Re: complexity for tellg()

"Carlo Capelli" <>
Tue, 20 Feb 2007 08:57:01 +0100
Of course you can approach the problem computing the position yourself, if
you know the size of the input read.
Not elegant, but it works for simple cases...

     std::ifstream in("Y:/Data/workspaces/tob4f/tob4f.dat");
     size_t pos = 0;
     std::string line;
        // int pos = in.tellg();
         pos += line.length() + 2; // account for line terminator...

Bye Carlo

"toton" <> ha scritto nel messaggio

  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;
int pos = in.tellg();
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

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