What is the most effective way to get the index of an iterator of an std::vector?

Question

I'm iterating over a vector and need the index the iterator is currently pointing at. AFAIK this can be done in two ways: - `it - vec.begin()` - `std::distance(vec.begin(), it)` What are the pros and cons of these methods?

Answer 1

Hello!

I strongly doubt that there is any effective difference (no pun intended) between the 2 ways; it's likely that they result in the same code. (No, I haven't verified this.) If your code doesn't need to calculate the index for every iteration, I'd feel free to use either, as they'll both boil down to a simple subtraction. If, on the other hand, you need the index more than just occasionally, and you're iterating across the whole vector, you might consider a 3rd alternative: incrementing (or decrementing, as required) a size_t variable, and use that as an index.