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classes - conventional Perl 5 classes


This document covers version 0.944


    package MyClass;
    use strict 'subs'; no warnings;
    use classes
        new             => 'classes::new_init',
        class_attrs     => [ 'Attr' ],
        class_attrs_ro  => { 'Read_Only_Attr'=>'yes' },
        class_attrs_pr  => { 'Priv_No_Accessors'=>'ok' },
        attrs           => [ 'attr', '_not_really_private' ],
        attrs_ro        => [ 'read_only_attr' ],
        attrs_pr        => [ 'attr_no_accessors' ],
        class_methods   => { 'Empty_Method'=>0 },
        methods         => { abstract_method => 'ABSTRACT' },
        throws          => 'X::Usage',
        exceptions      => 'X::MyOwn',


    package MyMixinMod;
    use classes

    package UsesMixins;
    use classes
        mixes => ['MyMixinMod','AnyPackage'],
        methods => {
                foo => 'SomePackage::a_foo_method',


    use classes name=>'MySuper', attrs=>['color'];

    package ExtendsMySuper;
    use classes
        extends => 'MySuper',

    package MultipleInheritance:
    use classes
        inherits => [ 'MySuper', 'AnotherPackage' ],

Package Methods (traditional export):

    package FunctionLib;
    use classes
        pkg_methods => [ 'foo', 'bar' ],

    use FunctionLib ':all';
    use FunctionLib qw( foo bar );

Dynamic Classes:

    package DynamicOne;
    use classes
        type => 'dynamic',
        class_methods => ['add_attr'],

    sub add_attr {
        my ($class, $attr_name) = @_;
        classes attrs => [$attr_name];
        return $class;


    name => 'MyClass',

    type => 'static',
    type => 'dynamic',
    type => 'mixable',

    extends  => 'SuperClass',
    inherits => 'SuperClass',
    inherits => ['Class1', 'Class2'],

    mixes => 'Module',
    mixes => { Module => ... },
    mixes => [
        { Module2 => [ 'method1', ... ] },
        { Module3 => 'ALL' | 'PUB' | 'SAFE' },
        { Module4 => qr/.../ },
    class_mixes => ...
    pkg_mixes => ...

    mixes_def => 'SAFE' | 'ALL' | 'PUB',

    attrs => [ 'attr1', 'attr2' ],
    attrs_ro => ...
    attrs_pr => ...

    class_attrs => [ 'class_attr1', 'class_attr2' ],
    class_attrs => {
        class_attr1 => undef,
        class_attr2 => 100,
        class_attr3 => 'string',
        class_attr4 => <ref>,
    class_attrs_ro => ...
    class_attrs_pr => ...
    unqualified => 1,
    unqualified => 0,

    noaccessors => 1,
    noaccessors => 0,

    justahash => 1,  # unqualified + noaccessors
    justahash => 0,  # unqualified + noaccessors

    methods => [ 'method1', 'method2' ],
    methods => {
        method1 => 'method1',
        method2 => 'local_method',
        method3 => 'Extern::library::method',
        method4 => 'ABSTRACT',
        method5 => <false> | 'EMPTY',
        method6 => sub { ... } | \&some::method,
    class_methods => ...
    pkg_methods => ...

    new   => 'new',
    new   => 'classes::new_args',
    new   => 'classes::new_only',
    new   => 'classes::new_init',
    new   => 'classes::new_fast',
    new   => 'MyModule::new_method',

    init  => 'initialize',
    init  => 'classes::init_args',
    init  => 'MyModule::initialize',

    clone => 'clone',
    clone => 'classes::clone',
    clone => 'MyModule::clone',

    dump  => 'classes::dump',

    throws => 'X::Usage',
    throws => [ 'X::Usage' ],

    exceptions => 'X::Doh',
    exceptions => [
        { name => 'X::FileOnFire', attrs=>['file'] },
    exceptions => { name => 'X::FileOnFire', attrs=>['file'] },

    base_exception => 'X::OtherClass',
    base_exception => 'X::classes',
    base_exception => {
        name    => 'X::BaseException',
        extends => 'X::Whatever',

    def_base_exception => 'X::classes',


    my $object = MyClass->new;
    my $object = MyClass->new(  attr1=>1  );
    my $object = MyClass->new({ attr1=>1 });

    $object->initialize(  attr1=>1  );
    $object->initialize({ attr1=>1 });

    my $deep_clone = $object->clone;



    my $value = MyClass->get_My_Class_Attr;
    my $value = $object->get_my_attr;







    MyClass->SUPER;  # $ISA[0]
    $object->SUPER;  # $ISA[0]



        ... ->classes::dump( $handle );
        ... ->classes::dump( \$buffer );

    classes::dump(['any', 'scalar', 'really']);

    classes::load MyClass;
    classes::load MyModule;
    classes::load MyPackage;

    MyClass->classes::set( My_Class_Attr=>1 );
    $object->classes::set( my_attr=>1 );

    my $value = MyClass->classes::get( 'My_Class_Attr' );
    my $value = $object->classes::get( 'my_attr1' );

    my $string = MyClass->classes::sprintf( '%s', 'My_Class_Attr');
    my $string = $object->classes::sprintf( '%s', 'my_attr1');

    MyClass->classes::printf( '%s', 'My_Class_Attr');
    $object->classes::printf( '%s', 'my_attr1');

    my $id = $object->classes::id; 



See X::classes, X::classes::traceable, classes::Throwable


A simple, stable, fast, and flexible way to use conventional Perl 5 classes in scripts, rapid prototypes, and full-scale applications.

