void setSemaphore(QSemaphore& sem) { semaphore = sem; } private: QSemaphore semaphore; };semaphore = sem为什么报错

QSemaphore is a class in the Qt framework that provides a way for threads to communicate with each other by controlling access to resources. A semaphore can be thought of as a counter that is used to limit the number of threads that can access a shared resource at the same time.

To set a semaphore, you need to create an instance of the QSemaphore class and initialize it with a value that represents the number of threads that are allowed to access the resource simultaneously. Here's an example:

QSemaphore semaphore(2); // Allow two threads to access the resource at the same time

In this example, we create a semaphore with an initial value of 2, which means that two threads can access the resource at the same time. If more than two threads try to access the resource simultaneously, the extra threads will have to wait until one of the existing threads releases the semaphore.

Once you have created a semaphore, you can use it in your code to control access to shared resources. For example, here's how you might use a semaphore to limit the number of threads that can write to a file:

void writeToFile(QFile& file, const QString& data, QSemaphore& semaphore)
{
    semaphore.acquire(); // Wait for permission to access the file
    QTextStream stream(&file);
    stream << data << endl;
    semaphore.release(); // Release the semaphore to allow other threads to access the file
}

In this example, the writeToFile function takes a QFile object, a QString containing the data to write, and a QSemaphore object. The function first calls the acquire method on the semaphore to wait for permission to access the file. Once permission is granted, the function writes the data to the file and then releases the semaphore using the release method to allow other threads to access the file.

Overall, using semaphores can help you ensure that your code is thread-safe and that multiple threads can access shared resources without causing conflicts or race conditions.