/ 
Class-Base-0.02
River stage two • 13 direct dependents • 16 total dependents
/ Class::Base

NAME

Class::Base - useful base class for deriving other modules

SYNOPSIS

    package My::Funky::Module;
    use base qw( Class::Base );

    # custom initialiser method

    sub init {
	my ($self, $config) = @_;

	# to indicate a failure
	return $self->error('bad constructor!');

	# or to indicate general happiness and well-being
	return $self;
    }

    package main;

    # new() constructor folds args into hash and calls init()
    my $object = My::Funky::Module->new( foo => 'bar', ... )
	  || die My::Funky::Module->error();

    # error() class/object method to get/set errors
    $object->error('something has gone wrong');
    print $object->error();

    # debugging() method (de-)activates the debug() method
    $object->debugging(1);

    # debug() prints to STDERR if debugging enabled
    $object->debug('The ', $animal, ' sat on the ', $place);

DESCRIPTION

This module implements a simple base class from which other modules can be derived, thereby inheriting a number of useful methods.

For a number of years, I found myself re-writing this module for practically every Perl project of any significant size. Or rather, I would copy the module from the last project and perform a global search and replace to change the names. Eventually, I decided to Do The Right Thing and release it as a module in it's own right.

It defines a base class which implements a number of useful methods like new(), init(), clone() and error(). Eventually, you will be able to mix-in other base class module to provide additional functionality to your objects in an easy and consistent manner. I just haven't got around to releasing those modules... yet.

Subclassing Class::Base

This module is what object-oriented afficionados would describe as an "abstract base class". That means that it's not designed to be used as a stand-alone module, rather as something from which you derive your own modules. Like this:

package My::Funky::Module
use base qw( Class::Base );

You can then use it like this:

use My::Funky::Module;

my $module = My::Funky::Module->new();

Construction and Initialisation Methods

If you want to apply any per-object initialisation, then simply write an init() method. This gets called by the new() method which passes a reference to a hash reference of configuration options.

    sub init {
	my ($self, $config) = @_;

	...

	return $self;
    }

When you create new objects using the new() method you can either pass a hash reference or list of named arguments. The new() method does the right thing to fold named arguments into a hash reference for passing to the init() method. Thus, the following are equivalent:

    # hash reference
    my $module = My::Funky::Module->new({ 
	foo => 'bar', 
	wiz => 'waz',
    });

    # list of named arguments (no enclosing '{' ... '}')
    my $module = My::Funky::Module->new(
	foo => 'bar', 
	wiz => 'waz'
    );

Error Handling

The init() method should return $self to indicate success or undef to indicate a failure. You can use the error() method to report an error within the init() method. The error() method returns undef, so you can use it like this:

    sub init {
	my ($self, $config) = @_;

	# let's make 'foobar' a mandatory argument
	$self->{ foobar } = $config->{ foobar }
	    || return $self->error("no foobar argument");

	return $self;
    }

When you create objects of this class via new(), you should now check the return value. If undef is returned then the error message can be retrieved by calling error() as a class method.

  my $module = My::Funky::Module->new()
	  || die My::Funky::Module->error();

Alternately, you can inspect the $ERROR package variable which will contain the same error message.

  my $module = My::Funky::Module->new()
	 || die $My::Funky::Module::ERROR;

Of course, being a conscientious Perl programmer, you will want to be sure that the $ERROR package variable is correctly defined.

package My::Funky::Module
use base qw( Class::Base );

our $ERROR;

You can also call error() as an object method. If you pass an argument then it will be used to set the internal error message for the object and return undef. Typically this is used within the module methods to report errors.

    sub another_method {
	my $self = shift;

	...

	# set the object error
	return $self->error('something bad happened');
    }

If you don't pass an argument then the error() method returns the current error value. Typically this is called from outside the object to determine its status. For example:

    my $object = My::Funky::Module->new()
        || die My::Funky::Module->error();

    $object->another_method()
	|| die $object->error();

Debugging Methods

The module implements two methods to assist in writing debugging code: debug() and debugging(). Debugging can be enabled on a per-object or per-class basis, or as a combination of the two.

When creating an object, you can set the DEBUG flag (or lower case debug if you prefer) to enable or disable debugging for that one object.

my $object = My::Funky::Module->new( debug => 1 )
      || die My::Funky::Module->error();

my $object = My::Funky::Module->new( DEBUG => 1 )
      || die My::Funky::Module->error();

If you don't explicitly specify a debugging flag then it assumes the value of the $DEBUG package variable in your derived class or 0 if that isn't defined.

You can also switch debugging on or off via the debugging() method.

