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 dbmo-
pen() to connect an on-disk database in the standard Unix
dbm(3x) format magically to a %HASH in their program. How-
ever, 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, access-
ing 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 implement-
ing 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 follow-
ing methods: TIESCALAR, FETCH, STORE, and possibly UNTIE
and/or 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', $$;
And now whenever either of those variables is accessed, its
current system priority is retrieved and returned. If those
variables are set, then the process's priority is changed!
We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource
class (not included) to access the PRIO_PROCESS, PRIO_MIN,
and PRIO_MAX constants from your system, as well as the get-
priority() and setpriority() system calls. Here's the
preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
TIESCALAR classname, LIST
This is the constructor for the class. That means it is
expected to return a blessed reference to a new scalar
(probably anonymous) that it's creating. For example:
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 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.
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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 any-
way.
FETCH this
This method will be triggered every time the tied vari-
able 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 vari-
able is set (assigned). Beyond its self reference, it
also expects one (and only one) argument--the new value
the user is trying to assign. Don't worry about return-
ing a value from STORE -- the semantic of assignment
returning the assigned value is implemented with FETCH.
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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
}
UNTIE this
This method will be triggered when the "untie" occurs.
This can be useful if the class needs to know when no
further calls will be made. (Except DESTROY of course.)
See "The "untie" Gotcha" below for more details.
DESTROY this
This method will be triggered when the tied variable
needs to be destructed. As with other object classes,
such a method is seldom necessary, because Perl deallo-
cates its moribund object's memory for you
automatically--this isn't C++, you know. We'll use a
DESTROY method here for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$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 possi-
ble.
Tying Arrays
A class implementing a tied ordinary array should define the
following methods: TIEARRAY, FETCH, STORE, FETCHSIZE,
STORESIZE and perhaps UNTIE and/or DESTROY.
FETCHSIZE and STORESIZE are used to provide $#array and
equivalent "scalar(@array)" access.
The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and
EXISTS 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 the first five of these in terms of the basic
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methods above. The default implementations of DELETE and
EXISTS in Tie::Array simply "croak".
In addition EXTEND will be called when perl would have pre-
extended allocation in a real array.
For this discussion, we'll implement an array whose elements
are a fixed size at creation. If you try to create an ele-
ment larger than the fixed size, you'll take an exception.
For example:
use FixedElem_Array;
tie @array, 'FixedElem_Array', 3;
$array[0] = 'cat'; # ok.
$array[1] = 'dogs'; # exception, length('dogs') > 3.
The preamble code for the class is as follows:
package FixedElem_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 "{ELEMSIZE}" field will
store the maximum element size allowed, and the
"{ARRAY}" field will hold the true ARRAY ref. If some-
one 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 $elemsize = shift;
if ( @_ || $elemsize =~ /\D/ ) {
croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
}
return bless {
ELEMSIZE => $elemsize,
ARRAY => [],
}, $class;
}
FETCH this, index
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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 = shift;
my $index = shift;
return $self->{ARRAY}->[$index];
}
If a negative array index is used to read from an array,
the index will be translated to a positive one inter-
nally by calling FETCHSIZE before being passed to FETCH.
You may disable this feature by assigning a true value
to the variable $NEGATIVE_INDICES in the tied array
class.
As you may have noticed, the name of the FETCH method
(et al.) is the same for all accesses, even though the
constructors differ in names (TIESCALAR vs TIEARRAY).
While in theory you could have the same class servicing
several tied types, in practice this becomes cumbersome,
and it's easiest to keep them at simply one tie type per
class.
STORE this, index, value
This method will be triggered every time an element in
the tied array is set (written). It takes two arguments
beyond its self reference: the index at which we're try-
ing to store something and the value we're trying to put
there.
In our example, "undef" is really "$self->{ELEMSIZE}"
number of spaces so we have a little more work to do
here:
sub STORE {
my $self = shift;
my( $index, $value ) = @_;
if ( length $value > $self->{ELEMSIZE} ) {
croak "length of $value is greater than $self->{ELEMSIZE}";
}
# fill in the blanks
$self->EXTEND( $index ) if $index > $self->FETCHSIZE();
# right justify to keep element size for smaller elements
$self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
}
Negative indexes are treated the same as with FETCH.
FETCHSIZE this
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Returns the total number of items in the tied array
associated with object this. (Equivalent to
"scalar(@array)"). For example:
sub FETCHSIZE {
my $self = shift;
return scalar @{$self->{ARRAY}};
}
STORESIZE this, count
Sets the total number of items in the tied array associ-
ated with object this to be count. If this makes the
array larger then class's mapping of "undef" should be
returned for new positions. If the array becomes smaller
then entries beyond count should be deleted.
