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Module Version: 0.302   Source  


Class::Closure - Encapsulated, declarative class style


version 0.302


 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 my $face;               # read-only

     public 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 FALLBACK => sub { print "Handling $AUTOLOAD" };

 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


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 will lie the declarations for the attributes and methods (and subclasses) for this class.


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

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

As of the moment, has and public only support scalar variables.

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(my $y)    = 2;
     public(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.


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) = @_;


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" ...


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.


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.

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 that 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.

Included in the distribution is a script which will test various aspects of Class::Closure objects against objects created with the traditional object model. In general, Class::Closure is quite a bit faster for plain method calls (the extra hash lookup for each attribute is more overhead than you'd think), but is slower for inherited methods and much slower for object creation. So it's not good to use Class::Closure for small, intermediate objects if you're worried about speed. Fortunately, Perl programs tend not to use these sorts of objects often.

accessor-like subs with arguments aren't yet supported, but there's nothing in the design that says they aren't allowed. I'm just lazy, and I'll happily add them upon request.

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.


Class::Struct, Class::Classless


Aristotle Pagaltzis <>

Documentation by Luke Palmer.


This documentation is copyright (c) 2004 by Luke Palmer.

This software is copyright (c) 2015 by Aristotle Pagaltzis.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

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