perlboot.1

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.

Only the name of the package and the specific sound change.  So can we
somehow share the definition for \f(CW\*(C`speak\*(C'\fR between the Cow and the
Horse?  Yes, with inheritance!
.Sh "Inheriting the windpipes"
.IX Subsection "Inheriting the windpipes"
We'll define a common subroutine package called \f(CW\*(C`Animal\*(C'\fR, with the
definition for \f(CW\*(C`speak\*(C'\fR:
.PP
.Vb 6
\&    { package Animal;
\&      sub speak {
\&        my $class = shift;
\&        print "a $class goes ", $class\->sound, "!\en";
\&      }
\&    }
.Ve
.PP
Then, for each animal, we say it \*(L"inherits\*(R" from \f(CW\*(C`Animal\*(C'\fR, along
with the animal-specific sound:
.PP
.Vb 4
\&    { package Cow;
\&      @ISA = qw(Animal);
\&      sub sound { "moooo" }
\&    }
.Ve
.PP
Note the added \f(CW@ISA\fR array.  We'll get to that in a minute.
.PP
But what happens when we invoke \f(CW\*(C`Cow\->speak\*(C'\fR now?
.PP
First, Perl constructs the argument list.  In this case, it's just
\&\f(CW\*(C`Cow\*(C'\fR.  Then Perl looks for \f(CW\*(C`Cow::speak\*(C'\fR.  But that's not there, so
Perl checks for the inheritance array \f(CW@Cow::ISA\fR.  It's there,
and contains the single name \f(CW\*(C`Animal\*(C'\fR.
.PP
Perl next checks for \f(CW\*(C`speak\*(C'\fR inside \f(CW\*(C`Animal\*(C'\fR instead, as in
\&\f(CW\*(C`Animal::speak\*(C'\fR.  And that's found, so Perl invokes that subroutine
with the already frozen argument list.
.PP
Inside the \f(CW\*(C`Animal::speak\*(C'\fR subroutine, \f(CW$class\fR becomes \f(CW\*(C`Cow\*(C'\fR (the
first argument).  So when we get to the step of invoking
\&\f(CW\*(C`$class\->sound\*(C'\fR, it'll be looking for \f(CW\*(C`Cow\->sound\*(C'\fR, which
gets it on the first try without looking at \f(CW@ISA\fR.  Success!
.ie n .Sh "A few notes about @ISA"
.el .Sh "A few notes about \f(CW@ISA\fP"
.IX Subsection "A few notes about @ISA"
This magical \f(CW@ISA\fR variable (pronounced \*(L"is a\*(R" not \*(L"ice-uh\*(R"), has
declared that \f(CW\*(C`Cow\*(C'\fR \*(L"is a\*(R" \f(CW\*(C`Animal\*(C'\fR.  Note that it's an array,
not a simple single value, because on rare occasions, it makes sense
to have more than one parent class searched for the missing methods.
.PP
If \f(CW\*(C`Animal\*(C'\fR also had an \f(CW@ISA\fR, then we'd check there too.  The
search is recursive, depth-first, left-to-right in each \f(CW@ISA\fR by
default (see mro for alternatives).  Typically, each \f(CW@ISA\fR has
only one element (multiple elements means multiple inheritance and
multiple headaches), so we get a nice tree of inheritance.
.PP
When we turn on \f(CW\*(C`use strict\*(C'\fR, we'll get complaints on \f(CW@ISA\fR, since
it's not a variable containing an explicit package name, nor is it a
lexical (\*(L"my\*(R") variable.  We can't make it a lexical variable though
(it has to belong to the package to be found by the inheritance mechanism),
so there's a couple of straightforward ways to handle that.
.PP
The easiest is to just spell the package name out:
.PP
.Vb 1
\&    @Cow::ISA = qw(Animal);
.Ve
.PP
Or allow it as an implicitly named package variable:
.PP
.Vb 3
\&    package Cow;
\&    use vars qw(@ISA);
\&    @ISA = qw(Animal);
.Ve
.PP
If you're bringing in the class from outside, via an object-oriented
module, you change:
.PP
.Vb 4
\&    package Cow;
\&    use Animal;
\&    use vars qw(@ISA);
\&    @ISA = qw(Animal);
.Ve
.PP
into just:
.PP
.Vb 2
\&    package Cow;
\&    use base qw(Animal);
.Ve
.PP
And that's pretty darn compact.
.Sh "Overriding the methods"
.IX Subsection "Overriding the methods"
Let's add a mouse, which can barely be heard:
.PP
.Vb 10
\&    # Animal package from before
\&    { package Mouse;
\&      @ISA = qw(Animal);
\&      sub sound { "squeak" }
\&      sub speak {
\&        my $class = shift;
\&        print "a $class goes ", $class\->sound, "!\en";
\&        print "[but you can barely hear it!]\en";
\&      }
\&    }
\&
\&    Mouse\->speak;
.Ve
.PP
which results in:
.PP
.Vb 2
\&    a Mouse goes squeak!
