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Re: OT: copy constructor

by chromatic (Archbishop)
on Jan 30, 2011 at 16:33 UTC ( #885126=note: print w/replies, xml ) Need Help??

in reply to OT: copy constructor
in thread scope and declaration in local packages

But what makes you say that this usage is almost always an error?

In the original code, $class went entirely unused. All of the shenanigans to figure out whether the invocant is a reference are useless.

The design problem with this technique is that it allows you to write:

my $object = Class->new(); my $otherobj = $object->new();

What relationship should $otherobj have with $object? Does it have any? It's easy to expect a prototypal relationship between the two, or one where the former's instance data somehow sets the latter's, but there's almost never any code in the copy-and-paste copy constructor to set this.

Why create an interface that does nothing?

Worst yet, consider what happens if someone invokes the constructor like this:

o my $object = Class::new( {} );

I know that's unlikely, but the copy constructor technique will silently create an object blessed into a package called HASH. Without the copy constructor technique, Perl will happily throw an exception about attempting to bless into a reference. This technique silences useful error messages for no good reason.

As far as I can tell, the only reason this code persists is because someone put it in the documentation as an example of a cool technique, and people started thinking it was necessary because the documentation explained Perl 5 objects as crazy black boxes that require a lot of arcana.

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