Provided by: libobject-insideout-perl_4.04-1_all 
NAME
Object::InsideOut - Comprehensive inside-out object support module
VERSION
This document describes Object::InsideOut version 4.04
SYNOPSIS
package My::Class; {
use Object::InsideOut;
# Numeric field
# With combined get+set accessor
my @data
:Field
:Type(numeric)
:Accessor(data);
# Takes 'INPUT' (or 'input', etc.) as a mandatory parameter to ->new()
my %init_args :InitArgs = (
'INPUT' => {
'Regex' => qr/^input$/i,
'Mandatory' => 1,
'Type' => 'numeric',
},
);
# Handle class-specific args as part of ->new()
sub init :Init
{
my ($self, $args) = @_;
# Put 'input' parameter into 'data' field
$self->set(\@data, $args->{'INPUT'});
}
}
package My::Class::Sub; {
use Object::InsideOut qw(My::Class);
# List field
# With standard 'get_X' and 'set_X' accessors
# Takes 'INFO' as an optional list parameter to ->new()
# Value automatically added to @info array
# Defaults to [ 'empty' ]
my @info
:Field
:Type(list)
:Standard(info)
:Arg('Name' => 'INFO', 'Default' => 'empty');
}
package Foo; {
use Object::InsideOut;
# Field containing My::Class objects
# With combined accessor
# Plus automatic parameter processing on object creation
my @foo
:Field
:Type(My::Class)
:All(foo);
}
package main;
my $obj = My::Class::Sub->new('Input' => 69);
my $info = $obj->get_info(); # [ 'empty' ]
my $data = $obj->data(); # 69
$obj->data(42);
$data = $obj->data(); # 42
$obj = My::Class::Sub->new('INFO' => 'help', 'INPUT' => 86);
$data = $obj->data(); # 86
$info = $obj->get_info(); # [ 'help' ]
$obj->set_info(qw(foo bar baz));
$info = $obj->get_info(); # [ 'foo', 'bar', 'baz' ]
my $foo_obj = Foo->new('foo' => $obj);
$foo_obj->foo()->data(); # 86
DESCRIPTION
This module provides comprehensive support for implementing classes using the inside-out
object model.
Object::InsideOut implements inside-out objects as anonymous scalar references that are
blessed into a class with the scalar containing the ID for the object (usually a sequence
number). For Perl 5.8.3 and later, the scalar reference is set as read-only to prevent
accidental modifications to the ID. Object data (i.e., fields) are stored within the
class's package in either arrays indexed by the object's ID, or hashes keyed to the
object's ID.
The virtues of the inside-out object model over the blessed hash object model have been
extolled in detail elsewhere. See the informational links under "SEE ALSO". Briefly,
inside-out objects offer the following advantages over blessed hash objects:
• Encapsulation
Object data is enclosed within the class's code and is accessible only through the
class-defined interface.
• Field Name Collision Avoidance
Inheritance using blessed hash classes can lead to conflicts if any classes use the
same name for a field (i.e., hash key). Inside-out objects are immune to this problem
because object data is stored inside each class's package, and not in the object
itself.
• Compile-time Name Checking
A common error with blessed hash classes is the misspelling of field names:
$obj->{'coment'} = 'Say what?'; # Should be 'comment' not 'coment'
As there is no compile-time checking on hash keys, such errors do not usually manifest
themselves until runtime.
With inside-out objects, text hash keys are not used for accessing field data. Field
names and the data index (i.e., $$self) are checked by the Perl compiler such that any
typos are easily caught using "perl -c".
$coment[$$self] = $value; # Causes a compile-time error
# or with hash-based fields
$comment{$$self} = $value; # Also causes a compile-time error
Object::InsideOut offers all the capabilities of other inside-out object modules with the
following additional key advantages:
• Speed
When using arrays to store object data, Object::InsideOut objects are as much as 40%
faster than blessed hash objects for fetching and setting data, and even with hashes
they are still several percent faster than blessed hash objects.
• Threads
Object::InsideOut is thread safe, and thoroughly supports sharing objects between
threads using threads::shared.
• Flexibility
Allows control over object ID specification, accessor naming, parameter name matching,
and much more.
• Runtime Support
Supports classes that may be loaded at runtime (i.e., using "eval { require ...; };").
This makes it usable from within mod_perl, as well. Also supports additions to class
hierarchies, and dynamic creation of object fields during runtime.
• Exception Objects
Object::InsideOut uses Exception::Class for handling errors in an OO-compatible
manner.
• Object Serialization
Object::InsideOut has built-in support for object dumping and reloading that can be
accomplished in either an automated fashion or through the use of class-supplied
subroutines. Serialization using Storable is also supported.
• Foreign Class Inheritance
Object::InsideOut allows classes to inherit from foreign (i.e., non-Object::InsideOut)
classes, thus allowing you to sub-class other Perl class, and access their methods
from your own objects.
• Introspection
Obtain constructor parameters and method metadata for Object::InsideOut classes.
CLASSES
To use this module, each of your classes will start with "use Object::InsideOut;":
package My::Class; {
use Object::InsideOut;
...
}
Sub-classes (child classes) inherit from base classes (parent classes) by telling
Object::InsideOut what the parent class is:
package My::Sub; {
use Object::InsideOut qw(My::Parent);
...
}
Multiple inheritance is also supported:
package My::Project; {
use Object::InsideOut qw(My::Class Another::Class);
...
}
Object::InsideOut acts as a replacement for the "base" pragma: It loads the parent
module(s), calls their "->import()" methods, and sets up the sub-class's @ISA array.
Therefore, you should not "use base ..." yourself, nor try to set up @ISA arrays.
Further, you should not use a class's @ISA array to determine a class's hierarchy: See
"INTROSPECTION" for details on how to do this.
If a parent class takes parameters (e.g., symbols to be exported via Exporter), enclose
them in an array ref (mandatory) following the name of the parent class:
package My::Project; {
use Object::InsideOut 'My::Class' => [ 'param1', 'param2' ],
'Another::Class' => [ 'param' ];
...
}
OBJECTS
Object Creation
Objects are created using the "->new()" method which is exported by Object::InsideOut to
each class, and is invoked in the following manner:
my $obj = My::Class->new();
Object::InsideOut then handles all the messy details of initializing the object in each of
the classes in the invoking class's hierarchy. As such, classes do not (normally)
implement their own "->new()" method.
Usually, object fields are initially populated with data as part of the object creation
process by passing parameters to the "->new()" method. Parameters are passed in as
combinations of "key => value" pairs and/or hash refs:
my $obj = My::Class->new('param1' => 'value1');
# or
my $obj = My::Class->new({'param1' => 'value1'});
# or even
my $obj = My::Class->new(
'param_X' => 'value_X',
'param_Y' => 'value_Y',
{
'param_A' => 'value_A',
'param_B' => 'value_B',
},
{
'param_Q' => 'value_Q',
},
);
Additionally, parameters can be segregated in hash refs for specific classes:
my $obj = My::Class->new(
'foo' => 'bar',
'My::Class' => { 'param' => 'value' },
'Parent::Class' => { 'data' => 'info' },
);
The initialization methods for both classes in the above will get 'foo' => 'bar',
"My::Class" will also get 'param' => 'value', and "Parent::Class" will also get
'data' => 'info'. In this scheme, class-specific parameters will override general
parameters specified at a higher level:
my $obj = My::Class->new(
'default' => 'bar',
'Parent::Class' => { 'default' => 'baz' },
);
"My::Class" will get 'default' => 'bar', and "Parent::Class" will get 'default' => 'baz'.
Calling "->new()" on an object works, too, and operates the same as calling "->new()" for
the class of the object (i.e., "$obj->new()" is the same as "ref($obj)->new()").
How the parameters passed to the "->new()" method are used to initialize the object is
discussed later under "OBJECT INITIALIZATION".
