NAME

Class::Declare - Declare classes with public, private and protected attributes and methods.

SYNOPSIS

  package My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare(
    public     => { public_attr     => 42         }  ,
    private    => { private_attr    => 'Foo'      }  ,
    protected  => { protected_attr  => 'Bar'      }  ,
    class      => { class_attr      => [ 3.141 ]  }
    static     => { static_attr     => { a => 1 } }  ,
    restricted => { restricted_attr => \'string'  }  ,
    abstract   => 'abstract_attr'                    ,
    friends    => 'main::trustedsub'                 ,
    new        => [ 'public_attr' , 'private_attr' ] ,
    init       => sub { # object initialisation
                    ...
                    1;
                  }                                  ,
    strict     => 0
  );
  sub publicmethod {
    my $self = __PACKAGE__->public( shift );
    ...
  }
  sub privatemethod {
    my $self = __PACKAGE__->private( shift );
    ...
  }
  sub protectedmethod {
    my $self = __PACKAGE__->protected( shift );
    ...
  }
  sub classmethod {
    my $self = __PACKAGE__->class( shift );
    ...
  }
  sub staticmethod {
    my $self = __PACKAGE__->static( shift );
    ...
  }
  sub restrictedmethod {
    my $self = __PACKAGE__->restricted( shift );
    ...
  }
  sub abstractmethod { __PACKAGE__->abstract }
  1;
  ...
  my $obj = My::Class->new( public_attr => 'fish' );

MOTIVATION

One of Perl's greatest strengths is it's flexible object model. You can turn anything (so long as it's a reference, or you can get a reference to it) into an object. This allows coders to choose the most appropriate implementation for each specific need, and still maintain a consistent object oriented approach.

A common paradigm for implementing objects in Perl is to use a blessed hash reference, where the keys of the hash represent attributes of the class. This approach is simple, relatively quick, and trivial to extend, but it's not very secure. Since we return a reference to the hash directly to the user they can alter hash values without using the class's accessor methods. This allows for coding "short-cuts" which at best reduce the maintainability of the code, and at worst may introduce bugs and inconsistencies not anticipated by the original module author.

On some systems, this may not be too much of a problem. If the developer base is small, then we can trust the users of our modules to Do The Right Thing. However, as a module's user base increases, or the complexity of the systems our module's are embedded in grows, it may become desirable to control what users can and can't access in our module to guarantee our code's behaviour. A traditional method of indicating that an object's data and methods are for internal use only is to prefix attribute and method names with underscores. However, this still relies on the end user Doing The Right Thing.

Class::Declare provides mechanisms for module developers to explicitly state where and how their class attributes and methods may be accessed, as well as hiding the underlying data store of the objects to prevent unwanted tampering with the data of the objects and classes. This provides a robust framework for developing Perl modules consistent with more strongly-typed object oriented languages, such as Java and C++, where classes provide "public", "private", and "protected" interfaces to object and class data and methods.

DESCRIPTION

Class::Declare allows class authors to specify public, private and protected attributes and methods for their classes, giving them control over how their modules may be accessed. The standard object oriented programming concepts of public, private and protected have been implemented for both class and instance (or object) attributes and methods.

Attributes and methods belong to either the class or an instance depending on whether they may be invoked via class instances (class and instance methods/attributes), or via classes (class methods/attributes only).

Class::Declare uses the following definitions for public, private and protected:

public: Public attributes and methods may be accessed by anyone from anywhere. The term public is used by Class::Declare to refer to instance attributes and methods, while the equivalent for class attributes and methods are given the term class attributes and methods.
private: Private attributes and methods may be accessed only by the class defining them and instances of that class. The term private is used to refer to instance methods and attributes, while the term static refers to class attributes and methods that exhibit the same properties.
protected: Protected attributes and methods may only be accessed by the defining class and it's instances, and classes and objects derived from the defining class. Protected attributes and methods are used to define the interface for extending a given class (through normal inheritance/derivation). The term protected is used to refer to protected instance methods and attributes, while protected class methods and attributes are referred to as restricted.
Note: since version 0.02, protected class methods and attributes are refered to as restricted, rather than shared. This change was brought about by the introduction of Class::Declare::Attributes and then clash with the existing Perl threading attribute :shared. The term restricted has been chosen to reflect that the use of these methods and attributes is restricted to the family of classes derived from the base class.

