fieldhash.pm

source of perl for linux application,

Garbage collection can be confusing when keys are created in a field hash
from normal scalars as well as references.  Once a reference is I<used> with
a field hash, the entry will be collected, even if it was later overwritten
with a plain scalar key (every positive integer is a candidate).  This
is true even if the original entry was deleted in the meantime.  In fact,
deletion from a field hash, and also a test for existence constitute
I<use> in this sense and create a liability to delete the entry when
the reference goes out of scope.  If you happen to create an entry
with an identical key from a string or integer, that will be collected
instead.  Thus, mixed use of references and plain scalars as field hash
keys is not entirely supported.

=head1 EXAMPLES

The examples show a very simple class that implements a I<name>, consisting
of a first and last name (no middle initial).  The name class has four
methods:

=over

=item * C<init()>

An object method that initializes the first and last name to its
two arguments. If called as a class method, C<init()> creates an
object in the given class and initializes that.

=item * C<first()>

Retrieve the first name

=item * C<last()>

Retrieve the last name

=item * C<name()>

Retrieve the full name, the first and last name joined by a blank.

=back

The examples show this class implemented with different levels of
support by C<Hash::Util::FieldHash>.  All supported combinations
are shown.  The difference between implementations is often quite
small.  The implementations are:

=over

=item * C<Name_hash>

A conventional (not inside-out) implementation where an object is
a hash that stores the field values, without support by
C<Hash::Util::FieldHash>.  This implementation doesn't allow
arbitrary inheritance.

=item * C<Name_id>

Inside-out implementation based on the C<id()> function.  It needs
a C<DESTROY> method.  For thread support a C<CLONE> method (not shown)
would also be needed.  Instead of C<Hash::Util::FieldHash::id()> the
function C<Scalar::Util::refaddr> could be used with very little
functional difference.  This is the basic pattern of an inside-out
class.

=item * C<Name_idhash>

Idhash-based inside-out implementation.  Like L<Name_id> it needs
a C<DESTROY> method and would need C<CLONE> for thread support.

=item * C<Name_id_reg>

Inside-out implementation based on the C<id()> function with explicit
object registry.  No destructor is needed and objects are thread safe.

=item * C<Name_idhash_reg>

Idhash-based inside-out implementation with explicit object registry.
No destructor is needed and objects are thread safe.

=item * C<Name_fieldhash>

FieldHash-based inside-out implementation.  Object registry happens
automatically.  No destructor is needed and objects are thread safe.

=back

These examples are realized in the code below, which could be copied
to a file F<Example.pm>.

=head2 Example 1

    use strict; use warnings;

    {
        package Name_hash; # standard implementation: the object is a hash

        sub init {
            my $obj = shift;
            my ($first, $last) = @_;
            # create an object if called as class method
            $obj = bless {}, $obj unless ref $obj;
            $obj->{ first} = $first;
            $obj->{ last} = $last;
            $obj;
        }

        sub first { shift()->{ first} }
        sub last { shift()->{ last} }

        sub name {
            my $n = shift;
            join ' ' => $n->first, $n->last;
        }

    }

    {
        package Name_id;
        use Hash::Util::FieldHash qw(id);

        my (%first, %last);

        sub init {
            my $obj = shift;
            my ($first, $last) = @_;
            # create an object if called as class method
            $obj = bless \ my $o, $obj unless ref $obj;
            $first{ id $obj} = $first;
            $last{ id $obj} = $last;
            $obj;
        }

        sub first { $first{ id shift()} }
        sub last { $last{ id shift()} }

        sub name {
            my $n = shift;
            join ' ' => $n->first, $n->last;
        }

        sub DESTROY {
            my $id = id shift;
            delete $first{ $id};
            delete $last{ $id};
        }

    }

    {
        package Name_idhash;
        use Hash::Util::FieldHash;