NOTE: You cannot create objects from Object::InsideOut itself:
# This is an error
# my $obj = Object::InsideOut->new();
In this way, Object::InsideOut is not an object class, but functions more like a pragma.
Object IDs
As stated earlier, this module implements inside-out objects as anonymous, read-only
scalar references that are blessed into a class with the scalar containing the ID for the
object.
Within methods, the object is passed in as the first argument:
sub my_method
{
my $self = shift;
...
}
The object's ID is then obtained by dereferencing the object: $$self. Normally, this is
only needed when accessing the object's field data:
my @my_field :Field;
sub my_method
{
my $self = shift;
...
my $data = $my_field[$$self];
...
}
At all other times, and especially in application code, the object should be treated as an
opaque entity.
ATTRIBUTES
Much of the power of Object::InsideOut comes from the use of attributes: Tags on variables
and subroutines that the attributes module sends to Object::InsideOut at compile time.
Object::InsideOut then makes use of the information in these tags to handle such
operations as object construction, automatic accessor generation, and so on.
(Note: The use of attributes is not the same thing as source filtering.)
An attribute consists of an identifier preceded by a colon, and optionally followed by a
set of parameters in parentheses. For example, the attributes on the following array
declare it as an object field, and specify the generation of an accessor method for that
field:
my @level :Field :Accessor(level);
When multiple attributes are assigned to a single entity, they may all appear on the same
line (as shown above), or on separate lines:
my @level
:Field
:Accessor(level);
However, due to limitations in the Perl parser, the entirety of any one attribute must be
on a single line:
# This doesn't work
# my @level
# :Field
# :Accessor('Name' => 'level',
# 'Return' => 'Old');
# Each attribute must be all on one line
my @level
:Field
:Accessor('Name' => 'level', 'Return' => 'Old');
For Object::InsideOut's purposes, the case of an attribute's name does not matter:
my @data :Field;
# or
my @data :FIELD;
However, by convention (as denoted in the attributes module), an attribute's name should
not be all lowercase.
FIELDS
Field Declarations
Object data fields consist of arrays within a class's package into which data are stored
using the object's ID as the array index. An array is declared as being an object field
by following its declaration with the ":Field" attribute:
my @info :Field;
Object data fields may also be hashes:
my %data :Field;
However, as array access is as much as 40% faster than hash access, you should stick to
using arrays. See "HASH ONLY CLASSES" for more information on when hashes may be
required.
Getting Data
In class code, data can be fetched directly from an object's field array (hash) using the
object's ID:
$data = $field[$$self];
# or
$data = $field{$$self};
Setting Data
Analogous to the above, data can be put directly into an object's field array (hash) using
the object's ID:
$field[$$self] = $data;
# or
$field{$$self} = $data;
However, in threaded applications that use data sharing (i.e., use "threads::shared"), the
above will not work when the object is shared between threads and the data being stored is
either an array, hash or scalar reference (this includes other objects). This is because
the $data must first be converted into shared data before it can be put into the field.
Therefore, Object::InsideOut automatically exports a method called "->set()" to each
class. This method should be used in class code to put data into object fields whenever
there is the possibility that the class code may be used in an application that uses
threads::shared (i.e., to make your class code thread-safe). The "->set()" method handles
all details of converting the data to a shared form, and storing it in the field.
The "->set()" method, requires two arguments: A reference to the object field array/hash,
and the data (as a scalar) to be put in it:
my @my_field :Field;
sub store_data
{
my ($self, $data) = @_;
...
$self->set(\@my_field, $data);
}
To be clear, the "->set()" method is used inside class code; not application code. Use it
inside any object methods that set data in object field arrays/hashes.
In the event of a method naming conflict, the "->set()" method can be called using its
fully-qualified name:
$self->Object::InsideOut::set(\@field, $data);
OBJECT INITIALIZATION
As stated in "Object Creation", object fields are initially populated with data as part of
the object creation process by passing "key => value" parameters to the "->new()" method.
These parameters can be processed automatically into object fields, or can be passed to a
class-specific object initialization subroutine.
Field-Specific Parameters
When an object creation parameter corresponds directly to an object field, you can specify
for Object::InsideOut to automatically place the parameter into the field by adding the
":Arg" attribute to the field declaration:
my @foo :Field :Arg(foo);
For the above, the following would result in $val being placed in "My::Class"'s @foo field
during object creation:
my $obj = My::Class->new('foo' => $val);
Object Initialization Subroutines
Many times, object initialization parameters do not correspond directly to object fields,
or they may require special handling. For these, parameter processing is accomplished
through a combination of an ":InitArgs" labeled hash, and an ":Init" labeled subroutine.
The ":InitArgs" labeled hash specifies the parameters to be extracted from the argument
list supplied to the "->new()" method. Those parameters (and only those parameters) which
match the keys in the ":InitArgs" hash are then packaged together into a single hash ref.
The newly created object and this parameter hash ref are then sent to the ":Init"
subroutine for processing.
Here is an example of a class with an automatically handled field and an :Init handled
field:
package My::Class; {
use Object::InsideOut;
# Automatically handled field
my @my_data :Field :Acc(data) :Arg(MY_DATA);
# ':Init' handled field
my @my_field :Field;
my %init_args :InitArgs = (
'MY_PARAM' => '',
);
sub _init :Init
{
my ($self, $args) = @_;
if (exists($args->{'MY_PARAM'})) {
$self->set(\@my_field, $args->{'MY_PARAM'});
}
}
...
}
An object for this class would be created as follows:
my $obj = My::Class->new('MY_DATA' => $dat,
'MY_PARAM' => $parm);
This results in, first of all, $dat being placed in the object's @my_data field because
the "MY_DATA" key is specified in the ":Arg" attribute for that field.
Then, "_init" is invoked with arguments consisting of the object (i.e., $self) and a hash
ref consisting only of "{ 'MY_PARAM' => $param }" because the key "MY_PARAM" is specified
in the ":InitArgs" hash. "_init" checks that the parameter "MY_PARAM" exists in the hash
ref, and then (since it does exist) adds $parm to the object's @my_field field.
Setting Data
Data processed by the ":Init" subroutine may be placed directly into the class's field
arrays (hashes) using the object's ID (i.e., $$self):
$my_field[$$self] = $args->{'MY_PARAM'};
However, as shown in the example above, it is strongly recommended that you use the
->set() method:
$self->set(\@my_field, $args->{'MY_PARAM'});
which handles converting the data to a shared format when needed for applications
using threads::shared.
All Parameters
The ":InitArgs" hash and the ":Arg" attribute on fields act as filters that constrain
which initialization parameters are and are not sent to the ":Init" subroutine. If,
however, a class does not have an ":InitArgs" hash and does not use the ":Arg"
attribute on any of its fields, then its ":Init" subroutine (if it exists, of course)
will get all the initialization parameters supplied to the "->new()" method.
Mandatory Parameters
Field-specific parameters may be declared mandatory as follows:
my @data :Field
:Arg('Name' => 'data', 'Mandatory' => 1);
If a mandatory parameter is missing from the argument list to "->new()", an error is
generated.
For ":Init" handled parameters, use:
my %init_args :InitArgs = (
'data' => {
'Mandatory' => 1,
},
);
"Mandatory" may be abbreviated to "Mand", and "Required" or "Req" are synonymous.
Default Values
For optional parameters, defaults can be specified for field-specific parameters using
either of these syntaxes:
my @data :Field
:Arg('Name' => 'data', 'Default' => 'foo');
my @info :Field :Arg(info) :Default('bar');
If an optional parameter with a specified default is missing from the argument list to
"->new()", then the default is assigned to the field when the object is created (before
the ":Init" subroutine, if any, is called).
