C++ iterators & loop optimization

ccoding-stylecompiler-constructioniteratoroptimization

I see a lot of c++ code that looks like this:

for( const_iterator it = list.begin(),
     const_iterator ite = list.end();
     it != ite; ++it)

As opposed to the more concise version:

for( const_iterator it = list.begin();
     it != list.end(); ++it)

Will there be any difference in speed between these two conventions? Naively the first will be slightly faster since list.end() is only called once. But since the iterator is const, it seems like the compiler will pull this test out of the loop, generating equivalent assembly for both.

Best Answer

The two versions are not the same though. In the second version it compares the iterator against list.end() every time, and what list.end() evaluates to might change over the course of the loop. Now of course, you cannot modify list through the const_iterator it; but nothing prevents code inside the loop from just calling methods on list directly and mutating it, which could (depending on what kind of data structure list is) change the end iterator. It might therefore be incorrect in some circumstances to store the end iterator beforehand, because that may no longer be the correct end iterator by the time you get to it.

Related Topic