Perl - Calling subclass constructor from superclass (OO)

Question

This may turn out to be an embarrassingly stupid question, but better than potentially creating embarrassingly stupid code. :-) This is an OO design question, really.

Let's say I have an object class 'Foos' that represents a set of dynamic configuration elements, which are obtained by querying a command on disk, 'mycrazyfoos -getconfig'. Let's say that there are two categories of behavior that I want 'Foos' objects to have:

• Existing ones: one is, query ones that exist in the command output I just mentioned (/usr/bin/mycrazyfoos -getconfig`. Make modifications to existing ones via shelling out commands.

• Create new ones that don't exist; new 'crazyfoos', using a complex set of /usr/bin/mycrazyfoos commands and parameters. Here I'm not really just querying, but actually running a bunch of system() commands. Affecting changes.

Here's my class structure:

Foos.pm

package Foos, which has a new($hashref->{name => 'myfooname',) constructor that takes a 'crazyfoo NAME' and then queries the existence of that NAME to see if it already exists (by shelling out and running the mycrazyfoos command above). If that crazyfoo already exists, return a Foos::Existing object. Any changes to this object requires shelling out, running commands and getting confirmation that everything ran okay.

If this is the way to go, then the new() constructor needs to have a test to see which subclass constructor to use (if that even makes sense in this context). Here are the subclasses:

Foos/Existing.pm

As mentioned above, this is for when a Foos object already exists.

Foos/Pending.pm

This is an object that will be created if, in the above, the 'crazyfoo NAME' doesn't actually exist. In this case, the new() constructor above will be checked for additional parameters, and it will go ahead and, when called using ->create() shell out using system() and create a new object... possibly returning an 'Existing' one...

OR

As I type this out, I am realizing it is perhaps it's better to have a single:

(an alternative arrangement)

Foos class, that has a

->new() that takes just a name

->create() that takes additional creation parameters

->delete(), ->change() and other params that affect ones that exist; that will have to just be checked dynamically.

So here we are, two main directions to go with this. I'm curious which would be the more intelligent way to go.

Answer 1

In general it's a mistake (design-wise, not syntax-wise) for the new method to return anything but a new object. If you want to sometimes return an existing object, call that method something else, e.g. new_from_cache().

I also find it odd that you're splitting up this functionality (constructing a new object, and returning an existing one) not just into separate namespaces, but also different objects. So in general, you're closer with your second approach, but you can still have the main constructor (new) handle a variety of arguments:

package Foos;
use strict;
use warnings;

sub new
{
    my ($class, %args) = @_;

    if ($args{name})
    {
        # handle the name => value option
    }

    if ($args{some_other_option})
    {
        # ...
    }

    my $this = {
        # fill in any fields you need...
    };

    return bless $this, $class;
}

sub new_from_cache
{
    my ($class, %args) = @_;

    # check if the object already exists...

    # if not, create a new object
    return $class->new(%args);
}

Note: I don't want to complicate things while you're still learning, but you may also want to look at Moose, which takes care of a lot of the gory details of construction for you, and the definition of attributes and their accessors.

Answer 2

It is generally speaking a bad idea for a superclass to know about its subclasses, a principle which extends to construction.[1] If you need to decide at runtime what kind of object to create (and you do), create a fourth class to have just that job. This is one kind of "factory".

Having said that in answer to your nominal question, your problem as described does not seem to call for subclassing. In particular, you apparently are going to be treating the different classes of Foos differently depending on which concrete class they belong to. All you're really asking for is a unified way to instantiate two separate classes of objects.

So how's this suggestion[3]: Make Foos::Exists and Foos::Pending two separate and unrelated classes and provide (in Foos) a method that returns the appropriate one. Don't call it new; you're not making a new Foos.

If you want to unify the interfaces so that clients don't have to know which kind they're talking about, then we can talk subclassing (or better yet, delegation to a lazily-created and -updated Foos::Handle).

[1]: Explaining why this is true is a subject hefty enough for a book[2], but the short answer is that it creates a dependency cycle between the subclass (which depends on its superclass by definition) and the superclass (which is being made to depend on its subclass by a poor design decision).
[2]: Lakos, John. (1996). Large-scale C++ Software Design. Addison-Wesley.
[3]: Not a recommendation, since I can't get a good enough handle on your requirements to be sure I'm not shooting fish in a dark ocean.

Answer 3

It is also a factory pattern (bad in Perl) if the object's constructor will return an instance blessed into more than one package.

I would create something like this. If the names exists than is_created is set to 1, otherwise it is set to 0.. I would merge the ::Pending, and ::Existing together, and if the object isn't created just put that into the default for the _object, the check happens lazily. Also, Foo->delete() and Foo->change() will defer to the instance in _object.

package Foo;
use Moose;

has 'name' => ( is => 'ro', isa => 'Str', required => 1 );
has 'is_created' => (
    is => 'ro'
    , isa => 'Bool'
    , init_arg => undef
    , default => sub {
        stuff_if_exists ? 1 : 0
    }
);

has '_object' => (
    isa => 'Object'
    , is => 'ro'
    , lazy => 1
    , init_arg => undef
    , default => sub {
        my $self = shift;
        $self->is_created
            ? Foo->new
            : Bar->new
    }
    , handles => [qw/delete change/]
);