Class::groups - Pragma to implement group of properties


Class::groups - Pragma to implement group of properties


Included in OOTools 2.21 distribution.

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

The distribution includes:

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


        Perl version >= 5.6.1
        perl -MCPAN -e 'install OOTools'
    Standard installation
    From the directory where this file is located, type:
        perl Makefile.PL
        make test
        make install



        package MyClass ;
        # implement group method without options
        use Class::groups qw(this that) ;
        # implement group method with properties
        use Class::groups { name  => 'myGroup' ,
                            props => [qw(prop1 prop2)]
                          } ;
        # with options
        use Class::groups
            { name      => 'myOtherGroup' ,
              no_strict => 1 ,
              default   => { aProp => 'some value' } ,
              pre_process=> sub
                             { if ( ref $_[1] eq 'ARRAY' )
                                { $_[1] = { map { $_=>$_ } @{$_[1]} }
                             } ,
              props     => [ { name    => [qw(prop3 prop4)] ,
                               default => 'something'
            } ;


        MyClass->myGroup(\%hash) ;
        # same thing
        MyClass->myGroup( prop1 => 1 ,
                          prop2 => 2 ) ;
        $hash_ref = MyClass->myGroup
        $value = MyClass->prop2 ;             # $value == 2
        $value = MyClass->myGroup('prop2') ;  # $value == 2
        $value = $MyClass::myGroup{prop2} ;   # $value == 2
        $value = $$hash_ref{prop2} ;          # $value == 2
        ($p1, $p2) = MyClass->myGroup(['prop1','prop2']) ;
        # the default will initialize the hash reference
        $other_hash_ref = MyClass->myOtherGroup
        $value = $other_hash_ref->{prop3}     # $value eq 'something'
        # adding an unknow property (see no_strict)
        MyClass->myOtherGroup(prop5 => 5) ;
        # copying the whole hash
        %myGroupHash = MyClass->myGroup


    Don't use the group accessor in list context in order to retrieve the hash keys: that way was deprecated and it doesn't work any more with this version.

        # WRONG
        @keys     = MyClass->myGroup ;
        # change it with
        @keys     = keys %{MyClass->myGroup} ;
        # new behaviour in list context
        %hash = MyClass->myGroup ;


    This pragma easily implements accessor methods for group of properties, 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).

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

    This module creates an accessor method with a hash in the class that implements it (e.g. %Class::any_group) and sets/gets it using the options you set; it also creates an accessor method for each property in the props option as you were using the Class::props pragma.

    Note: The grouped properties will be stored in e.g. $Class::any_group{any_property} instead of the usual $Class::any_property

    With the accessor method for the group you can:

    Note: The underlaying hash contains:

    IMPORTANT NOTE: If you write any script that rely on this module, you better send me an e-mail so I will inform you in advance about eventual planned changes, new releases, and other relevant issues that could speed-up your work.


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


    name => $name

    The name of the group accessor.

    no_strict => 0 | 1

    With no_strict option set to a true value, the accessor accepts and sets also unknown properties (i.e. not predeclared). You have to access the unknown properties without any accessor method. All the other options will work as expected. Without this option the method will croak if any property does not have an accessor method.

    Note: This option is on by default if you define an accessor group without any props option (i.e. in this case you can omit the 'no_strict' option).

    pre_process => \&code

    You can set a code reference to preprocess @_.

    The original @_ is passed to the referenced pre_process CODE. Modify @_ in the CODE to change the actual input value.

        # This code will transform the @_ on input
        # if it's passed a ref to an ARRAY
        # [ qw|a b c| ] will become
        # ( a=>'a', b=>'b', c=>'c')
        use Class::groups
            { name       => 'myGroup' ,
              pre_process=> sub
                             { if ( ref $_[1] eq 'ARRAY' )
                                { $_[1] = { map { $_=>$_ } @{$_[1]} }

    default => \%props | \&$method

    Use this option to set a default value. The default value must be a HASH reference or a CODE reference.

    If you pass a CODE reference as the default it will be evaluated only when the group will be accessed, and only if the group has no defined (HASH) value (this allows ``lazy'' data computing and may save some CPU); the group will be set to the HASH reference that the referenced CODE must return.

    You can reset a property to its default value by assigning an empty HASH reference ({}) to it.

    props => \@props

    This option creates the same properties accessor methods as you would use directly the Class::props pragma. It accepts a reference to an array, containing the same structured parameters as such accepted by the Class::props pragma.


    add_to( package, groups )

    This will add to the package package the accessors for the groups. It is useful to add properties in other packages.

       package Any::Package;
       Class::groups->('My::Package', { name => 'any_name', ... });
       # which has the same effect of
       package My::Package;
       use Class::groups { name => 'any_name', ... }


    If you need support or if you want just to send me some feedback or request, please use this link:


    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.


    Thanks to Juerd Waalboer ( that with its Attribute::Property inspired the creation of this distribution.

     Class::groups - Pragma to implement group of properties