Is std::unique_ptr an application of RAII?

Question

Is that an accurate description of it? Does it make sense?

Are you guaranteed that the object it points to won't be deleted until the unique_ptr goes out of scope [even if you're not using the unique_ptr]?

Answer 1

Yes, std::unique_ptr follows the RAII design principle.

No, std::unique_ptr does not prevent other code from doing something stupid, like calling delete on a pointer that belongs to the unique_ptr. The unique_ptr itself will call a deleter¹ on the object it owns when either:

1. it goes out of scope

or

2. the unique_ptr is reassigned (via operator= or reset) to point to a different object

One can also revoke unique_ptr's ownership of an object by moving to a different smart pointer, or using the release member function. This breaks the association between the object and the unique_ptr and unique_ptr will no longer clean up the object.

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¹ The default deleter will use either delete or delete [], depending on whether the target has array type. But unique_ptr is a template and its deleter can be customized, for example the cleanup operation for a FILE* can be chosen to be a call to fclose.

This capability can be used to schedule an arbitrary cleanup action to take place when the unique_ptr goes out of scope. RAII is used for keeping locks, closing files, and so forth -- clearly there would be major problems if the cleanup action were performed early just because the compiler didn't see any future usage of the smart pointer. Luckily the C++ object lifetime rules are completely deterministic (even the order of destruction for multiple automatic variables in the same scope is well defined), and you can count on the smart pointer cleaning up its owned object exactly when the smart pointer is itself destroyed.