perltie.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.

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.\" ========================================================================
.\"
.IX Title "PERLTIE 1"
.TH PERLTIE 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perltie \- how to hide an object class in a simple variable
.IX Xref "tie"
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& tie VARIABLE, CLASSNAME, LIST
\&
\& $object = tied VARIABLE
\&
\& untie VARIABLE
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Prior to release 5.0 of Perl, a programmer could use \fIdbmopen()\fR
to connect an on-disk database in the standard Unix \fIdbm\fR\|(3x)
format magically to a \f(CW%HASH\fR in their program.  However, their Perl was either
built with one particular dbm library or another, but not both, and
you couldn't extend this mechanism to other packages or types of variables.
.PP
Now you can.
.PP
The \fItie()\fR function binds a variable to a class (package) that will provide
the implementation for access methods for that variable.  Once this magic
has been performed, accessing a tied variable automatically triggers
method calls in the proper class.  The complexity of the class is
hidden behind magic methods calls.  The method names are in \s-1ALL\s0 \s-1CAPS\s0,
which is a convention that Perl uses to indicate that they're called
implicitly rather than explicitly\*(--just like the \s-1\fIBEGIN\s0()\fR and \s-1\fIEND\s0()\fR
functions.
.PP
In the \fItie()\fR call, \f(CW\*(C`VARIABLE\*(C'\fR is the name of the variable to be
enchanted.  \f(CW\*(C`CLASSNAME\*(C'\fR is the name of a class implementing objects of
the correct type.  Any additional arguments in the \f(CW\*(C`LIST\*(C'\fR are passed to
the appropriate constructor method for that class\*(--meaning \s-1\fITIESCALAR\s0()\fR,
\&\s-1\fITIEARRAY\s0()\fR, \s-1\fITIEHASH\s0()\fR, or \s-1\fITIEHANDLE\s0()\fR.  (Typically these are arguments
such as might be passed to the \fIdbminit()\fR function of C.) The object
returned by the \*(L"new\*(R" method is also returned by the \fItie()\fR function,
which would be useful if you wanted to access other methods in
\&\f(CW\*(C`CLASSNAME\*(C'\fR. (You don't actually have to return a reference to a right
\&\*(L"type\*(R" (e.g., \s-1HASH\s0 or \f(CW\*(C`CLASSNAME\*(C'\fR) so long as it's a properly blessed
object.)  You can also retrieve a reference to the underlying object
using the \fItied()\fR function.
.PP
Unlike \fIdbmopen()\fR, the \fItie()\fR function will not \f(CW\*(C`use\*(C'\fR or \f(CW\*(C`require\*(C'\fR a module
for you\*(--you need to do that explicitly yourself.
.Sh "Tying Scalars"
.IX Xref "scalar, tying"
.IX Subsection "Tying Scalars"
A class implementing a tied scalar should define the following methods:
\&\s-1TIESCALAR\s0, \s-1FETCH\s0, \s-1STORE\s0, and possibly \s-1UNTIE\s0 and/or \s-1DESTROY\s0.
.PP
Let's look at each in turn, using as an example a tie class for
scalars that allows the user to do something like:
.PP
.Vb 2
\&    tie $his_speed, \*(AqNice\*(Aq, getppid();
\&    tie $my_speed,  \*(AqNice\*(Aq, $$;
.Ve
.PP
And now whenever either of those variables is accessed, its current
system priority is retrieved and returned.  If those variables are set,
then the process's priority is changed!
.PP
We'll use Jarkko Hietaniemi <\[e-mail]\fR>'s BSD::Resource class (not
included) to access the \s-1PRIO_PROCESS\s0, \s-1PRIO_MIN\s0, and \s-1PRIO_MAX\s0 constants
from your system, as well as the \fIgetpriority()\fR and \fIsetpriority()\fR system
calls.  Here's the preamble of the class.
.PP
.Vb 5
\&    package Nice;
\&    use Carp;
\&    use BSD::Resource;
\&    use strict;
\&    $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
.Ve
.IP "\s-1TIESCALAR\s0 classname, \s-1LIST\s0" 4
.IX Xref "TIESCALAR"
.IX Item "TIESCALAR classname, LIST"
This is the constructor for the class.  That means it is
expected to return a blessed reference to a new scalar
(probably anonymous) that it's creating.  For example:
.Sp
.Vb 3
\&    sub TIESCALAR {
\&        my $class = shift;
\&        my $pid = shift || $$; # 0 means me
\&
\&        if ($pid !~ /^\ed+$/) {
\&            carp "Nice::Tie::Scalar got non\-numeric pid $pid" if $^W;
\&            return undef;
\&        }
\&
\&        unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
\&            carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
\&            return undef;
\&        }
\&
\&        return bless \e$pid, $class;
\&    }
.Ve
.Sp
This tie class has chosen to return an error rather than raising an
exception if its constructor should fail.  While this is how \fIdbmopen()\fR works,
other classes may well not wish to be so forgiving.  It checks the global
variable \f(CW$^W\fR to see whether to emit a bit of noise anyway.
