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Re: Dynamic names

by cdarke (Prior)
on Nov 16, 2010 at 21:17 UTC ( #871835=note: print w/replies, xml ) Need Help??


in reply to Dynamic names

You have a classic example of polymorphism. TMTOWTDI, here is one. In the loop we change the names of the animals to be blessed references of the relevant class. The trick here is that the second argument to bless is the class name, which can be a text string (otherwise we are into nasty evals).
use warnings; use strict; package Cow; { sub speak { print "Moo\n" } } package Horse; { sub speak { print "Hi, my name is Ed\n" } } package Sheep; { sub speak { print "Baaaah\n" } } my @pasture = qw(Cow Cow Horse Sheep Sheep); foreach my $animal (@pasture) { $animal = (bless \do{my $anon}, $animal); $animal->speak(); }
Produces:
Moo Moo Hi, my name is Ed Baaaah Baaaah

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Re^2: Dynamic names
by 7stud (Deacon) on Nov 16, 2010 at 21:31 UTC

    1) What does do{my $anon} accomplish?

    2) What does your code accomplish that this doesn't:

    my @pasture = qw(Cow Cow Horse Sheep Sheep); for my $beast (@pasture) { $beast->speak(); }
      The \do{my $anon} creates an anonymous reference which can be blessed. To be honest it doesn't do anything more than your simpler example in this case.

      Update: To clarify. Just using the plain text string is a class call - the first argument passed to each subroutine will be the name of the class (Cow, Horse, Sheep). My version, which creates a reference to an anonymous scalar, is an object call - the first argument passed to each argument is a unique reference referring to each animal, rather than each species. So this enables other magic to be performed because we can identify not just the type of animal (the class) the which Cow or Sheep, if we need to. It is also the basis of inside-out objects. I'm not clear if the questioner needs this extra level of control though.

        Hi, I have another question about that do{} construct. Is there any reason to use a do construct and not:

        bless \my $anon, $class;

        or the even easier to type:

        bless {}, $class;

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[hippo]: Make it an option?
[Corion]: hippo: Yes, but I'm not sure that it's even worth the effort of implementing it at all...
[Corion]: You'll only ever need that option if you have a long-running query whose results are not cached by your DB already, and in those cases I presume that the programmer will want to maintain the temporary tables themselves - I wouldn't know when to drop ...
[Corion]: ... the temporary tables, and also don't have a good idea on how to create unique table names for them
[hippo]: OIC. In that case leave it out but invite feature requests and see if any of the users suggest it. :-)
[Corion]: Talking about this makes me realize that it's likely only a half useful idea. But it still would be convenient to have as an option...
[Corion]: hippo: Hmmm - yeah, I could document it and wait for code implementing that option to show up ;-D

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