\&    [but you can barely hear it!]
.Ve
.PP
Here, \f(CW\*(C`Mouse\*(C'\fR has its own speaking routine, so \f(CW\*(C`Mouse\->speak\*(C'\fR
doesn't immediately invoke \f(CW\*(C`Animal\->speak\*(C'\fR.  This is known as
\&\*(L"overriding\*(R".  In fact, we didn't even need to say that a \f(CW\*(C`Mouse\*(C'\fR was
an \f(CW\*(C`Animal\*(C'\fR at all, since all of the methods needed for \f(CW\*(C`speak\*(C'\fR are
completely defined with \f(CW\*(C`Mouse\*(C'\fR.
.PP
But we've now duplicated some of the code from \f(CW\*(C`Animal\->speak\*(C'\fR,
and this can once again be a maintenance headache.  So, can we avoid
that?  Can we say somehow that a \f(CW\*(C`Mouse\*(C'\fR does everything any other
\&\f(CW\*(C`Animal\*(C'\fR does, but add in the extra comment?  Sure!
.PP
First, we can invoke the \f(CW\*(C`Animal::speak\*(C'\fR method directly:
.PP
.Vb 10
\&    # Animal package from before
\&    { package Mouse;
\&      @ISA = qw(Animal);
\&      sub sound { "squeak" }
\&      sub speak {
\&        my $class = shift;
\&        Animal::speak($class);
\&        print "[but you can barely hear it!]\en";
\&      }
\&    }
.Ve
.PP
Note that we have to include the \f(CW$class\fR parameter (almost surely
the value of \f(CW"Mouse"\fR) as the first parameter to \f(CW\*(C`Animal::speak\*(C'\fR,
since we've stopped using the method arrow.  Why did we stop?  Well,
if we invoke \f(CW\*(C`Animal\->speak\*(C'\fR there, the first parameter to the
method will be \f(CW"Animal"\fR not \f(CW"Mouse"\fR, and when time comes for it
to call for the \f(CW\*(C`sound\*(C'\fR, it won't have the right class to come back
to this package.
.PP
Invoking \f(CW\*(C`Animal::speak\*(C'\fR directly is a mess, however.  What if
\&\f(CW\*(C`Animal::speak\*(C'\fR didn't exist before, and was being inherited from a
class mentioned in \f(CW@Animal::ISA\fR?  Because we are no longer using
the method arrow, we get one and only one chance to hit the right
subroutine.
.PP
Also note that the \f(CW\*(C`Animal\*(C'\fR classname is now hardwired into the
subroutine selection.  This is a mess if someone maintains the code,
changing \f(CW@ISA\fR for \f(CW\*(C`Mouse\*(C'\fR and didn't notice \f(CW\*(C`Animal\*(C'\fR there in
\&\f(CW\*(C`speak\*(C'\fR.  So, this is probably not the right way to go.
.Sh "Starting the search from a different place"
.IX Subsection "Starting the search from a different place"
A better solution is to tell Perl to search from a higher place
in the inheritance chain:
.PP
.Vb 9
\&    # same Animal as before
\&    { package Mouse;
\&      # same @ISA, &sound as before
\&      sub speak {
\&        my $class = shift;
\&        $class\->Animal::speak;
\&        print "[but you can barely hear it!]\en";
\&      }
\&    }
.Ve
.PP
Ahh.  This works.  Using this syntax, we start with \f(CW\*(C`Animal\*(C'\fR to find
\&\f(CW\*(C`speak\*(C'\fR, and use all of \f(CW\*(C`Animal\*(C'\fR's inheritance chain if not found
immediately.  And yet the first parameter will be \f(CW$class\fR, so the
found \f(CW\*(C`speak\*(C'\fR method will get \f(CW\*(C`Mouse\*(C'\fR as its first entry, and
eventually work its way back to \f(CW\*(C`Mouse::sound\*(C'\fR for the details.
.PP
But this isn't the best solution.  We still have to keep the \f(CW@ISA\fR
and the initial search package coordinated.  Worse, if \f(CW\*(C`Mouse\*(C'\fR had
multiple entries in \f(CW@ISA\fR, we wouldn't necessarily know which one
had actually defined \f(CW\*(C`speak\*(C'\fR.  So, is there an even better way?
