Why can't I read and append with std::fstream on Mac OS X?

Question

Consider the following C++ program, which takes a file and prints each line. It's a slice of a larger program where I later append to the file, based on what I see.

#include <fstream>
using std::fstream;
#include <iostream>
#include <string>
using std::string;

int main()
{
 fstream file("file.txt", fstream::in | fstream::out | fstream::app);

 string line;
 while (std::getline(file, line))
  std::cerr << line << std::endl;

 return 0;
}

Now apply this version of file.txt (One word on the first line, followed by a newline):

Rain

On my machine (Snow Leopard), this prints out nothing. On closer inspection, the first call to getline fails. Strangely, it also fails if I add a second line: still nothing is printed!

Can anyone solve this mystery?

Answer 1

You should check if the file has actually been opened:

if (!file)
    std::cerr << "Oh dear" << std::endl;

Update: in fact the file likely has been opened, but is in append mode - see Neii's answer.

Update 2: okay, wrong again. In Leopard's g++ at least, the file will not be opened because the app flag is incompatible with the in flag. So the above check will print Oh dear.

In MSVC++, it goes ahead and opens the file, apparently with the read position at the start, which explains why other people saw it work and I apologise for doubting their veracity!

Answer 2

When you say:

fstream file("file.txt", fstream::in | fstream::out | fstream::app);

you open the file in append mode - i.e. at the end. Just open it in read mode:

fstream file("file.txt", fstream::in );

or use an ifstream:

ifstream file("file.txt" );

And of course as Earwicker suggests, you should always test that the open succeeded.

If you are determined to open in append mode, you can move the read pointer explicitly:

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main() {
    fstream file( "afile.txt", ios::in | ios::out | ios::app );
    if ( ! file.is_open()  ) {
        cerr << "open failed" << endl;
        return 1;
    }
    else {
        file.seekg( 0, ios::beg );   // move read pointer
        string line;
        while( getline( file, line ) ) {
            cout << line << endl;
        }
    }
}

Edit: It seems that the combination of flags used in the opening of the file leads to implementation specific behaviour. The above code works with g++ on Windows, but not with g++ on Linux.