vector's clear vs setsize

Question

I read on the net someplace that in a tight loop if you are clearing your vector repetitively, it might be better to use resize(0) as it might be faster. I am not sure about this. Anyone's got a definitive answer to this?

Answer 1

I assume you mean resize(0) instead of setsize, and calling that instead of clear(), and that you're talking about std::vector. IIRC a recent answer discussed this (can't find the link), and on modern STL implementations, clear() is likely identical to resize(0).

Previously clearing a vector might have freed all its memory (ie. its capacity also falls to zero), causing reallocations when you start adding elements again, in contrast to resize(0) keeping the capacity so there are fewer reallocations. However, I think in modern STL libraries there is no difference. If you're using an old STL implementation, or you're just paranoid, resize(0) might be faster.

Answer 2

This sound implementation specific, and is a job for you, your library and a profiler. But as I see it, I cannot see why resize(0) should be quicker when both will in effect have to call erase(begin(),end()).

Answer 3

Looking at Dinkumware STL source, both effectively call erase(begin(), end());

clear() is slightly more efficient, unsurprisingly., as it has just the one case to deal with. but I wouldn't expect it's measurable.