Basilides has asked for the wisdom of the Perl Monks concerning the following question:

Further to emilford's node today about tidy code , and my own query the other day about OO inheritance, here's a new question:

Say I've got a parent class, Word, with subclasses, Noun, Verb & Adjective, and I want to create a reference to one of the subclasses, whose type depends on a file input. To comply with use strict on lexical scope, I reckon I need to start off with a variable declaration, which *doesn't* include an assignment: my $word;. You see this in other languages but I've never seen it in Perl, hence I'm raising it now: 

my $word;  #this is it--are you supposed to do this?
if ($part eq 'n')
{   $word = new Noun(@blah);
}
elsif ($part eq 'v')
{   $word = new Verb(@blah);
}
else
{   $word = new Adjective(@blah);
}
print $word->get_english();
#etc etc

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If this isn't the done thing, please could you tell me how you *should* do it.

Replies are listed 'Best First'.
Re: Good practice: Variable declaration
by japhy (Canon) on Jun 27, 2002 at 16:43 UTC
    That's fine. You could be spiffy and write it like: 
    my %class = qw( n Noun v Verb );
my $word = ($class{$part} || "Adjective")->new(...);

    [download]
    if you feel like being spiffy.

    _____________________________________________________
    Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
    s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

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[d/l]

      That's fine. You could be spiffy and write it like:
      my %class = qw( n Noun v Verb );
      my $word = ($class{$part} || "Adjective")->new(...);

      Spiffier. 

      my $word = ({ qw/n Noun v Verb/ }->{$part} || 'Adjective')->new(...);

      [download]

      - Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

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[d/l]
        I changed my code from that because that doesn't lend itself to repetition. Easier to change one data structure than many lines of code. Although I un-abstracted from what I had originally: 
        my %class = qw( n Noun v Verb );
my $default = 'Adjective';
...

        [download]

        _____________________________________________________
        Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
        s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

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[d/l]
Re: Good practice: Variable declaration
by mfriedman (Monk) on Jun 27, 2002 at 16:40 UTC
    Yes, that's exactly how you're supposed to do it. :)

    If you want to be really anal, you could always write 

    my $word = undef;

    [download]

    Which accomplishes the same thing.

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[d/l]
Re: Deciding a class at runtime.
by frankus (Priest) on Jun 27, 2002 at 16:40 UTC
    This seems correct. Perhaps better to use: 
    $word = Verb->new(@blah);

    [download]
    But I guess that's a matter of preference unless you're doing multi-inheritence.

    --

    Brother Frankus.

    ¤

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[d/l]
Re: Good practice: Variable declaration
by Aristotle (Chancellor) on Jun 27, 2002 at 17:19 UTC
    To comply with use strict on lexical scope, I reckon I need to start off with a variable declaration, which *doesn't* include an assignment: my $word;
    No, you only need the my in there. Whether you assign something or not doesn't matter any. May I suggest though, that in your case it would be more readable to do it this way: 
    my $word =
    $part eq 'n' ? new Noun(@blah) :
    $part eq 'v' ? new Verb(@blah) :
                   new Adjective(@blah);

print $word->get_english();

    [download]
    Update: DOH! Ack. I don't know where I left my brain while I was posting the initial version of this. japhy++

    Makeshifts last the longest.

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[d/l]
      Ouch. Remove those extra $word ='s. Not only are they assigning to a package-scoped $word, they're screwing up your ?: code. And you have a ":" where you want a "?", beneath the first "?". 
      japhy% perl -MO=Deparse,-p
my $word =
    $part eq 'n' ? $word = new Noun(@blah) :
    $part eq 'v' ? $word = new Verb(@blah) :
                   $word = new Adjective(@blah);

      [download]
      Check that out and you'll see that $word ends up getting assigned as an Adjective object no matter what, because of the precedence of ?: compared with that of =.

      _____________________________________________________
      Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
      s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

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[d/l]
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