ios_base::ate and tellp()

[Alex Vinokur]

#include <fstream>
#include <iostream>

int main()
{
const char* fileName = "out1";
std::ofstream fs1(fileName);
fs1 << "AAAAAAAAAAA\n";
std::cout << fs1.tellp() << std::endl;
fs1.close();

std::ofstream fs2(fileName, std::ios_base::ate);
std::cout << fs2.tellp() << std::endl;
fs2.close();

return 0;
}

gcc version 4.4.6 20120305 (Red Hat 4.4.6-4) (GCC)

> g++ file02.cpp

> ./a.out

12
0


Why does fs2.tellp() print 0, but not 12?


Alex

[Victor Bazarov]

I think the 'ate' might actually count not from the start of the file
but from the initial position, one beyond the end. Since you're going
to be writing after the end, the contents of the file prior to your
writing are considered irrelevant. Maybe. I am not sure, this is just
my speculation.

V
--
I do not respond to top-posted replies, please don't ask

[Alex Vinokur]

Replacing std::ofstream fs2(fileName, std::ios_base::ate) with std::ofstream fs2(fileName, std::ios_base::in | std::ios_base::ate) produces expected output.
But why do we need std::ios_base::in here ?

[Melzzzzz]

On Wed, 23 Jan 2013 06:40:30 -0800, Alex Vinokur wrote:

>
> Replacing std::ofstream fs2(fileName, std::ios_base::ate) with
> std::ofstream fs2(fileName, std::ios_base::in | std::ios_base::ate)
> produces expected output.
> But why do we need std::ios_base::in here ?

Because file gets truncated if not. You can also try with std::ios::app.

[Alex Vinokur]

app -> (append) Set the stream's position indicator to the end of the stream before each output operation.


ate -> (at end) Set the stream's position indicator to the end of the stream on opening.

[Melzzzzz]

I think that app, tells stream not to truncate file.
You can try to reposition write pointer on app.