PERLTIE(1) Perl Programmers Reference Guide PERLTIE(1)NAMEperltie - how to hide an object class in a simple variable
SYNOPSIS
tie VARIABLE, CLASSNAME, LIST
$object = tied VARIABLE
untie VARIABLE
DESCRIPTION
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.
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Tying Scalars
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', $$;
And now whenever either of those variables is accessed,
its current system priority is retrieved and returned. If
those variables are set, then the process's priority is
changed!
We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource
class (not included) to access the PRIO_PROCESS, PRIO_MIN,
and PRIO_MAX constants from your system, as well as the
getpriority() and setpriority() system calls. Here's the
preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
TIESCALAR classname, LIST
This is the constructor for the class. That means it
is expected to return a blessed reference to a new
scalar (probably anonymous) that it's creating. For
example:
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 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;
}
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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 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;
$new_nicety = PRIO_MIN;
}
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if ($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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
return $new_nicety;
}
DESTROY this
This method will be triggered when the tied variable
needs to be destructed. As with other object
classes, such a method is seldom necessary, because
Perl deallocates its moribund object's memory for you
automatically--this isn't C++, you know. We'll use a
DESTROY method here for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$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
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
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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;
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 => [],
}, $class;
}
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FETCH this, index
This method will be triggered every time an
individual element the tied array is accessed (read).
It takes one argument beyond its self reference: the
index whose value we're trying to fetch.
sub FETCH {
my($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:
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:
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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
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 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};
}
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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.
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;
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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;
}
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
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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:
sub clobber {
my $self = shift;
$self->{CLOBBER} = @_ ? shift : 1;
}
DELETE this, key
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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);
}
}
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:
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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 $DEBUG;
}
Note that functions such as keys() and values() may return
huge lists when used on large objects, like DBM files.
You may prefer to use the each() function to iterate over
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such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
Tying FileHandles
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;
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 }
WRITE this, LIST
This method will be called when the handle is written
to via the syswrite function.
sub WRITE {
$r = shift;
my($buf,$len,$offset) = @_;
print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
}
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
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that was passed to the print function.
sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
PRINTF this, LIST
This method will be triggered every time the tied
handle is printed to with the printf() function.
Beyond its self reference it also expects the format
and list that was passed to the printf function.
sub PRINTF {
shift;
my $fmt = shift;
print sprintf($fmt, @_)."\n";
}
READ this, LIST
This method will be called when the handle is read
from via the read or sysread functions.
sub READ {
my $self = shift;
my $$bufref = \$_[0];
my(undef,$len,$offset) = @_;
print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
# add to $$bufref, set $len to number of characters read
$len;
}
READLINE this
This method will be called when the handle is read
from via <HANDLE>. The method should return undef
when there is no more data.
sub READLINE { $r = shift; "READLINE called $$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"; }
CLOSE this
This method will be called when the handle is closed
via the close function.
sub CLOSE { print "CLOSE called.\n" }
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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>;
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};
}
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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 $text, "\n";
}
And here is the previous example modified to use the
comment method (which requires the tied object):
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use strict;
use Remember;
my ($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 ALSO
See the DB_File manpage or the Config manpage for some
interesting tie() implementations.
16/Sep/1999 perl 5.005, patch 03 17
PERLTIE(1) Perl Programmers Reference Guide PERLTIE(1)BUGS
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.
AUTHOR
Tom Christiansen
TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and
Doug MacEachern <dougm@osf.org>
16/Sep/1999 perl 5.005, patch 03 18
PERLTIE(1) Perl Programmers Reference Guide PERLTIE(1)16/Sep/1999 perl 5.005, patch 03 19