In our example, 'undef' is really an element containing
"$self->{ELEMSIZE}" number of spaces. Observe:
sub STORESIZE {
my $self = shift;
my $count = shift;
if ( $count > $self->FETCHSIZE() ) {
foreach ( $count - $self->FETCHSIZE() .. $count ) {
$self->STORE( $_, '' );
}
} elsif ( $count < $self->FETCHSIZE() ) {
foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
$self->POP();
}
}
}
EXTEND this, count
Informative call that array is likely to grow to have
count entries. Can be used to optimize allocation. This
method need do nothing.
In our example, we want to make sure there are no blank
("undef") entries, so "EXTEND" will make use of
"STORESIZE" to fill elements as needed:
sub EXTEND {
my $self = shift;
my $count = shift;
$self->STORESIZE( $count );
}
EXISTS this, key
Verify that the element at index key exists in the tied
array this.
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In our example, we will determine that if an element
consists of "$self->{ELEMSIZE}" spaces only, it does not
exist:
sub EXISTS {
my $self = shift;
my $index = shift;
return 0 if ! defined $self->{ARRAY}->[$index] ||
$self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
return 1;
}
DELETE this, key
Delete the element at index key from the tied array
this.
In our example, a deleted item is "$self->{ELEMSIZE}"
spaces:
sub DELETE {
my $self = shift;
my $index = shift;
return $self->STORE( $index, '' );
}
CLEAR this
Clear (remove, delete, ...) all values from the tied
array associated with object this. For example:
sub CLEAR {
my $self = shift;
return $self->{ARRAY} = [];
}
PUSH this, LIST
Append elements of LIST to the array. For example:
sub PUSH {
my $self = shift;
my @list = @_;
my $last = $self->FETCHSIZE();
$self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
return $self->FETCHSIZE();
}
POP this
Remove last element of the array and return it. For
example:
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sub POP {
my $self = shift;
return pop @{$self->{ARRAY}};
}
SHIFT this
Remove the first element of the array (shifting other
elements down) and return it. For example:
sub SHIFT {
my $self = shift;
return shift @{$self->{ARRAY}};
}
UNSHIFT this, LIST
Insert LIST elements at the beginning of the array, mov-
ing existing elements up to make room. For example:
sub UNSHIFT {
my $self = shift;
my @list = @_;
my $size = scalar( @list );
# make room for our list
@{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
= @{$self->{ARRAY}};
$self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
}
SPLICE this, offset, length, LIST
Perform the equivalent of "splice" on the array.
offset is optional and defaults to zero, negative values
count back from the end of the array.
length is optional and defaults to rest of the array.
LIST may be empty.
Returns a list of the original length elements at
offset.
In our example, we'll use a little shortcut if there is
a LIST:
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sub SPLICE {
my $self = shift;
my $offset = shift || 0;
my $length = shift || $self->FETCHSIZE() - $offset;
my @list = ();
if ( @_ ) {
tie @list, __PACKAGE__, $self->{ELEMSIZE};
@list = @_;
}
return splice @{$self->{ARRAY}}, $offset, $length, @list;
}
UNTIE this
Will be called when "untie" happens. (See "The "untie"
Gotcha" below.)
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.
Tying Hashes
Hashes were the first Perl data type to be tied (see dbmo-
pen()). 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. SCALAR is
triggered when the tied hash is evaluated in scalar context.
UNTIE is called when "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 standard Tie::StdHash module for most of your
methods, redefining only the interesting ones. See
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 "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:
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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";
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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.
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 try-
ing 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";
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Another way to lay hands on a reference to the underly-
ing 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
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 hap-
pen.
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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:
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 %{ $self->{LIST} }
}
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SCALAR this
This is called when the hash is evaluated in scalar con-
text. In order to mimic the behaviour of untied hashes,
this method should return a false value when the tied
hash is considered empty. If this method does not exist,
perl will make some educated guesses and return true
when the hash is inside an iteration. If this isn't the
case, FIRSTKEY is called, and the result will be a false
value if FIRSTKEY returns the empty list, true other-
wise.
However, you should not blindly rely on perl always
doing the right thing. Particularly, perl will mistak-
enly return true when you clear the hash by repeatedly
calling DELETE until it is empty. You are therefore
advised to supply your own SCALAR method when you want
to be absolutely sure that your hash behaves nicely in
scalar context.
