NAME
Alter - Alter Ego Objects
Synopsis
package MyClass;
use Alter ego => {}; # Alter ego of type hash
# Put data in it
my $obj = \ do { my $o };
ego( $obj)->{a} = 1;
ego( $obj)->{b} = 2;
# Retrieve it again
print ego( $obj)->{ b}, "\n"; # prints 2
package OtherClass;
defined( ego $obj) or die; # dies, OtherClass hasn't set an alter ego
# Direct access to the corona of alter egos
my $crown = Alter::corona $obj;
Functions
Basic Functions
The functions described here accept a first argument named $obj. Despite
the name, $obj can be any reference, it doesn't have to be blessed
(though it usually will be). It is a fatal error if it is not a
reference or if the reference points to a read-only value.
"ego($obj)"
Retrieves the class-specific alter ego assigned to $obj by
"alter()" or by autovivification if that is enabled. If neither is
the case, an undefined value is returned. The class is the package
into which the call to "ego()" is compiled.
"alter($obj, $val)"
Assigns $val to the reference $obj as an alter ego for the
caller's class. The class is the package into which the call to
"alter" is compiled. Returns $obj (not the value assigned).
"Alter::corona( $obj)"
Direct access to the corona of alter ego's of $obj. The corona
is a hash keyed by class name in which the alter ego's of an object
are stored. Unlike "alter()" and "ego()", this function is not
caller-sensitive. Returns a reference to the corona hash, which is
created if necessary. This function is not exported, if needed it
must be called fully qualified.
"Alter::is_xs"
Returns a true value if the XS implementation of "Alter" is active,
false if the pure Perl fallback is in place.
Autovivification
You can set one of the types "SCALAR", "ARRAY", "HASH" or "GLOB" for
autovivification of the alter ego. This is done by specifying the type
in a "use" statement, as in
package MyClass;
use Alter 'ARRAY';
If the "ego()" function is later called from "MyClass" before an alter ego has been specified using "alter()", a new array reference will be created and returned. Autovivification happens only once per class and object. (You would have to delete the class entry from the object's corona to make it happen again.)
The type specification can also be a referece of the appropriate type, so "[]" can be used for "ARRAY" and "{}" for "HASH" (globrefs and scalar refs can also be used, but are less attractive).
Type specification can be combined with function imports. Thus
package MyClass;
use Alter ego => {};
imports the "ego()" function and specifies a hash tape for autovivification. With autovivification you will usually not need to import the "alter" function at all.
Specifying "NOAUTO" in place of a type specification switches autovivification off for the current class. This is also the default.
Serialization Support
Serialization is supported for human inspection in "Data::Dumper" style
and for disk storage and cloning in "Storable" style.
For "Data::Dumper" support "Alter" provides the class "Alter::Dumper" for use as a base class, which contains the single method "Dumper". "Dumper" returns a string that represents a hash in "Data::Dumper" format. The hash shows all alter egos that have been created for the object, keyed by class. An additional key "(body)" (which can't be a class name) holds the actual body of the object. Formatting- and other options of "Data::Dumper" will change the appearance of the dump string, with the exception of $Data::Dumper::Purity, which will always be 1. "Dumper" can also be imported from "Alter" directly.
Note that "eval()"-ing a dump string will not restore the object, but rather create a hash as described. Re-creation of an object is only available through "Storable".
For "Storable" support the class "Alter::Storable" is provided with the methods "STORABLE_freeze", "STORABLE_thaw" and "STORABLE_attach". The three functions are also exported by "Alter" Their interaction with "Storable" is described there.
Inheriting these methods allows "Storable"'s own functions "freeze()" and "thaw()" to save and restore an object's alter egos along with the actual object body. Other "Storable" functions, like "store", "nstore", "retrieve", etc. also become Alter-aware. There is one exception. "Storable::dclone" cannot be used on "Alter"-based objects. To clone an "Alter"-based object, "Storable::thaw(Storable::freeze($obj)" must be called explicitly.
Per default, both "Alter::Dumper" and "Alter::Storable" are made base classes of the current class (if necessary) by "use Alter". If the function "Dumper" is imported, or if "-dumper" is specified, "Alter::Dumper" is not made a base class. If any of the functions "STORABLE_freeze", "STORABLE_thaw" or "STORABLE_attach" is imported, or if "-storable" is specified, "Alter::Storable" is not made a base class.
Fallback Perl Implementation
"Alter" is properly an XS module and a suitable C compiler is required
to build it. If compilation isn't possible, the XS part is replaced with
a pure Perl implementation "Alter::AlterXS_in_perl". That happens
automatically at load time when loading the XS part fails. The boolean
function "Alter::is_xs" tells (in the obvious way) which implementation
is active. If, for some reason, you want to run the Perl fallback when
the XS version is available, set the environment variable
"PERL_ALTER_NO_XS" to a true value before "Alter" is loaded.