        Hash::Util::FieldHash::idhashes( \ my (%first, %last) );

        sub init {
            my $obj = shift;
            my ($first, $last) = @_;
            # create an object if called as class method
            $obj = bless \ my $o, $obj unless ref $obj;
            $first{ $obj} = $first;
            $last{ $obj} = $last;
            $obj;
        }

        sub first { $first{ shift()} }
        sub last { $last{ shift()} }

        sub name {
            my $n = shift;
            join ' ' => $n->first, $n->last;
        }

        sub DESTROY {
            my $n = shift;
            delete $first{ $n};
            delete $last{ $n};
        }

    }

    {
        package Name_id_reg;
        use Hash::Util::FieldHash qw(id register);

        my (%first, %last);

        sub init {
            my $obj = shift;
            my ($first, $last) = @_;
            # create an object if called as class method
            $obj = bless \ my $o, $obj unless ref $obj;
            register( $obj, \ (%first, %last) );
            $first{ id $obj} = $first;
            $last{ id $obj} = $last;
            $obj;
        }

        sub first { $first{ id shift()} }
        sub last { $last{ id shift()} }

        sub name {
            my $n = shift;
            join ' ' => $n->first, $n->last;
        }
    }

    {
        package Name_idhash_reg;
        use Hash::Util::FieldHash qw(register);

        Hash::Util::FieldHash::idhashes \ my (%first, %last);

        sub init {
            my $obj = shift;
            my ($first, $last) = @_;
            # create an object if called as class method
            $obj = bless \ my $o, $obj unless ref $obj;
            register( $obj, \ (%first, %last) );
            $first{ $obj} = $first;
            $last{ $obj} = $last;
            $obj;
        }

        sub first { $first{ shift()} }
        sub last { $last{ shift()} }

        sub name {
            my $n = shift;
            join ' ' => $n->first, $n->last;
        }
    }

    {
        package Name_fieldhash;
        use Hash::Util::FieldHash;

        Hash::Util::FieldHash::fieldhashes \ my (%first, %last);

        sub init {
            my $obj = shift;
            my ($first, $last) = @_;
            # create an object if called as class method
            $obj = bless \ my $o, $obj unless ref $obj;
            $first{ $obj} = $first;
            $last{ $obj} = $last;
            $obj;
        }

        sub first { $first{ shift()} }
        sub last { $last{ shift()} }

        sub name {
            my $n = shift;
            join ' ' => $n->first, $n->last;
        }
    }

    1;

To exercise the various implementations the script L<below|/"Example 2"> can
be used.

It sets up a class C<Name> that is a mirror of one of the implementation
classes C<Name_hash>, C<Name_id>, ..., C<Name_fieldhash>.  That determines
which implementation is run.

The script first verifies the function of the C<Name> class.

In the second step, the free inheritability of the implementation
(or lack thereof) is demonstrated.  For this purpose it constructs
a class called C<NamedFile> which is a common subclass of C<Name> and
the standard class C<IO::File>.  This puts inheritability to the test
because objects of C<IO::File> I<must> be globrefs.  Objects of C<NamedFile>
should behave like a file opened for reading and also support the C<name()>
method.  This class juncture works with exception of the C<Name_hash>
implementation, where object initialization fails because of the
incompatibility of object bodies.

=head2 Example 2

    use strict; use warnings; $| = 1;

    use Example;

    {
        package Name;
        use base 'Name_id';      # define here which implementation to run
    }


    # Verify that the base package works
    my $n = Name->init(qw(Albert Einstein));
    print $n->name, "\n";
    print "\n";

    # Create a named file handle (See definition below)
    my $nf = NamedFile->init(qw(/tmp/x Filomena File));
    # use as a file handle...
    for ( 1 .. 3 ) {
        my $l = <$nf>;
        print "line $_: $l";
    }
    # ...and as a Name object
    print "...brought to you by ", $nf->name, "\n";
    exit;


    # Definition of NamedFile
    package NamedFile;
    use base 'Name';
    use base 'IO::File';

    sub init {
        my $obj = shift;
        my ($file, $first, $last) = @_;
        $obj = $obj->IO::File::new() unless ref $obj;
        $obj->open($file) or die "Can't read '$file': $!";
        $obj->Name::init($first, $last);
    }
    __END__


=head1 GUTS

To make C<Hash::Util::FieldHash> work, there were two changes to
F<perl> itself.  C<PERL_MAGIC_uvar> was made avalaible for hashes,
and weak references now call uvar C<get> magic after a weakref has been
cleared.  The first feature is used to make field hashes intercept
their keys upon access.  The second one triggers garbage collection.

