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 vari
ables.
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 indi
cate that they're called implicitly rather than explic
itly--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 con
structor 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 prop
erly 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
A class implementing a tied scalar should define the fol
lowing 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', $$;
And now whenever either of those variables is accessed,
its current system priority is retrieved and returned. If
those variables are set, then the process's priority is
changed!
We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource
class (not included) to access the PRIO_PROCESS, PRIO_MIN,
and PRIO_MAX constants from your system, as well as the
getpriority() and setpriority() system calls. Here's the
preamble of the class.
package Nice;
use Carp;
use BSD::Resource;
use strict;
$Nice::DEBUG = 0 unless defined $Nice::DEBUG;
TIESCALAR classname, LIST
This is the constructor for the class. That means it
is expected to return a blessed reference to a new
scalar (probably anonymous) that it's creating. For
example:
sub TIESCALAR {
my $class = shift;
my $pid = shift || $$; # 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 repre
senting 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 excep
tion) 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 refer
ence, 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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
return $new_nicety;
}
UNTIE this
This method will be triggered when the "untie" occurs.
This can be useful if the class needs to know when no
further calls will be made. (Except DESTROY of
course.) See below for more details.
DESTROY this
This method will be triggered when the tied variable
needs to be destructed. As with other object classes,
such a method is seldom necessary, because Perl deal
locates its moribund object's memory for you automati
cally--this isn't C++, you know. We'll use a DESTROY
method here for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
}
That's about all there is to it. Actually, it's more than
all there is to it, because we've done a few nice things
here for the sake of completeness, robustness, and general
aesthetics. Simpler TIESCALAR classes are certainly pos
sible.
Tying Arrays
A class implementing a tied ordinary array should define
the following methods: TIEARRAY, FETCH, STORE, FETCHSIZE,
STORESIZE and perhaps UNTIE and/or DESTROY.
FETCHSIZE and STORESIZE are used to provide "$#array" and
equivalent "scalar(@array)" access.
The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and
EXISTS are required if the perl operator with the corre
sponding (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
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 ele
ments are a fixed size at creation. If you try to create
an element 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
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 $elemsize = shift;
if ( @_ || $elemsize =~ /\D/ ) {
croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
}
return bless {
ELEMSIZE => $elemsize,
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 = 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
internally by calling FETCHSIZE before being passed to
FETCH.
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 servic
ing several tied types, in practice this becomes cum
bersome, 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 argu
ments beyond its self reference: the index at which
we're trying 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
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 asso
ciated 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 contain
ing "$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( $count );
}
EXISTS this, key
Verify that the element at index key exists in the
tied array this.
In our example, we will determine that if an element
consists of "$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:
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,
moving existing elements up to make room. For exam
ple:
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 val
ues 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 off_
set.
In our example, we'll use a little shortcut if there
is a LIST:
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 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
dbmopen()). 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.
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::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 exist
ing in the hash, and the key existing in the hash but hav
ing a corresponding value of "undef". The two possibili
ties 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.
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;
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 argu
ment 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 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 argu
ments 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 under
lying 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() func
tion. 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 what
ever 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 success
fully 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 exam
ple, 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} }
}
UNTIE this
This is called when "untie" occurs.
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 $DEBUG;
}
Note that functions such as keys() and values() may return
huge lists when used on large objects, like DBM files.
You may prefer to use the each() function to iterate over
such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($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.
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 han
dle 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; $$r++; print join($,,map(uc($_),@_)),$\ }
PRINTF this, LIST
This method will be triggered every time the tied han
dle is printed to with the "printf()" function.
Beyond its self reference it also expects the format
and list that was passed to the printf function.
sub PRINTF {
shift;
my $fmt = shift;
print sprintf($fmt, @_)."\n";
}
READ this, LIST
This method will be called when the handle is read
from via the "read" or "sysread" functions.
sub READ {
my $self = shift;
my $$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" }
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.
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().
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 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;
}
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 atten
tion 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 $text, "\n";
}
And here is the previous example modified to use the "com
ment" method (which requires the tied object):
use strict;
use Remember;
my ($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 ref
erence, namely, the implicit reference from the tied vari
able. 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? 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 mes
sage:
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 $fred;
Now that UNTIE exists the class designer can decide which
parts of the class functionality are really associated
with "untie" and which with the object being destroyed.
What makes sense for a given class depends on whether the
inner references are being kept so that non-tie-related
methods can be called on the object. But in most cases it
probably makes sense to move the functionality that would
have been in DESTROY to the UNTIE method.
If the UNTIE method exists then the warning above does not
occur. Instead the UNTIE method is passed the count of
"extra" references and can issue its own warning if appro
priate. e.g. to replicate the no UNTIE case this method
can be used:
sub UNTIE
{
my ($obj,$count) = @_;
carp "untie attempted while $count inner references still exist" if $count;
}
SEE ALSO
See the DB_File manpage or the Config manpage for some
interesting tie() implementations. A good starting point
for many tie() implementations is with one of the modules
the Tie::Scalar manpage, the Tie::Array manpage, the
Tie::Hash manpage, or the Tie::Handle manpage.
BUGS
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 refer
ences 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.
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>
Tying Arrays by Casey Tweten <crt@kiski.net>
2001-03-18 perl v5.6.1 PERLTIE(1)
