Re: Postfix is slower than postfix: how likely?

The speed difference between prefix and postfix incrementing an
iterator can readily be measured with, say, a std::vector of
std::string's:

     #include <iostream>
     #include <vector>
     #include <string>

     using std::vector;
     using std::string;

     void Output(std::vector<std::string>::const_iterator iter);

     int main()
     {
         vector<string> v;

         v.resize(10000, "abc");

         vector<string>::const_iterator iter = v.begin();

         do {
             Output( ++iter);
         } while( iter != v.end());
     }

     void Output(std::vector<string>::const_iterator iter)
     {
         std::cout << *iter;
     }

The user and system time needed to run this program are:

     user 0m0.009s
     sys 0m0.007s

Replacing the do loop with this while loop:

     while (iter != v.end())
     {
         Output( iter++);
     }

takes measurably more time to complete:

     user 0m0.016s
     sys 0m0.026s

In other words, due to the extra copying required, the
postfix-incrementing iteration is on the order of 2.5 times as slow as
the same iteration performed with a prefix-incrementing iterator. A
difference in time that should not be at all surprising.