The separation of terms for class and instance methods and attributes has been adopted to simplify class declarations. See declare() below.

Class attributes are regarded as constant by Class::Declare: once declared they may not be modified. Instance attributes, on the other hand, are specific to each object, and may be modified at run-time.

Internally, Class::Declare uses hashes to represent the attributes of each of its objects, with the hashes remaining local to Class::Declare. To the user, the objects are represented as references to scalars which Class::Declare maps to object hashes in the object accessors. This prevents users from accessing object and class data without using the class's accessors.

The granting of access to attributes and methods is determined by examining the target of the invocation (the first parameter passed to the method, usually represented by $self), as well as the context of the invocation (where was the call made and who made it, determined by examining the caller() stack). This adds an unfortunate but necessary processing overhead for Class::Declare objects for each method and attribute access. While this overhead has been kept as low as possible, it may be desirable to turn it off in a production environment. Class::Declare permits disabling of the access control checks on a per-module basis, which may greatly improve the performance of an application. Refer to the strict parameter of declare() below for more information.

Class::Declare inherits from Exporter, so modules derived from Class::Declare can use the standard symbol export mechanisms. See Exporter for more information.

Defining Classes

To define a Class::Declare-derived class, a package must first "use" Class::Declare and inherit from it (either by adding it to the @ISA array, or through "use base"). Then Class::Declare::declare() must be called with the new class's name as its first parameter, followed by a list of arguments that actually defines the class. For example:

  package My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare( ... );
  1;

Class::Declare::declare() is a class method of Class::Declare and has the following call syntax and behaviour:

declare( [ param => value ] ): declare()'s primary task is to define the attributes of the class and its instances. In addition, it supports options for defining object initialisation code, friend methods and classes, and the application of strict access checking. param may have one of the following values:

public

public expects either a hash reference of attribute names and default values, an array reference containing attribute names whose default values will be "undef", or a single attribute name whose value will default to "undef". These represent the public attributes of this class. Class::Declare constructs accessor methods within the class, with the same name as the attributes. These methods are "lvalue" methods by default (see also Attribute Modifiers below), which means that the attributes may be assigned to, as well as being set by passing the new value as an accessor's argument.

For example:

  package My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare( public => { name => 'John' } );
  1;
  my $obj = My::Class->new;
  print $obj->name . "\n";    # prints 'John'
        $obj->name = 'Fred';  # the 'name' attribute is now 'Fred'
        $obj->name( 'Mary' ); # the 'name' attribute is now 'Mary'

The default value of each attribute is assigned during the object initialisation phase (see init and new() below). Public attributes may be set during the object creation call:

  my $obj = My::Class->new( name => 'Jane' );
  print $obj->name . "\n"; # prints 'Jane'

public attributes are instance attributes and therefore may only be accessed through class instances, and not through the class itself.

Note that the declare() call for "My::Class" from above could have been written as

  __PACKAGE__->declare( public => [ qw( name ) ] );

  __PACKAGE__->declare( public => 'name' );

In these cases, the attribute "name" would have had a default value of "undef".

private

As with public above, but the attributes are private (i.e. only accessible from within this class). If access is attempted from outside the defining class, then an error will be reported through die(). Private attributes may not be set in the call to the constructor, and as with public attributes, are instance attributes. See also strict and friends below.

protected

As with private above, but the attributes are protected (i.e. only accessible from within this class, and all classes that inherit from this class). Protected attributes are instance attributes, and they may not be set in the call to the constructor. See also strict and friends below.

class

This declares class attributes in the same manner as public above. class attributes are not restricted to object instances, and may be accessed via the class directly. The accessor methods created by Class::Declare, however, are not "lvalue" methods, and cannot, therefore, be assigned to. Nor can the values be set through the accessor methods. They behave in the same manner as values declared by "use constant" (except they must be called as class or instance methods). class attributes may not be set in the call to the constructor.

