Programming Perl, 4th Edition

Filehandle-Tying Methods

For our extended example, we’ll create a filehandle that uppercases strings that are printed to it. Just for kicks, we’ll begin the file with <SHOUT> when it’s opened and end with </SHOUT> when it’s closed. That way we can rant in well-formed XML.

Here’s the top of our Shout.pm file that will implement the class:

package Shout;
use Carp;                # So we can croak our errors

We’ll now list the method definitions in Shout.pm.

CLASSNAME–>TIEHANDLE(LIST)

This is the constructor for the class, which as usual should return a blessed reference.

sub TIEHANDLE {
    my $class = shift;
    my $form = shift;
    open(my $self, $form, @_)  || croak "can't open $form@_: $!";
    if ($form =~ />/) {
        print $self  "<SHOUT>\n";
        $$self–>{WRITING} = 1;     # Remember to do end tag
    }
    return bless $self, $class;    # $self is a glob ref
}

Here we open a new filehandle according to the mode and filename passed to the tie operator, write <SHOUT> to the file, and return a blessed reference to it. There’s a lot of stuff going on in that open statement, but we’ll just point out that, in addition to the usual “open or die” idiom, the my $self furnishes an undefined scalar to open, which knows to autovivify it into a typeglob. The fact that it’s a typeglob is also significant, because not only does the typeglob contain the real I/O object of the file, it also contains various other handy data structures that come along for free, like a scalar ($$$self), an array (@$$self), and a hash (%$$self). (We won’t mention the subroutine, ...