The format for ":Init" handled parameters is:
my %init_args :InitArgs = (
'data' => {
'Default' => 'foo',
},
);
In this case, if the parameter is missing from the argument list to "->new()", then the
parameter key is paired with the default value and added to the ":Init" argument hash ref
(e.g., "{ 'data' => 'foo' }").
Fields can also be assigned a default value even if not associated with an initialization
parameter:
my @hash :Field :Default({});
my @tuple :Field :Default([1, 'bar']);
Note that when using ":Default", the value must be properly structured Perl code (e.g.,
strings must be quoted as illustrated above).
"Default" and ":Default" may be abbreviated to "Def" and ":Def" respectively.
Generated Default Values
It is also possible to generate default values on a per object basis by using code in the
":Default" directive.
my @IQ :Field :Default(50 + rand 100);
my @ID :Field :Default(our $next; ++$next);
The above, for example, will initialize the "IQ" attribute of each new object to a
different random number, while its "ID" attribute will be initialized with a sequential
integer.
The code in a ":Default" specifier can also refer to the object being initialized, either
as $_[0] or as $self. For example:
my @unique_ID :Field :Default($self->gen_unique_ID);
Any code specified as a default will not have access to the surrounding lexical scope.
For example, this will not work:
my $MAX = 100;
my $MIN = 0;
my @bar :Field
:Default($MIN + rand($MAX-$MIX)); # Error
For anything lexical or complex, you should factor the initializer out into a utility
subroutine:
sub _rand_max :Restricted
{
$MIN + rand($MAX-$MIX)
}
my @bar :Field
:Default(_rand_max);
When specifying a generated default using the "Default" tag inside an ":Arg" directive,
you will need to wrap the code in a "sub { }", and $_[0] (but not $self) can be used to
access the object being initialized:
my @baz :Field
:Arg(Name => 'baz', Default => sub { $_[0]->biz });
System functions need to similarly be wrapped in "sub { }":
my @rand :Field
:Type(numeric)
:Arg(Name => 'Rand', Default => sub { rand });
Subroutines can be accessed using a code reference:
my @data :Field
:Arg(Name => 'Data', Default => \&gen_default);
On the other hand, the above can also be simplified by using the ":Default" directive
instead:
my @baz :Field :Arg(baz) :Default($self->biz);
my @rand :Field :Arg(Rand) :Default(rand) :Type(numeric);
my @data :Field :Arg(Data) :Default(gen_default);
Using generated defaults in the ":InitArgs" hash requires the use of the same types of
syntax as with the "Default" tag in an ":Arg" directive:
my %init_args :InitArgs = (
'Baz' => {
'Default' => sub { $_[0]->biz },
},
'Rand' => {
'Default' => sub { rand },
},
'Data' => {
'Default' => \&gen_default,
},
);
Sequential defaults
In the previous section, one of the examples is not as safe or as convenient as it should
be:
my @ID :Field :Default(our $next; ++$next);
The problem is the shared variable ($next) that's needed to track the allocation of "ID"
values. Because it has to persist between calls, that variable has to be a package
variable, except under Perl 5.10 or later where it could be a state variable instead:
use feature 'state';
my @ID :Field :Default(state $next; ++$next);
The version with the package variable is unsafe, because anyone could then spoof ID
numbers just by reassigning that universally accessible variable:
$MyClass::next = 0; # Spoof the next object
my $obj = MyClass->new; # Object now has ID 1
The state-variable version avoids this problem, but even that version is more complicated
(and hence more error-prone) than it needs to be.
The ":SequenceFrom" directive (which can be abbreviated to ":SeqFrom" or ":Seq") makes it
much easier to specify that an attribute's default value is taken from a linearly
increasing sequence. For instance, the ID example above could be rewritten as:
my @ID :Field :SequenceFrom(1);
This directive automatically creates a hidden variable, initializes it to the initial
value specified, generates a sequence starting at that initial value, and then uses
successive elements of that sequence each time a default value is needed for that
attribute during object creation.
If the initial value is a scalar, then the default sequence is generated by by computing
"$previous_value++". If it is an object, it is generated by calling "$obj->next()" (or by
calling "$obj++" if the object doesn't have a "next()" method).
This makes it simple to create a series of objects with attributes whose values default to
simple numeric, alphabetic, or alphanumeric sequences, or to the sequence specified by an
iterator object of some kind:
my @ID :Field :SeqFrom(1); # 1, 2, 3...
my @ID :Field :SeqFrom('AAA'); # 'AAA', 'AAB', 'AAC'...
my @ID :Field :SeqFrom('A01'); # 'A01', 'A02', 'A03'...
my @ID :Field :SeqFrom(ID_Iterator->new); # ->next, ->next, ->next...
In every other respect a ":SequenceFrom" directive is just like a ":Default". For
example, it can be used in conjunction with the ":Arg" directive as follows:
my @ID :Field :Arg(ID) :SeqFrom(1);
However, not as a tag inside the ":Arg" directive:
my @ID :Field :Arg('Name' => 'ID', 'SeqFrom' => 1) # WRONG
For the ":InitArgs" hash, you will need to roll your own sequential defaults if required:
use feature 'state';
my %init_args :InitArgs = (
'Counter' => {
'Default' => sub { state $next; ++$next }
},
);
Parameter Name Matching
Rather than having to rely on exact matches to parameter keys in the "->new()" argument
list, you can specify a regular expressions to be used to match them to field-specific
parameters:
my @param :Field
:Arg('Name' => 'param', 'Regexp' => qr/^PARA?M$/i);
In this case, the parameter's key could be any of the following: PARAM, PARM, Param, Parm,
param, parm, and so on. And the following would result in $data being placed in
"My::Class"'s @param field during object creation:
my $obj = My::Class->new('Parm' => $data);
For ":Init" handled parameters, you would similarly use:
my %init_args :InitArgs = (
'Param' => {
'Regex' => qr/^PARA?M$/i,
},
);
In this case, the match results in "{ 'Param' => $data }" being sent to the ":Init"
subroutine as the argument hash. Note that the ":InitArgs" hash key is substituted for
the original argument key. This eliminates the need for any parameter key pattern
matching within the ":Init" subroutine.
"Regexp" may be abbreviated to "Regex" or "Re".
Object Pre-initialization
Occasionally, a child class may need to send a parameter to a parent class as part of
object initialization. This can be accomplished by supplying a ":PreInit" labeled
subroutine in the child class. These subroutines, if found, are called in order from the
bottom of the class hierarchy upward (i.e., child classes first).
The subroutine should expect two arguments: The newly created (uninitialized) object
(i.e., $self), and a hash ref of all the parameters from the "->new()" method call,
including any additional parameters added by other ":PreInit" subroutines.
sub pre_init :PreInit
{
my ($self, $args) = @_;
...
}
The parameter hash ref will not be exactly as supplied to "->new()", but will be flattened
into a single hash ref. For example,
my $obj = My::Class->new(
'param_X' => 'value_X',
{
'param_A' => 'value_A',
'param_B' => 'value_B',
},
'My::Class' => { 'param' => 'value' },
);
would produce
{
'param_X' => 'value_X',
'param_A' => 'value_A',
'param_B' => 'value_B',
'My::Class' => { 'param' => 'value' }
}
as the hash ref to the ":PreInit" subroutine.
The ":PreInit" subroutine may then add, modify or even remove any parameters from the hash
ref as needed for its purposes. After all the ":PreInit" subroutines have been executed,
object initialization will then proceed using the resulting parameter hash.
The ":PreInit" subroutine should not try to set data in its class's fields or in other
class's fields (e.g., using set methods) as such changes will be overwritten during
initialization phase which follows pre-initialization. The ":PreInit" subroutine is only
intended for modifying initialization parameters prior to initialization.
Initialization Sequence
For the most part, object initialization can be conceptualized as proceeding from parent
classes down through child classes. As such, calling child class methods from a parent
class during object initialization may not work because the object will not have been
fully initialized in the child classes.
