Is it more efficient to set the size of a vector up front?

Question

If you are able to, is it more efficient to set the size of a vector up front? I intend to push_back values.

Answer 1

Yes, it's generally a little more efficient. Don't expect a huge improvement, but at worst it's harmless (obviously assuming you only reserve the space you actually need).

For a rather unusual situation, it may improve both the time taken and the amount of space consumed. When you use push_back it'l increase the size by some multiplicative factor when you run out of space, but if you use reserve, it may allocate exactly the amount you need instead of rounding up to the next multiple of whatever factor it uses.