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Hash::Util::FieldHash(3Perl Programmers Reference GuidHash::Util::FieldHash(3)

       Hash::Util::FieldHash - Support for Inside-Out Classes

	 ### Create fieldhashes
	 use Hash::Util qw(fieldhash fieldhashes);

	 # Create a single field hash
	 fieldhash my %foo;

	 # Create three at once...
	 fieldhashes \ my(%foo, %bar, %baz);
	 # ...or any number
	 fieldhashes @hashrefs;

	 ### Create an idhash and register it for garbage collection
	 use Hash::Util::FieldHash qw(idhash register);
	 idhash my %name;
	 my $object = \ do { my $o };
	 # register the idhash for garbage collection with $object
	 register($object, \ %name);
	 # the following entry will be deleted when $object goes out of scope
	 $name{$object} = 'John Doe';

	 ### Register an ordinary hash for garbage collection
	 use Hash::Util::FieldHash qw(id register);
	 my %name;
	 my $object = \ do { my $o };
	 # register the hash %name for garbage collection of $object's id
	 register $object, \ %name;
	 # the following entry will be deleted when $object goes out of scope
	 $name{id $object} = 'John Doe';

       "Hash::Util::FieldHash" offers a number of functions in support of "The
       Inside-out Technique" of class construction.


	   Returns the reference address of a reference $obj.  If $obj is not
	   a reference, returns $obj.

	   This function is a stand-in replacement for Scalar::Util::refaddr,
	   that is, it returns the reference address of its argument as a
	   numeric value.  The only difference is that "refaddr()" returns
	   "undef" when given a non-reference while "id()" returns its
	   argument unchanged.

	   "id()" also uses a caching technique that makes it faster when the
	   id of an object is requested often, but slower if it is needed only
	   once or twice.

	       $obj = id_2obj($id)

	   If $id is the id of a registered object (see "register"), returns
	   the object, otherwise an undefined value.  For registered objects
	   this is the inverse function of "id()".

	       register($obj, @hashrefs)

	   In the first form, registers an object to work with for the
	   function "id_2obj()".  In the second form, it additionally marks
	   the given hashrefs down for garbage collection.  This means that
	   when the object goes out of scope, any entries in the given hashes
	   under the key of "id($obj)" will be deleted from the hashes.

	   It is a fatal error to register a non-reference $obj.  Any non-
	   hashrefs among the following arguments are silently ignored.

	   It is not an error to register the same object multiple times with
	   varying sets of hashrefs.  Any hashrefs that are not registered yet
	   will be added, others ignored.

	   Registry also implies thread support.  When a new thread is
	   created, all references are replaced with new ones, including all
	   objects.  If a hash uses the reference address of an object as a
	   key, that connection would be broken.  With a registered object,
	   its id will be updated in all hashes registered with it.

	       idhash my %hash

	   Makes an idhash from the argument, which must be a hash.

	   An idhash works like a normal hash, except that it stringifies a
	   reference used as a key differently.	 A reference is stringified as
	   if the "id()" function had been invoked on it, that is, its
	   reference address in decimal is used as the key.

	       idhashes \ my(%hash, %gnash, %trash)
	       idhashes \ @hashrefs

	   Creates many idhashes from its hashref arguments.  Returns those
	   arguments that could be converted or their number in scalar

	       fieldhash %hash;

	   Creates a single fieldhash.	The argument must be a hash.  Returns
	   a reference to the given hash if successful, otherwise nothing.

	   A fieldhash is, in short, an idhash with auto-registry.  When an
	   object (or, indeed, any reference) is used as a fieldhash key, the
	   fieldhash is automatically registered for garbage collection with
	   the object, as if "register $obj, \ %fieldhash" had been called.

	       fieldhashes @hashrefs;

	   Creates any number of field hashes.	Arguments must be hash
	   references.	Returns the converted hashrefs in list context, their
	   number in scalar context.

       A word on terminology:  I shall use the term field for a scalar piece
       of data that a class associates with an object.	Other terms that have
       been used for this concept are "object variable", "(object) property",
       "(object) attribute" and more.  Especially "attribute" has some
       currency among Perl programmer, but that clashes with the "attributes"
       pragma.	The term "field" also has some currency in this sense and
       doesn't seem to conflict with other Perl terminology.

       In Perl, an object is a blessed reference.  The standard way of
       associating data with an object is to store the data inside the
       object's body, that is, the piece of data pointed to by the reference.

       In consequence, if two or more classes want to access an object they
       must agree on the type of reference and also on the organization of
       data within the object body.  Failure to agree on the type results in
       immediate death when the wrong method tries to access an object.
       Failure to agree on data organization may lead to one class trampling
       over the data of another.

       This object model leads to a tight coupling between subclasses.	If one
       class wants to inherit from another (and both classes access object
       data), the classes must agree about implementation details.
       Inheritance can only be used among classes that are maintained
       together, in a single source or not.

