A recurring pattern that occurs is that you want to construct instances from a class hierarchy based on some typeless data (eg, deserializing a string).

Taking a typical barnyard example: 

package Animal;
package Sheep; @ISA = ('Animal');
package Cow; @ISA = ('Animal');
package Pig; @ISA = ('Animal');

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A new animal wanders into your program and identifies itself with a noise. Either "baa", "moo" or "oink".

You could do this: 

sub make_animal
{
    my ($sound) = @_;
    if ($sound =~ /baa/)  { return Sheep->new; }
    if ($sound =~ /moo/)  { return Cow->new; }
    if ($sound =~ /oink/) { return Pig->new; }
}

[download]

The problem is that each time you define a new animal, you have to update your make_animal function.

What would be better is if within the class somehow the animal "registered" itself against the sound it makes.

We can do that by creating a global hash, and then each animal class enters itself into this hash: 

package Sheep;

BEGIN {
   our %::animal_sounds;
   $::animal_sounds{'baa'} = 'Sheep';
}

[download]

And then the make_animal function becomes: 

sub make_animal
{
    my ($sound) = @_;

    foreach my $sounds_like (keys %::animal_sounds)
    {
       if ($sound =~ /$sounds_like/)
       {
          return ($::animal_sounds{$sounds_like})->new;
       }
    }
}

[download]

This function then would not change when a new Animal is added to the hierarchy.

Can anyone recommend any ways of cleaning this up? Is there a more elegant way of doing what I am trying to do? Without using BEGIN blocks and a global hash perhaps? Or without breaking any strictures?

Is there a CPAN module that does something similiar to this?

-Andrew.

In reply to Factory Pattern by tomazos

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