how to open a txt file directly........

[Harlequin]

I wanted to write a program that can open a txt file directly through the operating system.

What i mean to say is that when you see a txt file i must be able to double click on it and it is directly opened by my program....like notepad can..........

[Bazzy]

The first argument passed to your program is the program filename, when opening a file also that argument is passed.
So you need to make your program recognise it:

int main ( int argc, char **argv )
{
    if ( argc==2 ) // one argument was passed
        fstream( argv[1] );//try to open that file
}

Then you need to make your system know that your program is the default for that file type