$object->debugging(0);	# debug off
$object->debugging(1);	# debug on

The debug() method examines the internal debugging flag (the _DEBUG member within the $self hash) and if it finds it set to any true value then it prints to STDERR all the arguments passed to it. The output is prefixed by a tag containing the class name of the object in square brackets (but see the id() method below for details on how to change that value).

For example, calling the method as:

$object->debug('foo', 'bar');

prints the following output to STDERR:

[My::Funky::Module] foobar

When called as class methods, debug() and debugging() instead use the $DEBUG package variable in the derived class as a flag to control debugging. This variable also defines the default DEBUG flag for any objects subsequently created via the new() method.

package My::Funky::Module
use base qw( Class::Base );

our $ERROR;
our $DEBUG = 0 unless defined $DEBUG;

# some time later, in a module far, far away
package main;

# debugging off (by default)
my $object1 = My::Funky::Module->new();

# turn debugging on for My::Funky::Module objects
$My::Funky::Module::DEBUG = 1;

# alternate syntax
My::Funky::Module->debugging(1);

# debugging on (implicitly from $DEBUG package var)
my $object2 = My::Funky::Module->new();

# debugging off (explicit override)
my $object3 = My::Funky::Module->new(debug => 0);

If you call debugging() without any arguments then it returns the value of the internal object flag or the package variable accordingly.

print "debugging is turned ", $object->debugging() ? 'on' : 'off';

METHODS

new()

Class constructor method which expects a reference to a hash array of parameters or a list of name => value pairs which are automagically folded into a hash reference. The method blesses a hash reference and then calls the init() method, passing the reference to the hash array of configuration parameters.

Returns a reference to an object on success or undef on error. In the latter case, the error() method can be called as a class method, or the $ERROR package variable (in the derived class' package) can be inspected to return an appropriate error message.

    my $object = My::Class->new( foo => 'bar' )	  # params list
	 || die $My::Class::$ERROR;               # package var

or

    my $object = My::Class->new({ foo => 'bar' }) # params hashref
	  || die My::Class->error;                # class method

init(\%config)

Object initialiser method which is called by the new() method, passing a reference to a hash array of configuration parameters. The method may be derived in a subclass to perform any initialisation required. It should return $self on success, or undef on error, via a call to the error() method.

    package My::Module;
    use base qw( Class::Base );

    sub init {
	my ($self, $config) = @_;

	# let's make 'foobar' a mandatory argument
	$self->{ foobar } = $config->{ foobar }
	    || return $self->error("no foobar argument");

	return $self;
    }

clone()

The clone() method performs a simple shallow copy of the object hash and creates a new object blessed into the same class. You may want to provide your own clone() method to perform a more complex cloning operation.

my $clone = $object->clone();

error($msg, ...)

General purpose method for getting and setting error messages. When called as a class method, it returns the value of the $ERROR package variable (in the derived class' package) if called without any arguments, or sets the same variable when called with one or more arguments. Multiple arguments are concatenated together.

# set error
My::Module->error('set the error string');
My::Module->error('set ', 'the ', 'error string');

# get error
print My::Module->error();
print $My::Module::ERROR;

When called as an object method, it operates on the _ERROR member of the object, returning it when called without any arguments, or setting it when called with arguments.

# set error
$object->error('set the error string');

# get error
print $object->error();

The method returns undef when called with arguments. This allows it to be used within object methods as shown:

    sub my_method {
	my $self = shift;

	# set error and return undef in one
	return $self->error('bad, bad, error')
	    if $something_bad;
    }

debug($msg, $msg, ...)

Prints all arguments to STDERR if the internal _DEBUG flag (when called as an object method) or $DEBUG package variable (when called as a class method) is set to a true value. Otherwise does nothing. The output is prefixed by a string of the form "[Class::Name]" where the name of the class is that returned by the id() method.

debugging($flag)

Used to get (no arguments) or set ($flag defined) the value of the internal _DEBUG flag (when called as an object method) or $DEBUG package variable (when called as a class method).

id($newid)

The debug() method calls this method to return an identifier for the object for printing in the debugging message. By default it returns the class name of the object (i.e. ref $self), but you can of course subclass the method to return some other value. When called with an argument it uses that value to set its internal _ID field which will be returned by subsequent calls to id().

AUTHOR

Andy Wardley <abw@kfs.org>

HISTORY

This module began life as the Template::Base module distributed as part of the Template Toolkit.

Thanks to Brian Moseley and Matt Sergeant for suggesting various enhancments, some of which went into version 0.02.

COPYRIGHT

Copyright (C) 1996-2002 Andy Wardley. All Rights Reserved.

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