Question

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.

Answer 1

I’ll just mention for the record that the C++ standard mandates that calling begin() and end() on any container type (be it vector, list, map etc.) must take only constant time. In practice, these calls will almost certainly be inlined to a single pointer comparison if you compile with optimisations turned on.

Note that this guarantee does not necessarily hold for additional vendor-supplied “containers” that do not actually obey the formal requirements of being a container laid out in the chapter 23 of the standard (e.g. the singly-linked list slist).