NAME
Object::LocalVars - Outside-in objects with local aliasing of $self and
object variables
SYNOPSIS
package My::Object;
use strict;
use Object::LocalVars;
give_methods our $self; # this exact line is required
our $field1 : Prop;
our $field2 : Prop;
sub as_string : Method {
return "$self has properties '$field1' and '$field2'";
}
DESCRIPTION
Do not use for production purposes!
*This is a experimental module I developed when exploring inside-out
objects. It is no longer supported, but is left on CPAN as an example of
the kind of strange OO approaches that are possible with Perl.*
This module helps developers create "outside-in" objects. Properties
(and $self) are declared as package globals. Method calls are wrapped
such that these globals take on a local value that is correct for the
specific calling object and the duration of the method call. I.e. $self
is locally aliased to the calling object and properties are locally
aliased to the values of the properties for that object. The package
globals themselves only declare properties in the package and hold no
data themselves. Data are stored in a separate namespace for each
property, keyed off the reference memory addresses of the objects.
Outside-in objects are similar to "inside-out" objects, which store data
in a single lexical hash (or closure) for each property, which is keyed
off the reference memory addresses of the objects. Both differ from
classic Perl objects, which hold data for the object directly using a
blessed reference or closure to a data structure, typically a hash. For
both outside-in and inside-out objects, data are stored centrally and
the blessed reference is simply a key to look up the right data in the
central data store.
The use of package variables for outside-in objects allows for the use
of dynamic symbol table manipulation and aliasing. As a result,
Object::LocalVars delivers a variety of features -- though with some
corresponding drawbacks.
Features
* Provides $self automatically to methods without '"my $self = shift"'
and the like
* Provides dynamic aliasing of properties within methods -- methods
can access properties directly as variables without the overhead of
calls to accessors or mutators, eliminating the overhead of these
calls in methods
* Array and hash properties may be accessed via direct dereference of
simple variables, allowing developers to push, pop, splice, etc.
without the usual tortured syntax to dereference an accessor call
* Properties no longer require accessors to have compile time syntax
checking under strictures (i.e. '"use strict"'); 'public' properties
have accessors automatically provided as needed
* Uses attributes to mark properties and methods, but only in the
BEGIN phase so should be mod_perl friendly (though this has not been
tested yet)
* Provides attributes for public, protected and private properties,
class properties, and methods
* Orthogonality -- can subclass just about any other class, regardless
of implementation.
* Multiple inheritance supported in initializers and destructors
(though only one superclass can be of a special, orthogonal type)
* Minimally thread-safe -- objects are safely cloned across thread
boundaries (or a pseudo-fork on Win32)
* Achieves these features without source filtering
Drawbacks
* Method inefficiency -- wrappers around methods create extra overhead
on method calls
* Accessor inefficiency -- privacy checks and extra indirection
through package symbol tables create extra overhead (compared to
direct hash dereferencing of classic Perl objects)
* Minimal encapsulation -- data are hidden but still publically
accessible, unlike approaches that use lexicals or closures to
create strong encapsulation; (will be addressed in a future release)
* Does not support threads::shared -- objects existing before a new
thread is created will persist into the new thread, but changes in
an object cannot be reflected in the corresponding object in the
other thread
Design principles
*Simplicity*
Object::LocalVars was written to simplify writing classes in Perl by
removing the need for redundant and awkward code. E.g.:
sub foo {
my $self = shift; # e.g. repetitive
push @{$self->some_list}, "foo"; # e.g. awkward
}
Instead, Object::LocalVars uses a more elegant, readable and minimalist
approach:
our $some_list : Prop;
sub foo : Method {
push @$some_list, "foo";
}
As with Perl, "easy things should be easy; difficult things should be
possible" and there should be a smooth learning curve from one to the
other.