       In particular, it is not possible to write general-purpose classes in
       this technique, classes that can advertise themselves as "Put me on
       your @ISA list and use my methods".  If the other class has different
       ideas about how the object body is used, there is trouble.

       For reference Name_hash in "Example 1" shows the standard
       implementation of a simple class "Name" in the well-known hash based
       way.  It also demonstrates the predictable failure to construct a
       common subclass "NamedFile" of "Name" and the class "IO::File" (whose
       objects must be globrefs).

       Thus, techniques are of interest that store object data not in the
       object body but some other place.

   The Inside-out Technique
       With inside-out classes, each class declares a (typically lexical) hash
       for each field it wants to use.	The reference address of an object is
       used as the hash key.  By definition, the reference address is unique
       to each object so this guarantees a place for each field that is
       private to the class and unique to each object.	See Name_id in
       "Example 1" for a simple example.

       In comparison to the standard implementation where the object is a hash
       and the fields correspond to hash keys, here the fields correspond to
       hashes, and the object determines the hash key.	Thus the hashes appear
       to be turned inside out.

       The body of an object is never examined by an inside-out class, only
       its reference address is used.  This allows for the body of an actual
       object to be anything at all while the object methods of the class
       still work as designed.	This is a key feature of inside-out classes.

   Problems of Inside-out
       Inside-out classes give us freedom of inheritance, but as usual there
       is a price.

       Most obviously, there is the necessity of retrieving the reference
       address of an object for each data access.  It's a minor inconvenience,
       but it does clutter the code.

       More important (and less obvious) is the necessity of garbage
       collection.  When a normal object dies, anything stored in the object
       body is garbage-collected by perl.  With inside-out objects, Perl knows
       nothing about the data stored in field hashes by a class, but these
       must be deleted when the object goes out of scope.  Thus the class must
       provide a "DESTROY" method to take care of that.

       In the presence of multiple classes it can be non-trivial to make sure
       that every relevant destructor is called for every object.  Perl calls
       the first one it finds on the inheritance tree (if any) and that's it.

       A related issue is thread-safety.  When a new thread is created, the
       Perl interpreter is cloned, which implies that all reference addresses
       in use will be replaced with new ones.  Thus, if a class tries to
       access a field of a cloned object its (cloned) data will still be
       stored under the now invalid reference address of the original in the
       parent thread.  A general "CLONE" method must be provided to re-
       establish the association.

       "Hash::Util::FieldHash" addresses these issues on several levels.

       The "id()" function is provided in addition to the existing
       "Scalar::Util::refaddr()".  Besides its short name it can be a little
       faster under some circumstances (and a bit slower under others).
       Benchmark if it matters.	 The working of "id()" also allows the use of
       the class name as a generic object as described further down.

       The "id()" function is incorporated in id hashes in the sense that it
       is called automatically on every key that is used with the hash.	 No
       explicit call is necessary.

       The problems of garbage collection and thread safety are both addressed
       by the function "register()".  It registers an object together with any
       number of hashes.  Registry means that when the object dies, an entry
       in any of the hashes under the reference address of this object will be
       deleted.	 This guarantees garbage collection in these hashes.  It also
       means that on thread cloning the object's entries in registered hashes
       will be replaced with updated entries whose key is the cloned object's
       reference address.  Thus the object-data association becomes thread-

       Object registry is best done when the object is initialized for use
       with a class.  That way, garbage collection and thread safety are
       established for every object and every field that is initialized.

       Finally, field hashes incorporate all these functions in one package.
       Besides automatically calling the "id()" function on every object used
       as a key, the object is registered with the field hash on first use.
       Classes based on field hashes are fully garbage-collected and thread
       safe without further measures.

   More Problems
       Another problem that occurs with inside-out classes is serialization.
       Since the object data is not in its usual place, standard routines like
       "Storable::freeze()", "Storable::thaw()" and "Data::Dumper::Dumper()"
       can't deal with it on their own.	 Both "Data::Dumper" and "Storable"
       provide the necessary hooks to make things work, but the functions or
       methods used by the hooks must be provided by each inside-out class.

       A general solution to the serialization problem would require another
       level of registry, one that that associates classes and fields.	So
       far, the functions of "Hash::Util::FieldHash" are unaware of any
       classes, which I consider a feature.  Therefore "Hash::Util::FieldHash"
       doesn't address the serialization problems.

   The Generic Object
       Classes based on the "id()" function (and hence classes based on
       "idhash()" and "fieldhash()") show a peculiar behavior in that the
       class name can be used like an object.  Specifically, methods that set
       or read data associated with an object continue to work as class
       methods, just as if the class name were an object, distinct from all
       other objects, with its own data.  This object may be called the
       generic object of the class.

       This works because field hashes respond to keys that are not references
       like a normal hash would and use the string offered as the hash key.
       Thus, if a method is called as a class method, the field hash is
       presented with the class name instead of an object and blithely uses it
       as a key.  Since the keys of real objects are decimal numbers, there is
       no conflict and the slot in the field hash can be used like any other.
       The "id()" function behaves correspondingly with respect to non-
       reference arguments.

