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Author image Juerd Waalboer


Attribute::Property - Easy lvalue accessors with validation. ($foo->bar = 42)



    use Attribute::Property;
    use Carp;

    package SomeClass;

    sub new : New { further initialization here ... }
    sub nondigits : Property { /^\D+\z/ }
    sub digits    : Property { /^\d+\z/ or croak "custom error message" }
    sub anyvalue  : Property;
    sub another   : Property;

    sub value     : Property {
        my $self = shift;  # Object is accessible as $_[0]
        s/^\s+//;          # New value can be altered through $_ or $_[1]

        $_ <= $self->maximum or croak "Value exceeds maximum";

    package Person;

    sub new  : New;
    sub name : Property;
    sub age  : Property { /^\d+\z/ and $_ > 0 }


    my $object = SomeClass->new(digits => '123');

    $object->nondigits = "abc";
    $object->digits    = "123";
    $object->anyvalue  = "abc123\n";

    $object->anyvalue('archaic style still works');

    my $john = Person->new(name => 'John Doe', age => 19);
    printf "%s is now %d years old", $john->name, $john->age;

    # These would croak
    $object->nondigits = "987";
    $object->digits    = "xyz";


This module introduces two attributes that make object oriented programming much easier. You can just define a constructor and some properties without having to write accessors.

    sub color : Property;
    sub color : Property { /^#[0-9A-F]{6}$/ }

The Property attribute turns a method into an object property. The original code block is used only to validate new values, the module croaks if it returns false. The method returns an lvalue, meaning that you can create a reference to it, assign to it and apply a regex to it.

Undefined subs (subs that have been declared but do not have a code block) with the Property attribute will be properties without any value validation.

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

Feel free to croak explicitly if you don't want the default error message.

    sub new : New;
    sub new : New { my $self = shift; ...; return $self; }

The New attribute turns a method into an object constructor. The original code block can be used for further initialization, but it is completely optional.

The constructor takes named arguments in property => value pairs and populates the hash with the given pairs. After validating them, of course.

The new object is passed to the initialization code block as $_[0]. Be sure to return the object if you use any initialization block. If there is no initialization code block, Attribute::Property takes care of returning the new object.


Your object must be a blessed hash reference. The property names will be used for the hash keys.

For class properties of Some::Module, the hash %Some::Module is used. For class properties of packages without ::, the behaviour is undefined.

In short: $foo->bar = 14 and $foo->bar(14) assign 14 to $foo->{bar} after positive validation. The same thing happens with my $foo = Class->new(bar => 14); given that Class::new uses the New property.

If you have the Want module installed, Attribute::Property will use it to make rvalue method calls more efficient.


Old fashioned $object->property(VALUE) is still available.

This module requires a modern Perl (5.6.0+), fossils like Perl 5.00x don't support our chicanery.


  • The New attribute should really be called Constructor, but that would conflict with the existing Attribute::Constructor module.


There is no license. This software was released into the public domain. Do with it what you want, but on your own risk. Both authors disclaim any responsibility.


Juerd Waalboer <juerd@cpan.org> <http://juerd.nl/>

Matthijs van Duin <xmath@cpan.org>