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man : Hash::Util::FieldHash(3p)
Hash::Util::FieldPerl(Programmers RefereHash::Util::FieldHash(3p)
NAME
Hash::Util::FieldHash - Support for Inside-Out Classes
SYNOPSIS
### 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 $object} = 'John Doe';
FUNCTIONS
"Hash::Util::FieldHash" offers a number of functions in
support of "The Inside-out Technique" of class
construction.
id
id($obj)
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.
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id_2obj
$obj = id_2obj($id)
If $id is the id of a registered object (see
"register"), returns the object, otherwise an
undefined value. For registered objects this is the
inverse function of "id()".
register
register($obj)
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
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
idhashes \ my(%hash, %gnash, %trash)
idhashes \ @hashrefs
Creates many idhashes from its hashref arguments.
Returns those arguments that could be converted or
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their number in scalar context.
fieldhash
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
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.
DESCRIPTION
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.
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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.
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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.
Solutions
"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-
safe.
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.
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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
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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 necessary.
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" as
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 function.
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"
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{
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.
EXAMPLES
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:
o "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.
o "first()"
Retrieve the first name
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o "last()"
Retrieve the last name
o "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:
o "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.
o "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 class.
o "Name_idhash"
Idhash-based inside-out implementation. Like Name_id
it needs a "DESTROY" method and would need "CLONE" for
thread support.
o "Name_id_reg"
Inside-out implementation based on the "id()" function
with explicit object registry. No destructor is
needed and objects are thread safe.
o "Name_idhash_reg"
Idhash-based inside-out implementation with explicit
object registry. No destructor is needed and objects
are thread safe.
o "Name_fieldhash"
FieldHash-based inside-out implementation. Object
registry happens automatically. No destructor is
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needed and objects are thread safe.
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;
$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;
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}
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( $obj, \ (%first, %last) );
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$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;
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}
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 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)
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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__
GUTS
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.
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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;
}
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.
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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.
AUTHOR
Anno Siegel (ANNO) wrote the xs code and the changes in
perl proper Jerry Hedden (JDHEDDEN) made it faster
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.
perl v5.10.0 2008-09-30 16
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