*Accessors and mutators*
A major objective of Object::LocalVars is a significant reduction in the
need for accessors (and mutators). In general, accessors break the OO
encapsulation paradigm by revealing or allowing changes to internal
object state. However, accessors are common in Perl for two big reasons:
* Accessors offer typo protection. Compare:
$self->{created}; # correct
$self->{craeted}; # typo
$self->craeted(); # typo, but caught at compile time
* Automatically generating accessors is easy
As a result, the proliferation of accessors opens up the class internals
unless additional protections are added to the accessors to make them
private.
With Object::LocalVars's aliasing, properties stay private by default
and don't *need* an accessor for typo safety. If protected or public
accessors are needed for subclasses or external code to check state,
these can be requested as needed.
Terminology
Object-oriented programming suffers from a plethora of terms used to
describe certain features and characteristics of classes and objects.
Perl further complicates this by using these or related terms for other
features entirely (e.g. attributes). (And Perl 6 swaps around these
definitions again.) Within this documentation, terms are used as
follows:
* *class* -- represents a model of associated states and behaviors in
terms of *properties* and *methods*; in Perl, a *class* is
represented by a "package"
* *object* -- represents a specific instance of a *class*; in Perl, an
*object* is represented by a reference to a data structure blessed
into a particular "package"
* *property* -- represents a particular state of a *class* or
*object*; *properties* which are common to all *objects* of a
*class* are referred to as *class properties*; *properties* which
can be unique to each *object* of a *class* are referred to as
*object properties*; in Object::LocalVars, *properties* are
represented by package variables marked with an appropriate
*attribute*
* *method* -- represents a behavior exhibited by a *class*; *methods*
which do not depend on *object properties* are referred to as *class
methods*; *methods* which depends on *object properties* are
referred to as *object methods*; in Object::LocalVars, *methods* are
represented by subroutines marked with an appropriate *attribute*
* *accessors* -- used generically to refer to both 'accessors' and
'mutators', *methods* which respectively read and change
*properties*.
* *attribute* -- code that modifies variable and subroutine
declarations; in Perl, *attributes* are separated from variable or
subroutine declarations with a colon (e.g. '"our $name : Prop"');
see perldoc for attributes for more details
USAGE
Getting Started
The most minimal usage of Object::LocalVars consists of importing it
with "use" and calling the "give_methods" routine:
use Object::Localvars;
give_methods our $self; # Required
This automatically imports attribute handlers to mark properties and
methods and imports several necessary, supporting subroutines that
provide basic class functionality such as object construction and
destruction. To support environments such as "mod_perl", which have no
"CHECK" or "INIT" phases, all attributes take effect during the "BEGIN"
phase when the module is compiled and executed. The "give_methods"
subroutine provides the run-time setup aspect of this and must always
appear as shown.
Declaring Object Properties
Properties are declared by specifying a package variable using the
keyword "our" and an appropriate attribute. There are several attributes
(and aliases for attributes) available which result in different degrees
of privacy and different resulting rules for creating accessors.
While properties are declared as an "our" variable, they are stored
elsewhere in a private package namespace. When methods are called, a
wrapper function temporarily *aliases* these package variables using
"local" to their proper class or object property values. This allows for
seamless access to properties, as if they were normal variables. For
example, dereferencing a list property:
our $favorites_list : Prop;
sub add_favorite : Method {
my $new_item = shift;
push @$favorites_list, $new_item;
}
Object::LocalVars provides the following attributes for object
properties:
* :Prop or :Priv
our $prop1 : Prop;
our $prop2 : Priv;
Either of these attributes declare a private property. Private
properties are aliased within methods, but no accessors are created.
This is the recommended default unless specific alternate
functionality is needed. Of course, developers are free to write
methods that act as accessors, and provide additional behavior such
as argument validation.
* :Prot
our $prop3 : Prot;
This attribute declares a protected property. Protected properties
are aliased within methods, and an accessor and mutator are created.
However, the accessor and mutator may only be called by the
declaring package or a subclass of it.
* :Pub
our $prop4 : Pub;
This attribute declares a public property. Public properties are
aliased within methods, and an accessor and mutator are created that
may be called from anywhere.
