AmigActive 2/AACD 2.iso/AACD/Programming/perlman/man/perltie.txt

Text File | 1999-09-09 | 25.1 KB | 651 lines

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 magically connect an on-disk database in the
       standard Unix dbm(3x) format 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.  All of 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(), or TIEHASH().  (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
       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 <[e-mail]>'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.  Since in
            this case we're just using 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;
                }

       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 ncessary, since 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, since 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, and perhaps
       DESTROY.

       WARNING: Tied arrays are incomplete.  They are also
       distinctly lacking something for the $#ARRAY access (which
       is hard, as it's an lvalue), as well as the other obvious
       array functions, like push(), pop(), shift(), unshift(),
       and splice().

       For this discussion, we'll implement an array whose
       indices are fixed at its creation.  If you try to access
       anything beyond those bounds, you'll take an exception.
       (Well, if you access an individual element; an aggregate
       assignment would be missed.) For example:

           require Bounded_Array;
           tie @ary, Bounded_Array, 2;
           $| = 1;
           for $i (0 .. 10) {
               print "setting index $i: ";
               $ary[$i] = 10 * $i;
               $ary[$i] = 10 * $i;
               print "value of elt $i now $ary[$i]\n";
           }

       The preamble code for the class is as follows:

           package Bounded_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
            {BOUND} field will store the maximum bound 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 $bound = shift;
                    confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
                        if @_ || $bound =~ /\D/;
                    return bless {
                        BOUND => $bound,
                        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($self,$idx) = @_;
                  if ($idx > $self->{BOUND}) {
                    confess "Array OOB: $idx > $self->{BOUND}";
                  }
                  return $self->{ARRAY}[$idx];
                }

            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 simply keep
            them at 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.  For example:

                sub STORE {
                  my($self, $idx, $value) = @_;
                  print "[STORE $value at $idx]\n" if _debug;
                  if ($idx > $self->{BOUND} ) {
                    confess "Array OOB: $idx > $self->{BOUND}";
                  }
                  return $self->{ARRAY}[$idx] = $value;
                }

       DESTROY this
            This method will be triggered when the tied variable
            needs to be destructed.  As with the sclar 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.

       The code we presented at the top of the tied array class
       accesses many elements of the array, far more than we've
       set the bounds to.  Therefore, it will blow up once they
       try to access beyond the 2nd element of @ary, as the
       following output demonstrates:

           setting index 0: value of elt 0 now 0
           setting index 1: value of elt 1 now 10
           setting index 2: value of elt 2 now 20
           setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
                   Bounded_Array::FETCH called at testba line 12


       Tying Hashes

       As the first Perl data type to be tied (see dbmopen()),
       associative arrays have the most complete and useful tie()
       implementation.  A class implementing a tied associative
       array 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.  And DESTROY is called when the
       tied variable is garbage collected.

       If this seems like a lot, then feel free to merely inherit
       from the standard Tie::Hash module for most of your
       methods, redefining only the interesting ones.  See the
       Tie::Hash manpage for details.

       Remember that Perl distinguishes between a key not
       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 dotfiles.  You index into the hash with the name of
       the file (minus the dot) and you get back that dotfile'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 dotfiles live

       CLOBBER
            whether we should try to change or remove those dot
            files

       LIST the hash of dotfile 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 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,
            since 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, since 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};
                    unlink($file) || carp "@{[&whowasi]}: can't unlink $file: $!";
                }

       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
            dotfiles!  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 dotfiles 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 our using a real hash so we'll
            just do the simple thing, but we'll have to indirect
            through the LIST field.

                sub NEXTKEY  {
                    carp &whowasi if $DEBUG;
                    my $self = shift;
                    return each %{ $self->{LIST} }
                }

       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 array values 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 isn't implemented yet.  Sorry; maybe someday.

SEE ALSO
       See the DB_File manpage or the Config manpage for some
       interesting tie() implementations.

BUGS
       Tied arrays are incomplete.  They are also distinctly
       lacking something for the $#ARRAY access (which is hard,
       as it's an lvalue), as well as the other obvious array
       functions, like push(), pop(), shift(), unshift(), and
       splice().

       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 partially address
       this need is the MLDBM module.  Check your nearest CPAN
       site as described in the perlmod manpage for source code
       to MLDBM.

AUTHOR
       Tom Christiansen