in reply to Re: Why do you need abstract classes in Perl?
in thread Why do you need abstract classes in Perl?

    package AbstractFoo; 
   ... 
    sub new { 
        my $package = shift; 
        die "Don't subclass AbstractFoo directly!" 
           if $package eq "AbstractFoo"; 
        bless { @_ }, $package; 
    }

(Bonus points to whoever sees a way to generalize this even further.) 

sub new {
  die "this is an abstract subclass" if $_[0] eq __PACKAGE__;
  shift->SUPER::new(@_);
}

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-- Randal L. Schwartz, Perl hacker

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Re: Re: Re: Why do you need abstract classes in Perl?
by dws (Chancellor) on Mar 05, 2001 at 10:41 UTC
    Next time I'll limit the bonus points to Monks or below :)

    The use of __PACKAGE__ was what I was looking for. I was thinking along the lines of 

        package AbstractFoo;
    ...
    sub new {
        my $package = shift;
        die __PACKAGE __ . " is an abstract class"
            if $package eq __PACKAGE__;
        bless { @_ }, $package;
    }

    [download]

    merlyn and I had a back and forth in the CB about the difference between his solution and mine. I noted that his would fail if nobody in AbstractFoo's superclass chain implemented new. merlyn agreed, saying (and here I'll ask him to correct me if I get the details wrong) that this was correct behavior, given that there's no generic way to implement new. I granted that point, but cited the Smalltalk image as evidence that there were common patterns. That, in essense, is where we left the discussion.

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      I think that's close to what I said. The problem is that there's no common "base class" for Perl objects as there is in the Smalltalk image, so you might want a hashref, or you might want an arrayref, or even a coderef. If the SUPER call fails, it's the right thing to have happen. Perhaps I could have put in a ->can and die call, but really, that's all the runtime system is going to do anyway. {grin}

      I will continue to argue that your bless is no more correct here than my die. It's really a personal framework choice.

      -- Randal L. Schwartz, Perl hacker

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