* :ReadOnly
our $prop5 : ReadOnly;
This attribute declares a readonly property. Readonly properties are
aliased within methods, and an accessor and mutator are created. The
accessor is public, but the mutator is protected.
Declaring Class Properties
Class properties work like object properties, but the value of a class
property is the same in all object or class methods.
Object::LocalVars provides the following attributes for class
properties:
* :Class or :ClassPriv
our $class1 : Class;
our $class2 : ClassPriv;
Either of these attributes declare a private class property. Private
class properties are aliased within methods, but no accessors are
created. This is the recommended default unless specific alternate
functionality is needed.
* :ClassProt
our $class3 : ClassProt;
This attribute declares a protected class property. Protected class
properties are aliased within methods, and an accessor and mutator
are created. However, the accessor and mutator may only be called by
the declaring package or a subclass of it.
* :ClassPub
our $class4 : ClassPub;
This attribute declares a public class property. Public class
properties are aliased within methods, and an accessor and mutator
are created that may be called from anywhere.
* :ClassReadOnly
our $class5 : ClassReadOnly;
This attribute declares a readonly class property. Readonly class
properties are aliased within methods, and an accessor and mutator
are created. The accessor is public, but the mutator is protected.
Declaring Methods
sub foo : Method {
my ($arg1, $arg2) = @_; # no need to shift $self
# $self and all properties automatically aliased
}
As with properties, methods are indicated by the addition of an
attribute to a subroutine declaration. When these marked subroutines are
called, a wrapper function ensures that $self and all properties are
aliased appropriately and passes only the remaining arguments to the
marked subroutine. Class properties are always aliased to the current
values of the class properties. If the method is called on an object,
all object properties are aliased to the state of that object. These
aliases are true aliases, not copies. Changes to the alias change the
underlying properties.
Object::LocalVars provides the following attributes for subroutines:
* :Method or :Pub
sub fcn1 : Method { }
sub fcn2 : Pub { }
Either of these attributes declare a public method. Public methods
may be called from anywhere. This is the recommended default unless
specific alternate functionality is needed.
* :Prot
sub fcn3 : Prot { }
This attribute declares a protected method. Protected methods may be
called only from the declaring package or a subclass of it.
* :Priv
sub fcn4 : Priv { }
This attribute declares a private method. Private methods may only
be called only from the declaring package. See "Hints and Tips" for
good style for calling private methods.
Accessors and Mutators
# property declarations
our $name : Pub; # :Pub creates an accessor and mutator
our $age : Pub;
# elsewhere in code
$obj->set_name( 'Fred' )->set_age( 23 );
print $obj->name;
Properties that are public or protected automatically have appropriate
accessors and mutators generated. By default, these use an Eiffel-style
syntax, e.g.: "$obj->x()" and "$obj->set_x()". Mutatators return the
calling object, allowing method chaining.
The prefixes for accessors and mutators may be altered using the
"accessor_style()" class method.
Constructors and Destructors
Object::LocalVars automatically provides the standard constructor,
"new", an initializer, "BUILDALL", and the standard destructor,
"DESTROY". Each calls a series of functions to manage initialization and
destruction within the inheritance model.
When "new" is called, a new blessed object is created. By default, this
object is an anonymous scalar. (See "CONFIGURATION OPTIONS" for how to
use another type of object as a base instead.)
After the object is created, "BUILDALL" is used to recursively
initialize superclasses using their "BUILDALL" methods. A user-defined
"PREBUILD" routine can modify the arguments passed to superclasses. The
object is then initialized using a user-defined "BUILD". (This approach
resembles the Perl6 object initialization model.)
A detailed program flow follows:
* Within "new": The name of the calling class is shifted off the
argument list
* Within "new": A reference to an anonymous scalar is blessed into the
calling class
* Within "new": "BUILDALL" is called as an object method on the
blessed reference with a copy of the arguments to "new"
* Within "BUILDALL": subroutine returns if initialization for the
current class has already been done for this object
* Within "BUILDALL": for each superclass listed in @ISA, if the
superclass can call "BUILDALL", then "PREBUILD" (if it exists) is
called with the name of the superclass and a copy of the remaining
argument list to "new". The superclass "BUILDALL" is then called as
an object method using the new blessed reference and the results of
the "PREBUILD". If "PREBUILD" does not exist, then any "BUILDALL" is
called with a copy of the arguments to "new".
