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>'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 representing the scalar we're dealing with. Because in this case we're using just a SCALAR ref for the tied scalar object, a simple $$self allows the method to get at the real value stored there. In our example below, that real value is the process ID to which we've tied our variable.

sub FETCH {
    my $self = shift;
    confess "wrong type" unless ref $self;
    croak "usage error" if @_;
    my $nicety;
    local($!) = 0;
    $nicety = getpriority(PRIO_PROCESS, $$self);
    if ($!) { croak "getpriority failed: $!" }
    return $nicety;
}

This time we've decided to blow up (raise an exception) if the renice fails--there's no place for us to return an error otherwise, and it's probably the right thing to do.

STORE this, value

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

sub STORE {
    my $self = shift;
    confess "wrong type" unless ref $self;
    my $new_nicety = shift;
    croak "usage error" if @_;

    if ($new_nicety < PRIO_MIN) {
        carp sprintf
          "WARNING: priority %d less than minimum system priority %d",
              $new_nicety, PRIO_MIN if $^W;
        $new_nicety = PRIO_MIN;
    }

    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 deallocates its moribund object's memory for you automatically--this isn't C++, you know. We'll use a DESTROY method here for debugging purposes only.

sub DESTROY {
    my $self = shift;
    confess "wrong type" unless ref $self;
    carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
}

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

Tying Arrays

A class implementing a tied ordinary array should define the following methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps 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 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 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 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 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 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 "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 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 reference: the index at which we're trying to store something, and the value we're trying to put there.

Here in our DotFiles example, we'll be careful not to let them try to overwrite the file unless they've called the clobber() method on the original object reference returned by tie().

    sub STORE {
	carp &whowasi if $DEBUG;
	my $self = shift;
	my $dot = shift;
	my $value = shift;
	my $file = $self->{HOME} . "/.$dot";
	my $user = $self->{USER};

	croak "@{[&whowasi]}: $file not clobberable"
	    unless $self->{CLOBBER};

	open(F, "> $file") || croak "can't open $file: $!";
	print F $value;
	close(F);
    }

If they wanted to clobber something, they might say:

$ob = tie %daemon_dots, 'daemon';
$ob->clobber(1);
$daemon_dots{signature} = "A true daemon\n";

Another way to lay hands on a reference to the underlying object is to use the tied() function, so they might alternately have set clobber using:

tie %daemon_dots, 'daemon';
tied(%daemon_dots)->clobber(1);

The clobber method is simply:

    sub clobber {
	my $self = shift;
	$self->{CLOBBER} = @_ ? shift : 1;
    }

DELETE this, key

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 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 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 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, @_)."\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 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 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 "untie attempted while $count inner references still exist" if $count;
}

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> and Doug MacEachern <dougm@osf.org>

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

Tying Arrays by Casey Tweten <crt@kiski.net>

To install lib, copy and paste the appropriate command in to your terminal.

cpanm

cpanm lib

CPAN shell

perl -MCPAN -e shell
install lib

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

SYNOPSIS

DESCRIPTION

Tying Scalars

Tying Arrays

Tying Hashes

Tying FileHandles

UNTIE this

The untie Gotcha

SEE ALSO

BUGS

AUTHOR

Module Install Instructions

The `untie` Gotcha