perlboot.pod

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

Using this simple syntax, we have class methods, (multiple)
inheritance, overriding, and extending.  Using just what we've seen so
far, we've been able to factor out common code, and provide a nice way
to reuse implementations with variations.  This is at the core of what
objects provide, but objects also provide instance data, which we
haven't even begun to cover.

=head2 A horse is a horse, of course of course -- or is it?

Let's start with the code for the C<Animal> class
and the C<Horse> class:

  { package Animal;
    sub speak {
      my $class = shift;
      print "a $class goes ", $class->sound, "!\n";
    }
  }
  { package Horse;
    @ISA = qw(Animal);
    sub sound { "neigh" }
  }

This lets us invoke C<< Horse->speak >> to ripple upward to
C<Animal::speak>, calling back to C<Horse::sound> to get the specific
sound, and the output of:

  a Horse goes neigh!

But all of our Horse objects would have to be absolutely identical.
If I add a subroutine, all horses automatically share it.  That's
great for making horses the same, but how do we capture the
distinctions about an individual horse?  For example, suppose I want
to give my first horse a name.  There's got to be a way to keep its
name separate from the other horses.

We can do that by drawing a new distinction, called an "instance".
An "instance" is generally created by a class.  In Perl, any reference
can be an instance, so let's start with the simplest reference
that can hold a horse's name: a scalar reference.

  my $name = "Mr. Ed";
  my $talking = \$name;

So now C<$talking> is a reference to what will be the instance-specific
data (the name).  The final step in turning this into a real instance
is with a special operator called C<bless>:

  bless $talking, Horse;

This operator stores information about the package named C<Horse> into
the thing pointed at by the reference.  At this point, we say
C<$talking> is an instance of C<Horse>.  That is, it's a specific
horse.  The reference is otherwise unchanged, and can still be used
with traditional dereferencing operators.

=head2 Invoking an instance method

The method arrow can be used on instances, as well as names of
packages (classes).  So, let's get the sound that C<$talking> makes:

  my $noise = $talking->sound;

To invoke C<sound>, Perl first notes that C<$talking> is a blessed
reference (and thus an instance).  It then constructs an argument
list, in this case from just C<($talking)>.  (Later we'll see that
arguments will take their place following the instance variable,
just like with classes.)

Now for the fun part: Perl takes the class in which the instance was
blessed, in this case C<Horse>, and uses that to locate the subroutine
to invoke the method.  In this case, C<Horse::sound> is found directly
(without using inheritance), yielding the final subroutine invocation:

  Horse::sound($talking)

Note that the first parameter here is still the instance, not the name
of the class as before.  We'll get C<neigh> as the return value, and
that'll end up as the C<$noise> variable above.

If Horse::sound had not been found, we'd be wandering up the
C<@Horse::ISA> list to try to find the method in one of the
superclasses, just as for a class method.  The only difference between
a class method and an instance method is whether the first parameter
is an instance (a blessed reference) or a class name (a string).

=head2 Accessing the instance data

Because we get the instance as the first parameter, we can now access
the instance-specific data.  In this case, let's add a way to get at
the name:

  { package Horse;
    @ISA = qw(Animal);
    sub sound { "neigh" }
    sub name {
      my $self = shift;
      $$self;
    }
  }

Now we call for the name:

  print $talking->name, " says ", $talking->sound, "\n";

Inside C<Horse::name>, the C<@_> array contains just C<$talking>,
which the C<shift> stores into C<$self>.  (It's traditional to shift
the first parameter off into a variable named C<$self> for instance
methods, so stay with that unless you have strong reasons otherwise.)
Then, C<$self> gets de-referenced as a scalar ref, yielding C<Mr. Ed>,
and we're done with that.  The result is:

  Mr. Ed says neigh.

