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PERLTIE
!!!PERLTIE
NAME
SYNOPSIS
DESCRIPTION
SEE ALSO
BUGS
AUTHOR
----
!!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 connect an on-disk database in the
standard Unix dbm(3x) format magically to a
%HASH in their program. However, their Perl was
either built with one particular dbm library or another, but
not both, and you couldn't extend this mechanism to other
packages or types of variables.


Now you can.


The ''tie()'' function binds a variable to a class
(package) that will provide the implementation for access
methods for that variable. Once this magic has been
performed, accessing a tied variable automatically triggers
method calls in the proper class. The complexity of the
class is hidden behind magic methods calls. The method names
are in ALL CAPS , which is a convention that
Perl uses to indicate that they're called implicitly rather
than explicitly--just like the ''BEGIN
()'' and ''END ()''
functions.


In the ''tie()'' call, VARIABLE is the name of
the variable to be enchanted. CLASSNAME is the name
of a class implementing objects of the correct type. Any
additional arguments in the LIST are passed to the
appropriate constructor method for that class--meaning
''TIESCALAR ()'',
''TIEARRAY ()'', ''TIEHASH
()'', or ''TIEHANDLE ()''. (Typically
these are arguments such as might be passed to the
''dbminit()'' function of C.) The object returned by the
``new'' method is also returned by the ''tie()''
function, which would be useful if you wanted to access
other methods in CLASSNAME. (You don't actually
have to return a reference to a right ``type'' (e.g.,
HASH or CLASSNAME) so long as it's a
properly blessed object.) You can also retrieve a reference
to the underlying object using the ''tied()''
function.


Unlike ''dbmopen()'', the ''tie()'' function will not
use or require a module for you--you need
to do that explicitly yourself.


__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''
''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
        unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
carp
        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 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
This time we've decided to blow up (raise an exception) if the renice fails--there's no place for us to return an error otherwise, and it's probably the right thing to do.


STORE this, value


This method will be triggered every time the tied variable
is set (assigned). Beyond its self reference, it also
expects one (and only one) argument--the new value the user
is trying to assign.


    sub STORE {
my $self = shift;
confess
        if ($new_nicety
        if ($new_nicety
        unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
confess


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 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


That's about all there is to it. Actually, it's more than
all there is to it, because we've done a few nice things
here for the sake of completeness, robustness, and general
aesthetics. Simpler TIESCALAR classes are
certainly possible.


__Tying Arrays__


A class implementing a tied ordinary array should define the
following methods: TIEARRAY ,
FETCH , STORE ,
FETCHSIZE , STORESIZE and
perhaps 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')
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


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-
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 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- number of spaces so we have a
little more work to do here:


    sub STORE {
my $self = shift;
my( $index, $value ) = @_;
if ( length $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-


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- number of spaces.
Observe:


    sub STORESIZE {
my $self  = shift;
my $count = shift;
if ( $count


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-


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- spaces only, it
does not exist:


    sub EXISTS {
my $self  = shift;
my $index = shift;
return 0 if ! defined $self-


DELETE this, key


Delete the element at index ''key'' from the tied array
''this''.


In our example, a deleted item is $self-{
ELEMSIZE }


    sub DELETE {
my $self  = shift;
my $index = shift;
return $self-


CLEAR this


Clear (remove, delete, ...) all values from the tied array
associated with object ''this''. For
example:


    sub CLEAR {
my $self = shift;
return $self-


PUSH this, LIST


Append elements of ''LIST'' to the array.
For example:


    sub PUSH {
my $self = shift;
my @list = @_;
my $last = $self-


POP this


Remove last element of the array and return it. For
example:


    sub POP {
my $self = shift;
return pop @{$self-


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-


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-


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-


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 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
Or here's another sample of using our tied class:


    tie %him, '!DotFiles', 'daemon';
foreach $f ( keys %him ) {
printf
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 $node = {
USER    =
        opendir(DIR, $dir)
croak
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
        unless (exists $self-
        if (defined $self-
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 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
        croak
        open(F,
If they wanted to clobber something, they might say:


    $ob = tie %daemon_dots, 'daemon';
$ob-
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)-
The clobber method is simply:


    sub clobber {
my $self = shift;
$self-


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
        my $self = shift;
my $dot = shift;
my $file = $self-
The value returned by DELETE becomes the return value of the call to ''delete()''. If you want to emulate the normal behavior of ''delete()'', you should return whatever FETCH would have returned for this key. In this example, we have chosen instead to return a value which tells the caller whether the file was successfully deleted.


CLEAR this


This method is triggered when the whole hash is to be
cleared, usually by assigning the empty list to
it.


In our example, that would remove all the user's dot files!
It's such a dangerous thing that they'll have to set
CLOBBER to something higher than 1 to make it
happen.


    sub CLEAR    {
carp


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


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


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


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


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),


__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


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


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


READLINE this


This method will be called when the handle is read from via
HANDLE


    sub READLINE { $r = shift;


GETC this


This method will be called when the getc function
is called.


    sub GETC { print


CLOSE this


This method will be called when the handle is closed via the
close function.


    sub CLOSE { print


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


Here's how to use our little example:


    tie(*FOO,'Shout');
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
        print $handle
    sub FETCH {
my $self = shift;
return $self-
    sub STORE {
my $self = shift;
my $value = shift;
my $handle = $self-
    sub DESTROY {
my $self = shift;
my $handle = $self-
    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
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-
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
When this code is executed there is no output. Here's why:


When a variable is tied, it is associated with the object
which is the return value of the TIESCALAR ,
TIEARRAY , or TIEHASH
function. This object normally has only one reference,
namely, the implicit reference from the tied variable. When
''untie()'' is called, that reference is destroyed. Then,
as in the first example above, the object's destructor (
DESTROY ) is called, which is normal for
objects that have no more valid references; and thus the
file is closed.


In the second example, however, we have stored another
reference to the tied object in $x. That means that
when ''untie()'' gets called there will still be a valid
reference to the object in existence, so the destructor is
not called at that time, and thus the file is not closed.
The reason there is no output is because the file buffers
have not been flushed to disk.


Now that you know what the problem is, what can you do to
avoid it? 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
!!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


You cannot easily tie a multilevel data structure (such as a
hash of hashes) to a dbm file. The first problem is that all
but GDBM and Berkeley DB have
size limitations, but beyond that, you also have problems
with how references are to be represented on disk. One
experimental module that does attempt to address this need
partially is the MLDBM module. Check your
nearest CPAN site as described in perlmodlib
for source code to MLDBM .


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''
''dougm@osf.org''''


UNTIE by Nick Ing-Simmons
nick@ing-simmons.net''''


Tying Arrays by Casey Tweten
crt@kiski.net''''
----
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Last edited on Tuesday, June 4, 2002 12:22:39 am by "perry"
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