=head2 The C<PERL_MAGIC_uvar> interface for hashes

C<PERL_MAGIC_uvar> I<get> magic is called from C<hv_fetch_common> and
C<hv_delete_common> through the function C<hv_magic_uvar_xkey>, which
defines the interface.  The call happens for hashes with "uvar" magic
if the C<ufuncs> structure has equal values in the C<uf_val> and C<uf_set>
fields.  Hashes are unaffected if (and as long as) these fields
hold different values.

Upon the call, the C<mg_obj> field will hold the hash key to be accessed.
Upon return, the C<SV*> value in C<mg_obj> will be used in place of the
original key in the hash access.  The integer index value in the first
parameter will be the C<action> value from C<hv_fetch_common>, or -1
if the call is from C<hv_delete_common>.

This is a template for a function suitable for the C<uf_val> field in
a C<ufuncs> structure for this call.  The C<uf_set> and C<uf_index>
fields are irrelevant.

    IV watch_key(pTHX_ IV action, SV* field) {
        MAGIC* mg = mg_find(field, PERL_MAGIC_uvar);
        SV* keysv = mg->mg_obj;
        /* Do whatever you need to.  If you decide to
           supply a different key newkey, return it like this
        */
        sv_2mortal(newkey);
        mg->mg_obj = newkey;
        return 0;
    }

=head2 Weakrefs call uvar magic

When a weak reference is stored in an C<SV> that has "uvar" magic, C<set>
magic is called after the reference has gone stale.  This hook can be
used to trigger further garbage-collection activities associated with
the referenced object.

=head2 How field hashes work

The three features of key hashes, I<key replacement>, I<thread support>,
and I<garbage collection> are supported by a data structure called
the I<object registry>.  This is a private hash where every object
is stored.  An "object" in this sense is any reference (blessed or
unblessed) that has been used as a field hash key.

The object registry keeps track of references that have been used as
field hash keys.  The keys are generated from the reference address
like in a field hash (though the registry isn't a field hash).  Each
value is a weak copy of the original reference, stored in an C<SV> that
is itself magical (C<PERL_MAGIC_uvar> again).  The magical structure
holds a list (another hash, really) of field hashes that the reference
has been used with.  When the weakref becomes stale, the magic is
activated and uses the list to delete the reference from all field
hashes it has been used with.  After that, the entry is removed from
the object registry itself.  Implicitly, that frees the magic structure
and the storage it has been using.

Whenever a reference is used as a field hash key, the object registry
is checked and a new entry is made if necessary.  The field hash is
then added to the list of fields this reference has used.

The object registry is also used to repair a field hash after thread
cloning.  Here, the entire object registry is processed.  For every
reference found there, the field hashes it has used are visited and
the entry is updated.

=head2 Internal function Hash::Util::FieldHash::_fieldhash

    # test if %hash is a field hash
    my $result = _fieldhash \ %hash, 0;

    # make %hash a field hash
    my $result = _fieldhash \ %hash, 1;

C<_fieldhash> is the internal function used to create field hashes.
It takes two arguments, a hashref and a mode.  If the mode is boolean
false, the hash is not changed but tested if it is a field hash.  If
the hash isn't a field hash the return value is boolean false.  If it
is, the return value indicates the mode of field hash.  When called with
a boolean true mode, it turns the given hash into a field hash of this
mode, returning the mode of the created field hash.  C<_fieldhash>
does not erase the given hash.

Currently there is only one type of field hash, and only the boolean
value of the mode makes a difference, but that may change.

=head1 AUTHOR

Anno Siegel (ANNO) wrote the xs code and the changes in perl proper
Jerry Hedden (JDHEDDEN) made it faster

=head1 COPYRIGHT AND LICENSE

Copyright (C) 2006-2007 by (Anno Siegel)

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself, either Perl version 5.8.7 or,
at your option, any later version of Perl 5 you may have available.

=cut