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
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
+We'll use Jarkko Hietaniemi <F<Jarkko.Hietaniemi@hut.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.
=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.
+As the first Perl data type to be tied (see dbmopen()), hashes have the
+most complete and useful tie() implementation. 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.
+And DESTROY is called when 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 interesting ones. See L<Tie::Hash> for details.
+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
+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
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 }
print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
print <FOO>;
+=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 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 $fred;
+ system "cat myfile.txt";
+
+When this code is executed there is no output. Here's why:
+
+When a variable is tied, it is associated with the object which is the
+return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
+object normally has only one reference, namely, the implicit reference
+from the tied variable. When untie() is called, that reference is
+destroyed. Then, as in the first example above, the object's
+destructor (DESTROY) is called, which is normal for objects that have
+no more valid references; and thus the file is closed.
+
+In the second example, however, we have stored another reference to
+the tied object in C<$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?
+Well, the good old C<-w> flag will spot any instances where you call
+untie() and there are still valid references to the tied object. If
+the second script above is 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;
+
=head1 SEE ALSO
See L<DB_File> or L<Config> for some interesting tie() implementations.
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>>
projects / p5sagit/p5-mst-13.2.git / blobdiff