Reserve on vector in c++

Question

I have always a dilema on reserve... We generally choose vector when size is not known and hence static array is not helpful. With this concept , is it good to reserve a big size earlier in starting itself to avoid copy constructor call ?

Answer 1

If you dont want ro reserve each time push_back do it. You can reserve for example 20-30% of full expected size.
When i write the code i reserve 20-30 elements if expect that it will be up to 100 elements, 400 for 1000 and etc.

Answer 2

Other variant, not use vector, use some list or thomething like list.

Answer 3

If you really don't know how many items your vector will hold then you can just let it auto-resize. It doubles the capacity on each resize* which is a reasonable tradeoff.

A lot of the time you have a pretty good estimate on how many items you need though so reserving then is a good idea.

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* It's probably implementation dependent but that's what I remember anyway.

Answer 4

coffeeunderrun oooo thanks, i read reverse, but now fix that it about reserve))!!!