Luke Palmer

NAME

Class::Closure - Encapsulated, declarative class style

SYNOPSIS

    package Dog;

    use Class::Closure;

    sub CLASS {
        extends Foo::Bar;           # Inherit from another class
        
        extends $some_object;       # Inherit from a single object (classless)
        
        my $hungry;                 # private
        
        has face => my $face;       # read-only
        
        public leash => my $leash;  # public
        
        accessor 'food',            # magical variable-like function
            get => sub { 'None' },
            set => sub { $hungry = 0; };
        
        method bark => sub { print "Woof!" };   # method (note the semicolon)
        
        method BUILD => sub { print "A new dog is born" };  # constructor

        destroy { print "Short is the life of a dog" };  # destructor

        method AUTOLOAD => sub { print "Huh?" };  # AUTOLOAD is allowed, too
    }

    my $fido = Dog->new;   # "A new dog is born"
    $fido->face;           # Get a read-only attribute
    $fido->leash = 'red';  # public attributes look like variables
    $fido->food = 20;      # This calls the food set accessor

DESCRIPTION

Class::Closure is a package that makes creating Perl classes less cumbersome. You can think of it as a more featureful Class::Struct.

To declare a class using Class::Closure, enter a new package, use Class::Closure, and define a sub called CLASS. Inside this sub lie the declarations for the attributes and methods (and subclasses) for this class.

Variables

To declare variables, you mark them as lexicals within the sub. You may prefix them with has and a name to make them read-only or public and a name to make them fully read-write public.

    sub CLASS {
        my $x;             # private
        has y => my $y;    # read-only
        public z => my $z; # public
    }

It's not necessary that their "external" name (the one before the =>) be the same as the variable's name, but it is recommended. Presently only scalars are handled by has and public; you have to define an accessor to specify the semantics you want.

You can give the variables a default value by assigning to the whole declaration. For simple private variables this is easy, for read-only and public variables, it requires extra parentheses:

    sub CLASS {
        my $x = 1;
        has(y => my $y) = 2;
        public(z => my $z) = 3;
    }

To use these variables within the class, use their plain lexical name with the sigil. To use them outside the class, call them as methods. Given the class above:

    $obj->x;      # Illegal; private
    $obj->y;      # Ok, 2.
    $obj->z;      # Ok, 3.
    $obj->z = 32; # Write to $z.

This may be a little different from the usual $obj->z(32) syntax you might be used to. Trust me, this will grow on you.

Methods

To declare methods, use the method keyword and pass a name and a reference to a sub:

    sub CLASS {
        method bark => sub {
            print "Woof!\n";
        };
    }

The invocant is still passed in as the first argument as in old-style OO. The fact is, though, that many times you won't need it, since you can reference the member variables without it. You still need it to call functions on yourself, though.

    sub CLASS {
        method chase_tail => sub {
            my ($self) = @_;
            $self->chase($self->find_tail);
        };
    }

Accessors

Sometimes a change of interface goes from using a public variable to a function with extra behavior. Some would say that's why you never make a member variable public. I disagree, since you can just fake one with the accessor keyword:

    sub CLASS {
        accessor 'number',
                get { print "Getting the number";  42; },
                set { print "Setting the number";  $_[0]->send($_[1]) };
    }
    print $obj->number;  # "Getting the number"  "42"
    $obj->number = 314;  # "Setting the number" ...

Inheritance

Unlike the standard Perl 5 object model, Class::Closure can inherit from both classes and variables (like Class::Classless). Also, it keeps their respective namespaces separate, so they don't accidentally stomp on each other's member variables, even if they're implemented with the standard object model.

To inherit, use the extends keyword. It can take as an argument either a class name (make sure you quote it lest you confuse Perl) or an object. If you need to pass construction parameters to your superclass, just inherit from it as an object:

    sub CLASS {
        extends MySuperClass->new(@params);
    }

Constructors and Destructors

The special method BUILD is called whenever a new object is created, with the blessed object in the first argument and the rest of the construction parameters in the remaining arguments.

Destructors are a little different. Because of the magic that Class::Closure has to do to get them to work with inheritance, they have a special syntax:

    sub CLASS {
        destroy { print "Destructing object"; }
    }

Yep, that's all. And you heard me correctly, they work right with inheritance, unlike the standard DESTROY method.

FALLBACK

Class::Closure supports an AUTOLOAD feature. But because it uses AUTOLOAD internally, it has to call it something else. It's called FALLBACK, and it works just like AUTOLOAD in every way (the name of the current sub is still even in $AUTOLOAD).

How does it work?

If you really want to get scary power out of this module, you have to understand how it works.

The CLASS sub that you defined in your package is actually called every time an object is created. That's right, so there's no need for a BUILD at all (but it makes things look cleaner). Class::Closure exports each one of these "keywords" into your namespace, and they are used right on the spot to construct the object each time.

This way each object's member hash is actually a lexical scratchpad, and it keeps track of where it is, so you don't have to reference $self all the time. It has the added plus that each object in an inheritance heirarchy has it's own scratchpad, so you don't get variable name conflicts.

In more detail, when you call new on your package, it derives a new anonymous package for only that object. Then when you use method (or has or public or attribute, which are really just wrappers around the same thing), it installs the sub you give into the symbol table position. These closure's aren't "cloned", but just referenced, so this doesn't take up the horrible amount of memory you might be thinking it does.

Then when all references to the object disappear, it uses Symbol's delete_package to clean out the anonymous package and free memory (and more importantly, call DESTROYs) associated with the object.

What does this all mean for you, the user? Since you understand that these "declarations" are just sub calls at object construction time, you can create your objects based on a dynamic template:

    sub CLASS {
        my ($class, $mode) = @_;

        if ($mode == 1) {
            method foo => sub { ... };
            method bar => sub { ... };
        }
        else {
            method foo => sub { ... };
            method bar => sub { ... };
        }
    }

That avoids a run-time check on each of the method calls, and makes things a little easier to read. There's all kinds of other fun stuff you can do.

Technical Notes / Bugs / Caveats / Etc.

The benchmarks say that Class::Closure has 50% less overhead than the traditional object model. Yes, that's right, Class::Closure, in addition to being super-cool, encapsulated, and easy to read, is faster than what you'd get if you did it the traditional way! It surprised me too. On the other hand, you sacrifice quite a lot of object construction time for this trade-off.

You might get in trouble if you try to define method names the same as the exported keyword names.

There are certainly more bugs, since this is complex, subtle, scary code. Bug reports/patches welcome.

SEE ALSO

Class::Struct, Class::Classless

AUTHOR

Luke Palmer - luke at luqui dot org

LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.