Knowing the order of events during object initialization may help in determining when this
can be done safely:
1. The scalar reference for the object is created, populated with an "Object ID", and
blessed into the appropriate class.
2. :PreInit subroutines are called in order from the bottom of the class hierarchy upward
(i.e., child classes first).
3. From the top of the class hierarchy downward (i.e., parent classes first), "Default
Values" are assigned to fields. (These may be overwritten by subsequent steps below.)
4. From the top of the class hierarchy downward, parameters to the "->new()" method are
processed for ":Arg" field attributes and entries in the ":InitArgs" hash:
a. "Parameter Preprocessing" is performed.
b. Checks for "Mandatory Parameters" are made.
c. "Default Values" specified in the ":InitArgs" hash are added for subsequent
processing by the ":Init" subroutine.
d. Type checking is performed.
e. "Field-Specific Parameters" are assigned to fields.
5. From the top of the class hierarchy downward, :Init subroutines are called with
parameters specified in the ":InitArgs" hash.
6. Checks are made for any parameters to "->new()" that were not handled in the above.
(See next section.)
Unhandled Parameters
It is an error to include any parameters to the "->new()" method that are not handled by
at least one class in the hierarchy. The primary purpose of this is to catch typos in
parameter names:
my $obj = Person->new('nane' => 'John'); # Should be 'name'
The only time that checks for unhandled parameters are not made is when at least one class
in the hierarchy does not have an ":InitArgs" hash and does not use the ":Arg" attribute
on any of its fields and uses an :Init subroutine for processing parameters. In such a
case, it is not possible for Object::InsideOut to determine which if any of the parameters
are not handled by the ":Init" subroutine.
If you add the following construct to the start of your application:
BEGIN {
no warnings 'once';
$OIO::Args::Unhandled::WARN_ONLY = 1;
}
then unhandled parameters will only generate warnings rather than causing exceptions to be
thrown.
Modifying ":InitArgs"
For performance purposes, Object::InsideOut normalizes each class's ":InitArgs" hash by
creating keys in the form of '_X' for the various options it handles (e.g., '_R' for
'Regexp').
If a class has the unusual requirement to modify its ":InitArgs" hash during runtime, then
it must renormalize the hash after making such changes by invoking
"Object::InsideOut::normalize()" on it so that Object::InsideOut will pick up the changes:
Object::InsideOut::normalize(\%init_args);
ACCESSOR GENERATION
Accessors are object methods used to get data out of and put data into an object. You
can, of course, write your own accessor code, but this can get a bit tedious, especially
if your class has lots of fields. Object::InsideOut provides the capability to
automatically generate accessors for you.
Basic Accessors
A get accessor is vary basic: It just returns the value of an object's field:
my @data :Field;
sub fetch_data
{
my $self = shift;
return ($data[$$self]);
}
and you would use it as follows:
my $data = $obj->fetch_data();
To have Object::InsideOut generate such a get accessor for you, add a ":Get" attribute to
the field declaration, specifying the name for the accessor in parentheses:
my @data :Field :Get(fetch_data);
Similarly, a set accessor puts data in an object's field. The set accessors generated by
Object::InsideOut check that they are called with at least one argument. They are
specified using the ":Set" attribute:
my @data :Field :Set(store_data);
Some programmers use the convention of naming get and set accessors using get_ and set_
prefixes. Such standard accessors can be generated using the ":Standard" attribute (which
may be abbreviated to ":Std"):
my @data :Field :Std(data);
which is equivalent to:
my @data :Field :Get(get_data) :Set(set_data);
Other programmers prefer to use a single combination accessors that performs both
functions: When called with no arguments, it gets, and when called with an argument, it
sets. Object::InsideOut will generate such accessors with the ":Accessor" attribute.
(This can be abbreviated to ":Acc", or you can use ":Get_Set" or ":Combined" or ":Combo"
or even "Mutator".) For example:
my @data :Field :Acc(data);
The generated accessor would be used in this manner:
$obj->data($val); # Puts data into the object's field
my $data = $obj->data(); # Fetches the object's field data
Set Accessor Return Value
For any of the automatically generated methods that perform set operations, the default
for the method's return value is the value being set (i.e., the new value).
You can specify the set accessor's return value using the "Return" attribute parameter
(which may be abbreviated to "Ret"). For example, to explicitly specify the default
behavior use:
my @data :Field :Set('Name' => 'store_data', 'Return' => 'New');
You can specify that the accessor should return the old (previous) value (or "undef" if
unset):
my @data :Field :Acc('Name' => 'data', 'Ret' => 'Old');
You may use "Previous", "Prev" or "Prior" as synonyms for "Old".
Finally, you can specify that the accessor should return the object itself:
my @data :Field :Std('Name' => 'data', 'Ret' => 'Object');
"Object" may be abbreviated to "Obj", and is also synonymous with "Self".
Method Chaining
An obvious case where method chaining can be used is when a field is used to store an
object: A method for the stored object can be chained to the get accessor call that
retrieves that object:
$obj->get_stored_object()->stored_object_method()
Chaining can be done off of set accessors based on their return value (see above). In
this example with a set accessor that returns the new value:
$obj->set_stored_object($stored_obj)->stored_object_method()
the set_stored_object() call stores the new object, returning it as well, and then the
stored_object_method() call is invoked via the stored/returned object. The same would
work for set accessors that return the old value, too, but in that case the chained method
is invoked via the previously stored (and now returned) object.
If the Want module (version 0.12 or later) is available, then Object::InsideOut also tries
to do the right thing with method chaining for set accessors that don't store/return
objects. In this case, the object used to invoke the set accessor will also be used to
invoke the chained method (just as though the set accessor were declared with
'Return' => 'Object'):
$obj->set_data('data')->do_something();
To make use of this feature, just add "use Want;" to the beginning of your application.
Note, however, that this special handling does not apply to get accessors, nor to
combination accessors invoked without an argument (i.e., when used as a get accessor).
These must return objects in order for method chaining to succeed.
:lvalue Accessors
As documented in "Lvalue subroutines" in perlsub, an ":lvalue" subroutine returns a
modifiable value. This modifiable value can then, for example, be used on the left-hand
side (hence "LVALUE") of an assignment statement, or a substitution regular expression.
For Perl 5.8.0 and later, Object::InsideOut supports the generation of ":lvalue" accessors
such that their use in an "LVALUE" context will set the value of the object's field. Just
add "'lvalue' => 1" to the set accessor's attribute. ('lvalue' may be abbreviated to
'lv'.)
Additionally, ":Lvalue" (or its abbreviation ":lv") may be used for a combined get/set
:lvalue accessor. In other words, the following are equivalent:
:Acc('Name' => 'email', 'lvalue' => 1)
:Lvalue(email)
Here is a detailed example:
package Contact; {
use Object::InsideOut;
# Create separate a get accessor and an :lvalue set accessor
my @name :Field
:Get(name)
:Set('Name' => 'set_name', 'lvalue' => 1);
# Create a standard get_/set_ pair of accessors
# The set_ accessor will be an :lvalue accessor
my @phone :Field
:Std('Name' => 'phone', 'lvalue' => 1);
# Create a combined get/set :lvalue accessor
my @email :Field
:Lvalue(email);
}
package main;
my $obj = Contact->new();
# Use :lvalue accessors in assignment statements
$obj->set_name() = 'Jerry D. Hedden';
$obj->set_phone() = '800-555-1212';
$obj->email() = 'jdhedden AT cpan DOT org';
# Use :lvalue accessor in substitution regexp
$obj->email() =~ s/ AT (\w+) DOT /\@$1./;
# Use :lvalue accessor in a 'substr' call
substr($obj->set_phone(), 0, 3) = '888';
print("Contact info:\n");
print("\tName: ", $obj->name(), "\n");
print("\tPhone: ", $obj->get_phone(), "\n");
print("\tEmail: ", $obj->email(), "\n");
The use of ":lvalue" accessors requires the installation of the Want module (version 0.12
or later) from CPAN. See particularly the section "Lvalue subroutines:" in Want for more
information.