This reference document covers syntax only. See the following for more:


Introductory primer of concepts, ideas and terms from object oriented programming without any particular implementation specifics in mind.


List of included tutorials aimed at taking a beginning Perl programmer from the basics to advanced techniques of object oriented programming with classes.


Cookbook collection of specific tasks and examples with lots of useable code.


Questions and answers about support, design decisions, justification, motivation, and other hype.


Declaration tags are passed to use classes at compile time or to the classes function at run time. All tags are optional depending on the context. Some have default values. Tags with undefined or otherwise negative values are usually ignored. A declaration representing the class is always available in a special DECL meta attribute best displayed with classes::dump.

Tag descriptions are ordered as you may expect to find them in a declaration.


Name of the class to define. If omitted will use the implied name of the calling package--including main, (which is just another class). Name must be valid Perl package name.

See: perlmod, perlobj


Specifies the type of classes usage:


Default. Indicates a class that is not going to change during its run time life:

  • Does not import the classes function
  • Defines and initializes declaration DECL
  • Defines the CLASS constant
  • Defines the SUPER method

Same as static but indicates the class/module/package can be used as a mixin. Like static a mixable can be a stand-alone class or not (unlike some other languages that support mixins). The attributes and methods declared in a mixable are "mixed into" other classes that use the mixes tag. The result is something between inheritance and having defined everything originally in the receiving classes.

Calls to the mixed in methods respond mostly as if they were inherited, they "see" functions and variables defined within the package in which mixable was declared. The only exception to this is the special $ATTR_foo and $CLASS_ATTR_foo keys which behave as expected pointing to the mixin from which they came. To help keep these straight they are included in the special MIXIN table along with every method that has been mixed in.

WARNING: Every object or class method., but not necessarily function, used by any declared object or class method in a mixable must also be declared in order for the declared method to work. Consider $self->_next_one called from a declared mixable method. See Can't locate object method under TROUBLESHOOTING for more.

Strictly speaking a mixin is not inherited. The special @ISA array is not updated and the normally inherited UNIVERSAL->isa method returns false if checked for the name of the mixin. Such an equivalent is not practical when dealing with mixins. Use MIXIN to assist with introspection if needed. It contains every method not from that immediate class and the package it came from.

See: mixes, $MIXIN, $DECL,


Indicates a class that can be created or redefined in some way at run time. dynamic classes behave exactly as static classes except they also import the classes function into the class itself allowing it to be used at run time to add to or redefine some part of the class.


Declares single class to extend that will be searched out of @INC and loaded exactly like the base pragma. Cannot be included in the same declaration with inherits.

Choose mixes over extends where possible.

Throws: X::InvalidName, X::Usage

See: base, "@INC" in perlvar, SUPER


Same as extends but for one or more classes (multiple inheritance).

SUPER will refer to the first inherited class. This tag cannot be included in the same class declaration as extends.

Choose mixes over inherits where possible.



"Mixes in" methods and attributes from another class or package. Modules are loaded if needed. The meta attributes $DECL and $MIXIN are updated.

Behavior differs depending on what is being mixed in.

If use classes type=>'mixable' (see type) was used to declare the mixin then all of the following from the declaration are mixed in:


For public attributes the special associated attribute key name strings are also mixed in (ex: $ATTR_foo, $CLASS_ATTR_foo, see attrs).

Everything else is seen as a simple package, a collection of methods or functions which might be a class declared with use classes (not type mixable), a traditional Perl class, a function library or any other package with subroutines. These can be selectively mixed in by name, regular expression, or one of the following aliases:


Default and safest. Matches any method name that is not all caps nor preceded with an underscore.


Matches any valid method name, including all caps and initial underscore except for the special:


WARNING: Other special all caps perl subroutines will be imported when using ALL. This includes DESTROY and AUTOLOAD if defined.


Matches any valid method name--including all caps--that does not begin with underscore except for the same special names listed for ALL above.

You can change the default alias by adding the mixes_def tag.

When in doubt check the symbol table with classes::dump(\%MyClass::).

Throws: X::InvalidName, X::Usage

See: class_mixes, pkg_mixes, mixes_def, methods, class_methods, pkg_methods, attrs, attrs_ro, class_attrs, class_attrs_ro, MIXIN, $MIXIN, classes::load, perlre, perlref, perlmod, perlobj, AutoLoader


Exactly the same as mixes but as if class_methods were used instead of methods.


Exactly the same as mixes but as if pkg_methods were used instead of methods.


Sets the default mixes filter for all mixins in that same declaration. Set to SAFE by default.


Declares object attributes. Attribute names must begin with [a-zA-Z_] followed by zero or more [a-zA-Z0-9_] characters.

