The Infinite Loop

Tales from a lean programmer.

Mutex lock guards in C++11


The new concurrency library of C++11 comes with two different classes for managing mutex locks: namely std::lock_guard and std::unique_lock. How do they compare with each other and in which situation which of the two classes should be preferably used?

The std::lock_guard class keeps its associated mutex locked during the entire life time by acquiring the lock on construction and releasing the lock on destruction. This makes it impossible to forget unlocking a critical section and it guarantees exception safety because any critical section is automatically unlocked when the stack is unwound after an exception was thrown. The std::lock_guard class should be used when a limited scope, like a class method, is to be locked.

void Foo::Bar()
    std::lock_guard<std::mutex> guard(this->Mutex);
    // mutex is locked now
}   // mutex is unlocked when lock guard goes out of scope

In contrast, the std::unique_lock class is a lot more flexible when dealing with mutex locks. It has the same interface as std::lock_guard but provides additional methods for explicitly locking and unlocking mutexes and deferring locking on construction. By passing std::defer_lock instead of std::adopt_lock the mutex remains unlocked when a std::unique_lock instance is constructed. The lock can then be obtained later by calling lock() on the std::unique_lock instance or alternatively, by passing it to the std::lock() function. To check if a std::unique_lock currently owns its associated mutex the owns_lock() method can be used. Hence, the mutex associated with a std::unique_lock doesn’t have to be locked (sometimes also referred to as owned) during the lock guard’s entire life time. As a consequence, the ownership of a std::unqiue_lock can be transferred between instances. This is why std::unique_lock is movable whereas std::lock_guard is not. Thus, more flexible locking schemes can be implemented by passing around locks between scopes.
For example a std::unique_lock can be returned from a function, or instances of a class containing a std::unique_lock attribute can be stored in containers. Consider the following example in which a mutex is locked in the function Foo(), returned to the function Bar() and only then unlocked on destruction.

std::mutex Mutex;

std::unique_lock<std::mutex> Foo()
    std::unique_lock<std::mutex> lock(Mutex);
    return lock;
    // mutex isn't unlocked here!

void Bar()
    auto lock = Foo();
}   // mutex is unlocked when lock goes out of scope

Keeping std::unique_lock‘s additional lock status up-to-date induces some additional, minimal space and speed overhead in comparison to std::lock_guard. Hence, as a general rule, std::lock_guard should be preferably used when the additional features of std::unique_lock are not needed.

2 thoughts on “Mutex lock guards in C++11

  1. Very informative. Thank you!

  2. Pingback: std::unique_lock vs std::lock_guard |

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