perltie.pod

MSYS在windows下模拟了一个类unix的终端

=head1 NAME

perltie - how to hide an object class in a simple variable

=head1 SYNOPSIS

 tie VARIABLE, CLASSNAME, LIST

 $object = tied VARIABLE

 untie VARIABLE

=head1 DESCRIPTION

Prior to release 5.0 of Perl, a programmer could use dbmopen()
to connect an on-disk database in the standard Unix dbm(3x)
format magically to a %HASH 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.

Now you can.

The tie() 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 ALL CAPS,
which is a convention that Perl uses to indicate that they're called
implicitly rather than explicitly--just like the BEGIN() and END()
functions.

In the tie() call, C<VARIABLE> is the name of the variable to be
enchanted.  C<CLASSNAME> is the name of a class implementing objects of
the correct type.  Any additional arguments in the C<LIST> are passed to
the appropriate constructor method for that class--meaning TIESCALAR(),
TIEARRAY(), TIEHASH(), or TIEHANDLE().  (Typically these are arguments
such as might be passed to the dbminit() function of C.) The object
returned by the "new" method is also returned by the tie() function,
which would be useful if you wanted to access other methods in
C<CLASSNAME>. (You don't actually have to return a reference to a right
"type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed
object.)  You can also retrieve a reference to the underlying object
using the tied() function.

Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
for you--you need to do that explicitly yourself.

=head2 Tying Scalars

A class implementing a tied scalar should define the following methods:
TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.

Let's look at each in turn, using as an example a tie class for
scalars that allows the user to do something like:

    tie $his_speed, 'Nice', getppid();
    tie $my_speed,  'Nice', $$;

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!

We'll use Jarkko Hietaniemi <F<[e-mail]>>'s BSD::Resource class (not
included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
from your system, as well as the getpriority() and setpriority() system
calls.  Here's the preamble of the class.

    package Nice;
    use Carp;
    use BSD::Resource;
    use strict;
    $Nice::DEBUG = 0 unless defined $Nice::DEBUG;

=over 4

=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:

    sub TIESCALAR {
        my $class = shift;
        my $pid = shift || $$; # 0 means me

        if ($pid !~ /^\d+$/) {
            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 \$pid, $class;
    }

This tie class has chosen to return an error rather than raising an
exception if its constructor should fail.  While this is how dbmopen() works,
other classes may well not wish to be so forgiving.  It checks the global
variable C<$^W> to see whether to emit a bit of noise anyway.

=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 SCALAR 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 ID to which we've tied our variable.

    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;
    }

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.

=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.

    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: $!";
        }
        return $new_nicety;
    }

=item UNTIE this

This method will be triggered when the C<untie> occurs. This can be useful
if the class needs to know when no further calls will be made. (Except DESTROY
of course.) See below for more details.

=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 DESTROY method here for debugging purposes only.

    sub DESTROY {
        my $self = shift;
        confess "wrong type" unless ref $self;
        carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
    }

=back

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
TIESCALAR classes are certainly possible.

=head2 Tying Arrays

A class implementing a tied ordinary array should define the following
methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.

FETCHSIZE and STORESIZE are used to provide C<$#array> and
equivalent C<scalar(@array)> access.

The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
required if the perl operator with the corresponding (but lowercase) name
is to operate on the tied array. The B<Tie::Array> class can be used as a
base class to implement the first five of these in terms of the basic
methods above.  The default implementations of DELETE and EXISTS in
B<Tie::Array> simply C<croak>.

In addition EXTEND will be called when perl would have pre-extended
allocation in a real array.

For this discussion, we'll implement an array whose elements are a fixed
size at creation.  If you try to create an element larger than the fixed
size, you'll take an exception.  For example:

    use FixedElem_Array;
    tie @array, 'FixedElem_Array', 3;
    $array[0] = 'cat';  # ok.
    $array[1] = 'dogs'; # exception, length('dogs') > 3.

