Hash::Util::FieldHash(3Perl Programmers Reference GuHash::Util::FieldHash(3pm)
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.
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 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.
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.
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.
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
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"
{
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
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 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;
}
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 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";
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 available 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
*/
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.
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 v5.16.3 2013-03-04 Hash::Util::FieldHash(3pm)
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