":lvalue" accessors also work like regular set accessors in being able to accept
arguments, return values, and so on:
my @pri :Field
:Lvalue('Name' => 'priority', 'Return' => 'Old');
...
my $old_pri = $obj->priority(10);
":lvalue" accessors can be used in method chains.
Caveats: While still classified as experimental, Perl's support for ":lvalue" subroutines
has been around since 5.6.0, and a good number of CPAN modules make use of them.
By definition, because ":lvalue" accessors return the location of a field, they break
encapsulation. As a result, some OO advocates eschew the use of ":lvalue" accessors.
":lvalue" accessors are slower than corresponding non-lvalue accessors. This is due to
the fact that more code is needed to handle all the diverse ways in which ":lvalue"
accessors may be used. (I've done my best to optimize the generated code.) For example,
here's the code that is generated for a simple combined accessor:
*Foo::foo = sub {
return ($$field[${$_[0]}]) if (@_ == 1);
$$field[${$_[0]}] = $_[1];
};
And the corresponding code for an ":lvalue" combined accessor:
*Foo::foo = sub :lvalue {
my $rv = !Want::want_lvalue(0);
Want::rreturn($$field[${$_[0]}]) if ($rv && (@_ == 1));
my $assign;
if (my @args = Want::wantassign(1)) {
@_ = ($_[0], @args);
$assign = 1;
}
if (@_ > 1) {
$$field[${$_[0]}] = $_[1];
Want::lnoreturn if $assign;
Want::rreturn($$field[${$_[0]}]) if $rv;
}
((@_ > 1) && (Want::wantref() eq 'OBJECT') &&
!Scalar::Util::blessed($$field[${$_[0]}]))
? $_[0] : $$field[${$_[0]}];
};
ALL-IN-ONE
Parameter naming and accessor generation may be combined:
my @data :Field :All(data);
This is syntactic shorthand for:
my @data :Field :Arg(data) :Acc(data);
If you want the accessor to be ":lvalue", use:
my @data :Field :LV_All(data);
If standard accessors are desired, use:
my @data :Field :Std_All(data);
Attribute parameters affecting the set accessor may also be used. For example, if you
want standard accessors with an ":lvalue" set accessor:
my @data :Field :Std_All('Name' => 'data', 'Lvalue' => 1);
If you want a combined accessor that returns the old value on set operations:
my @data :Field :All('Name' => 'data', 'Ret' => 'Old');
And so on.
If you need to add attribute parameters that affect the ":Arg" portion (e.g., "Default",
"Mandatory", etc.), then you cannot use ":All". Fall back to using the separate
attributes. For example:
my @data :Field :Arg('Name' => 'data', 'Mand' => 1)
:Acc('Name' => 'data', 'Ret' => 'Old');
READONLY FIELDS
If you want to declare a read-only field (i.e., one that can be initialized and retrieved,
but which doesn't have a set accessor):
my @data :Field :Arg(data) :Get(data);
there is a syntactic shorthand for that, too:
my @data :Field :ReadOnly(data);
or just:
my @data :Field :RO(data);
If a standard get accessor is desired, use:
my @data :Field :Std_RO(data);
For obvious reasons, attribute parameters affecting the set accessor cannot be used with
read-only fields, nor can ":ReadOnly" be combined with ":LValue".
As with ":All", if you need to add attribute parameters that affect the ":Arg" portion
then you cannot use the ":RO" shorthand: Fall back to using the separate attributes in
such cases. For example:
my @data :Field :Arg('Name' => 'data', 'Mand' => 1)
:Get('Name' => 'data');
DELEGATORS
In addition to autogenerating accessors for a given field, you can also autogenerate
delegators to that field. A delegator is an accessor that forwards its call to one of the
object's fields.
For example, if your Car object has an @engine field, then you might need to send all
acceleration requests to the Engine object stored in that field. Likewise, all braking
requests may need to be forwarded to Car's field that stores the Brakes object:
package Car; {
use Object::InsideOut;
my @engine :Field :Get(engine);
my @brakes :Field :Get(brakes);
sub _init :Init(private) {
my ($self, $args) = @_;
$self->engine(Engine->new());
$self->brakes(Brakes->new());
}
sub accelerate {
my ($self) = @_;
$self->engine->accelerate();
}
sub decelerate {
my ($self) = @_;
$self->engine->decelerate();
}
sub brake {
my ($self, $foot_pressure) = @_;
$self->brakes->brake($foot_pressure);
}
}
If the Car needs to forward other method calls to its Engine or Brakes, this quickly
becomes tedious, repetitive, and error-prone. So, instead, you can just tell
Object::InsideOut that a particular method should be automatically forwarded to a
particular field, by specifying a ":Handles" attribute:
package Car; {
use Object::InsideOut;
my @engine :Field
:Get(engine)
:Handles(accelerate, decelerate);
my @brakes :Field
:Get(brakes)
:Handles(brake);
sub _init :Init(private) {
my ($self, $args) = @_;
$self->engine(Engine->new());
$self->brakes(Brakes->new());
}
}
This option generates and installs a single delegator method for each of its arguments, so
the second example has exactly the same effect as the first example. The delegator simply
calls the corresponding method on the object stored in the field, passing it the same
argument list it received.
Sometimes, however, you may need to delegate a particular method to a field, but under a
different name. For example, if the Brake class provides an "engage()" method, rather
than a "brake()" method, then you'd need "Car::brake()" to be implemented as:
sub brake {
my ($self, $foot_pressure) = @_;
$self->brakes->engage($foot_pressure);
}
You can achieve that using the ":Handles" attribute, like so:
my @brakes :Field
:Get(brakes)
:Handles(brake-->engage);
The long arrow version still creates a delegator method "brake()", but makes that method
delegate to your Brakes object by calling its "engage()" method instead.
If you are delegating a large number of methods to a particular field, the ":Handles"
declarations soon become tedious:
my @onboard_computer :Field :Get(comp)
:Type(Computer::Onboard)
:Handles(engine_monitor engine_diagnostics)
:Handles(engine_control airbag_deploy)
:Handles(GPS_control GPS_diagnostics GPS_reset)
:Handles(climate_control reversing_camera)
:Handles(cruise_control auto_park)
:Handles(iPod_control cell_phone_connect);
And, of course, every time the interface of the "Computer::Onboard" class changes, you
have to change those ":Handles" declarations, too.
Sometimes, all you really want to say is: "This field should handle anything it can
handle". To do that, you write:
my @onboard_computer :Field :Get(comp)
:Type(Computer::Onboard)
:Handles(Computer::Onboard);
That is, if a ":Handles" directive is given a name that includes a "::", it treats that
name as a class name, rather than a method name. Then it checks that class's metadata
(see INTROSPECTION), retrieves a list of all the method names from the class, and uses
that as the list of method names to delegate.
Unlike an explicit ":Handles( method_name )", a ":Handles( Class::Name )" is tolerant of
name collisions. If any method of "Class::Name" has the same name as another method or
delegator that has already been installed in the current class, then ":Handles" just
silently ignores that particular method, and doesn't try to replace the existing one. In
other words, a ":Handles(Class::Name)" won't install a delegator to a method in
"Class::Name" if that method is already being handled somewhere else by the current class.
For classes that don't have a "::" in their name (e.g., "DateTime" and "POE"), just append
a "::" to the class name:
my @init_time :Field :Get(init_time)
:Type( DateTime )
:Default( DateTime->now() )
:Handles( DateTime:: );
Note that, when using the class-based version of ":Handles", every method is delegated
with its name unchanged. If some of the object's methods should be delegated under
different names, you have to specify that explicitly (and beforehand):
my @onboard_computer :Field :Get(comp) :Type(Computer::Onboard)
# rename this method when delegating...
:Handles( iPod_control-->get_iPod )
# delegate everything else with names unchanged...
:Handles( Computer::Onboard );
"Handles" may be abbreviated to "Handle" or "Hand".
