cpp core · high

RAII & lock_guard

Why scope-bound release (lock_guard) beats manual lock/unlock: the destructor frees the resource on every exit path, including exceptions.

The code

// RAII: lock_guard
{
    std::lock_guard<std::mutex> lk(m); // ctor: m.lock()
    doWork();                          // may throw
} // dtor: m.unlock() - always

// Manual (BUG): unlock skipped on early exit
m.lock();
if (err)
    return; // <-- leaks the lock!
m.unlock();

What this lesson walks through

1. 01RAII: acquire in ctor, release in dtor
2. 02Entering scope locks the mutex
3. 03Critical section runs
4. 04Scope exit unlocks automatically
5. 05Exceptions? Still released.
6. 06Manual lock/unlock: the leak
7. 07Why RAII wins

RAII binds a resource to an object's lifetime: the constructor acquires it, the destructor releases it. std::lock_guard is the canonical example for a mutex.