MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

PERLTIE

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

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 eitherbuilt with one particular dbm library or another, but not both, andyou 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 providethe implementation for access methods for that variable. Once this magichas been performed, accessing a tied variable automatically triggersmethod calls in the proper class. All of the complexity of the class ishidden behind magic methods calls. The method names are in ALL CAPS,which is a convention that Perl uses to indicate that they're calledimplicitly rather than explicitly---just like the BEGIN() and END()functions.

In the tie() call, VARIABLE is the name of the variable to beenchanted. CLASSNAME is the name of a class implementing objects ofthe correct type. Any additional arguments in the LIST are passed tothe appropriate constructor method for that class---meaning TIESCALAR(),TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are argumentssuch as might be passed to the dbminit() function of C.) The objectreturned by the ``new'' method is also returned by the tie() function,which would be useful if you wanted to access other methods inCLASSNAME. (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 blessedobject.) You can also retrieve a reference to the underlying objectusing the tied() function.

Unlike dbmopen(), the tie() function will not use or require a modulefor 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 forscalars 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 currentsystem priority is retrieved and returned. If those variables are set,then the process's priority is changed!

We'll use Jarkko Hietaniemi <jhiAATTiki.fi>'s BSD::Resource class (notincluded) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constantsfrom your system, as well as the getpriority() and setpriority() systemcalls. 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 isexpected 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 anexception if its constructor should fail. While this is how dbmopen() works,other classes may well not wish to be so forgiving. It checks the globalvariable $^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 theobject representing the scalar we're dealing with. Because in this casewe're using just a SCALAR ref for the tied scalar object, a simple $$selfallows the method to get at the real value stored there. In our examplebelow, 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 renicefails---there's no place for us to return an error otherwise, and it'sprobably 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 necessary, because Perldeallocates its moribund object's memory for you automatically---this isn'tC++, 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 thereis to it, because we've done a few nice things here for the sakeof completeness, robustness, and general aesthetics. SimplerTIESCALAR classes are certainly possible.

Tying Arrays

A class implementing a tied ordinary array should define the followingmethods: TIEARRAY, FETCH, STORE, and perhaps DESTROY.

WARNING: Tied arrays are incomplete. They are also distinctly lackingsomething for the $#ARRAY access (which is hard, as it's an lvalue), aswell 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 atits creation. If you try to access anything beyond those bounds, you'lltake an exception. (Well, if you access an individual element; anaggregate 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 toreturn a blessed reference through which the new array (probably ananonymous ARRAY ref) will be accessed.

In our example, just to show you that you don't really have to return anARRAY reference, we'll choose a HASH reference to represent our object.A HASH works out well as a generic record type: the {BOUND} field willstore the maximum bound allowed, and the {ARRAY} field will hold thetrue ARRAY ref. If someone outside the class tries to dereference theobject 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 arrayis accessed (read). It takes one argument beyond its self reference: theindex 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 samefor all accesses, even though the constructors differ in names (TIESCALARvs TIEARRAY). While in theory you could have the same class servicingseveral tied types, in practice this becomes cumbersome, and it's easiestto 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 atwhich we're trying to store something and the value we're trying to putthere. 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 scalar tie class, this is almost never needed in alanguage that does its own garbage collection, so this time we'lljust leave it out.

The code we presented at the top of the tied array class accesses manyelements 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, asthe 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()), hashes have themost complete and useful tie() implementation. A class implementing atied hash should define the following methods: TIEHASH is the constructor.FETCH and STORE access the key and value pairs. EXISTS reports whether akey is present in the hash, and DELETE deletes one. CLEAR empties thehash by deleting all the key and value pairs. FIRSTKEY and NEXTKEYimplement 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 inherit from merely thestandard Tie::Hash module for most of your methods, redefining only theinteresting 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 ofundef. The two possibilities can be tested with the exists() anddefined() functions.

Here's an example of a somewhat interesting tied hash class: it gives youa hash representing a particular user's dot files. You index into the hashwith the name of the file (minus the dot) and you get back that dot file'scontents. 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 regularhash for the object containing several importantfields, of which only the {LIST} field will be what theuser 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 tracingduring development. We keep also one convenience function aroundinternally to help print out warnings; whowasi() returns the function namethat 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 toreturn a blessed reference through which the new object (probably but notnecessarily 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 thereturn values out of a readdir, you'd better prepend the directoryin question. Otherwise, because we didn't chdir() there, it wouldhave been testing the wrong file.