NOTES: Failure to add the appropriate object to the delegation field will lead to errors
such as: Can't call method "bar" on an undefined value.
Typos in ":Handles" attribute declarations will lead to errors such as: Can't locate
object method "bat" via package "Foo". Adding an object of the wrong class to the
delegation field will lead to the same error, but can be avoided by adding a ":Type"
attribute for the appropriate class.
PERMISSIONS
Restricted and Private Accessors
By default, automatically generated accessors, can be called at any time. In other words,
their access permission is public.
If desired, accessors can be made restricted - in which case they can only be called from
within the class and any child classes in the hierarchy that are derived from it - or
private - such that they can only be called from within the accessors' class. Here are
examples of the syntax for adding permissions:
my @data :Field :Std('Name' => 'data', 'Permission' => 'private');
my @info :Field :Set('Name' => 'set_info', 'Perm' => 'restricted');
my @internal :Field :Acc('Name' => 'internal', 'Private' => 1);
my @state :Field :Get('Name' => 'state', 'Restricted' => 1);
When creating a standard pair of get_/set_ accessors, the permission setting is applied to
both accessors. If different permissions are required on the two accessors, then you'll
have to use separate ":Get" and ":Set" attributes on the field.
# Create a private set method
# and a restricted get method on the 'foo' field
my @foo :Field
:Set('Name' => 'set_foo', 'Priv' => 1)
:Get('Name' => 'get_foo', 'Rest' => 1);
# Create a restricted set method
# and a public get method on the 'bar' field
my %bar :Field
:Set('Name' => 'set_bar', 'Perm' => 'restrict')
:Get(get_bar);
"Permission" may be abbreviated to "Perm"; "Private" may be abbreviated to "Priv"; and
"Restricted" may be abbreviated to "Restrict".
Restricted and Private Methods
In the same vein as describe above, access to methods can be narrowed by use of
":Restricted" and ":Private" attributes.
sub foo :Restricted
{
my $self = shift;
...
}
Without either of these attributes, most methods have public access. If desired, you may
explicitly label them with the ":Public" attribute.
Exemptions
It is also possible to specify classes that are exempt from the Restricted and Private
access permissions (i.e., the method may be called from those classes as well):
my %foo :Field
:Acc('Name' => 'foo', 'Perm' => 'Restrict(Exempt::Class)')
:Get(get_bar);
sub bar :Private(Some::Class, Another::Class)
{
my $self = shift;
...
}
An example of when this might be needed is with delegation mechanisms.
Hidden Methods
For subroutines marked with the following attributes (most of which are discussed later in
this document):
:ID
:PreInit
:Init
:Replicate
:Destroy
:Automethod
:Dumper
:Pumper
:MOD_*_ATTRS
:FETCH_*_ATTRS
Object::InsideOut normally renders them uncallable (hidden) to class and application code
(as they should normally only be needed by Object::InsideOut itself). If needed, this
behavior can be overridden by adding the "Public", "Restricted" or "Private" attribute
parameters:
sub _init :Init(private) # Callable from within this class
{
my ($self, $args) = @_;
...
}
Restricted and Private Classes
Permission for object creation on a class can be narrowed by adding a ":Restricted" or
":Private" flag to its "use Object::InsideOut ..." declaration. This basically adds
":Restricted/:Private" permissions on the "->new()" method for that class. Exemptions are
also supported.
package Foo; {
use Object::InsideOut;
...
}
package Bar; {
use Object::InsideOut 'Foo', ':Restricted(Ping, Pong)';
...
}
In the above, class "Bar" inherits from class "Foo", and its constructor is restricted to
itself, classes that inherit from "Bar", and the classes "Ping" and "Pong".
As constructors are inherited, any class that inherits from "Bar" would also be a
restricted class. To overcome this, any child class would need to add its own permission
declaration:
package Baz; {
use Object::InsideOut qw/Bar :Private(My::Class)/;
...
}
Here, class "Baz" inherits from class "Bar", and its constructor is restricted to itself
(i.e., private) and class "My::Class".
Inheriting from a ":Private" class is permitted, but objects cannot be created for that
class unless it has a permission declaration of its own:
package Zork; {
use Object::InsideOut qw/:Public Baz/;
...
}
Here, class "Zork" inherits from class "Baz", and its constructor has unrestricted access.
(In general, don't use the ":Public" declaration for a class except to overcome
constructor permissions inherited from parent classes.)
TYPE CHECKING
Object::InsideOut can be directed to add type-checking code to the set/combined accessors
it generates, and to perform type checking on object initialization parameters.
Field Type Checking
Type checking for a field can be specified by adding the ":Type" attribute to the field
declaration:
my @count :Field :Type(numeric);
my @objs :Field :Type(list(My::Class));
The ":Type" attribute results in type checking code being added to set/combined accessors
generated by Object::InsideOut, and will perform type checking on object initialization
parameters processed by the ":Arg" attribute.
Available Types are:
'scalar'
Permits anything that is not a reference.
'numeric'
Can also be specified as "Num" or "Number". This uses
Scalar::Util::looks_like_number() to test the input value.
'list' or 'array'
'list(_subtype_)' or 'array(_subtype_)'
This type permits an accessor to accept multiple values (which are then placed in an
array ref) or a single array ref.
For object initialization parameters, it permits a single value (which is then placed
in an array ref) or an array ref.
When specified, the contents of the resulting array ref are checked against the
specified subtype:
'scalar'
Same as for the basic type above.
'numeric'
Same as for the basic type above.
A class name
Same as for the basic type below.
A reference type
Any reference type (in all caps) as returned by ref()).
'ARRAY_ref'
'ARRAY_ref(_subtype_)'
This specifies that only a single array reference is permitted. Can also be specified
as "ARRAYref".
When specified, the contents of the array ref are checked against the specified
subtype as per the above.
'HASH'
This type permits an accessor to accept multiple "key => value" pairs (which are then
placed in a hash ref) or a single hash ref.
For object initialization parameters, only a single ref is permitted.
'HASH_ref'
This specifies that only a single hash reference is permitted. Can also be specified
as "HASHref".
'SCALAR_ref'
This type permits an accessor to accept a single scalar reference. Can also be
specified as "SCALARref".
A class name
This permits only an object of the specified class, or one of its sub-classes (i.e.,
type checking is done using "->isa()"). For example, "My::Class". The class name
"UNIVERSAL" permits any object. The class name "Object::InsideOut" permits any object
generated by an Object::InsideOut class.
Other reference type
This permits only a reference of the specified type (as returned by ref()). The type
must be specified in all caps. For example, "CODE".
The ":Type" attribute can also be supplied with a code reference to provide custom type
checking. The code ref may either be in the form of an anonymous subroutine, or a fully-
qualified subroutine name. The result of executing the code ref on the input argument
should be a boolean value. Here's some examples:
package My::Class; {
use Object::InsideOut;
# Type checking using an anonymous subroutine
# (This checks that the argument is an object)
my @data :Field :Type(sub { Scalar::Util::blessed($_[0]) })
:Acc(data);
# Type checking using a fully-qualified subroutine name
my @num :Field :Type(\&My::Class::positive)
:Acc(num);
# The type checking subroutine may be made 'Private'
sub positive :Private
{
return (Scalar::Util::looks_like_number($_[0]) &&
($_[0] > 0));
}
}
Type Checking on ":Init" Parameters
For object initialization parameters that are sent to the ":Init" subroutine during object
initialization, the parameter's type can be specified in the ":InitArgs" hash for that
parameter using the same types as specified in the previous section. For example:
my %init_args :InitArgs = (
'COUNT' => {
'Type' => 'numeric',
},
'OBJS' => {
'Type' => 'list(My::Class)',
},
);
One exception involves custom type checking: If referenced in an ":InitArgs" hash, the
type checking subroutine cannot be made ":Private":
package My::Class; {
use Object::InsideOut;
sub check_type # Cannot be :Private
{
...