Each attribute receives both a public pair of accessor methods which begin with set_ and get_, unless specifically requested otherwise with noaccessors or justahash. A key variable of the form $ATTR_foo is also added to the class for use within the class. Use this when referring to your object attribute key since it observes things like unqualified. [It is a fraction of 1% slower according to benchmarks. There are other more advanced reasons explained in classescb].

    $self->{$ATTR_foo} = 'blah';

TIP: Vim users can add this macro line to your .vimrc file or equivalent to create your attributes quickly by typing the attribute name, escaping, then typing backslash (\a):

    map \a bi$self->{$ATTR_<ESC>ea}

All attribute values must be scalars. References to arrays, hashes and blessed objects are scalars.

WARNING: In your overriden get_ accessor use classes::clone or otherwise return a clone of attribute values that are references if you are concerned your class users might directly manipulate your attribute by using the returned reference. Better yet, don't make that attribute public, even as read-only, and use other methods that operate on the attribute values instead.

The get_ accessor always return the current value of the attribute, which is undef until some value is set. Initialize object attribute values from the new or initialize methods:

    package MyClass;
    use classes
        new   => 'new',
        attrs => ['color'],

    sub new {
        my $class = shift;
        my $self = {
            $ATTR_color => 'chartreuse',
        bless $self, $class;
        return $self->classes::init_args(@_);

    package main;
    my $object = MyClass->new;
    print $object->get_color;             # chartreuse
    print $object->get_color;             # blue

Or, if you intend to "recycle" and reinitialize existing objects rather than throwing them away and creating new ones:

    package MyClass;
    use classes
        new   => 'classes::new_init',
        attrs => ['color'],

    sub initialize {
        my $self = shift;
        $self->{$ATTR_color} = 'chartreuse';
        return $self->classes::init_args(@_);

    package main;
    my $object = MyClass->new;
    print $object->get_color;             # chartreuse
    print $object->get_color;             # blue

NOTE: The classes pragma follows the Perl best practice of adding the accessor prefixes (set_ and get_) to increase clarity, improve performance, catch bugs at compile time, and reduce the risk of attribute methods stomping on other methods. Attibute names can even be all capitals or other reserved names because the accessor method prefix prevents name collision.

WARNING: The set_ accessor (mutator) must always return void (return with no arguments). The return value of a set_ method should never be checked or used for anything. Throw and catch exceptions to handle bad values, etc.

    sub set_color { $_[0]->{$ATTR_color} = 'my:'. $_[1]; return }

Throws: X::Usage, X::InvalidName, X::classes::AttrAlreadyPublic

See: attrs_ro, class_attrs, class_attrs_ro, unqualified, initialize, perlsub, "return" in perldoc


Same as attrs but only get_ public accessor defined.

However, if an inherited read-write attribute with the same name is detected a read-only set_ public accessor is defined that does nothing more than throw a X::classes::ReadOnly exception.

WARNING: Beware of leaving behind custom overriden public set_ accessors when changing a read-write attribute (attrs) to read-only (attrs_ro).


Same as attrs but no public accessors are defined at all. The $ATTR_foo string is still created within the declaring class. These private/protected attributes are not inherited with extends, inherits, or use base since they have no accessor methods to inherit.

However, the object attribute hash key $ATTR_foo is mixed in if the attribute is in a explicitely mixable module allowing $self->{$ATTR_foo} from within the mixing class. This makes refactoring mixins from class code very easy since methods can literally be cut and paste without modification.


Sets internal use of unqualified attribute key names, which is usually a bad idea unless you really know what you are doing since classes could inadvertently stomp over each other's internal keys. Set to 1 to cause the internal object hash ref to not have each key prefixed with <CLASS>::.

NOTE: Use the $ATTR_foo and $CLASS_ATTR_foo key variables containing the corresponding names in order to avoid changing class code during refactoring. Methods can be cut and paste often without modification by following this convention. See attrs and class_attrs for more about this.


Disables creation of set_ and get_ accessor methods for object attributes expecting them to be set directly. Usually used in conjuction with unqualified. If so, consider setting justahash instead.


Same as unqualified, noaccessors, and new='classes::new_fast'> combined. Great for POPOs (plain old perl objects) that are first hashes that happen to have methods and class attributes associated with them. Objects from a class with this declaration fully expect to have their "internal" hash ref accessed directly.


Same as attrs but for attributes with class scope. In addition class attributes can be declared with initial values.

Class attributes declared and defined with the classes pragma behave like most OO programmers expect; changing a class attribute value anywhere changes it for all objects from that class as well as all objects from any class that inherits or mixes it in. Classes wishing to take over the class attribute must redeclare it (thereby overriding its accessors).

WARNING: This behavior is unlike Class::Data::Inheritable which obtusely allows any class to take over a "class attribute" by simply setting its value.