This fallback is not a full replacement for the XS implementation. Besides being markedly slower, it lacks key features in that it is not automatically garbage-collected and not thread-safe. Instead, "Alter::AlterXS_in_perl" provides a "CLONE" method for thread safety and a universal destructor "Alter::Destructor::DESTROY" for garbage collection. A class that uses the pure Perl implementation of "Alter" will obtain this destructor through inheritance (unless "-destroy" is specified with the "use" statement). So at the surface thread-safety and garbage-collection are retained. However, if you want to add your own destructor to a class, you must make sure that both (all) destructors are called as needed. Perl only calls the first one it meets on the @ISA tree and that's it.
Otherwise the fallback implementation works like the original. If compilation has problems, it should allow you to run test cases to help decide if it's worth trying. To make sure that production code doesn't inadvertently run with the Perl implementation
Alter::is_xs or die "XS implementation of Alter required";
can be used.
Exports
None by default, "alter()" and "ego()" upon request. Further available
are "STORABLE_freeze", "STORABLE_thaw" and "STORABLE_attach" as well as
"Dumper". ":all" imports all these functions.
Environment
The environment variable "PERL_ALTER_NO_XS" is inspected once at load
time to decide whether to load the XS version of "Alter" or the pure
Perl fallback. At run time it has no effect.
Description
The "Alter" module is meant to facilitate the creation of classes that
support black-box inheritance, which is to say that an "Alter" based
class can be a parent class for any other class, whether itself
"Alter" based or not. Inside-out classes also have that property.
"Alter" is thus an alternative to the inside-out technique of class
construction. In some respects, "Alter" objects are easier to handle.
Alter objects support the same data model as traditional Perl objects. To each class, an Alter object presents an arbitrary reference, the object's alter ego. The type of reference and how it is used are the entirely the class's business. In particular, the common practice of using a hash whose keys represent object fields still applies, only each class sees its individual hash.
"Alter" based objects are garbage-collected and thread-safe without additional measures.
"Alter" also supports "Data::Dumper" and "Storable" in a generic way, so that "Alter" based objects can be easily be viewed and made persistent (within the limitations of the respective modules).
"Alter" works by giving every object a class-specific alter ego, which can be any scalar, for its (the classe's) specific needs for data storage. The alter ego is set by the "alter()" function (or by autovivification), usually once per class and object at initialization time. It is retrieved by the "ego()" function in terms of which a class will define its accessors.
That works by magically (in the technical sense of "PERL_MAGIC_ext") assigning a hash keyed by classname, the corona, to every object that takes part in the game. The corona holds the individual alter ego's for each class. It is created when needed and stays with an object for its lifetime. It is subject to garbage collection when the object goes out of scope. Normally the corona is invisible to the user, but the "Alter::corona()" function (not exported) allows direct access if needed.
Example
The example first shows how a class "Name" is built from two classes
"First" and "Last" which implement the first and last names separately.
"First" treats its objects as hashes whereas "Last" uses them as arrays.
Nevertheless, the code in "Name" that joins the two classes via
subclassing is straightforward.
The second part of the example shows that "Alter" classes actually support black-box inheritance. Here, we use an object of class "IO::File" as the "carrier" object. This must be a globref to work. This object can be initialized to the class "Name", which in part sees it as a hash, in another part as an array. Methods of both classes now work on the object.
#!/usr/local/bin/perl
use strict; use warnings; $| = 1;
# Show that class Name works
my $prof = Name->new( qw( Albert Einstein));
print $prof->fname, "\n";
print $prof->lname, "\n";
print $prof->name, "\n";
# Share an object with a foreign class
{
package Named::Handle;
use base 'IO::File';
push our @ISA, qw( Name);
sub new {
my $class = shift;
my ( $file, $first, $last) = @_;
$class->IO::File::new( $file)->init( $first, $last);
}
sub init {
my $nh = shift;
$nh->Name::init( @_);
}
}
my $nh = Named::Handle->new( '/dev/null', 'Bit', 'Bucket');
print "okay, at eof\n" if $nh->eof; # IO::File methods work
print $nh->name, "\n"; # ...as do Name methods
exit;
#######################################################################
{
package First;
use Alter qw( alter ego);
sub new {
my $class = shift;
bless( \ my $o, $class)->init( @_);
}
sub init {
my $f = shift;
alter $f, { name => shift };
$f;
}
sub fname {
my $h = ego shift;
@_ ? $h->{ name} = shift : $h->{ name};
}
}
{
package Last;
use Alter qw( alter ego);
sub new {
my $class = shift;
bless( \ my $o, $class)->init( @_);
}
sub init {
my $l = shift;
alter $l, [ shift];
$l;
}
sub lname {
my $l = ego( shift);
@_ ? $l->[ 0] = shift : $l->[ 0];
}
}
{
package Name;
use base 'First';
use base 'Last';
sub init {
my $n = shift;
$n->First::init( shift);
$n->Last::init( shift);
}
sub name {
my $n = shift;
join ' ' => $n->fname, $n->lname;
}
}
__END__
Thanks
Thanks to Abigail who invented the inside-out technique, showhing what
the problem is with Perl inheritance and how it could be overcome with
just a little stroke of genius.
Thanks also to Jerry Hedden for making me aware of the possibilities of "ext" magic on which this implementation of "Alter" is built.
Author
Anno Siegel, <anno4000@zrz.tu-berlin.de>
COPYRIGHT AND LICENSE
Copyright (C) 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.