perltie(1)
NAME
perltie - 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 con-
nect 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 pro-
vide 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 "CLASS-
NAME". (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 mod-
ule for you--you need to do that explicitly yourself.
Tying Scalars
A class implementing a tied scalar should define the following 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 forgiv-
ing. 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.
Don't worry about returning 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;
$new_nicety = PRIO_MIN;
}
if ($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, $$self, $new_nicety)) {
confess "setpriority failed: $!";
}
}
UNTIE this
This method will be triggered when the "untie" occurs. This can be
useful if the class needs to know when no further calls will be
made. (Except DESTROY of course.) See "The "untie" Gotcha" below
for more details.
DESTROY this
This method will be triggered when the tied variable needs to be
destructed. As with other object classes, such a method is seldom
necessary, because Perl deallocates its moribund object's memory
for you automatically--this isn't C++, you know. We'll use a
DESTROY method here for debugging purposes only.
sub DESTROY {
my $self = shift;
confess "wrong type" unless ref $self;
carp "[ Nice::DESTROY pid $$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 com-
pleteness, 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 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 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 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 repre-
sent 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 (doubt-
less 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 FETCH-
SIZE before being passed to FETCH. You may disable this feature by
assigning a true value to the variable $NEGATIVE_INDICES in the
tied array class.
As you may have noticed, the name of the FETCH method (et al.) is
the same for all accesses, even though the constructors differ in
names (TIESCALAR vs TIEARRAY). While in theory you could have the
same class servicing several tied types, in practice this becomes
cumbersome, and it's easiest to keep them at simply one tie type
per class.
STORE this, index, value
This method will be triggered every time an element in the tied
array is set (written). It takes two arguments beyond its self
reference: the index at which we're trying to store something and
the value we're trying to put there.
In our example, "undef" is really "$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 associated 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( $count );
}
EXISTS this, key
Verify that the element at index key exists in the tied array this.
In our example, we will determine that if an element consists of
"$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 associ-
ated 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 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:
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 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. 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:
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 (proba-
bly 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 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
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 ref-
erence: 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
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, usu-
ally 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 some-
thing 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:
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 sim-
ple thing, but we'll have to go through the LIST field indirectly.
sub NEXTKEY {
carp &whowasi if $DEBUG;
my $self = shift;
return each %{ $self->{LIST} }
}
SCALAR this
This is called when the hash is evaluated in scalar context. In
order to mimic the behaviour of untied hashes, this method should
return a false value when the tied hash is considered empty. If
this method does not exist, perl will make some educated guesses
and return true when the hash is inside an iteration. If this isn't
the case, FIRSTKEY is called, and the result will be a false value
if FIRSTKEY returns the empty list, true otherwise.
However, you should not blindly rely on perl always doing the right
thing. Particularly, perl will mistakenly return true when you
clear the hash by repeatedly calling DELETE until it is empty. You
are therefore advised to supply your own SCALAR method when you
want to be absolutely sure that your hash behaves nicely in scalar
context.
In our example we can just call "scalar" on the underlying hash
referenced by "$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 debug-
ging 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.
When STDERR is tied, its PRINT method will be called to issue warnings
and error messages. This feature is temporarily 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";
}
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 <HAN-
DLE>. 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. 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 debug-
ging 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) 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 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 destruc-
tor (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 destruc-
tor 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
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() implementations. 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 avail-
able. 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 exiting 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 inefficient 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.
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 prob-
lems with how references are to be represented on disk. One experimen-
tal 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 ear-
lier 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(), truncate(), flock(),
fcntl(), stat() and -X can't currently be trapped.
AUTHOR
Tom Christiansen
TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and Doug MacEach-
ern <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 2006-06-14 PERLTIE(1)
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