[c++] Returning unique_ptr from functions

I think it's perfectly explained in item 25 of Scott Meyers' Effective Modern C++. Here's an excerpt:

The part of the Standard blessing the RVO goes on to say that if the conditions for the RVO are met, but compilers choose not to perform copy elision, the object being returned must be treated as an rvalue. In effect, the Standard requires that when the RVO is permitted, either copy elision takes place or std::move is implicitly applied to local objects being returned.

Here, RVO refers to return value optimization, and if the conditions for the RVO are met means returning the local object declared inside the function that you would expect to do the RVO, which is also nicely explained in item 25 of his book by referring to the standard (here the local object includes the temporary objects created by the return statement). The biggest take away from the excerpt is either copy elision takes place or std::move is implicitly applied to local objects being returned. Scott mentions in item 25 that std::move is implicitly applied when the compiler choose not to elide the copy and the programmer should not explicitly do so.

In your case, the code is clearly a candidate for RVO as it returns the local object p and the type of p is the same as the return type, which results in copy elision. And if the compiler chooses not to elide the copy, for whatever reason, std::move would've kicked in to line 1.

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