=head2 How to build a horse

Of course, if we constructed all of our horses by hand, we'd most
likely make mistakes from time to time.  We're also violating one of
the properties of object-oriented programming, in that the "inside
guts" of a Horse are visible.  That's good if you're a veterinarian,
but not if you just like to own horses.  So, let's let the Horse class
build a new horse:

  { package Horse;
    @ISA = qw(Animal);
    sub sound { "neigh" }
    sub name {
      my $self = shift;
      $$self;
    }
    sub named {
      my $class = shift;
      my $name = shift;
      bless \$name, $class;
    }
  }

Now with the new C<named> method, we can build a horse:

  my $talking = Horse->named("Mr. Ed");

Notice we're back to a class method, so the two arguments to
C<Horse::named> are C<Horse> and C<Mr. Ed>.  The C<bless> operator
not only blesses C<$name>, it also returns the reference to C<$name>,
so that's fine as a return value.  And that's how to build a horse.

We've called the constructor C<named> here, so that it quickly denotes
the constructor's argument as the name for this particular C<Horse>.
You can use different constructors with different names for different
ways of "giving birth" to the object (like maybe recording its
pedigree or date of birth).  However, you'll find that most people
coming to Perl from more limited languages use a single constructor
named C<new>, with various ways of interpreting the arguments to
C<new>.  Either style is fine, as long as you document your particular
way of giving birth to an object.  (And you I<were> going to do that,
right?)

=head2 Inheriting the constructor

But was there anything specific to C<Horse> in that method?  No.  Therefore,
it's also the same recipe for building anything else that inherited from
C<Animal>, so let's put it there:

  { package Animal;
    sub speak {
      my $class = shift;
      print "a $class goes ", $class->sound, "!\n";
    }
    sub name {
      my $self = shift;
      $$self;
    }
    sub named {
      my $class = shift;
      my $name = shift;
      bless \$name, $class;
    }
  }
  { package Horse;
    @ISA = qw(Animal);
    sub sound { "neigh" }
  }

Ahh, but what happens if we invoke C<speak> on an instance?

  my $talking = Horse->named("Mr. Ed");
  $talking->speak;

We get a debugging value:

  a Horse=SCALAR(0xaca42ac) goes neigh!

Why?  Because the C<Animal::speak> routine is expecting a classname as
its first parameter, not an instance.  When the instance is passed in,
we'll end up using a blessed scalar reference as a string, and that
shows up as we saw it just now.

=head2 Making a method work with either classes or instances

All we need is for a method to detect if it is being called on a class
or called on an instance.  The most straightforward way is with the
C<ref> operator.  This returns a string (the classname) when used on a
blessed reference, and an empty string when used on a string (like a
classname).  Let's modify the C<name> method first to notice the change:

  sub name {
    my $either = shift;
    ref $either
      ? $$either # it's an instance, return name
      : "an unnamed $either"; # it's a class, return generic
  }

Here, the C<?:> operator comes in handy to select either the
dereference or a derived string.  Now we can use this with either an
instance or a class.  Note that I've changed the first parameter
holder to C<$either> to show that this is intended:

  my $talking = Horse->named("Mr. Ed");
  print Horse->name, "\n"; # prints "an unnamed Horse\n"
  print $talking->name, "\n"; # prints "Mr Ed.\n"

and now we'll fix C<speak> to use this:

  sub speak {
    my $either = shift;
    print $either->name, " goes ", $either->sound, "\n";
  }

And since C<sound> already worked with either a class or an instance,
we're done!

=head2 Adding parameters to a method

Let's train our animals to eat:

  { package Animal;
    sub named {
      my $class = shift;
      my $name = shift;
      bless \$name, $class;
    }
    sub name {
      my $either = shift;
      ref $either
	? $$either # it's an instance, return name
	: "an unnamed $either"; # it's a class, return generic
    }
    sub speak {
      my $either = shift;
      print $either->name, " goes ", $either->sound, "\n";
    }
    sub eat {
      my $either = shift;
      my $food = shift;
      print $either->name, " eats $food.\n";
    }
  }
  { package Horse;
    @ISA = qw(Animal);
    sub sound { "neigh" }
  }
  { package Sheep;
    @ISA = qw(Animal);
    sub sound { "baaaah" }
  }

And now try it out:

  my $talking = Horse->named("Mr. Ed");
  $talking->eat("hay");
  Sheep->eat("grass");

which prints:

  Mr. Ed eats hay.
  an unnamed Sheep eats grass.

