C++ Condition Variables

Introduction to Condition Variables

Condition variables in C++ are synchronization primitives that enable threads to wait until a particular condition is met. They work with std::mutex to manage the synchronization of shared resources. The std::condition_variable class is part of the C++ standard library and is essential in multi-threaded programming.

Basic Usage of std::condition_variable

The primary operations for a condition variable are wait, notify_one, and notify_all. The wait operation blocks the current thread until the condition variable is notified. The notify_one operation wakes up one waiting thread, and notify_all wakes up all waiting threads.

Understanding the Wait Function

The wait function is crucial for condition variables. It releases the mutex while the thread is waiting and automatically re-acquires it once the thread is notified and the wait condition is satisfied. This is typically used in a loop to check the condition repeatedly.

Using notify_one and notify_all

The notify_one function wakes up a single waiting thread, which can be useful when only one thread should proceed. In contrast, notify_all resumes all waiting threads, which is useful when multiple threads need to proceed after a condition is met. Choosing between these depends on the specific needs of your application.

Best Practices and Considerations

• Always use a std::unique_lock with a condition variable. This is necessary for the wait function to work properly.
• Be cautious of spurious wakeups, which are false wakeups that can occur. Always use a loop to check the condition.
• Avoid using notify_all if only one thread needs to be notified, as it can lead to inefficiencies.
• Ensure the condition variable and the corresponding mutex are accessible to all threads that need to use them.