use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
-=over
+=over 4
=item TIESCALAR classname, LIST
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.
+of course.) See L<The C<untie> Gotcha> below for more details.
=item DESTROY this
In addition EXTEND will be called when perl would have pre-extended
allocation in a real array.
-This means that tied arrays are now I<complete>. The example below needs
-upgrading to illustrate this. (The documentation in B<Tie::Array> is more
-complete.)
+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:
-For this discussion, we'll implement an array whose indices are fixed at
-its creation. If you try to access anything beyond those bounds, you'll
-take an exception. For example:
-
- require Bounded_Array;
- tie @ary, 'Bounded_Array', 2;
- $| = 1;
- for $i (0 .. 10) {
- print "setting index $i: ";
- $ary[$i] = 10 * $i;
- $ary[$i] = 10 * $i;
- print "value of elt $i now $ary[$i]\n";
- }
+ 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 Bounded_Array;
+ package FixedElem_Array;
use Carp;
use strict;
-=over
+=over 4
=item TIEARRAY classname, LIST
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<{BOUND}> field will
-store the maximum bound allowed, and the C<{ARRAY}> field will hold the
+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 $bound = shift;
- confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
- if @_ || $bound =~ /\D/;
- return bless {
- BOUND => $bound,
- ARRAY => [],
- }, $class;
+ my $class = shift;
+ my $elemsize = shift;
+ if ( @_ || $elemsize =~ /\D/ ) {
+ croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
+ }
+ return bless {
+ ELEMSIZE => $elemsize,
+ ARRAY => [],
+ }, $class;
}
=item FETCH this, index
index whose value we're trying to fetch.
sub FETCH {
- my($self,$idx) = @_;
- if ($idx > $self->{BOUND}) {
- confess "Array OOB: $idx > $self->{BOUND}";
- }
- return $self->{ARRAY}[$idx];
+ 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.
+before being passed to FETCH. You may disable this feature by
+assigning a true value to the variable C<$NEGATIVE_INDICES> in the
+tied array class.
As you may have noticed, the name of the FETCH method (et al.) is the same
for all accesses, even though the constructors differ in names (TIESCALAR
This method will be triggered every time an element in the tied array is set
(written). It takes two arguments beyond its self reference: the index at
which we're trying to store something and the value we're trying to put
-there. For example:
+there.
+
+In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
+spaces so we have a little more work to do here:
sub STORE {
- my($self, $idx, $value) = @_;
- print "[STORE $value at $idx]\n" if _debug;
- if ($idx > $self->{BOUND} ) {
- confess "Array OOB: $idx > $self->{BOUND}";
+ my $self = shift;
+ my( $index, $value ) = @_;
+ if ( length $value > $self->{ELEMSIZE} ) {
+ croak "length of $value is greater than $self->{ELEMSIZE}";
}
- return $self->{ARRAY}[$idx] = $value;
+ # 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-E<gt>{ELEMSIZE}> spaces:
+
+ sub DELETE {
+ my $self = shift;
+ my $index = shift;
+ return $self->STORE( $index, '' );
+ }
+
+=item CLEAR this
+
+Clear (remove, delete, ...) all values from the tied array associated with
+object I<this>. For example:
+
+ sub CLEAR {
+ my $self = shift;
+ return $self->{ARRAY} = [];
+ }
+
+=item PUSH this, LIST
+
+Append elements of I<LIST> to the array. For example:
+
+ sub PUSH {
+ my $self = shift;
+ my @list = @_;
+ my $last = $self->FETCHSIZE();
+ $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
+ return $self->FETCHSIZE();
+ }
+
+=item POP this
+
+Remove last element of the array and return it. For example:
+
+ sub POP {
+ my $self = shift;
+ return pop @{$self->{ARRAY}};
+ }
+
+=item SHIFT this
+
+Remove the first element of the array (shifting other elements down)
+and return it. For example:
+
+ sub SHIFT {
+ my $self = shift;
+ return shift @{$self->{ARRAY}};
+ }
+
+=item UNSHIFT this, LIST
+
+Insert LIST elements at the beginning of the array, moving existing elements
+up to make room. For example:
+
+ sub UNSHIFT {
+ my $self = shift;
+ my @list = @_;
+ my $size = scalar( @list );
+ # make room for our list
+ @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
+ = @{$self->{ARRAY}};
+ $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
+ }
+
+=item SPLICE this, offset, length, LIST
+
+Perform the equivalent of C<splice> on the array.
+
+I<offset> is optional and defaults to zero, negative values count back
+from the end of the array.
+
+I<length> is optional and defaults to rest of the array.
+
+I<LIST> may be empty.
+
+Returns a list of the original I<length> elements at I<offset>.
+
+In our example, we'll use a little shortcut if there is a I<LIST>:
+
+ sub SPLICE {
+ my $self = shift;
+ my $offset = shift || 0;
+ my $length = shift || $self->FETCHSIZE() - $offset;
+ my @list = ();
+ if ( @_ ) {
+ tie @list, __PACKAGE__, $self->{ELEMSIZE};
+ @list = @_;
+ }
+ return splice @{$self->{ARRAY}}, $offset, $length, @list;
+ }
+
=item UNTIE this
-Will be called when C<untie> happens. (See below.)
+Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.)
=item DESTROY this
=back
-The code we presented at the top of the tied array class accesses many
-elements of the array, far more than we've set the bounds to. Therefore,
-it will blow up once they try to access beyond the 2nd element of @ary, as
-the following output demonstrates:
-
- setting index 0: value of elt 0 now 0
- setting index 1: value of elt 1 now 10
- setting index 2: value of elt 2 now 20
- setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
- Bounded_Array::FETCH called at testba line 12
-
=head2 Tying Hashes
Hashes were the first Perl data type to be tied (see dbmopen()). A class
the tied variable is garbage collected.
If this seems like a lot, then feel free to inherit from merely the
-standard Tie::Hash module for most of your methods, redefining only the
+standard Tie::StdHash module for most of your methods, redefining only the
interesting ones. See L<Tie::Hash> for details.
Remember that Perl distinguishes between a key not existing in the hash,
Here are the methods for the DotFiles tied hash.
-=over
+=over 4
=item TIEHASH classname, LIST
=item UNTIE this
-This is called when C<untie> occurs.
+This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
=item DESTROY this
package Shout;
-=over
+=over 4
=item TIEHANDLE classname, LIST
sub READ {
my $self = shift;
- my $$bufref = \$_[0];
+ my $bufref = \$_[0];
my(undef,$len,$offset) = @_;
print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
# add to $$bufref, set $len to number of characters read
=item UNTIE this
As with the other types of ties, this method will be called when C<untie> happens.
-It may be appropriate to "auto CLOSE" when this occurs.
+It may be appropriate to "auto CLOSE" when this occurs. See
+L<The C<untie> Gotcha> below.
=item DESTROY this
=head2 UNTIE this
You can define for all tie types an UNTIE method that will be called
-at untie().
+at untie(). See L<The C<untie> Gotcha> below.
=head2 The C<untie> Gotcha
Tied filehandles are still incomplete. sysopen(), truncate(),
flock(), fcntl(), stat() and -X can't currently be trapped.
+The bucket usage information provided by C<scalar(%hash)> is not
+available. If C<%hash> is tied, this will currently result in a
+fatal error.
+
+Counting the number of entries in a hash via C<scalar(keys(%hash))> or
+C<scalar(values(%hash)>) is inefficient since it needs to iterate
+through all the entries with FIRSTKEY/NEXTKEY.
+
=head1 AUTHOR
Tom Christiansen
UNTIE by Nick Ing-Simmons <F<nick@ing-simmons.net>>
+Tying Arrays by Casey West <F<casey@geeknest.com>>
https://perl5.git.perl.org / perl5.git / blobdiff