.IP "\s-1FETCH\s0 this" 4
.IX Xref "FETCH"
.IX Item "FETCH this"
This method will be triggered every time the tied variable is accessed
(read).  It takes no arguments beyond its self reference, which is the
object representing the scalar we're dealing with.  Because in this case
we're using just a \s-1SCALAR\s0 ref for the tied scalar object, a simple $$self
allows the method to get at the real value stored there.  In our example
below, that real value is the process \s-1ID\s0 to which we've tied our variable.
.Sp
.Vb 10
\&    sub FETCH {
\&        my $self = shift;
\&        confess "wrong type" unless ref $self;
\&        croak "usage error" if @_;
\&        my $nicety;
\&        local($!) = 0;
\&        $nicety = getpriority(PRIO_PROCESS, $$self);
\&        if ($!) { croak "getpriority failed: $!" }
\&        return $nicety;
\&    }
.Ve
.Sp
This time we've decided to blow up (raise an exception) if the renice
fails\*(--there's no place for us to return an error otherwise, and it's
probably the right thing to do.
.IP "\s-1STORE\s0 this, value" 4
.IX Xref "STORE"
.IX Item "STORE this, value"
This method will be triggered every time the tied variable is set
(assigned).  Beyond its self reference, it also expects one (and only one)
argument\*(--the new value the user is trying to assign. Don't worry about
returning a value from \s-1STORE\s0 \*(-- the semantic of assignment returning the
assigned value is implemented with \s-1FETCH\s0.
.Sp
.Vb 5
\&    sub STORE {
\&        my $self = shift;
\&        confess "wrong type" unless ref $self;
\&        my $new_nicety = shift;
\&        croak "usage error" if @_;
\&
\&        if ($new_nicety < PRIO_MIN) {
\&            carp sprintf
\&              "WARNING: priority %d less than minimum system priority %d",
\&                  $new_nicety, PRIO_MIN if $^W;
\&            $new_nicety = PRIO_MIN;
\&        }
\&
\&        if ($new_nicety > PRIO_MAX) {
\&            carp sprintf
\&              "WARNING: priority %d greater than maximum system priority %d",
\&                  $new_nicety, PRIO_MAX if $^W;
\&            $new_nicety = PRIO_MAX;
\&        }
\&
\&        unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
\&            confess "setpriority failed: $!";
\&        }
\&    }
.Ve
.IP "\s-1UNTIE\s0 this" 4
.IX Xref "UNTIE"
.IX Item "UNTIE this"
This method will be triggered when the \f(CW\*(C`untie\*(C'\fR occurs. This can be useful
if the class needs to know when no further calls will be made. (Except \s-1DESTROY\s0
of course.) See "The \f(CW\*(C`untie\*(C'\fR Gotcha" below for more details.
.IP "\s-1DESTROY\s0 this" 4
.IX Xref "DESTROY"
.IX Item "DESTROY this"
This method will be triggered when the tied variable needs to be destructed.
As with other object classes, such a method is seldom necessary, because Perl
deallocates its moribund object's memory for you automatically\*(--this isn't
\&\*(C+, you know.  We'll use a \s-1DESTROY\s0 method here for debugging purposes only.
.Sp
.Vb 5
\&    sub DESTROY {
\&        my $self = shift;
\&        confess "wrong type" unless ref $self;
\&        carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
\&    }
.Ve
.PP
That's about all there is to it.  Actually, it's more than all there
is to it, because we've done a few nice things here for the sake
of completeness, robustness, and general aesthetics.  Simpler
\&\s-1TIESCALAR\s0 classes are certainly possible.
.Sh "Tying Arrays"
.IX Xref "array, tying"
.IX Subsection "Tying Arrays"
A class implementing a tied ordinary array should define the following
methods: \s-1TIEARRAY\s0, \s-1FETCH\s0, \s-1STORE\s0, \s-1FETCHSIZE\s0, \s-1STORESIZE\s0 and perhaps \s-1UNTIE\s0 and/or \s-1DESTROY\s0.
.PP
\&\s-1FETCHSIZE\s0 and \s-1STORESIZE\s0 are used to provide \f(CW$#array\fR and
equivalent \f(CW\*(C`scalar(@array)\*(C'\fR access.
.PP
The methods \s-1POP\s0, \s-1PUSH\s0, \s-1SHIFT\s0, \s-1UNSHIFT\s0, \s-1SPLICE\s0, \s-1DELETE\s0, and \s-1EXISTS\s0 are
required if the perl operator with the corresponding (but lowercase) name