Is it legal to have a pointer to a reserved vector element? [duplicate]

Question

I'm curious if this sort of thing is legal:

std::vector<some_class_type> vec;
vec.reserve(10);
some_class_type* ptr = vec.data() + 3; // that object doesn't exist yet

Note that I'm not attempting to access the value pointed to.

This is what the standard says about data(), but I'm not sure if it's relevant:

Returns: A pointer such that [data(),data() + size()) is a valid range. For a non-empty vector, data() == &front().

Answer 1

1. data must return a pointer to a valid range ([vector.data]). Pointers that point into that range, including the one that points one past the last element of the range, must be stable until next reallocation. When size() is zero, data() points past the end of an empty range, which is a perfectly valid pointer to keep (but not to dereference of course).
2. Since data must remain stable for the next 10 insertions, we can assume that a storage region suitably sized and aligned for placing an array of 10 elements in it was obtained, and data points to the beginning of that region. That is, the implementation must call an allocation function and set the internal data pointer to its return value. (There is no direct indication in the standard that this must be true, but I cannot imagine a situation where it might be false. I assume that reallocation mandated ([vector.capacity]) by the standard actually calls one of the allocation functions such as ::operator new or std::malloc, otherwise calling this operation "reallocation" would be rather dubious. At any rate, there seems to be no way to avoid such allocation on any current architecture).
3. Any such storage region actually contains a live array of an appropriate size ([intro.object]) (though not necessarily live array elements, depending on the element type). Pointer arithmetic within such an array is valid, even though dereferencing resulting pointers is not necessarily valid.