Factory

A class that generates instances of other classes, choosing which class to build on the basis of run-time information.

Façade

A class that acts as a front-end for one or more other classes, delegating the actual work to those classes as appropriate.

In your particular example, what you want is a factory: a class that generates objects according to where the sun rises. For example:

# Rational helper class...

package Astro::Data::Scientific;

use base Solar::Data;
use base Lunar::Data;

# <define methods here>



# "Differently rational" helper class...

package Astro::Data::Mythological;

use base Astrological::Data;        # Pre-empt normal methods
use base Astro::Data::Scientific;

# <overload any methods here that the 
#  heavens dictate should behave differently>



# Factory class...

package Astro::Data;

# Constructor returns instances of helper classes,
# depending on run-time astro[nl]o[mg]ical conditions

sub new {
    shift; # off class name

    if ($sun_rises_in_west) {
        return Astro::Data::Mythological->new(@_);
    }
    else {
        return Astro::Data::Scientific->new(@_);
    }
}

# <no methods of its own, since it's never actually instantiated>
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Thanks, Damian and all the others who took the time to reply.

I think I was mostly thrown by the unfamiliar nomenclature. It turns out that I've already used the factory concept (in projects other than the current one) without knowing the fancy name. So I shouldn't have too much trouble implementing one now.

Jim Keenan

Here is a cleaned-up version of Damian's example, along with a test.
Read more... (2 kB)

I hope this example is (a) correct and (b) useful to the next person who wonders about this.

Jim Keenan

If $sun_rises_in_west is a global condition, encapsulate it.

Sometimes you want to calculate things for the parallel universe you don't live in. What is your user going to do then?

To emulate global behavior, just make your default instantiation take into account the fact that the default $sun_rises_in_west_flag is x and not y, and save it as instance data.

To dispatch based on instance data, the most useful technique is delegation - encapsulate your instance data in an object, and have your object as that object to polymorphically get_sunrise_time based on the kind of object it is.

Playing with multiple inheritence implies that your object hierarchy is not only a definition, it also encapsulates state.

While this feels very cool and meta-meta-meta-fun, it's usually not very good for:

• testing
• stability
• readability
• maintainability

Update: blazar requrested that I added an example, based on TheDamian's reply..

## this starts like damian's example:

# Rational helper class...

package Astro::Data::Scientific;

use base Solar::Data;
use base Lunar::Data;

# <define methods here>



# "Differently rational" helper class...

package Astro::Data::Mythological;

use base Astrological::Data;        # Pre-empt normal methods
use base Astro::Data::Scientific;

# <overload any methods here that the 
#  heavens dictate should behave differently>



# Delegating class

package Astro::Data;

sub new {
    my $class = shift;
    my $sun_rises_in_west = @_ ? shift : $default_sun_rises_in_west;

    my $delegate = ("Astro::Data::" . ($sun_rises_in_west ? "Mythologi
+cal" : "Scientific"))->new;

    bless {
        delegate => $delegate,
    }, $class;
}

# define like this or use Class::Accessor
sub sunrise_delegate {
    $_[0]{delegate};
}

# define like this or use Class::Delegation
sub get_sunrise_time {
    my $self = shift;

    $self->sunrise_delegate->get_sunrise_time(@_);
}
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Update 2: I should mention what I really like about delegates - they allow you to keep things better separated.

Instead of your entire set of capabilities (methods) being mushed into one object, you get a higher degree of separation. Whether this boundry is later blurred for convenience as far as code using the delegating object is concerned is irrelevant - the implementation of the delegating object stays cleaner.

This is useful when you have conceptual objects that perform many roles. For example, I have a system where data can be used as resources in a resource allocator.

Instead of making the data, which already has the mess involved with persistence and what not added in, inherit yet another class (the one for a resource), i simply make a new resource class, that is a thin wrapper around this object. This object then becomes a delegate of the resource class.

This has a conceptual simiarity to the MVC model. The model part shows you what the is structured like. The control decides what to do with the data, andn the view, through the controller, displays the model. The delegating object is a bit like a controller, and the delegate is a bit like a model in this sense.

Apropos of the recent discussion "How to differentiate hash reference and object reference?", how do you recommend handling isa when using delegation like this? Do you also define an isa that passes the isa call through to the delegate?

sub isa {
  my ($self, $class) = @_;
  return $self->{delegate}->isa($class);
}
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If so, it's another reason why people should avoid this:

if ( UNIVERSAL::isa( $obj, 'Astro::Data::Mythological' ) ) {
  # whatever
}
[download]

-xdg

Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

Well, the reimplementation of isa is probably the way to go, but normally I've found that the greater the all-knowingness of an object, the less I need to introspect it.

In a sense the behavior is transparent, instead of using a delegate there could have been an if/else on the member data in the single classes get_sunrise_time

My point is that when designing in this direction, usually you want to pretend that the object is the same, and the delegate is just an implementation detail. In situations when the role is truely different you don't want to instantiate from the same class anyway, you have have two top level classes, one for west-rising suns, and the other for east rising ones.

In fact, that is exactly what the delegation model is - it's two independant classes, completely unbound to each other, but interchangable, and an object which can reuse them to give you an alternative with a single point of entry.

Good examples of this kind of reuse can be dug up by querying Any... These are modules which usually wrap around several modules which do the same role, but have different features. The wrapper modules hide the details of the difference from the user, letting the user give the generic module more kinds of input, without caring which bunch of code really does the job at the end.

To wit, after saying

my $a = Archive::Any->new($archive_file);
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I don't care if what I got was reblessed into a different class, or wheter it has a delegate, or whether it uses MMD inside. In fact, it probably isn't Archive::Tar at all, in any sense, since it doesn't provide it's full interface.

When ->isa checks are used with respect to the delegate for the prupose of type checking (what kind of archive format do you encapsulate, mr Archive::Any instance?), then it's perhaps a design problem, because not caring is the reason we wrapped it in the first place.

-nuffin
zz zZ Z Z #!perl

In this case, what you really should do is not to mess with the inheritance at the run time, but to have a class called SolarSystemDataFactory.

This class should have a method that returns you an instance of the right class based on your input. For example, it takes a parameter saying whether the sun rises on the east on west, and base on that, it either returns you an object of Solar::Data or Nemesis::Data.

Hello,

your last example look a lot like a goto!

In fact, it works a bit like the goto considered harmfull. It all comes down to code reuse. If your object/class modifies, under certain unusual conditions, it's inheritance tree and thus its behaviour, it becomes very difficult to inherit from such class. You'd have to know in advance all possible scenarios of behaviour of your object/class before reuse. Checking wheter or not your object is of a certain type becomes very difficult because its type depends on an internal 'if'.

It's better (easier for maintenance, easier to comprehend, easier for reuse) to fix your inheritance first, at compile time maybe, or by convention by never changing @ISA.