static

As with class attributes, except access to "static" attributes is limited to the defining class and its objects. static attributes are the class-equivalent of private instance attributes. See also friends.

restricted

As with class attributes, except access to "restricted" attributes is limited to the defining class and all classes that inherit from the defining class, and their respective objects. restricted attributes are the class-equivalent of protected instance attributes. See also friends.

abstract

This declares the set of abstract methods provided by this class, and will cause the generation of stub routines that die() when invoked, ensuring derived classes define these methods.

friends

Here you may specify classes and methods that may be granted access to the defining classes private, protected, static and restricted attributes and methods. friends expects either a single value, or a reference to a list of values. These values may either be class names, or fully-qualified method names (i.e. class and method name). When a call is made to a private or protected method or attribute accessor, and a friend has been declared, a check is performed to see if the caller is within a friend package or is a friend method. If so, access is granted. Otherwise, access is denied through a call to die().

Note that friend status may not be inherited. This is to avoid scenarios such as the following:

  package My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare( ...
                        friends => 'My::Trusted::Class' );
  1;
  package My::Trusted::Class;
  ...
  1;
  package Spy::Class;
  use strict;
  use warnings;
  use base qw( My::Trusted::Class );
  sub infiltrate {
    .. do things here to My::Class objects that we shouldn't
  }
  1;

init

This defines the object initialisation code, which is executed as the last phase of object creation by new(). init expects a "CODEREF" which is called with the first argument being the new object being created by the call to new(). The initialisation routine is expected to return a true value to indicate success. A false value will cause new() to die() with an error. The initialisation routines are invoked during object creation by new(), after default and constructor attribute values have been assigned.

If the inheritance tree of a class contains multiple init methods, then these will be executed in reverse @ISA order to ensure the primary base-class of the new class has the final say on object initialisation (i.e. the class left-most in the @ISA array will have it's init routine executed last). If a class appears multiple times in an @ISA array, either through repetition or inheritance, then it will only be executed once, and as early in the init execution chain as possible.

Class::Declare uses a "CODEREF" rather than specifying a default initialisation subroutine (e.g. "sub INIT { ... }") to avoid unnecessary pollution of class namespaces. There is generally no need for initialisation routines to be accessible outside of new().

new

If new is defined, then it should contain a list (either a single value or an array reference) of the instance attributes (public, private or protected) that may be set in the call to the constructor new(). This permits the exposure of protected and private attributes during construction (and thus permitting read-only protected and private attributes). new makes it possible to do the following:

  package My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare(
    public  => { mypublic  => undef } ,
    private => { myprivate => undef } ,
    new     => [ qw( myprivate ) ]
  );
  1;
  ...
  my  $obj  = My::Class->new( myprivate => 1 );

Note that if new is specified in a call to declare() then only those attributes listed in the value of new may be defined in the call to the constructor new() (overriding the default behaviour of allowing public attributes). In addition, the attributes must be defined in this class, and not inherited. This prevents unintended access such as:

  public My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare(
  
    private => { myprivate => undef }
  );
  1;
  ...
  public Bad::Class;
  use strict;
  use warnings;
  use base qw( My::Class );
  __PACKAGE__->declare(
    # this will throw an error with die()
    new => 'myprivate'
  );
  1;

strict

If strict is set to true, then Class::Declare will define class(), static(), restricted(), public(), private(), and protected() methods (see "Class Methods" and "Object Methods" below) within the current package that enforce the class/static/restricted/public/private/protected relationships in method calls.

If strict is set to false and defined (e.g. 0, not "undef"), then Class::Declare will convert the above method calls to no-ops, and no invocation checking will be performed. Note that this conversion is performed for this class only.

By setting strict to "undef" (or omitting it from the call to declare() altogether), Class::Declare will not create these methods in the current package, but will rather let them be inherited from the parent class. In this instance, if the parent's methods are no-ops, then the child class will inherit no-ops. Note that the public(), private(), etc methods from Class::Declare enforce the public/private/etc relationships.