* Within "BUILDALL": if a "BUILD" method exists, it is called as a
method using a copy of the arguments to "new"
During object destruction, the process works in reverse. In "DESTROY",
user-defined cleanup for the object's class is handled with "DEMOLISH"
(if it exists). Then, memory for object properties is freed. Finally,
"DESTROY" is called for each superclass in @ISA which can do "DESTROY".
Both "BUILDALL" and "DESTROY" handle "diamond" inheritance patterns
appropriately. Initialization and destruction will only be done once for
each superclass for any given object.
Hints and Tips
*Calling private methods*
Good style for private method calling in traditional Perl
object-oriented programming is to call private methods directly,
"foo($self,@args)", rather than with method lookup, "$self->foo(@args)".
This avoids unintentionally calling a subclass method of the same name
if a subclass happens to provide one.
*Avoiding hidden internal data*
For a package using Object::LocalVars, e.g. "My::Package", object
properties are stored in "My::Package::DATA", class properties are
stored in "My::Package::CLASSDATA", methods are stored in
"My::Package::METHODS", and objects are tracked for cloning in
"My::Package::TRACKER". Do not access these areas directly or overwrite
them with other global data or unexpected results are guaranteed to
occur.
(In a future release of this module, this storage approach should be
replaced by fully-encapsulated anonymous symbol tables.)
METHODS TO BE WRITTEN BY A DEVELOPER
"PREBUILD()"
sub PREBUILD {
my ($superclass, @args) = @_;
# filter @args in some way
return @args;
}
This subroutine may be written to filter arguments given to "BUILDALL"
before passing them to a superclass "BUILDALL". *This must not be tagged
with a ":Method" attribute* or equivalent as it is called before the
object is fully initialized. The primary purpose of this subroutine is
to strip out any arguments that would cause the superclass initializer
to die and/or to add any default arguments that should always be passed
to the superclass.
"BUILD()"
# Assuming our $counter : Class;
sub BUILD : Method {
my %init = ( %defaults, @_ );
$prop1 = $init{prop1};
$counter++;
}
This method may be written to initialize the object after it is created.
If available, it is called at the end of "BUILDALL". The @_ array
contains the original array passed to "BUILDALL".
"DEMOLISH()"
# Assume our $counter : Class;
sub DEMOLISH : Method {
$counter--;
}
This method may be defined to provide some cleanup actions when the
object goes out of scope and is destroyed. If available, it is called at
the start of the destructor (i.e "DESTROY").
METHODS AUTOMATICALLY EXPORTED
These methods will be automatically exported for use. This export can be
prevented by passing the method name preceded by a "!" in a list after
the call to "use Object::LocalVars". E.g.:
use Object::LocalVars qw( !new );
This is generally not needed and is strongly discouraged, but is
available should developers need some very customized behavior in "new"
or "DESTROY" that can't be achieved with "BUILD" and "DEMOLISH".
"give_methods()"
give_methods our $self;
Installs wrappers around all subroutines tagged as methods. This
function (and the declaration of "our $self") *must* be used in all
classes built with Object::LocalVars. It should only be called once for
any class.
"new()"
my $obj = Some::Class->new( @arguments );
The constructor. Classes built with Object::LocalVars have this
available by default and do not need their own constructor.
"caller()"
my $caller = caller(0);
This subroutine is exported automatically and emulates the built-in
"caller" with the exception that if the caller is Object::LocalVars
(i.e. from a wrapper function), it will continue to look upward in the
calling stack until the first non-Object::LocalVars package is found.
"BUILDALL()"
The initializer. It is initially called by "new" and then recursively
calls "BUILDALL" for all superclasses. Arguments for superclass
initialization are filtered through "PREBUILD". It should not be called
by users.
