Concept difference between pre and post increment operator for STL

Question

Supposedly:

for (vector<int>::iterator iter = ivec.begin(); iter != ivec.end(); ++iter)
{}

I do understand the difference when it comes to pre/post increment for built-in types like int etc but in terms of an iterator, what's the difference here between ++iter and iter++? (Bear in mind that I do know that both yield the same result here).

Answer 1

++iter is most likely to be faster but never slower than iter++.

Implementing the post increment operator iter++ needs to generate an additional temporary(this temporary is returned back while the original iter is incremented ++) over implementing the post increment operator ++iter, So unless the compiler can optimize(yes it can) the post increment, then ++iter will most likely be faster than iter++.

Given the above, It is always preferable to use ++iter in the looping conditions.

Answer 2

The difference is they do not yield the same result, while this particular example will do the same regardless of the increment form used. The pre-increment form first increments the value, and then returns it; while the post-increment form increments the result but returns the value previous to incrementation. This is usually a no cost for fundamental types, but for things like iterators it requires creating a temporary value to hold the non-incremented value, in order to be later returned.