An instance method with parameters gets invoked with the instance,
and then the list of parameters.  So that first invocation is like:

  Animal::eat($talking, "hay");

=head2 More interesting instances

What if an instance needs more data?  Most interesting instances are
made of many items, each of which can in turn be a reference or even
another object.  The easiest way to store these is often in a hash.
The keys of the hash serve as the names of parts of the object (often
called "instance variables" or "member variables"), and the
corresponding values are, well, the values.

But how do we turn the horse into a hash?  Recall that an object was
any blessed reference.  We can just as easily make it a blessed hash
reference as a blessed scalar reference, as long as everything that
looks at the reference is changed accordingly.

Let's make a sheep that has a name and a color:

  my $bad = bless { Name => "Evil", Color => "black" }, Sheep;

so C<< $bad->{Name} >> has C<Evil>, and C<< $bad->{Color} >> has
C<black>.  But we want to make C<< $bad->name >> access the name, and
that's now messed up because it's expecting a scalar reference.  Not
to worry, because that's pretty easy to fix up:

  ## in Animal
  sub name {
    my $either = shift;
    ref $either ?
      $either->{Name} :
      "an unnamed $either";
  }

And of course C<named> still builds a scalar sheep, so let's fix that
as well:

  ## in Animal
  sub named {
    my $class = shift;
    my $name = shift;
    my $self = { Name => $name, Color => $class->default_color };
    bless $self, $class;
  }

What's this C<default_color>?  Well, if C<named> has only the name,
we still need to set a color, so we'll have a class-specific initial color.
For a sheep, we might define it as white:

  ## in Sheep
  sub default_color { "white" }

And then to keep from having to define one for each additional class,
we'll define a "backstop" method that serves as the "default default",
directly in C<Animal>:

  ## in Animal
  sub default_color { "brown" }

Now, because C<name> and C<named> were the only methods that
referenced the "structure" of the object, the rest of the methods can
remain the same, so C<speak> still works as before.

=head2 A horse of a different color

But having all our horses be brown would be boring.  So let's add a
method or two to get and set the color.

  ## in Animal
  sub color {
    $_[0]->{Color}
  }
  sub set_color {
    $_[0]->{Color} = $_[1];
  }

Note the alternate way of accessing the arguments: C<$_[0]> is used
in-place, rather than with a C<shift>.  (This saves us a bit of time
for something that may be invoked frequently.)  And now we can fix
that color for Mr. Ed:

  my $talking = Horse->named("Mr. Ed");
  $talking->set_color("black-and-white");
  print $talking->name, " is colored ", $talking->color, "\n";

which results in:

  Mr. Ed is colored black-and-white

=head2 Summary

So, now we have class methods, constructors, instance methods,
instance data, and even accessors.  But that's still just the
beginning of what Perl has to offer.  We haven't even begun to talk
about accessors that double as getters and setters, destructors,
indirect object notation, subclasses that add instance data, per-class
data, overloading, "isa" and "can" tests, C<UNIVERSAL> class, and so
on.  That's for the rest of the Perl documentation to cover.
Hopefully, this gets you started, though.

=head1 SEE ALSO

For more information, see L<perlobj> (for all the gritty details about
Perl objects, now that you've seen the basics), L<perltoot> (the
tutorial for those who already know objects), L<perltooc> (dealing
with class data), L<perlbot> (for some more tricks), and books such as
Damian Conway's excellent I<Object Oriented Perl>.

Some modules which might prove interesting are Class::Accessor,
Class::Class, Class::Contract, Class::Data::Inheritable,
Class::MethodMaker and Tie::SecureHash

=head1 COPYRIGHT

Copyright (c) 1999, 2000 by Randal L. Schwartz and Stonehenge
Consulting Services, Inc.  Permission is hereby granted to distribute
this document intact with the Perl distribution, and in accordance
with the licenses of the Perl distribution; derived documents must
include this copyright notice intact.

Portions of this text have been derived from Perl Training materials
originally appearing in the I<Packages, References, Objects, and
Modules> course taught by instructors for Stonehenge Consulting
Services, Inc. and used with permission.

Portions of this text have been derived from materials originally
appearing in I<Linux Magazine> and used with permission.