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. 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.
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:
tie $his_speed, 'Nice', getppid();
tie $my_speed, 'Nice', $$;
And now whenever either of those variables is accessed, its current system priority is retrieved and returned. If those variables are set, then the process's priority is changed!
We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource class (not included) to access the
PRIO_PROCESS,
PRIO_MIN, and
PRIO_MAX constants from your system, as well as the
getpriority() and
setpriority() system calls. Here's the preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 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.
$$self allows the method to get at the real value stored there. In our example below, that real value is the process
ID to which we've tied our variable.
sub FETCH {
my $self = shift;
confess "wrong type" unless ref $self;
croak "usage error" if @_;
my $nicety;
local($!) = 0;
$nicety = getpriority(PRIO_PROCESS, $$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.
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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
return $new_nicety;
}
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$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.
A class implementing a tied ordinary array should define the following methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps DESTROY.
FETCHSIZE and
STORESIZE are used to provide
$#array and equivalent scalar(@array) access. The methods
POP,
PUSH,
SHIFT,
UNSHIFT,
SPLICE are required if the perl operator with the corresponding (but lowercase) name is to operate on the tied array. The
Tie::Array class can be used as a base class to implement these in terms of the basic
five methods above.
In addition EXTEND will be called when perl would have pre-extended allocation in a real array.
This means that tied arrays are now complete. The example below needs upgrading to illustrate this. (The documentation in Tie::Array is more complete.)
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";
}
The preamble code for the class is as follows:
package Bounded_Array;
use Carp;
use strict;
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 => [],
}, $class;
}
sub FETCH {
my($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.
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;
}
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
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 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 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 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.
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.
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.
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;
}
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.
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:
sub clobber {
my $self = shift;
$self->{CLOBBER} = @_ ? shift : 1;
}
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.
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);
}
}
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};
}
keys() or
each() call.
sub FIRSTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
my $a = keys %{$self->{LIST}}; # reset each() iterator
each %{$self->{LIST}}
}
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} }
}
sub DESTROY {
carp &whowasi if $DEBUG;
}
Note that functions such as
keys() and
values() may return huge lists when used on large objects, like
DBM files. You may prefer to use the
each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
This is partially implemented now.
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 and 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;
sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
sub WRITE {
$r = shift;
my($buf,$len,$offset) = @_;
print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
}
sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
sub PRINTF {
shift;
my $fmt = shift;
print sprintf($fmt, @_)."\n";
}
sub READ {
$r = shift;
my($buf,$len,$offset) = @_;
print "READ called, \$buf=$buf, \$len=$len, \$offset=$offset";
}
sub READLINE { $r = shift; "PRINT called $$r times\n"; }
sub GETC { print "Don't GETC, Get Perl"; return "a"; }
sub CLOSE { print "CLOSE called.\n" }
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>;
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;
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.
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>
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.