Nigel I have been looking at the "decorator" model, and I would do it differently.
Instead of using inheritance, I would use AUTOLOAD to implement
quasi-inheritance. The relation is "Emulates" rather than "ISA". Does your
design model have to be language independent, or can we use perl specific
stuff like AUTOLOAD?
Attached is an example (factor.pl, bignum.pm, strnum.pm) of this
quasi-inheritance. As an exercise and alternative to Math::BigInt, I wanted
a quickly implementable representation for arbitrarily large positive
integers. I knocked this up first as strnum (objects are digit strings), but
wanted something more fast and juicy: bignum (objects are arrays of ints),
which required more human programming effort.
Having strnum available first meant that I could prototype all the methods
needed for a bignum object, and code them one by one, while having a working
application at all stages. This is called the evo methodology. bignum has an
AUTOLOAD method which makes a strnum object, calls the corresponding method
and unwraps the return value if this is an object reference.
There are a few assumptions here, but it would be possible to do something
more generic, particularly for methods which pass and return different
object of different classes, but you get the idea.
Coming back to your "decorator" model. I suggest implementing your debug
class as follows. This is generic and can be used with any classes, and
copes with arbitrarily complex arguments and return values.
#-------------------------------
# calling code (all you need to do for each class)
package foo::Debug;
use MethodTracer;
@ISA = qw(MethodTracer);
.
.
$fred = new foo:Debug(@blah); # was new foo(@blah)
foo:Debug->debug(1) # Turn on debugging of foo objects
#-------------------------------
MethodTracer.pm
# This is called via inheritance to construct an object
# of class blah::blah::Debug. This assumes that the "real"
# class is called blah::blah
package MethodTracer;
use Data::Dumper;
sub new
{
my ($pkg) = @_;
$pkg =~ /\:\:\w+$/;
my $underly = $`;
$underly::new(@_); # relies on constructor respecting package
param
}
sub debug # get and set debug at a class level
{
my $pkg = shift;
@_ ? (${$pkg}::_debug = shift) : ${$pkg}::_debug;
}
sub DESTROY
{
1; # to stop AUTOLOAD being called
}
sub AUTOLOAD
{
my ($self,@args) = @_;
my $method = $AUTOLOAD;
$method =~ s/\:\:Debug//;
my $pkg = ref $self;
print "called $method. Parameters:",Dumper(@args) if
${$pkg}::_debug;
if (wantarray)
{
my @rv = &$method(@_);
print "Return from $method. List context,
values:",Dumper(@rv)
if ${$pkg}::_debug;
@rv;
} else
my $rv = &$method(@_);
print "Return from $method. Scalar context,
returned:",Dumper($rv)
if ${$pkg}::_debug;
$rv;
}
}
1;
#----------------------------------
I intend to go to tonight's meeting.
C U There.
Ivor.
-----Original Message-----
From: Nigel Wetters [mailto:[EMAIL PROTECTED]]
Sent: 08 January 2002 17:12
To: [EMAIL PROTECTED]
Subject: Patterns in Perl reminder
Study group starts tomorrow. For details, see
http://london.pm.org/pipermail/london.pm/2002-January/007171.html
_________________________________________________________________
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bignum.pm
Description: Binary data
factor.pl
Description: Binary data
strnum.pm
Description: Binary data
