Programming Perl, 4th Edition

Saving Data Structures

If you want to save your data structures for use by another program later, there are many ways to do it. The easiest way is to use Perl’s Data::Dumper module, which turns a (possibly self-referential) data structure into a string that can be saved externally and later reconstituted with eval or do.

use Data::Dumper;
$Data::Dumper::Purity = 1;       # since %TV is self–referential
open (FILE, "> tvinfo.perldata") || die "can't open tvinfo: $!";
print FILE Data::Dumper–>Dump([\%TV], ['*TV']);
close(FILE)                      || die "can't close tvinfo: $!";

A separate program (or the same program) can then read in the file later:

open (FILE, "< tvinfo.perldata") || die "can't open tvinfo: $!";
undef $/;                        # read in file all at once
eval <FILE>;                     # recreate %TV
die "can't recreate tv data from tvinfo.perldata: $@" if $@;
close(FILE)                      || die "can't close tvinfo: $!";
print $TV{simpsons}{members}[2]{age};

or simply:

do "tvinfo.perldata"            || die "can't recreate tvinfo: $! $@";
print $TV{simpsons}{members}[2]{age};

Storable, another standard module, writes out data structures in very fast, packed binary format. It also supports automatic file locking (provided your system implements the flock function), and it even has fancy hooks so object classes can handle their own serialization. Here’s how you might save that same structure using Storable:

use Storable qw(lock_nstore);
lock_nstore(\%TV, "tvdata.storable");

And here’s how to restore it into a variable that will hold a reference to the retrieved hash: ...