| Class::Base - useful base class for deriving other modules |
new()init(\%config)clone()debugging($flag)id($newid)
Class::Base - useful base class for deriving other modules
package My::Funky::Module;
use base qw( Class::Base );
# custom initialiser method
sub init {
my ($self, $config) = @_;
# copy various params into $self
$self->params($config, qw( FOO BAR BAZ ))
|| return undef;
# to indicate a failure
return $self->error('bad constructor!')
if $something_bad;
# 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);
This module implements a simple base class from which other modules
can be derived, thereby inheriting a number of useful methods such as
new(), init(), params(), clone(), error() and
debug().
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. Each time it got a little more polished and eventually, I decided to Do The Right Thing and release it as a module in it's own right.
It doesn't pretend to be an all-encompassing solution for every kind
of object creation problem you might encounter. In fact, it only
supports blessed hash references that are created using the popular,
but by no means universal convention of calling new() with a list
or reference to a hash array of named parameters. Constructor failure
is indicated by returning undef and setting the $ERROR package
variable in the module's class to contain a relevant message (which
you can also fetch by calling error() as a class method).
e.g.
my $object = My::Module->new(
file => 'myfile.html',
msg => 'Hello World'
) || die $My::Module::ERROR;
or:
my $object = My::Module->new({
file => 'myfile.html',
msg => 'Hello World',
}) || die My::Module->error();
The new() method handles the conversion of a list of arguments
into a hash array and calls the init() method to perform any
initialisation. In many cases, it is therefore sufficient to define
a module like so:
package My::Module;
use Class::Base;
use base qw( Class::Base );
sub init {
my ($self, $config) = @_;
# copy some config items into $self
$self->params($config, qw( FOO BAR )) || return undef;
return $self;
}
# ...plus other application-specific methods
1;
Then you can go right ahead and use it like this:
use My::Module;
my $object = My::Module->new( FOO => 'the foo value',
BAR => 'the bar value' )
|| die $My::Module::ERROR;
Despite its limitations, Class::Base can be a surprisingly useful
module to have lying around for those times where you just want to
create a regular object based on a blessed hash reference and don't
want to worry too much about duplicating the same old code to bless a
hash, define configuration values, provide an error reporting
mechanism, and so on. Simply derive your module from Class::Base
and leave it to worry about most of the detail. And don't forget, you
can always redefine your own new(), error(), or other method, if
you don't like the way the Class::Base version works.
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 $module = My::Funky::Module->new();
If you want to apply any per-object initialisation, then simply write
an init() method. This gets called by the new() method which
passes a reference to a hash reference of configuration options.
sub init {
my ($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 $module = My::Funky::Module->new(
foo => 'bar',
wiz => 'waz'
);
Within the init() method, you can either handle the configuration
yourself:
sub init {
my ($self, $config) = @_;
$self->{ file } = $config->{ file }
|| return $self->error('no file specified');
return $self;
}
or you can call the params() method to do it for you:
sub init {
my ($self, $config) = @_;
$self->params($config, 'file')
|| return $self->error('no file specified');
return $self;
}
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();
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';
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 $self;
}
The params() method accept a reference to a hash array as the
first argument containing configuration values such as those passed
to the init() method. The second argument can be a reference to
a list of parameter names or a reference to a hash array mapping
parameter names to default values. If the second argument is not
a reference then all the remaining arguments are taken as parameter
names. Thus the method can be called as follows:
sub init {
my ($self, $config) = @_;
# either...
$self->params($config, qw( foo bar ));
# or...
$self->params($config, [ qw( foo bar ) ]);
# or...
$self->params($config, { foo => 'default foo value',
bar => 'default bar value' } );
return $self;
}
The method looks for values in $config corresponding to the keys specified and copies them, if defined, into $self.
Keys can be specified in UPPER CASE and the method will look for
either upper or lower case equivalents in the $config hash. Thus
you can call params() from init() like so:
sub init {
my ($self, $config) = @_;
$self->params($config, qw( FOO BAR ))
return $self;
}
but use either case for parameters passed to new():
my $object = My::Module->new( FOO => 'the foo value',
BAR => 'the bar value' )
|| die My::Module->error();
my $object = My::Module->new( foo => 'the foo value',
bar => 'the bar value' )
|| die My::Module->error();
Note however that the internal key within $self used to store the
value will be in the case provided in the call to params() (upper
case in this example). The method doesn't look for upper case
equivalents when they are specified in lower case.
When called in list context, the method returns a list of all the values corresponding to the list of keys, some of which may be undefined (allowing you to determine which values were successfully set if you need to). When called in scalar context it returns a reference to the same list.
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();
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;
}
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().
Andy Wardley <abw@kfs.org>
This is version 0.03 of Class::Base.
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 (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.
| Class::Base - useful base class for deriving other modules |