chen li wrote:
>
> --- "Mumia W." <[EMAIL PROTECTED]>
> wrote:
>
>
>>On 09/22/2006 07:50 AM, chen li wrote:
>>
>>>--- "Mumia W."
>>
>><[EMAIL PROTECTED]>
>>
>>>wrote:
>>>
>>>>That "redundancy" allows you to use inheritance.
>>
>>If
>>
>>>>you have a class
>>>>Employee, with an expanded set of fields, this
>>>>method makes it possible
>>>>to add the fields without having to modify each
>>>>method that uses them.
>>>>
>>>
>>>Where can I find more about this kind of syntax?
>>>[...]
>>
>>Read the "Class Data" section in perltoot.
>>
>
> Hi Mumia,
>
> I don't think I get the point. If my purpose of using
> AUTOLOAD is to handle
> 1) many attributes in construct
> 2) create an un-predined method on the fly
> what will happen if I make the following changes:
>
> in sub new change
> my $self = {_permitted => \%fields,%fields,};
> to
> my $self =\%fields;
>
> in sub AUTOLOAD comment out the unless statement.
>
> Do you think these change will affect inheritance?
>
> Thanks in advacne,
>
> Li
>
>
> ########### copy from perltoot
> package Person;
> use Carp;
> our $AUTOLOAD; # it's a package global
>
> my %fields = (
> name => undef,
> age => undef,
> peers => undef,
> );
>
> sub new {
> my $class = shift;
> my $self = {
> _permitted => \%fields,
> %fields,
> };
> bless $self, $class;
> return $self;
> }
>
>
> sub AUTOLOAD {
> my $self = shift;
> my $type = ref($self)
> or croak "$self is not an
> object";
>
> my $name = $AUTOLOAD;
> $name =~ s/.*://; # strip
> fully-qualified portion
>
> unless (exists
> $self->{_permitted}->{$name} ) {
> croak "Can't access `$name' field in
> class $type";
> }
>
> if (@_) {
> return $self->{$name} = shift;
> } else {
> return $self->{$name};
> }
> }
Hello Li
About the _permitted hash key, the perltoot tutorial says
+ I could have avoided the "_permitted" field entirely, but I wanted to
+ demonstrate how to store a reference to class data on the object so you
+ wouldn't have to access that class data directly from an object method.
So the point of the class data %fields is to restrict autoloaded methods to the
expected ones, while the point of putting a reference to it in the object data
is, as Tom says, to demonstrate how class data can be accessed indirectly from
within an object method.
Preventing the module from autoloading non-permitted methods has the same
purpose as 'use strict "vars"', i.e. to prevent a misspelled name producing
obscure runtime bugs. Without the _permitted hash, I could write:
my $me = new Person;
$me->nane('Rob');
die unless $me->name eq 'Rob';
and I could be perplexed for a while over why it kept dying. Adding the check,
the AUTOLOAD method would croak and immediately explain the problem.
With regard to the reference to class data, later on in the tutorial there is a
class Person with a class scalar $Census which keeps track of the number of
objects that have been created. This scalar can be accessed via the 'population'
method. Using the same technique, the 'new' and 'population' methods could be
rewritten:
my $Census = 0;
sub new {
my $class = shift;
my $self = {
NAME => undef,
AGE => undef,
PEERS => [],
_census => \$Census,
};
${$self->{_census}}++;
bless ($self, $class);
}
sub population {
my $self = shift;
return ${$self->{_census}};
}
so that the access method has no need to refer directly to the package variable,
in the same way that your AUTOLOAD method needn't use the %fields hash
explicitly. I believe this is what Mumia meant about inheritance: if the method
had directly accessed %fields hash then a subclass like Employee would have no
way of reaching it, but once a reference to it appears in the object data then
all is well once more.
I hope this makes things clearer for you.
Rob
(After reading the tutorial further I realise that Tom Christiansen has made a
very similar adaptation to the one I have described above. The main difference
is that the 'population' method is written so that it can also be called as the
class method Person::population as well as an object method $person->population.
I hope this doesn't confuse you.)
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