}
my %init_args :InitArgs = (
'ARG' => {
'Type' => \&check_type,
},
);
...
}
Also, as shown, it also doesn't have to be a fully-qualified name.
CUMULATIVE METHODS
Normally, methods with the same name in a class hierarchy are masked (i.e., overridden) by
inheritance - only the method in the most-derived class is called. With cumulative
methods, this masking is removed, and the same-named method is called in each of the
classes within the hierarchy. The return results from each call (if any) are then
gathered together into the return value for the original method call. For example,
package My::Class; {
use Object::InsideOut;
sub what_am_i :Cumulative
{
my $self = shift;
my $ima = (ref($self) eq __PACKAGE__)
? q/I was created as a /
: q/My top class is /;
return ($ima . __PACKAGE__);
}
}
package My::Foo; {
use Object::InsideOut 'My::Class';
sub what_am_i :Cumulative
{
my $self = shift;
my $ima = (ref($self) eq __PACKAGE__)
? q/I was created as a /
: q/I'm also a /;
return ($ima . __PACKAGE__);
}
}
package My::Child; {
use Object::InsideOut 'My::Foo';
sub what_am_i :Cumulative
{
my $self = shift;
my $ima = (ref($self) eq __PACKAGE__)
? q/I was created as a /
: q/I'm in class /;
return ($ima . __PACKAGE__);
}
}
package main;
my $obj = My::Child->new();
my @desc = $obj->what_am_i();
print(join("\n", @desc), "\n");
produces:
My top class is My::Class
I'm also a My::Foo
I was created as a My::Child
When called in a list context (as in the above), the return results of cumulative methods
are accumulated, and returned as a list.
In a scalar context, a results object is returned that segregates the results by class for
each of the cumulative method calls. Through overloading, this object can then be
dereferenced as an array, hash, string, number, or boolean. For example, the above could
be rewritten as:
my $obj = My::Child->new();
my $desc = $obj->what_am_i(); # Results object
print(join("\n", @{$desc}), "\n"); # Dereference as an array
The following uses hash dereferencing:
my $obj = My::Child->new();
my $desc = $obj->what_am_i();
while (my ($class, $value) = each(%{$desc})) {
print("Class $class reports:\n\t$value\n");
}
and produces:
Class My::Class reports:
My top class is My::Class
Class My::Child reports:
I was created as a My::Child
Class My::Foo reports:
I'm also a My::Foo
As illustrated above, cumulative methods are tagged with the ":Cumulative" attribute (or
":Cumulative(top down)"), and propagate from the top down through the class hierarchy
(i.e., from the parent classes down through the child classes). If tagged with
":Cumulative(bottom up)", they will propagated from the object's class upward through the
parent classes.
CHAINED METHODS
In addition to ":Cumulative", Object::InsideOut provides a way of creating methods that
are chained together so that their return values are passed as input arguments to other
similarly named methods in the same class hierarchy. In this way, the chained methods act
as though they were piped together.
For example, imagine you had a method called "format_name" that formats some text for
display:
package Subscriber; {
use Object::InsideOut;
sub format_name {
my ($self, $name) = @_;
# Strip leading and trailing whitespace
$name =~ s/^\s+//;
$name =~ s/\s+$//;
return ($name);
}
}
And elsewhere you have a second class that formats the case of names:
package Person; {
use Lingua::EN::NameCase qw(nc);
use Object::InsideOut;
sub format_name
{
my ($self, $name) = @_;
# Attempt to properly case names
return (nc($name));
}
}
And you decide that you'd like to perform some formatting of your own, and then have all
the parent methods apply their own formatting. Normally, if you have a single parent
class, you'd just call the method directly with "$self->SUPER::format_name($name)", but if
you have more than one parent class you'd have to explicitly call each method directly:
package Customer; {
use Object::InsideOut qw(Person Subscriber);
sub format_name
{
my ($self, $name) = @_;
# Compress all whitespace into a single space
$name =~ s/\s+/ /g;
$name = $self->Subscriber::format_name($name);
$name = $self->Person::format_name($name);
return $name;
}
}
With Object::InsideOut, you'd add the ":Chained" attribute to each class's "format_name"
method, and the methods will be chained together automatically:
package Subscriber; {
use Object::InsideOut;
sub format_name :Chained
{
my ($self, $name) = @_;
# Strip leading and trailing whitespace
$name =~ s/^\s+//;
$name =~ s/\s+$//;
return ($name);
}
}
package Person; {
use Lingua::EN::NameCase qw(nc);
use Object::InsideOut;
sub format_name :Chained
{
my ($self, $name) = @_;
# Attempt to properly case names
return (nc($name));
}
}
package Customer; {
use Object::InsideOut qw(Person Subscriber);
sub format_name :Chained
{
my ($self, $name) = @_;
# Compress all whitespace into a single space
$name =~ s/\s+/ /g;
return ($name);
}
}
So passing in someone's name to "format_name" in "Customer" would cause leading and
trailing whitespace to be removed, then the name to be properly cased, and finally
whitespace to be compressed to a single space. The resulting $name would be returned to
the caller:
my ($name) = $obj->format_name($name_raw);
Unlike ":Cumulative" methods, ":Chained" methods always returns an array - even if there
is only one value returned. Therefore, ":Chained" methods should always be called in an
array context, as illustrated above.
The default direction is to chain methods from the parent classes at the top of the class
hierarchy down through the child classes. You may use the attribute ":Chained(top down)"
to make this more explicit.
If you label the method with the ":Chained(bottom up)" attribute, then the chained methods
are called starting with the object's class and working upward through the parent classes
in the class hierarchy, similar to how ":Cumulative(bottom up)" works.
ARGUMENT MERGING
As mentioned under "Object Creation", the "->new()" method can take parameters that are
passed in as combinations of "key => value" pairs and/or hash refs:
my $obj = My::Class->new(
'param_X' => 'value_X',
'param_Y' => 'value_Y',
{
'param_A' => 'value_A',
'param_B' => 'value_B',
},
{
'param_Q' => 'value_Q',
},
);
The parameters are merged into a single hash ref before they are processed.
Adding the ":MergeArgs" attribute to your methods gives them a similar capability. Your
method will then get two arguments: The object and a single hash ref of the merged
arguments. For example:
package Foo; {
use Object::InsideOut;
...
sub my_method :MergeArgs {
my ($self, $args) = @_;
my $param = $args->{'param'};
my $data = $args->{'data'};
my $flag = $args->{'flag'};
...
}
}
package main;
my $obj = Foo->new(...);
$obj->my_method( { 'data' => 42,
'flag' => 'true' },
'param' => 'foo' );
ARGUMENT VALIDATION
A number of users have asked about argument validation for methods:
<http://www.cpanforum.com/threads/3204>. For this, I recommend using Params::Validate:
package Foo; {
use Object::InsideOut;
use Params::Validate ':all';
sub foo
{
my $self = shift;
my %args = validate(@_, { bar => 1 });
my $bar = $args{bar};
...
}
}
Using Attribute::Params::Validate, attributes are used for argument validation
specifications:
package Foo; {
use Object::InsideOut;
use Attribute::Params::Validate;
sub foo :method :Validate(bar => 1)
{
my $self = shift;
my %args = @_;
my $bar = $args{bar};
...
}
}
Note that in the above, Perl's ":method" attribute (in all lowercase) is needed.
