Cloning Objects

Problem

You want to write a constructor method that might be called on an existing object.

Solution

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.

Discussion

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 can be called on the class or an existing object. 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 called 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, $proto);
}
bless($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 called as an instance method.

See Also

perlobj (1) and of Perl Developing; ; ;