.Sh "The \s-1SUPER\s0 way of doing things"
.IX Subsection "The SUPER way of doing things"
By changing the \f(CW\*(C`Animal\*(C'\fR class to the \f(CW\*(C`SUPER\*(C'\fR class in that
invocation, we get a search of all of our super classes (classes
listed in \f(CW@ISA\fR) automatically:
.PP
.Vb 9
\&    # same Animal as before
\&    { package Mouse;
\&      # same @ISA, &sound as before
\&      sub speak {
\&        my $class = shift;
\&        $class\->SUPER::speak;
\&        print "[but you can barely hear it!]\en";
\&      }
\&    }
.Ve
.PP
So, \f(CW\*(C`SUPER::speak\*(C'\fR means look in the current package's \f(CW@ISA\fR for
\&\f(CW\*(C`speak\*(C'\fR, invoking the first one found. Note that it does \fInot\fR look in
the \f(CW@ISA\fR of \f(CW$class\fR.
.Sh "Where we're at so far..."
.IX Subsection "Where we're at so far..."
So far, we've seen the method arrow syntax:
.PP
.Vb 1
\&  Class\->method(@args);
.Ve
.PP
or the equivalent:
.PP
.Vb 2
\&  $a = "Class";
\&  $a\->method(@args);
.Ve
.PP
which constructs an argument list of:
.PP
.Vb 1
\&  ("Class", @args)
.Ve
.PP
and attempts to invoke
.PP
.Vb 1
\&  Class::method("Class", @Args);
.Ve
.PP
However, if \f(CW\*(C`Class::method\*(C'\fR is not found, then \f(CW@Class::ISA\fR is examined
(recursively) to locate a package that does indeed contain \f(CW\*(C`method\*(C'\fR,
and that subroutine is invoked instead.
.PP
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.
.Sh "A horse is a horse, of course of course \*(-- or is it?"
.IX Subsection "A horse is a horse, of course of course  or is it?"
Let's start with the code for the \f(CW\*(C`Animal\*(C'\fR class
and the \f(CW\*(C`Horse\*(C'\fR class:
.PP
.Vb 10
\&  { package Animal;
\&    sub speak {
\&      my $class = shift;
\&      print "a $class goes ", $class\->sound, "!\en";
\&    }
\&  }
\&  { package Horse;
\&    @ISA = qw(Animal);
\&    sub sound { "neigh" }
\&  }
.Ve
.PP
This lets us invoke \f(CW\*(C`Horse\->speak\*(C'\fR to ripple upward to
\&\f(CW\*(C`Animal::speak\*(C'\fR, calling back to \f(CW\*(C`Horse::sound\*(C'\fR to get the specific
sound, and the output of:
.PP
.Vb 1
\&  a Horse goes neigh!
.Ve
.PP
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.
.PP
We can do that by drawing a new distinction, called an \*(L"instance\*(R".
An \*(L"instance\*(R" 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.
.PP
.Vb 2
\&  my $name = "Mr. Ed";
\&  my $talking = \e$name;
.Ve
.PP
So now \f(CW$talking\fR 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 \f(CW\*(C`bless\*(C'\fR:
.PP
.Vb 1
\&  bless $talking, Horse;
.Ve
.PP
This operator stores information about the package named \f(CW\*(C`Horse\*(C'\fR into
the thing pointed at by the reference.  At this point, we say
\&\f(CW$talking\fR is an instance of \f(CW\*(C`Horse\*(C'\fR.  That is, it's a specific
horse.  The reference is otherwise unchanged, and can still be used
with traditional dereferencing operators.
.Sh "Invoking an instance method"
.IX Subsection "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 \f(CW$talking\fR makes:
.PP
.Vb 1
\&  my $noise = $talking\->sound;
.Ve
.PP
To invoke \f(CW\*(C`sound\*(C'\fR, Perl first notes that \f(CW$talking\fR is a blessed
reference (and thus an instance).  It then constructs an argument
list, in this case from just \f(CW\*(C`($talking)\*(C'\fR.  (Later we'll see that
arguments will take their place following the instance variable,
just like with classes.)
.PP
Now for the fun part: Perl takes the class in which the instance was
blessed, in this case \f(CW\*(C`Horse\*(C'\fR, and uses that to locate the subroutine
to invoke the method.  In this case, \f(CW\*(C`Horse::sound\*(C'\fR is found directly
(without using inheritance), yielding the final subroutine invocation:
.PP
.Vb 1
\&  Horse::sound($talking)
.Ve
.PP
Note that the first parameter here is still the instance, not the name
of the class as before.  We'll get \f(CW\*(C`neigh\*(C'\fR as the return value, and
that'll end up as the \f(CW$noise\fR variable above.
.PP
If Horse::sound had not been found, we'd be wandering up the
\&\f(CW@Horse::ISA\fR 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).
.Sh "Accessing the instance data"
.IX Subsection "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:
.PP
.Vb 8
\&  { package Horse;
\&    @ISA = qw(Animal);
\&    sub sound { "neigh" }