Perl Cd Bookshelf [Electronic resources] نسخه متنی

13.4.1. Problem

Normally, each object has its own complete state stored within
itself. The value of a data attribute in one object is unrelated to
the value that attribute might have in another instance of the same
class. For example, setting her gender here does
nothing to his gender, because they are
different objects with distinct states:

$him = Person->new( );
$him->gender("male");
$her = Person->new( );
$her->gender("female");

Imagine a classwide attribute where changing the attribute for one
instance changes it for all of them. Just as some programmers prefer
capitalized global variables, some prefer uppercase names when the
method affects class data instead of instance data. Here's an example
of using a class method named Max_Bounds:

FixedArray->Max_Bounds(100); # set for whole class
$alpha = FixedArray->new( );
printf "Bound on alpha is %d\n", $alpha->Max_Bounds( );
100
$beta = FixedArray->new( );
$beta->Max_Bounds(50); # still sets for whole class
printf "Bound on alpha is %d\n", $alpha->Max_Bounds( );
50

The implementation is simple:

package FixedArray;
$Bounds = 7; # default
sub new { bless( { }, shift ) }
sub Max_Bounds {
my $proto = shift;
$Bounds = shift if @_; # allow updates
return $Bounds;
}

To make the value effectively read-only, simply remove the update
possibility, as in:

sub Max_Bounds { $Bounds }

If you're deeply paranoid, make $Bounds a lexical
variable private to the scope of the file containing the class. Then
no one could say $FixedArray::Bounds to discover
its values. They'd be forced to go through the method interface
instead.

Here's a tip to help build scalable classes: store object data on the
object's namespace (in the hash), and store class data in the class
namespace (package variables or file-scoped lexicals). Only class
methods should directly access classwide attributes. Object methods
should access only instance data. If the object method needs access
to class data, its constructor should store a reference to that data
in the object. Here's an example:

sub new {
my $class = shift;
my $self = bless({ }, $class);
$self->{Max_Bounds_ref} = \$Bounds;
return $self;
}