[Kranar]

I would generally suggest going for correctness over performance and the solution I provided is correct in the general case. Using an offset is only correct in the special case where objects will not be inserted or removed at an index less than the offset, otherwise you will end up with bugs as the offset becomes invalid upon such operations.

Furthermore, depending on the size of T, the performance penalty of the extra heap allocations is amortized over the cost of resizing the vector. That is vector reallocation is significantly faster for a unique_ptr<T> than it is for T when T is large and almost all memory allocators are tuned to allocate objects close together in space when they are allocated close together in time, so you don't lose the cache locality or need to worry about memory fragmentation.