One possible use of this feature is as follows:

  package My::Class;
  use strict;
  use warnings;
  use base qw( Class::Declare );
  __PACKAGE__->declare( public    => ...                ,
                        private   => ...                ,
                        protected => ...                ,
                        strict    => $ENV{ USE_STRICT } );
  ...
  1;

Here, during development and testing the environment variable "USE_STRICT" may be left undefined, or set to true to help ensure correctness of the code, but then set to false (e.g. 0) in production to avoid the additional computational overhead.

Setting strict to false does not interfere with the friends() method (see below). Turning strict access checking off simply stops the checks from being performed and does not change the logic of whether a class or method as been declared as a friend of a given class.

Note:

declare() may be called only once per class to prevent class redefinitions
attribute names specified in the call to declare() may not be the same as class and instance methods already defined in the class
attribute names must be unique for a class

If any of the above rules are violated, then declare() will raise an error with die().

Creating Objects

Once a Class::Declare-derived class has been declared, instances of that class may be created through the new() method supplied by Class::Declare. new() may be called either as a class or an instance method. If called as a class method, a new instance will be created, using the class's default attribute values as the default values for this instance. If new() is called as an instance method, the default attribute values for the new instance will be taken from the invoking instance. This may be used to clone Class::Declare-derived objects.

Class::Declare::new() has the following call syntax and behaviour:

new( [ param => value ] )

new() creates instances of Class::Declare objects. If a problem occurs during the creation of an object, such as the failure of an object initialisation routine, then new() will raise an error through die().

When called as a class method, new() will create new instances of the specified class, using the class's default attribute values. If it's called as an instance method, then new() will clone the invoking object.

new() accepts named parameters as arguments, where param corresponds to a public attribute of the class of the object being created. If an unknown attribute name, or a non-public attribute name is specified, then new() will die() with an error. Public attribute values specified in the call to new() are assigned after the creation of the object, to permit over-riding of default values (either class-default attributes or attributes cloned from the invoking object).

new() can be extended to accept non-public instance attributes as parameters through the specification of the new attribute of declare() (see above). In this instance, only the attributes listed in the definition of new in declare() will be accepted, and all public attributes will only be accepted if contained within this list.

If the calling class, or any of its base classes, has an object initialisation routine defined (specified by the init parameter of declare()), then these routines will be invoked in reverse @ISA order, once the object's attribute values have been set. An initialisation routine may only be called once per class per object, so if a class appears multiple times in the @ISA array of the new object's class, then the base class's initialisation routine will be called as early in the initialisation chain as possible, and only once (i.e. as a result of the right-most occurrence of the base class in the @ISA array).

The initialisation routines should return a true value to indicate success. If any of the routines fail (i.e. return a false value), then new() will die() with an error.

When a new instance is created, instance attributes (i.e. public, private and protected attributes) are cloned, so that the new instance has a copy of the default values. For values that are not references, this amounts to simply copying the value through assignment. For values that are references, Storable::dclone() is used to ensure each instance has it's own copy of the references data structure (the structures are local to each instance).

However, if an instance attribute value is a "CODEREF", then new() simply copies the reference to the new object, since "CODEREF"s cannot be cloned.

Class attributes are not cloned as they are assumed to be constant across all object instances.

Class Access Control Methods

Class::Declare provides the following class methods for implementing class, static and restricted access control in class methods. These methods may be called either through a Class::Declare-derived class, or an instance of such a class.

Note that a class method is a public class method, a static method is a private class method, and a restricted method is a protected class method.

abstract( target )

Ensure a method is implemented, but throwing a fatal error (i.e. die()'ing if called).

class( target )

Ensure a method is called as a class method of this package via the target.

  sub myclasssub {
    my $self = __PACKAGE__->class( shift );
    ...
  }

A class method may be called from anywhere, and target must inherit from this class (either an object or instance). If class() is not invoked in this manner, then class() will die() with an error.

See also the strict parameter for declare() above.

static( target )

Ensure a method is called as a static method of this package via target.

  sub mystaticsub {
    my $self = __PACKAGE__->static( shift );
    ...
  }

A static method may only be called from within the defining class, and target must inherit from this class (either an object or instance). If static() is not invoked in this manner, then static() will die() with an error.