       Two possible uses (besides ignoring the property) come to mind.	A
       singleton class could be implemented this using the generic object.  If
       necessary, an "init()" method could die or ignore calls with actual
       objects (references), so only the generic object will ever exist.

       Another use of the generic object would be as a template.  It is a
       convenient place to store class-specific defaults for various fields to
       be used in actual object initialization.

       Usually, the feature can be entirely ignored.  Calling object methods
       as class methods normally leads to an error and isn't used routinely
       anywhere.  It may be a problem that this error isn't indicated by a
       class with a generic object.

   How to use Field Hashes
       Traditionally, the definition of an inside-out class contains a bare
       block inside which a number of lexical hashes are declared and the
       basic accessor methods defined, usually through
       "Scalar::Util::refaddr".	 Further methods may be defined outside this
       block.  There has to be a DESTROY method and, for thread support, a
       CLONE method.

       When field hashes are used, the basic structure remains the same.  Each
       lexical hash will be made a field hash.	The call to "refaddr" can be
       omitted from the accessor methods.  DESTROY and CLONE methods are not

       If you have an existing inside-out class, simply making all hashes
       field hashes with no other change should make no difference.  Through
       the calls to "refaddr" or equivalent, the field hashes never get to see
       a reference and work like normal hashes.	 Your DESTROY (and CLONE)
       methods are still needed.

       To make the field hashes kick in, it is easiest to redefine "refaddr"

	   sub refaddr { shift }

       instead of importing it from "Scalar::Util".  It should now be possible
       to disable DESTROY and CLONE.  Note that while it isn't disabled,
       DESTROY will be called before the garbage collection of field hashes,
       so it will be invoked with a functional object and will continue to

       It is not desirable to import the functions "fieldhash" and/or
       "fieldhashes" into every class that is going to use them.  They are
       only used once to set up the class.  When the class is up and running,
       these functions serve no more purpose.

       If there are only a few field hashes to declare, it is simplest to

	   use Hash::Util::FieldHash;

       early and call the functions qualified:

	   Hash::Util::FieldHash::fieldhash my %foo;

       Otherwise, import the functions into a convenient package like "HUF"
       or, more general, "Aux"

	       package Aux;
	       use Hash::Util::FieldHash ':all';

       and call

	   Aux::fieldhash my %foo;

       as needed.

   Garbage-Collected Hashes
       Garbage collection in a field hash means that entries will
       "spontaneously" disappear when the object that created them disappears.
       That must be borne in mind, especially when looping over a field hash.
       If anything you do inside the loop could cause an object to go out of
       scope, a random key may be deleted from the hash you are looping over.
       That can throw the loop iterator, so it's best to cache a consistent
       snapshot of the keys and/or values and loop over that.  You will still
       have to check that a cached entry still exists when you get to it.

       Garbage collection can be confusing when keys are created in a field
       hash from normal scalars as well as references.	Once a reference is
       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 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.

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

       ·   "init()"

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

       ·   "first()"

	   Retrieve the first name

       ·   "last()"

	   Retrieve the last name

       ·   "name()"

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

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

       ·   "Name_hash"

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

       ·   "Name_id"

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

       ·   "Name_idhash"

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

       ·   "Name_id_reg"

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

       ·   "Name_idhash_reg"

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

       ·   "Name_fieldhash"

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

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

   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;

	       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;

	       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;

	       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;

	       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;

	       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;

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

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


       To exercise the various implementations the script below can be used.

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

       The script first verifies the function of the "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 "NamedFile" which is a common subclass of "Name" and the
       standard class "IO::File".  This puts inheritability to the test
       because objects of "IO::File" must be globrefs.	Objects of "NamedFile"
       should behave like a file opened for reading and also support the
       "name()" method.	 This class juncture works with exception of the
       "Name_hash" implementation, where object initialization fails because
       of the incompatibility of object bodies.

   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";

	   # 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);

       To make "Hash::Util::FieldHash" work, there were two changes to perl
       itself.	"PERL_MAGIC_uvar" was made avalaible for hashes, and weak
       references now call uvar "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.

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

       Upon the call, the "mg_obj" field will hold the hash key to be
       accessed.  Upon return, the "SV*" value in "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 "action" value from
       "hv_fetch_common", or -1 if the call is from "hv_delete_common".

       This is a template for a function suitable for the "uf_val" field in a
       "ufuncs" structure for this call.  The "uf_set" and "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
	       mg->mg_obj = newkey;
	       return 0;

   Weakrefs call uvar magic
       When a weak reference is stored in an "SV" that has "uvar" magic, "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.

   How field hashes work
       The three features of key hashes, key replacement, thread support, and
       garbage collection are supported by a data structure called the 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 "SV" that
       is itself magical ("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.

   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;

       "_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.	 "_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.

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

       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.

perl v5.10.1			  2009-07-07	      Hash::Util::FieldHash(3)

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