The preamble code for the class is as follows:

    package FixedElem_Array;
    use Carp;
    use strict;

=over 4

=item TIEARRAY classname, LIST

This is the constructor for the class.  That means it is expected to
return a blessed reference through which the new array (probably an
anonymous ARRAY ref) will be accessed.

In our example, just to show you that you don't I<really> have to return an
ARRAY reference, we'll choose a HASH reference to represent our object.
A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
store the maximum element size allowed, and the C<{ARRAY}> field will hold the
true ARRAY ref.  If someone outside the class tries to dereference the
object returned (doubtless thinking it an ARRAY ref), they'll blow up.
This just goes to show you that you should respect an object's privacy.

    sub TIEARRAY {
      my $class    = shift;
      my $elemsize = shift;
      if ( @_ || $elemsize =~ /\D/ ) {
        croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
      }
      return bless {
        ELEMSIZE => $elemsize,
        ARRAY    => [],
      }, $class;
    }

=item FETCH this, index

This method will be triggered every time an individual element the tied array
is accessed (read).  It takes one argument beyond its self reference: the
index whose value we're trying to fetch.

    sub FETCH {
      my $self  = shift;
      my $index = shift;
      return $self->{ARRAY}->[$index];
    }

If a negative array index is used to read from an array, the index
will be translated to a positive one internally by calling FETCHSIZE
before being passed to FETCH.

As you may have noticed, the name of the FETCH method (et al.) is the same
for all accesses, even though the constructors differ in names (TIESCALAR
vs TIEARRAY).  While in theory you could have the same class servicing
several tied types, in practice this becomes cumbersome, and it's easiest
to keep them at simply one tie type per class.

=item STORE this, index, value

This method will be triggered every time an element in the tied array is set
(written).  It takes two arguments beyond its self reference: the index at
which we're trying to store something and the value we're trying to put
there.

In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
spaces so we have a little more work to do here:

    sub STORE {
      my $self = shift;
      my( $index, $value ) = @_;
      if ( length $value > $self->{ELEMSIZE} ) {
        croak "length of $value is greater than $self->{ELEMSIZE}";
      }
      # fill in the blanks
      $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
      # right justify to keep element size for smaller elements
      $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
    }

Negative indexes are treated the same as with FETCH.

=item FETCHSIZE this

Returns the total number of items in the tied array associated with
object I<this>. (Equivalent to C<scalar(@array)>).  For example:

    sub FETCHSIZE {
      my $self = shift;
      return scalar @{$self->{ARRAY}};
    }

=item STORESIZE this, count

Sets the total number of items in the tied array associated with
object I<this> to be I<count>. If this makes the array larger then
class's mapping of C<undef> should be returned for new positions.
If the array becomes smaller then entries beyond count should be
deleted. 

In our example, 'undef' is really an element containing
C<$self-E<gt>{ELEMSIZE}> number of spaces.  Observe:

    sub STORESIZE {
      my $self  = shift;
      my $count = shift;
      if ( $count > $self->FETCHSIZE() ) {
        foreach ( $count - $self->FETCHSIZE() .. $count ) {
          $self->STORE( $_, '' );
        }
      } elsif ( $count < $self->FETCHSIZE() ) {
        foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
          $self->POP();
        }
      }
    }

=item EXTEND this, count

Informative call that array is likely to grow to have I<count> entries.
Can be used to optimize allocation. This method need do nothing.

In our example, we want to make sure there are no blank (C<undef>)
entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
as needed:

    sub EXTEND {   
      my $self  = shift;
      my $count = shift;
      $self->STORESIZE( $count );
    }

=item EXISTS this, key

Verify that the element at index I<key> exists in the tied array I<this>.

In our example, we will determine that if an element consists of
C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:

    sub EXISTS {
      my $self  = shift;
      my $index = shift;
      return 0 if ! defined $self->{ARRAY}->[$index] ||
                  $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
      return 1;
    }

=item DELETE this, key

Delete the element at index I<key> from the tied array I<this>.

In our example, a deleted item is C<$self->{ELEMSIZE}> spaces: