=head1 DESCRIPTION
Prior to release 5.0 of Perl, a programmer could use dbmopen()
-to magically connect an on-disk database in the standard Unix dbm(3x)
-format to a %HASH in their program. However, their Perl was either
+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.
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. All of the complexity of the class is
+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()
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
+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
+"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.
=head2 Tying Scalars
A class implementing a tied scalar should define the following methods:
-TIESCALAR, FETCH, STORE, and possibly DESTROY.
+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:
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<lt>Jarkko.Hietaniemi@hut.fiE<gt>>'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.
+We'll use Jarkko Hietaniemi <F<jhi@iki.fi>>'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 strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
-=over
+=over 4
=item TIESCALAR classname, LIST
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. Since in this case
-we're just using a SCALAR ref for the tied scalar object, a simple $$self
+object representing the scalar we're dealing with. Because in this case
+we're using just a SCALAR ref for the tied scalar object, a simple $$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.
return $new_nicety;
}
+=item UNTIE this
+
+This method will be triggered when the C<untie> occurs. This can be useful
+if the class needs to know when no further calls will be made. (Except DESTROY
+of course.) See L<The C<untie> Gotcha> below for more details.
+
=item DESTROY this
This method will be triggered when the tied variable needs to be destructed.
-As with other object classes, such a method is seldom necessary, since Perl
+As with other object classes, such a method is seldom necessary, because Perl
deallocates its moribund object's memory for you automatically--this isn't
C++, you know. We'll use a DESTROY method here for debugging purposes only.
=back
That's about all there is to it. Actually, it's more than all there
-is to it, since we've done a few nice things here for the sake
+is to it, because we've done a few nice things here for the sake
of completeness, robustness, and general aesthetics. Simpler
TIESCALAR classes are certainly possible.
=head2 Tying Arrays
A class implementing a tied ordinary array should define the following
-methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.
+methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
-B<WARNING>: Tied arrays are I<incomplete>. They are also distinctly lacking
-something for the C<$#ARRAY> access (which is hard, as it's an lvalue), as
-well as the other obvious array functions, like push(), pop(), shift(),
-unshift(), and splice().
+FETCHSIZE and STORESIZE are used to provide C<$#array> and
+equivalent C<scalar(@array)> access.
-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. (Well, if you access an individual element; an
-aggregate assignment would be missed.) For example:
+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>.
- 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";
- }
+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 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. You may disable this feature by
+assigning a true value to the variable C<$NEGATIVE_INDICES> in the
+tied array class.
+
As you may have noticed, the name of the FETCH method (et al.) is the same
for all accesses, even though the constructors differ in names (TIESCALAR
vs TIEARRAY). While in theory you could have the same class servicing
several tied types, in practice this becomes cumbersome, and it's easiest
-to simply keep them at one tie type per class.
+to keep them at simply one tie type per class.
=item STORE this, index, value
This method will be triggered every time an element in the tied array is set
(written). It takes two arguments beyond its self reference: the index at
which we're trying to store something and the value we're trying to put
-there. For example:
+there.
+
+In our example, C<undef> is really C<$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->{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 L<The C<untie> Gotcha> below.)
+
=item DESTROY this
This method will be triggered when the tied variable needs to be destructed.
=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
-As the first Perl data type to be tied (see dbmopen()), associative arrays
-have the most complete and useful tie() implementation. A class
-implementing a tied associative array should define the following
-methods: TIEHASH is the constructor. FETCH and STORE access the key and
-value pairs. EXISTS reports whether a key is present in the hash, and
-DELETE deletes one. CLEAR empties the hash by deleting all the key and
-value pairs. FIRSTKEY and NEXTKEY implement the keys() and each()
-functions to iterate over all the keys. And DESTROY is called when the
-tied variable is garbage collected.
+Hashes were the first Perl data type to be tied (see dbmopen()). A class
+implementing a tied hash should define the following methods: TIEHASH is
+the constructor. FETCH and STORE access the key and value pairs. EXISTS
+reports whether a key is present in the hash, and DELETE deletes one.
+CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
+and NEXTKEY implement the keys() and each() functions to iterate over all
+the keys. UNTIE is called when C<untie> happens, and DESTROY is called when
+the tied variable is garbage collected.
-If this seems like a lot, then feel free to merely inherit
-from the standard Tie::Hash module for most of your methods, redefining only
-the interesting ones. See L<Tie::Hash> for details.
+If this seems like a lot, then feel free to inherit from merely 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,
and the key existing in the hash but having a corresponding value of
C<defined()> functions.
Here's an example of a somewhat interesting tied hash class: it gives you
-a hash representing a particular user's dotfiles. You index into the hash
-with the name of the file (minus the dot) and you get back that dotfile's
+a hash representing a particular user's dot files. You index into the hash
+with the name of the file (minus the dot) and you get back that dot file's
contents. For example:
use DotFiles;
$dot{login} =~ /MANPATH/ ||
$dot{cshrc} =~ /MANPATH/ )
{
- print "you seem to set your manpath\n";
+ print "you seem to set your MANPATH\n";
}
Or here's another sample of using our tied class:
=item HOME
-where those dotfiles live
+where those dot files live
=item CLOBBER
=item LIST
-the hash of dotfile names and content mappings
+the hash of dot file names and content mappings
=back
my $DEBUG = 0;
sub debug { $DEBUG = @_ ? shift : 1 }
-For our example, we want to able to emit debugging info to help in tracing
+For our example, we want to be able to emit debugging info to help in tracing
during development. We keep also one convenience function around
internally to help print out warnings; whowasi() returns the function name
that calls it.
Here are the methods for the DotFiles tied hash.
-=over
+=over 4
=item TIEHASH classname, LIST
It's probably worth mentioning that if you're going to filetest the
return values out of a readdir, you'd better prepend the directory
-in question. Otherwise, since we didn't chdir() there, it would
-have been testing the wrong file.
+in question. Otherwise, because we didn't chdir() there, it would
+have been testing the wrong file.
=item FETCH this, key
It was easy to write by having it call the Unix cat(1) command, but it
would probably be more portable to open the file manually (and somewhat
-more efficient). Of course, since dot files are a Unixy concept, we're
+more efficient). Of course, because dot files are a Unixy concept, we're
not that concerned.
=item STORE this, key, value
This method is triggered when the whole hash is to be cleared, usually by
assigning the empty list to it.
-In our example, that would remove all the user's dotfiles! It's such a
+In our example, that would remove all the user's dot files! It's such a
dangerous thing that they'll have to set CLOBBER to something higher than
1 to make it happen.
sub CLEAR {
carp &whowasi if $DEBUG;
my $self = shift;
- croak "@{[&whowasi]}: won't remove all dotfiles for $self->{USER}"
+ croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
unless $self->{CLOBBER} > 1;
my $dot;
foreach $dot ( keys %{$self->{LIST}}) {
useful if you're carrying about ordering or calling the iterator from more
than one sequence, or not really storing things in a hash anywhere.
-For our example, we're using a real hash so we'll just do the simple
-thing, but we'll have to indirect through the LIST field.
+For our example, we're using a real hash so we'll do just the simple
+thing, but we'll have to go through the LIST field indirectly.
sub NEXTKEY {
carp &whowasi if $DEBUG;
return each %{ $self->{LIST} }
}
+=item UNTIE this
+
+This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
+
=item DESTROY this
This method is triggered when a tied hash is about to go out of
=back
-Note that functions such as keys() and values() may return huge array
-values when used on large objects, like DBM files. You may prefer to
-use the each() function to iterate over such. Example:
+Note that functions such as keys() and values() may return huge lists
+when used on large objects, like DBM files. You may prefer to use the
+each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
This is partially implemented now.
-A class implementing a tied filehandle should define the following methods:
-TIEHANDLE, PRINT and/or READLINE, and possibly DESTROY.
+A class implementing a tied filehandle should define the following
+methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
+READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
+OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
+used on the handle.
It is especially useful when perl is embedded in some other program,
where output to STDOUT and STDERR may have to be redirected in some
package Shout;
-=over
+=over 4
=item TIEHANDLE classname, LIST
This is the constructor for the class. That means it is expected to
return a blessed reference of some sort. The reference can be used to
-hold some internal information. We won't use it in out example.
+hold some internal information.
sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
+=item WRITE this, LIST
+
+This method will be called when the handle is written to via the
+C<syswrite> function.
+
+ sub WRITE {
+ $r = shift;
+ my($buf,$len,$offset) = @_;
+ print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
+ }
+
=item PRINT this, LIST
-This method will be triggered every time the tied handle is printed to.
+This method will be triggered every time the tied handle is printed to
+with the C<print()> function.
Beyond its self reference it also expects the list that was passed to
the print function.
sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
+=item PRINTF this, LIST
+
+This method will be triggered every time the tied handle is printed to
+with the C<printf()> function.
+Beyond its self reference it also expects the format and list that was
+passed to the printf function.
+
+ sub PRINTF {
+ shift;
+ my $fmt = shift;
+ print sprintf($fmt, @_)."\n";
+ }
+
+=item READ this, LIST
+
+This method will be called when the handle is read from via the C<read>
+or C<sysread> functions.
+
+ sub READ {
+ my $self = shift;
+ my $bufref = \$_[0];
+ my(undef,$len,$offset) = @_;
+ print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
+ # add to $$bufref, set $len to number of characters read
+ $len;
+ }
+
=item READLINE this
-This method will be called when the handle is read from. The method
-should return undef when there is no more data.
+This method will be called when the handle is read from via <HANDLE>.
+The method should return undef when there is no more data.
+
+ sub READLINE { $r = shift; "READLINE called $$r times\n"; }
+
+=item GETC this
+
+This method will be called when the C<getc> function is called.
+
+ sub GETC { print "Don't GETC, Get Perl"; return "a"; }
+
+=item CLOSE this
+
+This method will be called when the handle is closed via the C<close>
+function.
- sub READLINE { $r = shift; "PRINT called $$r times\n"; }
+ sub CLOSE { print "CLOSE called.\n" }
+
+=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. See
+L<The C<untie> Gotcha> below.
=item DESTROY this
print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
print <FOO>;
+=head2 UNTIE this
+
+You can define for all tie types an UNTIE method that will be called
+at untie(). See L<The C<untie> Gotcha> below.
+
+=head2 The C<untie> Gotcha
+
+If you intend making use of the object returned from either tie() or
+tied(), and if the tie's target class defines a destructor, there is a
+subtle gotcha you I<must> guard against.
+
+As setup, consider this (admittedly rather contrived) example of a
+tie; all it does is use a file to keep a log of the values assigned to
+a scalar.
+
+ package Remember;
+
+ use strict;
+ use warnings;
+ use IO::File;
+
+ sub TIESCALAR {
+ my $class = shift;
+ my $filename = shift;
+ my $handle = new IO::File "> $filename"
+ or die "Cannot open $filename: $!\n";
+
+ print $handle "The Start\n";
+ bless {FH => $handle, Value => 0}, $class;
+ }
+
+ sub FETCH {
+ my $self = shift;
+ return $self->{Value};
+ }
+
+ sub STORE {
+ my $self = shift;
+ my $value = shift;
+ my $handle = $self->{FH};
+ print $handle "$value\n";
+ $self->{Value} = $value;
+ }
+
+ sub DESTROY {
+ my $self = shift;
+ my $handle = $self->{FH};
+ print $handle "The End\n";
+ close $handle;
+ }
+
+ 1;
+
+Here is an example that makes use of this tie:
+
+ use strict;
+ use Remember;
+
+ my $fred;
+ tie $fred, 'Remember', 'myfile.txt';
+ $fred = 1;
+ $fred = 4;
+ $fred = 5;
+ untie $fred;
+ system "cat myfile.txt";
+
+This is the output when it is executed:
+
+ The Start
+ 1
+ 4
+ 5
+ The End
+
+So far so good. Those of you who have been paying attention will have
+spotted that the tied object hasn't been used so far. So lets add an
+extra method to the Remember class to allow comments to be included in
+the file -- say, something like this:
+
+ sub comment {
+ my $self = shift;
+ my $text = shift;
+ my $handle = $self->{FH};
+ print $handle $text, "\n";
+ }
+
+And here is the previous example modified to use the C<comment> method
+(which requires the tied object):
+
+ use strict;
+ use Remember;
+
+ my ($fred, $x);
+ $x = tie $fred, 'Remember', 'myfile.txt';
+ $fred = 1;
+ $fred = 4;
+ comment $x "changing...";
+ $fred = 5;
+ untie $fred;
+ system "cat myfile.txt";
+
+When this code is executed there is no output. Here's why:
+
+When a variable is tied, it is associated with the object which is the
+return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
+object normally has only one reference, namely, the implicit reference
+from the tied variable. When untie() is called, that reference is
+destroyed. Then, as in the first example above, the object's
+destructor (DESTROY) is called, which is normal for objects that have
+no more valid references; and thus the file is closed.
