Perl Cd Bookshelf [Electronic resources] نسخه متنی

13.6.1. Problem

Start your constructor like this:

my $proto = shift;
my $class = ref($proto) || $proto;
my $parent = ref($proto) && $proto;

The $class variable will contain the class to
bless into, and the $parent variable will either
be false, or else the object you're cloning.

Sometimes you need another object of the same type as the current
one. You could do this:

$ob1 = SomeClass->new( );
# later on
$ob2 = (ref $ob1)->new( );

but that's not very clear. It's clearer to have a single constructor
that behaves correctly, regardless of whether its invocant is a class
name or an existing object of that class. As a class method, it
should return a new object with the default initialization. As an
instance method, it should return a new object initialized from the
object it was invoked on:

$ob1 = Widget->new( );
$ob2 = $ob1->new( );

Here's a version of new that takes this into
consideration:

sub new {
my $proto = shift;
my $class = ref($proto) || $proto;
my $parent = ref($proto) && $proto;
my $self;
# check whether we're shadowing a new from @ISA
if (@ISA && $proto->SUPER::can("new") ) {
$self = $proto->SUPER::new(@_);
} else {
$self = { };
bless ($self, $class);
}
$self->{PARENT} = $parent;
$self->{START} = time( ); # init data fields
$self->{AGE} = 0;
return $self;
}

Initializing doesn't have to mean simply copying values from the
parent. If you're writing a linked list or binary tree class, your
constructor can return a new object linked into the list or tree,
when invoked as an instance method.