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. 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 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. 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->