Class::props(3)

NAME
Class::props - Pragma to implement lvalue accessors with options

VERSION 2.41
Included in OOTools 2.41 distribution.

The latest versions changes are reported in the Changes file in this distribution.

The distribution includes:

Package::props
    Pragma to implement lvalue accessors with options

Package::groups
    Pragma to implement groups of properties accessors with options

Class::constr
    Pragma to implement constructor methods

Class::props
    Pragma to implement lvalue accessors with options

Class::groups
    Pragma to implement groups of properties accessors with options

Class::Error
    Delayed checking of object failure

Class::Util
    Class utility functions

Object::props
    Pragma to implement lvalue accessors with options

Object::groups
    Pragma to implement groups of properties accessors with options

INSTALLATION

Prerequisites
        Perl version >= 5.6.1

CPAN
        perl -MCPAN -e 'install OOTools'

Standard installation
    From the directory where this file is located, type:

        perl Makefile.PL
        make
        make test
        make install

SYNOPSIS

  Class
    package MyClass ;

    # implement constructor without options
    use Class::constr ;

    # just accessors without options (list of strings)
    use Class::props @prop_names ;                       # @prop_names (1)

    # a property with validation and default (list of hash refs)
    use Class::props { name       => 'digits',
                       validation => sub{ /^\d+\z/ } ,    # just digits
                       default    => 10
                     } ;

    # a group of properties with common full options
    use Class::props { name       => \@prop_names2,      # @prop_names2 (1)
                       default    => sub{$_[0]->other_default} ,
                       validation => sub{ /\w+/ } ,
                       protected  => 1 ,
                       no_strict  => 1 ,
                       allowed    => qr/::allowed_sub$/
                     } ;

    # all the above in just one step (list of strings and hash refs)
    use Class::props @prop_names ,                       # @prop_names (1)
                     { name       => 'digits',
                       validation => sub{ /^\d+\z/ } ,
                       default    => 10
                     } ,
                     { name       => \@prop_names2,      # @prop_names2 (1)
                       default    => sub{$_[0]->other_default} ,
                       validation => sub{ /\w+/ } ,
                       protected  => 1 ,
                       no_strict  => 1 ,
                       allowed    => qr/::allowed_sub$/
                     } ;

    # (1) must be set in a BEGIN block to have effect at compile time

  Usage
    $object = MyClass->new(digits => '123');

    $object->digits    = '123';
    MyClass->digits    = '123';  # same thing

    $object->digits('123');      # old way supported

    $d = $object->digits;     # $d == 123
    $d = $MyClass::digits     # $d == 123

    undef $object->digits        # $object->digits == 10 (default)

    # these would croak
    $object->digits    = "xyz";
    MyClass->digits    = "xyz";

    # this will bypass the accessor whithout croaking
    $MyClass::digits   = "xyz";

DESCRIPTION
This pragma easily implements lvalue accessor methods for the properties of your Class (lvalue means that you can create a reference to it, assign to it and apply a regex to it; see also "KNOWN ISSUE"), which are very efficient function templates that your modules may import at compile time. "This technique saves on both compile time and memory use, and is less error-prone as well, since syntax checks happen at compile time." (quoted from "Function Templates" in the perlref manpage).

You can completely avoid to write the accessor by just declaring the names and eventually the default value, validation code and other option of your properties.

The accessor method creates a scalar in the class that implements it (e.g. $Class::any_property) and sets/gets it using the options you set.

This module allows also "lazy" data computing (see the "default" option).

IMPORTANT NOTE: Since the version 1.7 the options are ignored if you access the underlaying scalar without using the accessor, so you can directly access it when you need to bypass the options.

  Package, Class or Object properties?
The main difference between "Packages::props", "Object::props" and "Class::props" is the underlaying scalar that holds the value of the property.

Look at this example:

   package BaseClass;
   use Object::props  'an_object_prop';
   use Class::props   'a_class_prop';
   use Package::props 'a_package_prop';

   use Class::constr;

   package SubClass;
   our @ISA = 'BaseClass';

   package main;
   $obj = SubClass->new;

   # object
   $obj->an_object_prop;        # accessor callable through the object
   $obj->{an_object_prop};      # underlaying scalar in object $obj

   # class
   $obj->a_class_prop;          # accessor callable through the object
   ref($obj)->a_class_prop;     # accessor callable through the object class
   $SubClass::a_class_prop;     # underlaying scalar in class 'SubClass'

   # package
   $obj->a_package_prop;        # accessor callable through the object
   ref($obj)->a_package_prop;   # accessor callable through the object class
                                # accessible through @ISA
   BaseClass->a_package_prop;   # accessor callable through the package
   $BaseClass::a_package_prop;  # underlaying scalar in package 'BaseClass'

The object property is stored into the object itself, a class property is stored in a global scalar in the object class itself, while a package property is sotored in the package that implements it.

Different underlaying scalars are suitable for different usages depending on the need to access them and to inherit the defaults.

  Example
   package MyClass;
   use Class::constr ;
   use Object::props 'obj_prop' ;
   use Class::props qw( class_prop1
                        class_prop2 ) ;

   package main ;
   my $object1 = MyClass->new( obj_prop    => 1   ,
                               class_prop1 => 11 ) ;
   my $object2 = MyClass->new( obj_prop    => 2    ,
                               class_prop2 => 22 ) ;

   print $object1->obj_prop    ; # would print 1
   print $$object1{obj_prop}   ; # would print 1

   print $object2->obj_prop    ; # would print 2
   print $$object2{obj_prop}   ; # would print 2

   print $object1->class_prop1 ; # would print 11
   print $object2->class_prop1 ; # would print 11
   print $MyClass::class_prop1 ; # would print 11

   print $object1->class_prop2 ; # would print 22
   print $object2->class_prop2 ; # would print 22
   print $MyClass::class_prop2 ; # would print 22

   $object2->class_prop1 = 100 ; # object method
   MyClass->class_prop2  = 200 ; # static method works as well

   print $object1->class_prop1 ; # would print 100
   print $object2->class_prop1 ; # would print 100
   print $object1->class_prop2 ; # would print 200
   print $object2->class_prop2 ; # would print 200

Note: If you want to see some working example of this module, take a look at the source of my other distributions.