Class attributes are implemented as package variables. A key variable containing the qualified name of the attribute is available in the $CLASS_ATTR_foo form. Use it within the class to refer to class attribute package variables (with strict 'vars' off of course):

    no strict 'refs';
    sub set_Color { $$CLASS_ATTR_color = $_[1]; return }

TIP: Vim users can add the following macro line to their .vimrc to quickly create this by typing the name of the class attribute, then escape, then backslash capital A (\A):

    map \A bi$$CLASS_ATTR_<ESC>ea

The following are identical within the class. Pick the one you prefer, but keep in mind that if your class package name changes, you might have a lot of find and replace to do on your class:


WARNING: Don't attempt to initialize a class attribute value during during new or initialize since it blows away any changes to the class attribute over the previous life of the class affecting all its derived classes. Provide an initial value in the hash ref form of the class_attrs declaration:

    class_attr => {
        foo => 'initial value',

Same as class_attrs but no set_ accessor like attrs_ro.


Same as class_attrs but private (no accessors) like attrs_pr.


Declares member methods (more strictly, "operations") and the actual methods to which they refer. Usually the two will be the same, which seems redundant and is the default for the ARRAY ref form, but the actual method may refer to any method in any class, module, or package. Like mixes the module containing the external method will be loaded if it has not been.

The following special anonymous methods are also available:


Assigns a nearly empty anonymous method that will throw an X::Unimplemented exception if called before being overriden. This is useful for defining an abstract class or interface which expects to have methods mixed in or inherited to realize the abstract ones.


Assigns an empty anonymous method sub {}. Useful for nullifying a method without breaking the interface.

Method values can also be CODE refs with the disadvantage of only appearing as CODE in the declaration DECL instead of the local or qualified method name.

Use methods, class_methods, or pkg_methods instead of mixes if you only need to pull in a few specific methods.

Per perlstyle guidelines, name your public methods with an initial lowercase letter. Join multiword methods with underscores. Begin your private methods with an underscore, if you declare them at all.

Throws: X::Usage, X::InvalidName,

See: class_methods, pkg_methods, mixes, extends, inherits, perlstyle


Same as methods but with class scope. However, since perl currently makes no distinction there is no difference between this tag and methods other than the class_methods section of the declaration DECL.

Avoid bimodal methods that can be called from both a class or object. You cannot declare a class and object method with the same name, (although should you wish you could declare a method with the same name as an attribute because of the attribute accessor prefixes set/get).

See: methods, pkg_methods


Package methods are just functions. Using this tag is optimized shorthand for what you might do using the Exporter module's EXPORT_OK hash. Package methods are automatically available for import on request:

    package MyPackage;
    use classes pkg_methods=>['ini2hash', 'hash2ini'];

    package main;
    use MyPackage 'ini2hash';
    my $hash = ini2hash($ini);

Or, use the special ':all' tag:

    use MyPackage ':all';
    my $hash = ini2hash($ini);
    my $ini  = hash2ini($hash);

Under the hood the import routine added to your package is a slimmed down equivalent to what would be added by the Exporter module. Obviously you can override the automatic import with your own if you want to do something fancier with your pkg_methods when they are requested.

Unlike class_methods and methods, pkg_methods do not expect a first argument to be a class or object reference.

See: methods, class_methods, pkg_mixes


Declares exceptions that are thrown by the class but defined elsewhere. Loads the module containing the exception class if needed and found.

Use throws or exceptions to quickly add common exceptions to your shell scripts and prototypes. Even if you are not using OO the base X::classes and X::classes::traceable classes, which both mix in classes::Throwable, can be useful.

See: exceptions, EXCEPTIONS, classes::load


Declares exception classes be automatically defined. Exception classes are listed by name or declaration. By default each is a subclass of a automatically defined exception class matching the class name of the form X::MyClass.

    package MyClass;
    use classes
        exceptions => ['X::MyException','X::MyOther'],

The above is exactly equivalent to the following long hand:

        { name=>X::MyClass,     extends=>'X::classes::traceable' },
        { name=>X::MyException, extends=>'X::MyClass' },
        { name=>X::MyOther,     extends=>'X::MyClass' },

This preserves a convenient exception inheritance tree useful for catching exceptions in user code.

If the hash ref form is used and extends is omitted it is implied to be whatever the base exception class is, by default the X::MyClass exception.

Use base_exception and def_base_exception to change the default base exception class from the X::MyClass one dynamically created matching the class name.

Inheritance is strongly preferred over mixins for exceptions in order to trap them at different scope levels.

TIP: To save further hassle declaring exception classes, use the X::classes::traceable message and item generic attributes instead of declaring your own additional exception attributes where practical.

Declaring an exception class that already appears to exist causes a X::NameUnavailable exception to be thrown. To avoid this, change the declaration from exceptions to throws; or, use a different name in the hash ref declaration of the exception and specify it as extending the one that already exists; or, use a different name and just don't worry about extending the other. X::Usage is a good example of this. It is predeclared and used by classes itself and therefore available to every class that uses the classes pragma.

