Mutexes & Condition Variables

std::mutex protects shared data from simultaneous access by multiple threads; std::lock_guard applies RAII to locking, so the mutex is released automatically — even if an exception is thrown — rather than needing a manual unlock:

#include <mutex>

std::mutex mtx;
int counter = 0;

void increment() {
    std::lock_guard<std::mutex> lock(mtx); // locked here
    counter++;
} // automatically unlocked here, whatever happens

std::condition_variable cv;
bool ready = false;

void waitForReady() {
    std::unique_lock<std::mutex> lock(mtx);
    cv.wait(lock, [] { return ready; }); // sleeps until notified AND ready
}

A condition variable lets a thread sleep efficiently until another thread signals a change, rather than repeatedly checking a flag in a busy loop.

Common mistakes

• Locking a mutex manually with .lock()/.unlock() instead of std::lock_guard — an exception thrown between the two leaves the mutex locked forever, deadlocking every other thread that needs it.