Re: complexity for tellg()

Hi,
  I am reading a big file , and need to have a flag for current file
position so that I can store the positions for later direct access.
 However it looks tellg is a very costly function ! But it's code says
it should just return the current buffer position , thus should be a
very low cost function.
To explain,
{
        boost::progress_timer t;
        std::ifstream in("Y:/Data/workspaces/tob4f/tob4f.dat");
        std::string line;
        while(in){
            int pos = in.tellg();
            std::getline(in,line);
        }
    }
This code takes 0.58 sec in my computer, while if I uncomment the line
in.tellg() , it takes 120.8 sec (varies a little )

can anyone say the reason & the possible workout ?
I amusing MS Visual Studio 7.1 and the std library provided by visual
studio 7.1