Contemplating std::lock_guard vs. std::scoped_lock

C 17 marked the introduction of a novel lock class, std::scoped_lock, which shares similarities with the venerable std::lock_guard. This article delves into the distinctions between these two locking mechanisms, guiding you in choosing the optimal tool for your specific needs.

When to Utilize std::lock_guard

For scenarios where you require the locking of a single mutex for the entirety of a scope, std::lock_guard offers a safer API than std::scoped_lock. Consider the following example:

{
    std::scoped_lock lock;  // protect this block
    ...
}

This snippet inadvertently results in a runtime error because it engages in "invisible behavior." It appears to compile smoothly but ultimately effects no locking. The intended functionality likely required:

{
    std::scoped_lock lock{mut};  // protect this block
    ...
}

Clarifying the target mutex for locking resolves this issue. Conversely, employing std::lock_guard would prevent the first example from compiling, rendering it a clear compile-time error.

When to Utilize std::scoped_lock

When confronted with situations where multiple mutexes need locking, std::scoped_lock shines. For instance:

{
    std::scoped_lock(lock1, lock2);  // protect this block
    ...
}

This snippet succinctly locks two mutexes for the duration of the block.

Conclusion

Both std::lock_guard and std::scoped_lock cater to distinct scenarios:

• std::lock_guard for locking a single mutex throughout a scope.
• std::scoped_lock for locking multiple mutexes or for unlocking within the enclosed scope.

While std::scoped_lock provides more comprehensive functionality, std::lock_guard offers enhanced API safety in specific cases. Consider the requirements of your code carefully when selecting the most appropriate approach.