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I have question to correct my understanding of efficiency of accessing elements of a vector by using index access (with operator []) or using an iterator. My understanding is "iterator" is more efficient than "index access". (also I think vector::end() is more efficient than vector::size() ). Now I wrote sample code measure it (under Windows 7 using Cygwin, with g++ 4.5.3) The index access loop version (formerly labeled as random access): int main() { std::vector< size_t > vec ( 10000000 ); size_t value = 0; for( size_t x=0; x<10; ++x ) { for ( size_t idx = 0; idx < vec.size(); ++idx ) { value += vec[idx]; } return value; } } The iterator loop code is this: for (std::vector< size_t >::iterator iter = vec.begin(); iter != vec.end(); ++iter) { value = *iter; } I am surprised to see that the "index access" version is much quicker. I used the time command to "measure". The numbers were : results using g++ source.cpp (no optimizations) index access real 800ms iterator access real 2200ms Do these numbers make sense? (I repeated the runs multiple times) And I wondered what details I miss and why I am mistaken... results using g++ -O2 index access, time real: ~200ms iterator access, time real: ~200ms I repeated tests on different platform (amd64 w/ g++ and power7 w xlC) and see that all the time I used optimized code the example programs have similar execution time. edit changed code to add values ( value += *iter ) instead of just using assignment. Added details about compiler options. Added new numbers for using -O2. * edit2 changed title correcting "iterator efficiency" to

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