+
+In the second example, however, we have stored another reference to
+the tied object in $x. That means that when untie() gets called
+there will still be a valid reference to the object in existence, so
+the destructor is not called at that time, and thus the file is not
+closed. The reason there is no output is because the file buffers
+have not been flushed to disk.
+
+Now that you know what the problem is, what can you do to avoid it?
+Prior to the introduction of the optional UNTIE method the only way
+was the good old C<-w> flag. Which will spot any instances where you call
+untie() and there are still valid references to the tied object. If
+the second script above this near the top C<use warnings 'untie'>
+or was run with the C<-w> flag, Perl prints this
+warning message:
+
+ untie attempted while 1 inner references still exist
+
+To get the script to work properly and silence the warning make sure
+there are no valid references to the tied object I<before> untie() is
+called:
+
+ undef $x;
+ untie $fred;
+
+Now that UNTIE exists the class designer can decide which parts of the
+class functionality are really associated with C<untie> and which with
+the object being destroyed. What makes sense for a given class depends
+on whether the inner references are being kept so that non-tie-related
+methods can be called on the object. But in most cases it probably makes
+sense to move the functionality that would have been in DESTROY to the UNTIE
+method.
+
+If the UNTIE method exists then the warning above does not occur. Instead the
+UNTIE method is passed the count of "extra" references and can issue its own
+warning if appropriate. e.g. to replicate the no UNTIE case this method can
+be used:
+
+ sub UNTIE
+ {
+ my ($obj,$count) = @_;
+ carp "untie attempted while $count inner references still exist" if $count;
+ }
+
=head1 SEE ALSO
See L<DB_File> or L<Config> for some interesting tie() implementations.
+A good starting point for many tie() implementations is with one of the
+modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
=head1 BUGS
-Tied arrays are I<incomplete>. They are also distinctly lacking something
-for the C<$#ARRAY> access (which is hard, as it's an lvalue), as well as
-the other obvious array functions, like push(), pop(), shift(), unshift(),
-and splice().
-
You cannot easily tie a multilevel data structure (such as a hash of
hashes) to a dbm file. The first problem is that all but GDBM and
Berkeley DB have size limitations, but beyond that, you also have problems
with how references are to be represented on disk. One experimental
-module that does attempt to partially address this need is the MLDBM
-module. Check your nearest CPAN site as described in L<perlmod> for
+module that does attempt to address this need partially is the MLDBM
+module. Check your nearest CPAN site as described in L<perlmodlib> for
source code to MLDBM.
+Tied filehandles are still incomplete. sysopen(), truncate(),
+flock(), fcntl(), stat() and -X can't currently be trapped.
+
=head1 AUTHOR
Tom Christiansen
-TIEHANDLE by Sven Verdoolaege E<lt>F<skimo@dns.ufsia.ac.be>E<gt>
+TIEHANDLE by Sven Verdoolaege <F<skimo@dns.ufsia.ac.be>> and Doug MacEachern <F<dougm@osf.org>>
+
+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