_flag)
        {
            std::cout << "LED is blinking every 1 second\n";
            sleep(1);
        }
        std::cout << "\t\tLED STOP BLINKS\n";
    } };

    // while the main function is running
    std::cout << "\t\Main thread starts...\n";
    std::cout << "I'm gonna do 1\n";
    std::cout << "And I'm gonna do 2\n";
    std::cout << "And I'm gonna sleep for 10s\n";
    sleep(10);
    std::cout << "I just wake up after 10 seconds\n";
    std::cout << "I'm gonna go back to sleep for 3 seconds\n";
    sleep(3);
    std::cout << "I just wake up after 3 seconds\n";
    std::cout << "\t\tSystem stops. Now the LED should stop blinking\n";
    stop_blink_flag = true;
    LED_thread.join(); // main waits for t to finish
}

The result shows that the stop_blink_flag was not updated from main thread to LED_thread , so the LED keeps blinking. The detail result is as followed:

LED is blinking every 1 second
    Main thread starts...
I'm gonna do 1
And I'm gonna do 2
And I'm gonna sleep for 10s
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
I just wake up after 10 seconds
I'm gonna go back to sleep for 3 seconds
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
I just wake up after 3 seconds
        System stops. Now the LED should stop blinking
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second
LED is blinking every 1 second

Any help or advice would be very helpful and appreciated!

Answer 1

Right now you are passing the variable by value - the lambda makes a copy.

To pass it by reference instead - so the lambda accesses the original variable - just put & before it:

// before
std::thread LED_thread{ [stop_blink_flag] {

// after                 v
std::thread LED_thread{ [&stop_blink_flag] {

Note that since the lambda now accesses the original variable, it's not safe for the lambda to use it after main returns (which destroys the variable). In your case, this is safe because LED_thread.join() waits for the lambda to return.