OPTIONS

  name => $name
The name of the property is used as the identifier to create the accessor method, and the scalar that contains it.

Given 'my_prop' as the class property name:

    MyClass->my_prop = 10 ;  # assign 10 to $MyClass::my_prop
    MyClass->my_prop( 10 );  # assign 10 to $MyClass::my_prop

    # assuming $object is an object of class MyClass
    $object->my_prop = 10 ;  # assign 10 to $MyClass::my_prop
    $object->my_prop( 10 );  # assign 10 to $MyClass::my_prop

    # same thing if MyClass::constr is implemented
    # by the Class::constr pragma

    $object = MyClass->new( my_prop => 10 );

You can group properties that have the same set of option by passing a reference to an array containing the names. If you don't use any option you can pass a list of plain names as well. See "SYNOPSYS".

  default => $value | \&code
Use this option to set a default value. If any "validation" option is set, then the default value is validated as well (the "no_strict" option override this).

If you pass a CODE reference as the default it will be evaluated only when the property will be accessed, and only if the property has no defined value (this allows "lazy" data computing and may save some CPU); the property will be set to the result of the referenced CODE.

You can reset a property to its default value by assigning it the undef value.

  no_strict => 0 | 1
With "no_strict" option set to a true value, the "default" value will not be validate even if a validation option is set. Without this option the method will croak if the "default" are not valid.

  validation => \&code
You can set a code reference to validate a new value. If you don't set any "validation" option, no validation will be done on the assignment.

In the validation code, the object or class is passed in $_[0] and the value to be validated is passed in $_[1] and for regexing convenience it is aliased in $_.

    # web color validation
    use Class::props { name       => 'web_color'
                     , validation => sub { /^#[0-9A-F]{6}$/ }
                     } ;
    # this would croak
    $object->web_color = 'dark gray' ;

You can alse use the "validation" code as a sort of pre_process or filter for the input values: just assign to $_ in the validation code in order to change the actual imput value.

    # this will uppercase all input values
    use Class::props { name       => 'uppercase_it'
                     , validation => sub { $_ = uc }
                     } ;
    # when used
    $object->uppercase_it = 'abc' ; # stored value will be 'ABC'

The validation code should return true on success and false on failure. Croak explicitly if you don't like the default error message.

  post_process => \&code
You can set a code reference to transform the stored value, just before it is returned. If you don't set any "post_process" option, no transformation will be done on the returned value, so in that case the returned value will be the same stored value.

In the post_process code, the object or class is passed in $_[0] and the value to be transformed is passed in $_[1], and for regexing convenience it is aliased in $_; the accessor will return the value returned from the post_process code

    # this will uppercase all output values
    use Class::props { name         => 'uppercase_it'
                     , post_process => sub { uc }
                     }

    # when used
    Class->uppercase_it = 'aBc'; # stored value will be 'aBc'
    print Class->uppercase_it  ; # would print 'ABC'

Warning: The post_process code is ALWAYS executed in SCALAR context regardless the execution context of the accessor itself.

  allowed => $re | \@res
The property is settable only by the caller sub that matches with the content of this option. The content can be a compiled RE or a simple string that will be used to check the caller. (Pass an array ref for multiple items)

    use Class::props { name    => 'restricted'
                       allowed => [ qr/::allowed_sub1$/ ,
                                    qr/::allowed_sub2$/ ]
                     }

You can however force the assignation from not matching subs by setting $Class::props::force to a true value.

  protected => 0 | 1
Set this option to a true value and the property will be turned read-only when used from outside its class or sub-classes. This allows you to normally read and set the property from your class but it will croak if your user tries to set it.

You can however force the protection and set the property from outside the class that implements it by setting $Class::props::force to a true value.

METHODS

  add_to( package, properties )
This will add to the package package the accessors for the properties. It is useful to add properties in other packages.

   package Any::Package;
   Class:props->('My::Package', { name => 'any_name', ... });

   # which has the same effect of
   package My::Package;
   use Class::props { name => 'any_name', ... }

KNOWN ISSUE
Due to the perl bug #17663 (Perl 5 Debugger doesn't handle properly lvalue sub assignment), you must know that under the -d switch the lvalue sub assignment will not work, so your program will not run as you expect.

In order to avoid the perl-bug you have 3 alternatives:

1.  patch perl itself as suggested in this post: http://www.talkaboutprogramming.com/group/comp.lang.perl.moderated/messages/13142.html (See also the cgi-builder-users mailinglist about that topic)

2.  use the lvalue sub assignment (e.g. "$s->any_property = 'something'") only if you will never need -d

3.  if you plan to use -d, use only standard assignments (e.g. "$s->any_property('something')")

Maybe a next version of perl will fix the bug, or maybe lvalue subs will be banned forever, meanwhile be careful with lvalue sub assignment.

SUPPORT and FEEDBACK
If you need support or if you want just to send me some feedback or request, please use this link: http://perl.4pro.net/?Class::props.

AUTHOR and COPYRIGHT
Copyright 2004-2005 by Domizio Demichelis.

All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the same terms as perl itself.

CREDITS
Thanks to Juerd Waalboer (http://search.cpan.org/author/JUERD) that with its Attribute::Property inspired the creation of this distribution.