"CLONE()"
When threads are used, this subroutine is called by perl when a new
thread is created to ensure objects are properly cloned to the new
thread. Users shouldn't call this function directly and it must not be
overridden.
"DESTROY()"
A destructor. This is not used within Object::LocalVars directly but is
exported automatically when Object::LocalVars is imported. "DESTROY"
calls "DEMOLISH" (if it exists), frees object property memory, and then
calls "DESTROY" for every superclass in @ISA. It should not be called by
users.
CONFIGURATION OPTIONS
"accessor_style()"
package My::Class;
use Object::LocalVars;
BEGIN {
Object::LocalVars->accessor_style( {
get => 'get_',
set => 'set_'
});
}
This class method changes the prefixes for accessors and mutators. When
called from within a "BEGIN" block before properties are declared, it
will change the style of all properties subsequently declared. It takes
as an argument a hash reference with either or both of the keys 'get'
and 'set' with the values indicating the accessor/mutator prefix to be
used.
If the prefix is the same for both, an combined accessor/mutator will be
created that sets the value of the property if an argument is passed and
always returns the value of the property. E.g.:
package My::Class;
use Object::LocalVars;
BEGIN {
Object::LocalVars->accessor_style( {
get => q{},
set => q{}
});
}
our $age : Pub;
# elsewhere
$obj->age( $obj->age() + 1 ); # increment age by 1
Combined accessor/mutators are treated as mutators for the
interpretation of privacy settings.
"base_object()"
package My::Class;
use Object::LocalVars;
Object::LocalVars->base_object( 'Another::Class' );
give_methods our $self;
This class method changes the basic blessed object type for the calling
package from being an anonymous scalar to a fully-fledged object of the
given type. This allows classes build with Object::LocalVars to subclass
any type of class, regardless of its underlying implementation (e.g. a
hash) -- though only a single class can be subclassed in such a manner.
"PREBUILD" (if it exists) is called on the arguments to "new" before
generating the base object using its constructor. The object is then
re-blessed into the proper class. Other initializers are run as normal
based on @ISA, but the base class is not intialized again.
If the given base class does not already exist in @ISA, it is imported
with "require" and pushed onto the @ISA stack, similar to the pragma
base.
BENCHMARKING
Forthcoming. In short, Object::LocalVars can be faster than traditional
approaches if the ratio of property access within methods is high
relative to number of method calls. It is slower than traditional
approaches if there are many method calls that individually do little
property access. In general, Object::LocalVars trades off coding
elegance and clarity for speed of execution.
SEE ALSO
These other modules provide similiar functionality and/or inspired this
one. Quotes are from their respective documentations.
* Attribute::Property -- "easy lvalue accessors with validation"; uses
attributes to mark object properties for accessors; validates lvalue
usage with a hidden tie
* Class::Std -- "provides tools that help to implement the 'inside out
object' class structure"; based on the book *Perl Best Practices*;
nice support for multiple-inheritance and operator overloading
* Lexical::Attributes -- "uses a source filter to hide the details of
the Inside-Out technique from the user"; API based on Perl6 syntax;
provides $self automatically to methods
* Spiffy -- "combines the best parts of Exporter.pm, base.pm, mixin.pm
and SUPER.pm into one magic foundation class"; "borrows ideas from
other OO languages like Python, Ruby, Java and Perl 6"; optionally
uses source filtering to provide $self automatically to methods
INSTALLATION
The following commands will build, test, and install this module:
perl Build.PL
perl Build
perl Build test
perl Build install
BUGS
Please report bugs using the CPAN Request Tracker at
<
http://rt.cpan.org/NoAuth/Bugs.html?Dist=/home/david/projects/Object-Lo
calVars>
AUTHOR
David A Golden (DAGOLDEN)
[email protected]
http://dagolden.com/
COPYRIGHT
Copyright (c) 2005 by David A Golden
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
The full text of the license can be found in the LICENSE file included
with this module.