FETCH this, key

This method will be triggered every time an element in the tied hash isaccessed (read). It takes one argument beyond its self reference: the keywhose 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 itwould probably be more portable to open the file manually (and somewhatmore efficient). Of course, because dot files are a Unixy concept, we'renot 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 atwhich we're trying to store something, and the value we're trying to putthere.

Here in our DotFiles example, we'll be careful not to letthem 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 touse the tied() function, so they might alternately have set clobberusing:

    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'llbe 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 callto 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 tellsthe caller whether the file was successfully deleted.

CLEAR this

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

In our example, that would remove all the user's dot files! It's such adangerous thing that they'll have to set CLOBBER to something higher than1 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() functionon 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 goingto 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 asecond argument which is the last key that had been accessed. This isuseful if you're carrying about ordering or calling the iterator from morethan 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 simplething, but we'll have to go through the LIST field indirectly.

    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 ofscope. You don't really need it unless you're trying to add debuggingor 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 arrayvalues when used on large objects, like DBM files. You may prefer touse 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 followingmethods: TIEHANDLE, at least one of PRINT, PRINTF, READLINE, GETC, or READ,and possibly DESTROY.

It is especially useful when perl is embedded in some other program,where output to STDOUT and STDERR may have to be redirected in somespecial 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 toreturn a blessed reference of some sort. The reference can be used tohold some internal information.

    sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }

PRINT this, LIST

This method will be triggered every time the tied handle is printed towith the print() function.Beyond its self reference it also expects the list that was passed tothe 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 towith the printf() function.Beyond its self reference it also expects the format and list that waspassed 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 reador sysread functions.

    sub READ {        $r = shift;        my($buf,$len,$offset) = @_;        print "READ called, \$buf=$buf, \$len=$len, \$offset=$offset";    }

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; "PRINT 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"; }

DESTROY this

As with the other types of ties, this method will be called when thetied handle is about to be destroyed. This is useful for debugging andpossibly 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>;

The `untie` Gotcha

If you intend making use of the object returned from either tie() ortied(), and if the tie's target class defines a destructor, there is asubtle gotcha you must guard against.

As setup, consider this (admittedly rather contrived) example of atie; all it does is use a file to keep a log of the values assigned toa scalar.

    package Remember;

    use strict;    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 havespotted that the tied object hasn't been used so far. So lets add anextra method to the Remember class to allow comments to be included inthe 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 thereturn value of the TIESCALAR, TIEARRAY, or TIEHASH function. Thisobject normally has only one reference, namely, the implicit referencefrom the tied variable. When untie() is called, that reference isdestroyed. Then, as in the first example above, the object'sdestructor (DESTROY) is called, which is normal for objects that haveno more valid references; and thus the file is closed.

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

Now that you know what the problem is, what can you do to avoid it?Well, the good old -w flag will spot any instances where you calluntie() and there are still valid references to the tied object. Ifthe second script above is run with the -w flag, Perl prints thiswarning message:

    untie attempted while 1 inner references still exist

To get the script to work properly and silence the warning make surethere are no valid references to the tied object before untie() iscalled:

    undef $x;    untie $fred;

BUGS

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

You cannot easily tie a multilevel data structure (such as a hash ofhashes) to a dbm file. The first problem is that all but GDBM andBerkeley DB have size limitations, but beyond that, you also have problemswith how references are to be represented on disk. One experimentalmodule that does attempt to address this need partially is the MLDBMmodule. Check your nearest CPAN site as described in the perlmodlib manpage forsource code to MLDBM.

AUTHOR

Tom Christiansen

TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and Doug MacEachern <dougmAATTosf.org>

This document was created byman2html,using the manual pages.

PERLTIE

NAME

SYNOPSIS

DESCRIPTION

Tying Scalars

Tying Arrays

Tying Hashes

Tying FileHandles

The untie Gotcha

SEE ALSO

BUGS

AUTHOR

Index

The `untie` Gotcha