In our example we can just call "scalar" on the underly-
ing hash referenced by "$self->{LIST}":
sub SCALAR {
carp &whowasi if $DEBUG;
my $self = shift;
return scalar %{ $self->{LIST} }
}
UNTIE this
This is called when "untie" occurs. See "The "untie"
Gotcha" below.
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 $DEBUG;
}
Note that functions such as keys() and values() may return
huge lists when used on large objects, like DBM files. You
may prefer to use the each() function to iterate over such.
Example:
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# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($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, UNTIE and
DESTROY. The class can also provide: BINMODE, OPEN, EOF,
FILENO, SEEK, TELL - if the corresponding perl operators are
used on the handle.
When STDERR is tied, its PRINT method will be called to
issue warnings and error messages. This feature is tem-
porarily disabled during the call, which means you can use
"warn()" inside PRINT without starting a recursive loop.
And just like "__WARN__" and "__DIE__" handlers, STDERR's
PRINT method may be called to report parser errors, so the
caveats mentioned under "%SIG" in perlvar 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.
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";
}
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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; $$r++; print join($,,map(uc($_),@_)),$\ }
PRINTF this, LIST
This method will be triggered every time the tied handle
is printed to with the "printf()" function. Beyond its
self reference it also expects the format and list that
was passed to the printf function.
sub PRINTF {
shift;
my $fmt = shift;
print sprintf($fmt, @_);
}
READ this, LIST
This method will be called when the handle is read from
via the "read" or "sysread" functions.
sub READ {
my $self = shift;
my $bufref = \$_[0];
my(undef,$len,$offset) = @_;
print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
# add to $$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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UNTIE this
As with the other types of ties, this method will be
called when "untie" happens. It may be appropriate to
"auto CLOSE" when this occurs. See "The "untie" Gotcha"
below.
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>;
UNTIE this
You can define for all tie types an UNTIE method that will
be called at untie(). See "The "untie" Gotcha" below.
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) exam-
ple 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 warnings;
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;
}
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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:
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sub comment {
my $self = shift;
my $text = shift;
my $handle = $self->{FH};
print $handle $text, "\n";
}
And here is the previous example modified to use the "com-
ment" method (which requires the tied object):
use strict;
use Remember;
my ($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 refer-
ence, 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 (DES-
TROY) 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? Prior to the introduction of the optional UNTIE
method the only way was the good old "-w" flag. Which will
spot any instances where you call untie() and there are
still valid references to the tied object. If the second
script above this near the top "use warnings 'untie'" or was
run with the "-w" flag, Perl prints this warning message:
untie attempted while 1 inner references still exist
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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 $fred;
Now that UNTIE exists the class designer can decide which
parts of the class functionality are really associated with
"untie" and which with the object being destroyed. What
makes sense for a given class depends on whether the inner
references are being kept so that non-tie-related methods
can be called on the object. But in most cases it probably
makes sense to move the functionality that would have been
in DESTROY to the UNTIE method.
If the UNTIE method exists then the warning above does not
occur. Instead the UNTIE method is passed the count of
"extra" references and can issue its own warning if
appropriate. e.g. to replicate the no UNTIE case this method
can be used:
sub UNTIE
{
my ($obj,$count) = @_;
carp "untie attempted while $count inner references still exist" if $count;
}
SEE ALSO
See DB_File or Config for some interesting tie() implementa-
tions. A good starting point for many tie() implementations
is with one of the modules Tie::Scalar, Tie::Array,
Tie::Hash, or Tie::Handle.
BUGS
The bucket usage information provided by "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 exit-
ing the scope the arrays or the hashes are not restored.
Counting the number of entries in a hash via
"scalar(keys(%hash))" or "scalar(values(%hash)") is ineffi-
cient 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.
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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 is DBM::Deep. Check your
nearest CPAN site as described in perlmodlib for source
code. Note that despite its name, DBM::Deep does not use
dbm. Another earlier attempt at solving the problem is
MLDBM, which is also available on the CPAN, but which has
some fairly serious limitations.
Tied filehandles are still incomplete. sysopen(), trun-
cate(), flock(), fcntl(), stat() and -X can't currently be
trapped.
AUTHOR
Tom Christiansen
TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and
Doug MacEachern <dougm@osf.org>
UNTIE by Nick Ing-Simmons <nick@ing-simmons.net>
SCALAR by Tassilo von Parseval
<tassilo.von.parseval@rwth-aachen.de>
Tying Arrays by Casey West <casey@geeknest.com>
perl v5.8.8 2006-06-30 23