WARNING: Always use the conventional X:: namespace in your exception class names. This practice makes exceptions easy to spot in code while reducing name conflicts with other legitimate classes and base exceptions. If you are really concerned with exception class namespace clashes that are out of your control then add the full class name after the X:: to qualify it further, long for sure, but safe from conflict. The following vim syntax hilighting macro makes spotting exceptions even easier in the code. Add it to your ~/.vim/syntax/perl.vim:

    syn match  perlOperator "X\:\:[a-zA-Z:_0-9]*"

See: throws, X::classes, X::classes::traceable, classes::Throwable, EXCEPTIONS, "DECLARATION TAGS"


Sets the base exception class to use for all exceptions declared. By default becomes a dynamically create exception class matching the name of the class in the form X::MyClass. Applys only to the specified or implied class associated with the declaration.


Same as base_exception but applies to any and all declarations that use the classes pragma from that time forward. Remember that this applies at compile time when using use classes.

TIP: If you never need or want traceablity in your exceptions set this to X::classes in some master class to create the lightest exceptions possible. Then when debugging, you can change in the master class back to X::classes::traceable or something like it.


Declares the method to use for the standard new constructor. Shortcut for class_methods. The following are equivalent:

    new => 'new',
    class_methods => [qw( new )],
    class_methods => { new => 'new'},
    new => 'classes::new_only',
    class_methods => { new => 'classes::new_only' }

NOTE: Athough using the name new for the constructor is not required it is recommended and a well-established best practice.

See also: new, clone, initialize, new_args, new_init, new_only, new_fast, class_methods


Declares the method to use for the standard initialize method. Shortcut for methods. The following are equivalent:

    init => 'initialize',
    methods => [qw( initialize )],
    methods => { initialize => 'initialize'},
    init => 'classes::init_args',
    methods => { initialize => 'classes::init_args' }

NOTE: Athough an initialize method is not required it is recommended for classes with objects that would prefer to be (re)initialized than thrown away and replaced with a new one.

See also: initialize, new, new_init, init_args, methods


Declares the method to use for the common clone method. Shortcut for methods. The following are equivalent:

    clone => 'clone',
    methods => [qw( clone )],
    methods => { clone => 'clone'},

    clone => 'classes::clone',
    methods => { clone => 'classes::clone' }

See also: clone, new, methods


Declares the method to use for the dump method commonly defined during development to help with debugging. The following are equivalent:

    dump => 'classes::dump',
    methods => { dump => 'classes::dump' }

See also: classes::dump, methods


The following methods are either defined into the classes and mixins that are created using classes or are available with the fully qualifed classes:: prefix and can be mixed into your code:

    use classes type=>'dynamic';
    classes ... ;

    use classes ();
    classes::classes ... ;
    classes::define ... ;

Main classes command function. The dynamic (run time) variant of use classes. The classes function is imported into classes with the type => 'dynamic' to allow manipulation of classes at run time. The classes::define function is an identical (symbol) alias to classes::classes that is never exported and always available in its fully qualified form.

Throws: every exception listed under EXCEPTIONS

See: type


The standard constructor method. Defined by most all classes but often missing from mixables. When called from a class returns a new instance (object) of the class. classes::new_args and classes::new_only are good defaults where no constructor customization is needed. classes::new_init hands all arguments to an expected initialize method. classes::new_fast expects a single hash ref as argument and uses it for the internal object storage.

Use clone instead of new to create copies of objects.

Often you will need a custom new method to initialize object attributes. See attrs for a small example of this.

Consider overriding initialize before new if your class' objects may need to be reinitialized rather than thrown away and replaced with new ones.

See: new, new_args, new_only, new_init, new_fast, initialize, class_methods, perlobj, perlref


Constructor implementation. Fulfills new. Creates object and then hands off with any arguments to classes::init_args:

    sub new_args {
        my $class = shift;
        my $self = {};
        bless $self, $class;
        return $self->classes::init_args(@_);

See: init_args, new, new_init, new_only, new_fast, class_methods


Constructor implementation. Fulfills new. Ignores any arguments (since it does not call the initializer):

    sub new_only { return bless {}, $_[0] }

See: new, new_args, new_init, new_fast, class_methods


Constructor implementation. Fulfills new. Creates object and then hands off with any arguments to initialize:

    sub new_init {
        my $class = shift;
        my $self = {};
        bless $self, $class;
        return $self->initialize(@_);

See: new, new_args, new_only, new_fast, class_methods


Fastest constructor implementation. Fulfills new. Expects a compatible hash ref as the first and only argument. Blesses that ref into class.

    sub new_fast { return bless $_[1]||{}, $_[0] }

Useful when you have a hash that you absolutely trust and need the speed. Particularly useful when 'inflating' thousands of record objects from parsed lines and the like and have every intention of directly manipulating the internal hash ref rather than burdening it with accessors but still want to associate that record with a class. In short, a good way to tack a class onto your structure with the least amount of OO bloat.

NOTE: The only way faster to associate a hash with a class is to bypass any accessor altogether and call bless on the hash. Using bless alone completely trusts that the class will never use the constructor in any other way, which is a pretty big leap in most OO code.

See: new, new_only, new_init, class_methods


The initializer. Commonly defined by classes instead of a custom new constructor so that objects from the class can be reinitialized rather than thrown away and recreated. Usually called by new constructor to setup the initial object state including aggregations from other classes. classes::new_init expects an initialize.

