Question

This code:

#include <iostream>
#include <cstdio>
#include <fstream>
#include <string>

int main()
{   
    std::remove("test.txt");
    std::fstream f("test.txt",std::ios::in | std::ios::out | std::ios::binary | std::ios::trunc);
    std::cout << f.good() << std::endl;
    f<<"test"<< std::flush;
    std::cout << f.tellg() << " " << f.tellp() << std::endl;
    f.seekg(0);
    std::string s;
    f>>s;
    std::cout << f.tellg() << " " << f.tellp() << std::endl;
}   

Gives following output in gcc-4.4.5

1
4 4
4 4

i.e. both tellg and tellp returned expected stream position 4.

While gcc-4.6.0

Gives:

1
4 4
-1 4

Where can I find a reference to tell:

1. 1st case is correct (bug in gcc-4.6)
2. 2nd case is correct (bug in gcc < gcc-4.6)
3. Both case are correct the behavior is undefined

Answer 1

Ok, it is not a bug, even it seems that it is required behavior:

According to C++ 2003 standard:

• tellg(): (27.6.1.3)

  After constructing a sentry object, if fail() != false, returns pos_type(-1) to indicate failure. Otherwise, returns rdbuf()->pubseekoff(0, cur, in).

• sentry (27.6.1.1.2):

  if noskipws is zero and is.flags() & ios_base::skipws is nonzero, the func-
  tion extracts and discards each character as long as the next available input character c is a whitespace character. If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits::eof(), the function calls setstate(failbit | eofbit) (which may throw ios_base::failure).

So basically

• tellg() creates sentry object:
• sentry extracts white space characters and should set failbit after getting to eof.
• tellg() sees failbit should return eof() (-1)

So gcc-4.6 seems to behave correctly…