Question

Someone told me that I can write

for (iterator it = somecontainer.begin(); it != somecontainer.end(); ++it)

instead of

for (iterator it = somecontainer.begin(); it != somecontainer.end(); it++)

…since the latter one has the cost of an extra unused temporary variable. Is this optimization useful for modern compiler? Do I need to consider this optimization when writing code?

Answer 1

It’s a good habit to get into, since iterators may be arbitrarily complex. For vector::iterator or int indexes, no, it won’t make a difference.

The compiler can never eliminate (elide) the copy because copy elision only eliminates intermediate temporaries, not unused ones. For lightweight objects including most iterators, the compiler can optimize out the code implementing the copy. However, it isn’t always obvious when it can’t. For example, post-incrementing istream_iterator<string> is guaranteed to copy the last string object read. On a non-reference-counted string implementation, that will allocate, copy, and immediately free memory. The issue is even more likely to apply to heavier, non-iterator classes supporting post-increment.

There is certainly no disadvantage. It seems to have become the predominant style over the past decade or two.