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 $object} = 'John Doe';
Hash::Util::FieldHash
offers a number of functions in support of
The Inside-out Technique of class construction.
- id($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.
- $obj = id_2obj($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)
- 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 context.
- 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.
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.
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-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.
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 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.
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.
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 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
class.
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 safe.
These examples are realized in the code below, which could be copied to a file Example.pm.
- 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.
- use strict; use warnings; $| = 1;
- use Example;
- {
- package Name;
- use parent '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 parent 'Name';
- use parent '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__
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.
PERL_MAGIC_uvar
interface for hashesPERL_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;
- }
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.
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.
_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.