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.
+argument--the new value the user is trying to assign. Don't worry about
+returning a value from STORE -- the semantic of assignment returning the
+assigned value is implemented with FETCH.
sub STORE {
my $self = shift;
unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
- return $new_nicety;
}
=item UNTIE this
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 keys. SCALAR is triggered when the tied hash is evaluated in scalar
+context. 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 inherit from merely the
return each %{ $self->{LIST} }
}
+=item SCALAR this
+
+This is called when the hash is evaluated in scalar context. In order
+to mimic the behaviour of untied hashes, this method should return a
+false value when the tied hash is considered empty. If this method does
+not exist, perl will make some educated guesses and return true when
+the hash is inside an iteration. If this isn't the case, FIRSTKEY is
+called, and the result will be a false value if FIRSTKEY returns the empty
+list, true otherwise.
+
+However, you should B<not> blindly rely on perl always doing the right
+thing. Particularly, perl will mistakenly return true when you clear the
+hash by repeatedly calling DELETE until it is empty. You are therefore
+advised to supply your own SCALAR method when you want to be absolutely
+sure that your hash behaves nicely in scalar context.
+
+In our example we can just call C<scalar> on the underlying hash
+referenced by C<$self-E<gt>{LIST}>:
+
+ sub SCALAR {
+ carp &whowasi if $DEBUG;
+ my $self = shift;
+ return scalar %{ $self->{LIST} }
+ }
+
=item UNTIE this
This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
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
-special way. See nvi and the Apache module for examples.
+When STDERR is tied, its PRINT method will be called to issue warnings
+and error messages. This feature is temporarily disabled during the call,
+which means you can use C<warn()> inside PRINT without starting a recursive
+loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT
+method may be called to report parser errors, so the caveats mentioned under
+L<perlvar/%SIG> apply.
+
+All of this is especially useful when perl is embedded in some other
+program, where output to STDOUT and STDERR may have to be redirected
+in some special way. See nvi and the Apache module for examples.
In our example we're going to create a shouting handle.
sub PRINTF {
shift;
my $fmt = shift;
- print sprintf($fmt, @_)."\n";
+ print sprintf($fmt, @_);
}
=item READ this, LIST
=head1 BUGS
+The bucket usage information provided by C<scalar(%hash)> is not
+available. What this means is that using %tied_hash in boolean
+context doesn't work right (currently this always tests false,
+regardless of whether the hash is empty or hash elements).
+
+Localizing tied arrays or hashes does not work. After exiting the
+scope the arrays or the hashes are not restored.
+
+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.
+
+Tied hash/array slices cause multiple FETCH/STORE pairs, there are no
+tie methods for slice operations.
+
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
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>>
+SCALAR by Tassilo von Parseval <F<tassilo.von.parseval@rwth-aachen.de>>
+
Tying Arrays by Casey West <F<casey@geeknest.com>>
projects / p5sagit/p5-mst-13.2.git / blobdiff