Is it safe to use emplace_back with a container of unique_ptrs?

问题

Consider the following:

std::vector<std::unique_ptr<int>> ptrsToInts;
ptrsToInts.emplace_back(new int);

If reallocation occurs in the vector, and that fails (throwing std::bad_alloc), am I "safe" or will I leak an int?

C++11 23.3.6.5 [vector.modifiers]/1 says:

If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T or by any InputIterator operation there are no effects.

which seems to indicate that this is a potential problem. That is, if there are "no effects", then no unique_ptr ever was constructed, and therefore the destructor behavior one would rely on to delete that pointer would not occur. (Which might indicate that emplace_back should be banned for containers of unique_ptrs)

回答1:

If reallocation is required and it fails, then yes, your object never went into the container and will thus be lost.

However, it should be noted that this is pure user error. emplace_back should not be "banned" for containers of unique_ptr, because there are perfectly safe ways of doing this (such as reserveing the space beforehand, so you know it will always be there). Also, you can pass in whole unique_ptrs, since it's perfectly capable of using a move constructor.

So really, it's your fault for not properly wrapping your non-RAII object (the int*) in a RAII object before the point where you could throw exceptions.

来源：https://stackoverflow.com/questions/13172888/is-it-safe-to-use-emplace-back-with-a-container-of-unique-ptrs