See also the strict and friends parameters for declare() above.

restricted( target )

Ensure a method is called as a restricted method of this package via target.

  sub myrestrictedsub {
    my $self = __PACKAGE__->restricted( shift );
    ...
  }

A restricted method may only be called from within the defining class or a class that inherits from the defining class, and target must inherit from this class (either an object or instance). If restricted() is not invoked in this manner, then restricted() will die() with an error.

See also the strict and friends parameters for declare() above.

Note: restricted() was called shared() in the first release of Class::Declare. However, with the advent of Class::Declare::Attributes, there was a clash between the use of :shared as an attribute by Class::Declare::Attributes, and the Perl use of :shared attributes for threading.

Instance Access Control Methods

Class::Declare provides the following instance methods for implementing public, private and protected access control in instance methods. These methods may only be called through a Class::Declare-derived instance.

public( target )

Ensure a method is called as a public method of this class via target.

  sub mypublicsub {
    my $self = __PACKAGE__->public( shift );
    ...
  }

A public method may be called from anywhere, and target must be an object that inherits from this class. If public() is not invoked in this manner, then public() will die() with an error.

See also the strict parameter for declare() above.

private( target )

Ensure a method is called as a private method of this class via target.

  sub myprivatesub {
    my $self = __PACKAGE__->private( shift );
    ...
  }

A private method may only be called from within the defining class, and target must be an instance that inherits from this class. If private() is not invoked in this manner, then private() will die() with an error.

See also the strict and friends parameters for declare() above.

protected( target )

Ensure a method is called as a protected method of this class via target.

  sub myprotectedsub {
    my $self = __PACKAGE__->protected( shift );
    ...
  }

A protected method may only be called from within the defining class or a class that inherits from the defining class, and target must be an instance that inherits from this class. If protected() is not invoked in this manner, then protected() will die() with an error.

See also the strict and friends parameters for declare() above.

Destroying Objects

Object destruction is handled via the normal Perl DESTROY() method. Class::Declare implements a DESTROY() method that performs clean-up and house keeping, so it is important that any class derived from Class::Declare that requires a DESTROY() method ensures that it invokes it's parent's DESTROY() method, using a paradigm similar to the following:

  sub DESTROY
  {
    my $self = __PACKAGE__->public( shift );
    ... do local clean-up here ..
    # call the parent clean-up
       $self->SUPER::DESTROY( @_ );
  } # DESTROY()

Attribute Modifiers

By default Class::Declare class attributes ("class", "static", and "restricted") are read-only, while instance attributes ("public", "private", and "protected") are read-write. Class::Declare provides two attribute modifiers, rw and ro for changing this behaviour, allowing class attributes to be read-write, and instance attributes to be read only.

The modifiers may be imported separately,

  use Class::Declare qw( :read-only  );

  use Class::Declare qw( ro );

  use Class::Declare qw( :read-write );

  use Class::Declare qw( rw );

or collectively, using the ":modifiers" tag.

  use Class::Declare qw( :modifiers );

To use the modifiers, they must be incorporated into the attribute definition for the class. For example:

  package My::Class;
  use strict;
  use Class::Declare qw( :modifiers );
  use vars           qw( @ISA );
      @ISA = qw( Class::Declare );
  __PACKAGE__->declare( class  => { my_class  => rw undef } ,
                        public => { my_public => ro 1234  } );

Here, the attribute "my_class" has been declared read-write by rw, permitting it's value to be changed at run time. The public attribute "my_public" has been declared read-only by ro, preventing it from being changed once set. Please note that although they may be marked as read-only, public attributes may still be set during object creation (i.e. in the call to new()). However, once set, the value may not be changed.

rw: Declare a class attribute to be read-write, instead of defaulting to read-only. Note that this has no effect on instance attributes as they are read-write by default.
ro: Declare an instance attribute to be read-only, instead of defaulting to read-write. Note that this has no effect on class attributes as they are read-only by default.

