C++ find size of ofstream

Question

I have code that currently does something like the following:

ofstream fout;
fout.open("file.txt");
fout<<"blah blah "<<100<<","<<3.14;
//get ofstream length here
fout<<"write more stuff"<<endl;

Is there a convenient way to find out the length of that line that is written at the stage I specified above? (in real life, the int 100 and float 3.14 are not constant and can change). Is there a good way to do what I want?

EDIT: by length, I mean something that can be used using fseek, e.g.

fseek(pFile, -linelength, SEEK_END);

score 8 · Accepted Answer · answered May 29 '13 at 22:50

You want tellp. This is available for output streams (e.g., ostream, ofstream, stringstream).

There's a matching tellg that's available for input streams (e.g., istream, ifstream, stringstream). Note that stringstream supports both input and output, so it has both tellp and tellg.

As to keeping the two straight, the p means put and the g means get, so if you want the "get position" (i.e., the read position) you use tellg. If you want the put (write) position, you use tellp.

Note that fstream supports both input and output, so it includes both tellg and tellp, but you can only call one of them at any given time. If the most recent operation was a write, then you can call tellp. If the most recent operation was a read, you can call tellg. Otherwise, you don't get meaningful results.

I have made a related question about how to handle the outputs from tellg/tellp since they may overflow a long long. I'm adding a link here to help others in the future: http://stackoverflow.com/questions/16825351/c-how-to-add-subtract-tellp-tellg-return — user788171, May 29 '13 at 23:04

Kornel Kisielewicz · Answer 2 · 2013-05-29T22:49:43.033

3

You can get the current position using tellg or tellp:

fin.tellg();  // postition of "get" in istream or iostream (fstream)
fout.tellp(); // position of "put"  in ostream or iostream (fstream)

edited May 29 '13 at 22:49

answered May 29 '13 at 22:43

Kornel Kisielewicz

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why, i'm printing to find in the code sample, it is output not input – user788171 May 29 '13 at 22:50

score 3 · Answer 3 · answered May 29 '13 at 22:49

3

Use ostream::tellp(), before and after

answered May 29 '13 at 22:49

AlexK

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score 2 · Answer 4 · answered May 29 '13 at 22:49

2

fout::tellp() will give you position of the next byte to be written to.

answered May 29 '13 at 22:49

Tony Hopkinson

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C++ find size of ofstream

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