The initialize method always takes an initial self-reference to the object being initialized as the first argument, the rest of the arguments are dependent on the class itself, but usually a hash and/or hash ref of attribute keys and values are accepted. Usually new and initialize should accept the same argument signature and initialize must always return the same self reference passed to it to preserve the identify of the object.

See attrs and dump for examples.

See: init_args, new, new_init, new_only, methods


Initializer implementation. Takes a hash or HASH ref as arguments--usually passed from the new constructor--and uses the argument keys as attribute names setting each attribute value by calling the corresponding accessor:

    # $object->set_attr1(1) implied
    $object->initialize(  attr=>1  );
    $object->initialize({ attr=>1 });

Combined with a new constructor:

    my $object = MyClass->new(  attr1=>1  );
    my $object = MyClass->new({ attr1=>1 });

Here is the actual code for quick reference:

    sub init_args {
        my $self  = shift;
        my $attrs = $_[0];
        $attrs = {@_} if ref $attrs ne 'HASH';

        while ( my ( $attr, $value ) = each %$attrs ) {
            my $setter = $self->can("set_$attr")
                or X::UnknownAttr->throw([$attr]);

        return $self;

NOTE: Dispatching to attribute accessor methods not only supports encapsulation but also is the only reliable method of generically setting attributes during construction and initialization, despite the extra subroutine call. This is because Perl 5 does not do any attribute inheritance, only method inheritance.

See: attrs, class_attrs

Throws X::UnknownAttr if can't find a setter for the attr/key passed.


When called from an object returns a new object with the current state of the original, a deep clone. No clone method is defined by default, but it is recommended.

Use clone instead of a bimodal new:

    my $object = MyClass->new;    # good
    my $clone  = $object->clone;  # good
    my $clone  = $object->new;    # not so good

The classes::clone method can be mixed into your classes:

    clone => 'classes::clone',

The classes::clone method is modeled after the Clone_PP and Clone modules to create the best clone reasonably possible with Perl 5. It returns deeply cloned copies of the original objects, but makes shallow copies of attributes that are globs, regx objects and anything other than the basic HASH, ARRAY, SCALAR, and REF, which themselves are cleanly and recursively cloned. Attributes that are objects are cloned by their primitive blessed ref type--not their own clone methods--and are then blessed into the same class.

classes::clone can also serve as a standalone function for cloning structures besides objects:

    my $array = [ 'some', {thing=>'a'}, \$little, qr/komplex/ ];
    my $cloned_array = classes::clone $array;

If you need to reference the actual classes::clone code consider perldoc -m classes.

Throws: X::Usage

See: Clone_PP, Clone,


Returns the numeric, unique memory address of the object (or any ref) that is passed. Shortcut to Scalar::Util::refaddr:

    sub id { Scalar::Util::refaddr($_[0]) }

Useful when comparing clones in testing and what not:

    my $event = Event->new;
    my $clone = $event->clone;
    if ($event->classes::id == $clone->classes::id) {
        print 'come on, that is not a _real_ clone';

Can be combined with the CORE::time and/or the current process ID to make a pretty unique object identifier for persistence and the like:

    package MyClass;
    use classes
        new      => 'new',
        attrs_ro => ['id'],

    sub new {
        my $class = shift;
        my $self = {};
        bless $self, $class;
        $self->{$ATTR_id} = CORE::time . "-$$-" . $self->classes::id;
        return $self;

See: "refaddr" in Scalar::Util


A mutator dispatch method. Companion to classes::get. When called from an object or class sets and returns a new value for a named attribute by calling (dispatching) the object's setter/writer/mutator method. The minimal speed loss for the dispatch pays for the flexibility of allowing attributes to be set without knowing their names before the code is executed.

    $crayon->classes::set( 'color' => 'purple' );

Can be mixed into your class to give them public dispatchers:

    methods => { set => 'classes::set' },

Here is the actual code for quick reference:

    sub set {
        my ( $self, $name, $value ) = @_;
        my $accessor = $self->can("set_$name")
            || X::MethodNotFound->throw("set_$name");
        return $self->$accessor($value);

Throws: X::MethodNotFound if the attribute was defined read-only or not defined at all.


The accessor dispatch method. Companion to classes::set. Returns the current value of the named attribute but does not set a new value.

Here is the actual code for quick reference:

    sub get {
        my ( $self, $name ) = @_;
        my $accessor = $self->can("get_$name")
            || X::MethodNotFound->throw("get_$name");
        return $self->$accessor;

Throws: X::MethodNotFound if the attribute was not defined.


Mixable object or class method that takes a standard sprintf FORMAT and a list of ATTRNAMES and simply looks up the attribute values by their get_foo equivalents and returns a formatted string with the values.


Same as sprintf but prints the string instead of just returning it.


Using Data::Dumper dumps a visual representation of a class, object, or any scalar to a handle (STDERR by default) or string buffer.

