Re: complexity for tellg()

"Alf P. Steinbach" <>
Tue, 20 Feb 2007 07:46:26 +0100
* toton:

  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

Most likely the cause is conversion of CRLF to LF, which you've
specified by (1) opening the file in text mode and (2) compiling with a
Windows compiler.

One cure could then be to open the file in binary mode, and handle
newlines as appropriate (or not).

A: Because it messes up the order in which people normally read text.
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