Serialising Objects

Class::Declare objects may be serialised (and therefore cloned) by using Storable. Class::Declare uses Storable::dclone() itself during object creation to copy instance attribute values. However, Storable is unable to serialise "CODEREF"s, and attempts to do so will fail. This causes the failure of serialisation of Class::Declare objects that have "CODEREF"s as attribute values. However, for cloning, Class::Declare avoids this problem by simply copying "CODEREF"s from the original object to the clone.

Miscellaneous Methods

The following methods are class methods of Class::Declare provided to simplify the creation of classes. They are provided as convenience methods, and may be called as either class or instance methods.

friend()

Returns true if the calling class or method is a friend of the given class or object. That is, for a given object or class, friend() will return true if it is called within the context of a class or method that has been granted friend status by the object or class (see friend in declare() above). A friend may access private, protected, static and restricted methods and attributes of a class and it's instances, but not of derived classes.

friend() will return true for a given class or object if called within that class. That is, a class is always it's own friend.

In all other circumstances, friend() will return false.

  package Class::A;
  my $object = Class::B;
  sub somesub {
    ...
    $object->private_method   if ( $object->friend );
    ...
  }

dump( [ param => value ] )

Generate a textual representation of an object or class. Since Class::Declare objects are represented as references to scalars, Data::Dumper is unable to generate a meaningful dump of Class::Declare-derived objects. dump() pretty-prints objects, showing their attributes and their values. dump() obeys the access control imposed by Class::Declare on it's objects and classes, limiting it's output to attributes a caller has been granted access to see or use.

dump() will always observe the access control mechanisms as specified by Class::Declare::class(), Class::Declare::private(), etc, and it's behaviour is not altered by the setting of strict in declare() to be false (see declare() above). This is because strict is designed as a mechanism to accelerate the execution of Class::Declare-derived modules, not circumvent the intended access restrictions of those modules.

dump() accepts the following optional named parameters:

all: If all is true (the default value), and none of the attribute/method type parameters (e.g. public, static, etc) have been set, then dump() will display all attributes the caller has access to. If any of the attribute type parameters have been set to true, then all will be ignored, and only those attribute types specified in the call to dump() will be displayed.
class: If class is true, then dump() will display only class attributes of the invocant and their values, and all other types of attributes explicitly requested in the call to dump() (the all parameter is ignored). If the caller doesn't have access to class methods, then dump() will die() with an error. If no class attributes exist, and no other attributes have been requested then "undef" is returned.
static: As with class, but displaying static attributes and their values.
restricted: As with class, but displaying restricted attributes and their values.
public: As with class, but displaying public attributes and their values. Note that public attributes can only be displayed for class instances. Requesting the dump() of public attributes of a class will result in dump() die()ing with an error.
private: As with public, but displaying private attributes and their values.
protected: As with public, but displaying protected attributes and their values.
friends: If friends is true, then dump() will display the list of friends of the invoking class or object.
depth: By default, dump() operates recursively, creating a dump of all requested attribute values, and their attribute values (if they themselves are objects). If depth is set, then dump() will limit it's output to the given recursive depth. A depth of 0 will display the target's attributes, but will not expand those attribute values.
indent: indent specifies the indentation used in the output of dump(), and defaults to 4 spaces.
backtrace: If backtrace is true, the dump() will back-trace references if they are encountered multiple times in the generation of the dump() output. The back-trace is similar to the default behaviour of Data::Dumper, where only the first instance of a reference is shown in full, and all other occurences are displayed as a link back to the original occurrence of that reference. By default, backtrace is true.

If an attribute type parameter, such as static or private, is set in the call to dump() then this only has effect on the target object of the dump() call, and not any subsequent recursive calls to dump() used to display nested objects.

hash( [ param => value ] )

Return a hash representing the values of the attributes of the class or object (depending on how hash() is called. hash() supports the same calling parameters as dump(), except for "friends" and "indent"). hash() observes normal access control, only returning attributes that the caller would normally have access to. "abstract" attributes are returned with a value of "undef".

If called in a list context, hash() will return a hash, otherwise a hash reference is returned.