NOTE: Be sure to use parens or indirect notation when dumping objects since context is important--especially when dumping the self-referencing return value or most methods:

    $ini->read->classes::dump;   # right
    classes::dump $ini->read;    # not what you would expect

Can also be mixed into your classes to give them their own dump methods:

    dump => 'classes::dump',

Dumps the implied caller if no argument passed:

   package MyClass;

Dumping a class displays three things: the current declaration DECL, the current state of any class_attrs and the list of any methods that have been mixed in. Dumping an object also displays its internal hash ref:

    package MixMeSimple;
    sub mixed_in {'yes'};

    package MixMeDeclared;
    use classes
        type           => 'mixable',
        attrs          => [qw( a_mixmedecl )],
        attrs_pr       => [ 'private_attr' ],
        class_attrs    => [qw( ca_mixmedecl )],
        class_attrs_pr => { PrivateAttr=>'yep' },
        methods        => [qw( m_mixmedecl )],
        class_methods  => [qw( cm_mixmedecl )],

    package main;
    use classes
        name  => 'MySuper',
        attrs => ['color'],
        new   => 'classes::new_args',

    use classes
        name            => 'MyClass',
        extends         => 'MySuper',
        new             => 'classes::new_init',
        init            => 'classes::init_args',
        throws          => 'X::Usage',
        exceptions      => 'X::MyOwn',
        mixes           => [qw( MixMeSimple MixMeDeclared )],
        class_attrs     => { Attr=>1 },
        class_attrs_ro  => { Read_Only_Attr=>'yes' },
        attrs           => [ 'attr' ],
        attrs_ro        => [ 'read_only_attr' ],
        class_methods   => { Empty_Method=>0 },
        methods         => { abstract_method=>'ABSTRACT' },

    my $object = MyClass->new(attr=>'ok');


    ###########################  MyClass  ############################

    $DECL1 = {
      'attrs' => [
      'exceptions' => [
      'class_attrs' => {
        'ca_mixmedecl' => undef,
        'Attr' => 1
      'name' => 'MyClass',
      'class_attrs_ro' => {
        'Read_Only_Attr' => 'yes'
      'class_methods' => {
        'Empty_Method' => 'EMPTY',
        'new' => 'classes::new_init',
        'cm_mixmedecl' => 'MixMeDeclared::cm_mixmedecl'
      'inherits' => [
      'attrs_ro' => [
      'methods' => {
        'mixed_in' => 'MixMeSimple::mixed_in',
        'abstract_method' => 'ABSTRACT',
        'initialize' => 'classes::init_args',
        'm_mixmedecl' => 'MixMeDeclared::m_mixmedecl'
      'attrs_pr' => [
      'type' => 'static',
      'class_attrs_pr' => {
        'PrivateAttr' => 'yep'
      'throws' => [

    $MIXIN1 = {
      '$ATTR_a_mixmedecl' => 'MixMeDeclared',
      '$CLASS_ATTR_PrivateAttr' => 'MixMeDeclared',
      '$ATTR_private_attr' => 'MixMeDeclared',
      'initialize' => 'classes',
      'get_ca_mixmedecl' => 'MixMeDeclared',
      'cm_mixmedecl' => 'MixMeDeclared',
      'set_a_mixmedecl' => 'MixMeDeclared',
      'mixed_in' => 'MixMeSimple',
      'get_a_mixmedecl' => 'MixMeDeclared',
      'set_ca_mixmedecl' => 'MixMeDeclared',
      'new' => 'classes',
      '$CLASS_ATTR_ca_mixmedecl' => 'MixMeDeclared',
      'm_mixmedecl' => 'MixMeDeclared'

    $CLASS_STATE1 = {
      'Read_Only_Attr' => 'yes',
      'ca_mixmedecl' => undef,
      'Attr' => 1

    $OBJECT_STATE1 = bless( {
      'MySuper::color' => 'green',
      'MyClass::attr' => 'ok'
    }, 'MyClass' );

Throws: X::Usage


Loads a module with use MyModule using use base compatibility, but without the @ISA updates> and fields stuff. Used internally by the classes pragma itself to load packages and modules.

Throw a X::Empty exception (somewhat like Base class package Foo is empty error use base throws) when no symbols whatsoever are found for the loaded module.


Returns a constant (inlined subroutine) containing the class name. Exactly the same as __PACKAGE__ and literally stolen from CLASS module (Michael G. Schwern). Defined into anything that uses classes.


Returns a hash reference of the class declaration kept current as the class is altered at run time with classes or classes::define.

