Especially in non trivial iterator types, the prefix versions of the ++ and -- operators are more efficient than the postfix versions. Only use the postfix version when you actually need to use the previous value.

        set<CClass1> set1;
        Fill(set1);
        set<CClass1>::iterator theEnd = set1.end();

        // use ++i, not i++
        for (set<CClass1>::iterator i = set1.begin(); i != theEnd; ++i)
        {
            // Do something with *i
        }

Note that a pointer is indeed a simple kind of iterator where usually no significant performance difference will exist between the prefix and postfix versions.

Other simple types are the built-in character types, integer types, floating point types and enumerations.