CODEWITHSUNDEEP
c++c++11language-designstd
Paolo M
Paolo M

Reputation: 12757

Why is std::unique_lock not derived from std::lock_guard

std::lock_guard and std::unique_lock interfaces look very similar, in their common part (constructors and destructor).

Why there is no hierarchical relationship between them?

Upvotes: 3

Views: 328

Answers (1)

MikeMB
MikeMB

Reputation: 21156

They have non substitutable semantics:
lock_guard is guaranteed to be locked through all of it's lifetime.
unique_lock doesn't guarantee that so it doesn't follow the "IS A"-rule (unique_lock cannot be a lock_guard, as it offers fewer guarantees).

Also implementing unique_lock based on lock_guard wouldn't be trivial (maybe even impossible) for that reason.

Obviously the same is true the other way round: Although you can implement a lock_guard in terms of a unique_lock (private inheritance), lock_guard doesn't provide the same functionality (lock()/unlock()) as unique_lock so it cannot be publicly derived from it.

Upvotes: 8

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