WARNING: Do not directly alter the DECL hash. Take a copy if needed instead:

    my %own_decl = %$MyClass::DECL;
    my %own_decl = %$DECL;

See: example output of DECL from classes::dump method


Returns the name of the super (parent) class in which it is defined ($<blessed_as>::ISA[0]). This fills the gap left by the SUPER:: construct that only refers to the super class of the current package. Without a SUPER that refers to the actual superclass mixes that deal with inheritance would be much more difficult to code:

    package Parent;
    sub foo {'foo'};

    package MixMod;
    sub bar1 { shift->SUPER->foo };
    sub bar2 { shift->SUPER::foo };  # BAD

    package MyClass;
    use classes extends=>'Parent', mixes=>'MixMod';
    print MyClass->SUPER->foo . "\n";  # foo
    print MyClass->SUPER::foo . "\n";  # foo
    print MyClass->bar1 . "\n";        # foo
    print MyClass->bar2 . "\n";        # ERROR

See: "SUPER" in perlobj


Defined when the mixes tag is used. Contains a dynamically updated hash of mixed in method and attribute names and the package and name from which they came. This is the only way to identify if a method or attribute was mixed in rather than simply declared:

    package MyMixin;
    sub bar {'bar'};

    package MyClass;
    use classes

    package main;
    print "bar is mixin\n" if MyClass->MIXIN->{bar};
    print "foo is mixin\n" if MyClass->MIXIN->{foo};

The MIXIN hash is displayed in the classes::dump output.

WARNING: Testing for defined $MIXIN is not sufficient since the ref will usually be defined even if the HASH it refers to contains nothing. Use defined %$MIXIN instead.

See: mixes, classes::dump


Constant referring to the current perl version ($]).

See: "$]" in perlvar


The classes pragma defines and uses the following exception classes that any code with use classes can immediately use:


Minimal base exception class. Base class of all other exception classes. See X::classes.


Subclass of X::classes. Adds light traceability to similar to Exception::Class. Base class of all other exception classes. See X::classes::traceable.


Thrown when somehow unexpectedly an attribute accessor is called where the attribute was initially declared with greater scope and then redeclared with a more limited scope, ro or pr:

    package WideOpen;
    use strict 'subs'; no warnings;
    use classes
        type  => 'mixable',
        new   => 'classes::new_args',
        attrs => ['foo'],

    package MorePrivate;
    use strict 'subs'; no warnings;
    use classes
        mixes    => 'WideOpen',
        attrs_ro => ['foo'],

    sub do_something_involving_foo {
        my $self = shift;
        $self->{$ATTR_foo} = 'something new';
        return $self;

    package main;
    my $o = MorePrivate->new; 
    print $o->get_foo;          # ok
    $o->set_foo('something');   # throws X::AttrScope

Even though the redeclaration causes the correct update to DECL the inherited accessor method is overriden as a safety precaution. This is not a problem if the attribute with the same name was never declared with greater scope in the first place since the public accessor (the setter in this case) won't exist, causing a different Perl compile-time error. See attrs for more.


Something was empty that shouldn't be, a package being loaded, a variable, etc.


Class or attribute name is invalid.


Attempt to import a package method detected where the method is not defined or defined as a class or object method instead.


Accessor method not found when set or get dispatch methods are called.


A call to an unimplemented ABSTRACT method is detected.


Any invalid syntax usage.


Attribute value undefined.


Attribute not known to the class in question.

See: X::classes, X::classes::traceable, classes::Throwable


See classesfaq


See classes::dump, classescb, classestut



Dependencies have been all but left out to improve portability.

Perl 5.6.1 is required.

Scalar::Utils and CORE::time are required and supported since 5.6 and standard from 5.8.

Data::Dumper is required for classes::dump to work and has been part of Perl standard since pre 5.6.1.


classestut, classescb, classesfaq, X::classes, X::classes::traceable, classes::Throwable, classes::test

The object oriented Perl related pages:

perlobj, perlboot, perltoot, perltooc, perlbot, perlstyle

The object oriented modules that most influenced the creation of the classes pragma:

base, fields, CLASS, Class::Struct, Exception::Class, Clone, Clone::PP, Class::MethodMaker, Class::MakeMethods, Class::Base, Class::Contract, Class::Accessor, Class::Meta, Class::Std, Class::Data::Inheritable, Class::Maker

All the rest of the Class:: (and related) namespace on CPAN including, but by no means limited to, the following:

Attribute::Deprecated, Attribute::Unimplemented, Class::Container, Class::Field, Class::Generate, Class::HPL00::Class, Class::Inspector, Class::MOP, Clone::Clonable, Class::Class


Robert S Muhlestein (rmuhle at cpan dot org)


The classes pragma was built from many other great modules and ideas with a lot of feedback and testing. Here are a few specific individuals who directly or indirectly contributed to its creation:

Matthew Simon Cavalletto, Damian Conway, Derek Cordon, Ray Finch, A. (Pete) Fontenot, C. Garrett Goebel, Erik Johnson, Jim Miner, ken1, Dave Rolsky, Matt Sargent, David Muir Sharnoff, Dean Roehrich, Michael G Schwern, Casey West, David Wheeler


Copyright 2005, 2006 Robert S. Muhlestein (rob at All rights reserved. This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself. [See perlartistic.]


The Perl 'strict' pragma was not seeing some cases of $$foo, specifically when used as an argument to defined. When they fixed that bug in the 'strict' pragma they caused some of the classes pragma unit tests to fail, but only because of strict's normal overzealousness rejection of good perl in an advanced context, which is required to create other simplifications in Perl OO programming using classes.

If you use accessor methods use strict should not give you problems, and when it does remember you can turn it off for sections of code or the whole thing with 'no strict', if you are careful and use it properly no one will burn you at the stake, at least not on my team.

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