Programming Perl, 4th Edition

Hash-Tying Methods

Here’s an example of an interesting tied-hash class: it gives you a hash representing a particular user’s dot files (that is, files whose names begin with a period, which is a naming convention for initialization files under Unix). You index into the hash with the name of the file (minus the period) and get back that dot file’s contents. For example:

use DotFiles;
tie %dot, "DotFiles";
if ( $dot{profile} =~ /MANPATH/ ||
     $dot{login}   =~ /MANPATH/ ||
     $dot{cshrc}   =~ /MANPATH/    ) {
    print "you seem to set your MANPATH\n";
}

Here’s another way to use our tied class:

# Third argument is the name of a user whose dot files we will tie to.
tie %him, "DotFiles", "daemon";
foreach $f (keys %him) {
    printf "daemon dot file %s is size %d\n", $f, length $him{$f};
}

In our DotFiles example we implement the object as a regular hash containing several important fields, of which only the {CONTENTS} field will contain what the user thinks of as the hash. Table 14-1 gives the object’s actual fields.

Table 14-1. Object fields in DotFiles

Field	Contents
USER	Whose dot files this object represents
HOME	Where those dot files live
CLOBBER	Whether we are allowed to change or remove those dot files
CONTENTS	The hash of dot file names and content mappings

Here’s the start of DotFiles.pm:

package DotFiles;
use Carp;
sub whowasi { (caller(1))[3] . "()" }
my $DEBUG = 0;
sub debug { $DEBUG = @_ ? shift : 1 }

For our example we want to be able to turn on debugging output to help in tracing during development, ...