Note: As of v0.10, hash() supports the depth parameter, and will, by default, recurse to generate a hash of the entire object tree (if derived from Class::Declare). If depth is set, then hash() will limit it's output to the given recursive depth. A depth of 0 will display the target's attributes, but will not expand those attribute values. hash() will descend "ARRAY" and "HASH" references if asked to recurse.

arguments( args => default )

A class helper method for handling named argument lists. In Perl, named argument lists are supported by coercing a list into a hash by assuming a key/value pairing. For example, named arguments may be implemented as

  sub mysub {
    my  %args = @_;
    ...
  }

and called as

  mysub( name => 'John' , age => 34 );

%args is now the hash with keys "name" and "age" and corresponding values 'John' and 34 respectively.

So if named arguments are so easy to implement, why go to the trouble of calling arguments()? To make your code more robust. The above example failed to test whether there was an even number of elements in the argument list (needed to flatten the list into a hash), and it made no checks to ensure the supplied arguments were expected. Does mysub() really want a name and age, or does it want some other piece of information?

arguments() ensures the argument list can be safely flattened into a hash, and raises an error indicating the point at which the original method was called if it can't. Also, it ensures the arguments passed in are those expected by the method. Note that this does not check the argument values themselves, but merely ensures unknown named arguments are flagged as errors.

arguments() also enables you to define default values for your arguments. These values will be assigned when a named argument is not supplied in the list of arguments.

The calling convention of arguments() is as follows (note, we assume here that the method is in a Class::Declare-derived class):

  sub mysub {
    ...
    my %args = $self->arguments( \@_ => { name => 'Guest user' ,
                                          age  => undef        } );
    ...
  }

Here, mysub() will accept two arguments, "name" and "age", where the default value for "name" is 'Guest user', while "age" defaults to "undef".

Alternatively, arguments() may be called in either of the following ways:

    my %args = $self->arguments( \@_ => [ qw( name age ) ] );

    my %args = $self->arguments( \@_ => 'name' );

Here, the default argument values are "undef", and in the second example, only the the single argument name will be recognized.

If default is not given (or is undef), then arguments() will simply flatten the argument list into a hash and assume that all named arguments are valid. If default is the empty hash (i.e. "{}"), then no named arguments will be accepted.

If called in a list context, arguments() returns the argument hash, while if called in a scalar context, arguments() will return a reference to the hash. arguments() may be called as either a class or instance method.

REVISION()

Extract the revision number from CVS revision strings. REVISION() looks for the package variable $REVISION for a valid CVS revision strings, and if found, will return the revision number from the string. If $REVISION is not defined, or does not contain a CVS revision string, then REVISION() returns "undef".

  package My::Class;
  use strict;
  use base qw( Class::Declare );
  use vars qw( $REVISION      );
               $REVISION = '$Revision: 1518 $';
  ...
  1;
  print My::Class->REVISION;  # prints the revision number

VERSION( [ required ] )

Replacement for UNIVERSAL::VERSION(), that falls back to REVISION() to report the CVS revision number as the version number if the package variable $VERSION is not defined. If required is given, then VERSION() will die if the required version is not less than or equal to the current package version (or revision, if VERSION() falls back to REVISION()). VERSION() will die if required is not a valid version string.

has( method )

If this class directly implements the given method(), then return a reference to this method. Otherwise, return false. This is similar to UNIVERSAL::can(), which will return a reference if this class either directly implements method(), or inherits it.

strict()

If this class is operating with strict access checking (i.e. strict from declare() was not explicitly set to false in this class or one of its parent classes) then strict() will return true, otherwise return false.

CAVEAT

Class::Declare has been designed to be thread-safe, and as such is suitable for such environments as "mod_perl". However, it has not been proven to be thread-safe. If you are coding in a threaded environment, and experience problems with Class::Declare's behaviour, please let me know.

BUGS

The name. I don't really like Class::Declare as a name, but I can't think of anything more appropriate. I guess it really doesn't matter too much. Suggestions welcome.

Apart from the name, Class::Declare has no known bugs. That is not to say the bugs don't exist, rather they haven't been found. The testing for this module has been quite extensive (there are over 3000 test cases in the module's test suite), but patches are always welcome if you discover any problems.

AUTHOR

Ian Brayshaw, <ian@onemore.org>

COPYRIGHT AND LICENSE

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