Is it faster to cache the size of std::vector

Question

I'm looking at a C++ class that wraps around std::vector and stores the size of the vector. The class updates this size everytime the vector is modified leading to a lot of redundant code (as an up-to-date size is always available from the size() method). Is this truly faster or would the compiler optimize iterated usage of size() anyway?

Answer 1

std::vector::size has constant time complexity, so yes, it is already stored in the vector and the code is redundant.

Answer 2

First, unlike what is stated in the previous postings, the implementations of std::vector that I've seen (MS and g++) do not cache the size. On the other hand, it can be calculated by a simple pointer subtraction, and anything you do to cache the size will almost certainly add more overhead than it saves.

Answer 3

On all implementations I've seen, caching would be slower. It's just wasting memory bandwidth.

Answer 4

As far as I know, the standard library implementation of vector::size is just returning a cached size, not recalculating it each time.