PERLTIE(1) PERLTIE(1)
perltie - how to hide an object class in a simple variable
tie VARIABLE, CLASSNAME, LIST
$object = tied VARIABLE
untie VARIABLE
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. All of 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, VARIABLE is the name of the variable to be enchanted.
CLASSNAME is the name of a class implementing objects of the correct
type. Any additional arguments in the 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 CLASSNAME. (You don't
actually have to return a reference to a right "type" (e.g., HASH or
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 use or require a module for
you--you need to do that explicitly yourself.
Tying Scalars [Toc] [Back]
A class implementing a tied scalar should define the following methods:
TIESCALAR, FETCH, STORE, and possibly 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:
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tie $his_speed, 'Nice', getppid();
tie $my_speed, 'Nice', $$;
And now whenever either of those variables is accessed, its current
system priority is retrieved and returned. If those variables are set,
then the process's priority is changed!
We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource class (not
included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
from your system, as well as the getpriority() and setpriority() system
calls. Here's the preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
TIESCALAR classname, LIST
This is the constructor for the class. That means it is expected to
return a blessed reference to a new scalar (probably anonymous) that
it's creating. For example:
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 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 $^W to see whether to emit a bit of
noise anyway.
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
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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.
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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
return $new_nicety;
}
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DESTROY this
This method will be triggered when the tied variable needs to be
destructed. As with other object classes, such a method is seldom
necessary, because Perl deallocates its moribund object's memory for
you automatically--this isn't C++, you know. We'll use a DESTROY
method here for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
}
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.
Tying Arrays [Toc] [Back]
A class implementing a tied ordinary array should define the following
methods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.
WARNING: Tied arrays are incomplete. They are also distinctly lacking
something for the $#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().
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:
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";
}
The preamble code for the class is as follows:
package Bounded_Array;
use Carp;
use strict;
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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 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 {BOUND} field will store the maximum bound allowed, and
the {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 => [],
}, $class;
}
FETCH this, index
This method will be triggered every time an individual element the
tied array is accessed (read). It takes one argument beyond its
self reference: the index whose value we're trying to fetch.
sub FETCH {
my($self,$idx) = @_;
if ($idx > $self->{BOUND}) {
confess "Array OOB: $idx > $self->{BOUND}";
}
return $self->{ARRAY}[$idx];
}
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.
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:
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sub STORE {
my($self, $idx, $value) = @_;
print "[STORE $value at $idx]\n" if _debug;
if ($idx > $self->{BOUND} ) {
confess "Array OOB: $idx > $self->{BOUND}";
}
return $self->{ARRAY}[$idx] = $value;
}
DESTROY this
This method will be triggered when the tied variable needs to be
destructed. As with the scalar tie class, this is almost never
needed in a language that does its own garbage collection, so this
time we'll just leave it out.
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
Tying Hashes [Toc] [Back]
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 the Tie::Hash manpage 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
undef. The two possibilities can be tested with the exists() and
defined() functions.
Here's an example of a somewhat interesting tied hash class: it gives
you 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
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dot file's contents. For example:
use DotFiles;
tie %dot, 'DotFiles';
if ( $dot{profile} =~ /MANPATH/ ||
$dot{login} =~ /MANPATH/ ||
$dot{cshrc} =~ /MANPATH/ )
{
print "you seem to set your MANPATH\n";
}
Or here's another sample of using our tied class:
tie %him, 'DotFiles', 'daemon';
foreach $f ( keys %him ) {
printf "daemon dot file %s is size %d\n",
$f, length $him{$f};
}
In our tied hash DotFiles example, we use a regular hash for the object
containing several important fields, of which only the {LIST} field will
be what the user thinks of as the real hash.
USER whose dot files this object represents
HOME where those dot files live
CLOBBER
whether we should try to change or remove those dot files
LIST the hash of dot file names and content mappings
Here's the start of Dotfiles.pm:
package DotFiles;
use Carp;
sub whowasi { (caller(1))[3] . '()' }
my $DEBUG = 0;
sub debug { $DEBUG = @_ ? shift : 1 }
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.
TIEHASH classname, LIST
This is the constructor for the class. That means it is expected to
return a blessed reference through which the new object (probably
but not necessarily an anonymous hash) will be accessed.
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Here's the constructor:
sub TIEHASH {
my $self = shift;
my $user = shift || $>;
my $dotdir = shift || '';
croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
$user = getpwuid($user) if $user =~ /^\d+$/;
my $dir = (getpwnam($user))[7]
|| croak "@{[&whowasi]}: no user $user";
$dir .= "/$dotdir" if $dotdir;
my $node = {
USER => $user,
HOME => $dir,
LIST => {},
CLOBBER => 0,
};
opendir(DIR, $dir)
|| croak "@{[&whowasi]}: can't opendir $dir: $!";
foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
$dot =~ s/^\.//;
$node->{LIST}{$dot} = undef;
}
closedir DIR;
return bless $node, $self;
}
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, because we didn't chdir() there, it would
have been testing the wrong file.
FETCH this, key
This method will be triggered every time an element in the tied hash
is accessed (read). It takes one argument beyond its self
reference: the key whose value we're trying to fetch.
Here's the fetch for our DotFiles example.
sub FETCH {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
my $dir = $self->{HOME};
my $file = "$dir/.$dot";
unless (exists $self->{LIST}->{$dot} || -f $file) {
carp "@{[&whowasi]}: no $dot file" if $DEBUG;
return undef;
}
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if (defined $self->{LIST}->{$dot}) {
return $self->{LIST}->{$dot};
} else {
return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
}
}
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, because dot files are a Unixy
concept, we're not that concerned.
