Inheritance via class::derived

How09 has asked for the wisdom of the Perl Monks concerning the following question:

Hi, just starting in perl. In this perlboot example I have a question:

{ package Animal;
  sub sound { "WOOOAAARRRR" }
  sub speak {
    my $class = shift;
    print "a $class goes ", $class->sound, "!\n";
  }
}

{ package Horse; 
  @Horse::ISA = qw(Animal);
  sub sound { "neigh" }
  sub p1 { print "p1: ". shift() ."\n"; }
}

Horse->speak();         # ok, works as expected

Horse::p1("Horse");     # ok, prints "Horse"

Horse::speak("Horse");  # Message:
                        # "Undefined subroutine &Horse::speak called..
+."
                        # So, the inheritance mechanism does not work 
+for this?
[download]

Comment on Inheritance via class::derived_method Question Download Code

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Re: Inheritance via class::derived_method Question by ikegami (Patriarch) on Nov 18, 2010 at 18:33 UTC
Inheritance only applies to classes and objects. If you're not going to use either, how could inheritance possibly "work". `Horse->speak(); # Class method $ed->speak(); # Object method` [download]	[reply] [d/l]
Re: Inheritance via class::derived_method Question by AnomalousMonk (Archbishop) on Nov 18, 2010 at 22:03 UTC
So, the inheritance mechanism does not work for this? The 'inheritance mechanism' is the `->` operator, which is used in `Horse->speak();` but entirely absent (and so cannot work) in `Horse::speak("Horse");` which is an invocation of the `Horse` package's `speak` subroutine, which is undefined in the OPed example.	[reply] [d/l] [select]
Re^2: Inheritance via class::derived_method Question by ikegami (Patriarch) on Nov 18, 2010 at 22:55 UTC
The 'inheritance mechanism' is the `->` operator, No. The inheritance mechanism applies to method calls. Few of the `->` operators have anything to do with methods. For example, `$x->[0]` [download] Furthermore, not all method calls use the `->` operator. For example, `new Foo` [download] Forms of method calls: `$class->method(LIST) # Static method call. $object->method(LIST) # Instance method call. $x->Class::method(LIST) # Pretend the LHS is class "Class". $x->SUPER::method(LIST) # Pretend the LHS is its super class. method $x LIST; # Indirect method syntax` [download] I think that's all of them. `Horse::speak("Horse")` is a sub call. Subroutines aren't inherited. Subroutines don't have classes, so subroutines don't have parent classes, so subroutines have nothing from which they could inherit.	[reply] [d/l] [select]
Re^3: Inheritance via class::derived_method Question by How09 (Novice) on Nov 20, 2010 at 06:26 UTC
Thanks for the help. I think I'm getting a feel for it now. `class::method();` Is more like a name which is either defined or not. There is no @class::ISA array search for "method". There is no "automatic" first argument=class passed to the method. `class->method();` dereferences class for "method" and if not found will then use the "special method lookup mechanism" to search the classes listed in the @ISA array for "method" The class name is automatically passed to "method" as the 1st argument. ----- Fairly close? Thanks again.	[reply] [d/l] [select]
Re^4: Inheritance via class::derived_method Question by ikegami (Patriarch) on Nov 22, 2010 at 17:31 UTC