By default, std::allocator just wraps new, so it doesn't make things any harder to measure. vector isn't "allocating extra space" for no reason - its capacity grows geometrically in order to provide amortized O(1) push_back. If you use reserve() incorrectly, you can actually trigger quadratic complexity (as I've done in the past when I didn't know any better). Even with unused capacity, vector is quite space-efficient. For example, with GCC's 2x growth, on average a vector will be using 33% extra space (that's 0.5/1.5). With VC's 1.5x growth, on average a vector will be using 20% extra space (that's 0.25/1.25). That's a pretty small cost to pay for vector's optimal locality. And in practice, many containers will actually be exactly-sized (range construction and copy construction will produce exact sizing), so the overhead is even lower.