STORE this, key, value
This method will be triggered every time an element in the tied hash
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.
Here in our DotFiles example, we'll be careful not to let them try
to overwrite the file unless they've called the clobber() method on
the original object reference returned by tie().
sub STORE {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
my $value = shift;
my $file = $self->{HOME} . "/.$dot";
my $user = $self->{USER};
croak "@{[&whowasi]}: $file not clobberable"
unless $self->{CLOBBER};
open(F, "> $file") || croak "can't open $file: $!";
print F $value;
close(F);
}
If they wanted to clobber something, they might say:
$ob = tie %daemon_dots, 'daemon';
$ob->clobber(1);
$daemon_dots{signature} = "A true daemon\n";
Another way to lay hands on a reference to the underlying object is
to use the tied() function, so they might alternately have set
clobber using:
tie %daemon_dots, 'daemon';
tied(%daemon_dots)->clobber(1);
The clobber method is simply:
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sub clobber {
my $self = shift;
$self->{CLOBBER} = @_ ? shift : 1;
}
DELETE this, key
This method is triggered when we remove an element from the hash,
typically by using the delete() function. Again, we'll be careful
to check whether they really want to clobber files.
sub DELETE {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
my $file = $self->{HOME} . "/.$dot";
croak "@{[&whowasi]}: won't remove file $file"
unless $self->{CLOBBER};
delete $self->{LIST}->{$dot};
my $success = unlink($file);
carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
$success;
}
The value returned by DELETE becomes the return value of the call to
delete(). If you want to emulate the normal behavior of delete(),
you should return whatever FETCH would have returned for this key.
In this example, we have chosen instead to return a value which
tells the caller whether the file was successfully deleted.
CLEAR this
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 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 dot files for $self->{USER}"
unless $self->{CLOBBER} > 1;
my $dot;
foreach $dot ( keys %{$self->{LIST}}) {
$self->DELETE($dot);
}
}
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EXISTS this, key
This method is triggered when the user uses the exists() function on
a particular hash. In our example, we'll look at the {LIST} hash
element for this:
sub EXISTS {
carp &whowasi if $DEBUG;
my $self = shift;
my $dot = shift;
return exists $self->{LIST}->{$dot};
}
FIRSTKEY this
This method will be triggered when the user is going to iterate
through the hash, such as via a keys() or each() call.
sub FIRSTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
my $a = keys %{$self->{LIST}}; # reset each() iterator
each %{$self->{LIST}}
}
NEXTKEY this, lastkey
This method gets triggered during a keys() or each() iteration. It
has a second argument which is the last key that had been accessed.
This is 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 do just the simple
thing, but we'll have to go through the LIST field indirectly.
sub NEXTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
return each %{ $self->{LIST} }
}
DESTROY this
This method is triggered when a tied hash is about to go out of
scope. You don't really need it unless you're trying to add
debugging or have auxiliary state to clean up. Here's a very simple
function:
sub DESTROY {
carp &whowasi if $DEBUG;
}
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Note that functions such as keys() and values() may return huge array
values when used on large objects, like DBM files. You may prefer to use
the each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
Tying FileHandles [Toc] [Back]
This is partially implemented now.
A class implementing a tied filehandle should define the following
methods: TIEHANDLE, at least one of PRINT, PRINTF, READLINE, GETC, or
READ, and possibly DESTROY.
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.
In our example we're going to create a shouting handle.
package Shout;
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.
sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
PRINT this, LIST
This method will be triggered every time the tied handle is printed
to with the print() function. Beyond its self reference it also
expects the list that was passed to the print function.
sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
PRINTF this, LIST
This method will be triggered every time the tied handle is printed
to with the printf() function. Beyond its self reference it also
expects the format and list that was passed to the printf function.
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sub PRINTF {
shift;
my $fmt = shift;
print sprintf($fmt, @_)."\n";
}
READ this LIST
This method will be called when the handle is read from via the read
or sysread functions.
sub READ {
$r = shift;
my($buf,$len,$offset) = @_;
print "READ called, \$buf=$buf, \$len=$len, \$offset=$offset";
}
READLINE this
This method will be called when the handle is read from via
<HANDLE>. The method should return undef when there is no more
data.
sub READLINE { $r = shift; "PRINT called $$r times\n"; }
GETC this
This method will be called when the getc function is called.
sub GETC { print "Don't GETC, Get Perl"; return "a"; }
DESTROY this
As with the other types of ties, this method will be called when the
tied handle is about to be destroyed. This is useful for debugging
and possibly cleaning up.
sub DESTROY { print "</shout>\n" }
Here's how to use our little example:
tie(*FOO,'Shout');
print FOO "hello\n";
$a = 4; $b = 6;
print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
print <FOO>;
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The 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 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;
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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 $text, "\n";
}
And here is the previous example modified to use the comment method
(which requires the tied object):
use strict;
use Remember;
my ($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.
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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?
Well, the good old -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 -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 before untie() is
called:
undef $x;
untie $fred;
See the DB_File manpage or the Config manpage for some interesting tie()
implementations.
Tied arrays are incomplete. They are also distinctly lacking something
for the $#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 address this need partially is
the MLDBM module. Check your nearest CPAN site as described in the
perlmodlib manpage for source code to MLDBM.
Tom Christiansen
TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and Doug MacEachern
<dougm@osf.org>
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