There is some incompatibility between Attribute::Params::Validate and some of
Object::InsideOut's attributes. Namely, you cannot use ":Validate" with ":Private",
":Restricted", ":Cumulative", ":Chained" or ":MergeArgs". In these cases, use the
"validate()" function from Params::Validate instead.
AUTOMETHODS
There are significant issues related to Perl's "AUTOLOAD" mechanism that cause it to be
ill-suited for use in a class hierarchy. Therefore, Object::InsideOut implements its own
":Automethod" mechanism to overcome these problems.
Classes requiring "AUTOLOAD"-type capabilities must provided a subroutine labeled with the
":Automethod" attribute. The ":Automethod" subroutine will be called with the object and
the arguments in the original method call (the same as for "AUTOLOAD"). The ":Automethod"
subroutine should return either a subroutine reference that implements the requested
method's functionality, or else just end with "return;" to indicate that it doesn't know
how to handle the request.
Using its own "AUTOLOAD" subroutine (which is exported to every class), Object::InsideOut
walks through the class tree, calling each ":Automethod" subroutine, as needed, to fulfill
an unimplemented method call.
The name of the method being called is passed as $_ instead of $AUTOLOAD, and is not
prefixed with the class name. If the ":Automethod" subroutine also needs to access the $_
from the caller's scope, it is available as $CALLER::_.
Automethods can also be made to act as "CUMULATIVE METHODS" or "CHAINED METHODS". In
these cases, the ":Automethod" subroutine should return two values: The subroutine ref to
handle the method call, and a string designating the type of method. The designator has
the same form as the attributes used to designate ":Cumulative" and ":Chained" methods:
':Cumulative' or ':Cumulative(top down)'
':Cumulative(bottom up)'
':Chained' or ':Chained(top down)'
':Chained(bottom up)'
The following skeletal code illustrates how an ":Automethod" subroutine could be
structured:
sub _automethod :Automethod
{
my $self = shift;
my @args = @_;
my $method_name = $_;
# This class can handle the method directly
if (...) {
my $handler = sub {
my $self = shift;
...
return ...;
};
### OPTIONAL ###
# Install the handler so it gets called directly next time
# no strict refs;
# *{__PACKAGE__.'::'.$method_name} = $handler;
################
return ($handler);
}
# This class can handle the method as part of a chain
if (...) {
my $chained_handler = sub {
my $self = shift;
...
return ...;
};
return ($chained_handler, ':Chained');
}
# This class cannot handle the method request
return;
}
Note: The OPTIONAL code above for installing the generated handler as a method should not
be used with ":Cumulative" or ":Chained" automethods.
OBJECT SERIALIZATION
Basic Serialization
my $array_ref = $obj->dump();
my $string = $obj->dump(1);
Object::InsideOut exports a method called "->dump()" to each class that returns either
a Perl or a string representation of the object that invokes the method.
The Perl representation is returned when "->dump()" is called without arguments. It
consists of an array ref whose first element is the name of the object's class, and
whose second element is a hash ref containing the object's data. The object data hash
ref contains keys for each of the classes that make up the object's hierarchy. The
values for those keys are hash refs containing "key => value" pairs for the object's
fields. For example:
[
'My::Class::Sub',
{
'My::Class' => {
'data' => 'value'
},
'My::Class::Sub' => {
'life' => 42
}
}
]
The name for an object field (data and life in the example above) can be specified by
adding the ":Name" attribute to the field:
my @life :Field :Name(life);
If the ":Name" attribute is not used, then the name for a field will be either the
name associated with an ":All" or ":Arg" attribute, its get method name, its set
method name, or, failing all that, a string of the form "ARRAY(0x...)" or
"HASH(0x...)".
When called with a true argument, "->dump()" returns a string version of the Perl
representation using Data::Dumper.
Note that using Data::Dumper directly on an inside-out object will not produce the
desired results (it'll just output the contents of the scalar ref). Also, if inside-
out objects are stored inside other structures, a dump of those structures will not
contain the contents of the object's fields.
In the event of a method naming conflict, the "->dump()" method can be called using
its fully-qualified name:
my $dump = $obj->Object::InsideOut::dump();
my $obj = Object::InsideOut->pump($data);
"Object::InsideOut->pump()" takes the output from the "->dump()" method, and returns
an object that is created using that data. If $data is the array ref returned by
using "$obj->dump()", then the data is inserted directly into the corresponding fields
for each class in the object's class hierarchy. If $data is the string returned by
using "$obj->dump(1)", then it is "eval"ed to turn it into an array ref, and then
processed as above.
Caveats: If any of an object's fields are dumped to field name keys of the form
"ARRAY(0x...)" or "HASH(0x...)" (see above), then the data will not be reloadable
using "Object::InsideOut->pump()". To overcome this problem, the class developer must
either add ":Name" attributes to the ":Field" declarations (see above), or provide a
":Dumper"/":Pumper" pair of subroutines as described below.
Dynamically altering a class (e.g., using ->create_field()) after objects have been
dumped will result in "undef" fields when pumped back in regardless of whether or not
the added fields have defaults.
Modifying the output from "->dump()", and then feeding it into
"Object::InsideOut->pump()" will work, but is not specifically supported. If you know
what you're doing, fine, but you're on your own.
":Dumper" Subroutine Attribute
If a class requires special processing to dump its data, then it can provide a
subroutine labeled with the ":Dumper" attribute. This subroutine will be sent the
object that is being dumped. It may then return any type of scalar the developer
deems appropriate. Usually, this would be a hash ref containing "key => value" pairs
for the object's fields. For example:
my @data :Field;
sub _dump :Dumper
{
my $obj = $_[0];
my %field_data;
$field_data{'data'} = $data[$$obj];
return (\%field_data);
}
Just be sure not to call your ":Dumper" subroutine "dump" as that is the name of the
dump method exported by Object::InsideOut as explained above.
":Pumper" Subroutine Attribute
If a class supplies a ":Dumper" subroutine, it will most likely need to provide a
complementary ":Pumper" labeled subroutine that will be used as part of creating an
object from dumped data using "Object::InsideOut->pump()". The subroutine will be
supplied the new object that is being created, and whatever scalar was returned by the
":Dumper" subroutine. The corresponding ":Pumper" for the example ":Dumper" above
would be:
sub _pump :Pumper
{
my ($obj, $field_data) = @_;
$obj->set(\@data, $field_data->{'data'});
}
Storable
Object::InsideOut also supports object serialization using the Storable module. There are
two methods for specifying that a class can be serialized using Storable. The first
method involves adding Storable to the Object::InsideOut declaration in your package:
package My::Class; {
use Object::InsideOut qw(Storable);
...
}
and adding "use Storable;" in your application. Then you can use the "->store()" and
"->freeze()" methods to serialize your objects, and the "retrieve()" and "thaw()"
subroutines to de-serialize them.
package main;
use Storable;
use My::Class;
my $obj = My::Class->new(...);
$obj->store('/tmp/object.dat');
...
my $obj2 = retrieve('/tmp/object.dat');
The other method of specifying Storable serialization involves setting a "::storable"
variable inside a "BEGIN" block for the class prior to its use:
package main;
use Storable;
BEGIN {
$My::Class::storable = 1;
}
use My::Class;
NOTE: The caveats discussed above for the "->pump()" method are also applicable when using
the Storable module.
OBJECT COERCION
Object::InsideOut provides support for various forms of object coercion through the
overload mechanism. For instance, if you want an object to be usable directly in a
string, you would supply a subroutine in your class labeled with the ":Stringify"
attribute:
sub as_string :Stringify
{
my $self = $_[0];
my $string = ...;
return ($string);
}
Then you could do things like:
print("The object says, '$obj'\n");
For a boolean context, you would supply:
sub as_bool :Boolify
{
my $self = $_[0];
my $true_or_false = ...;
return ($true_or_false);
}
and use it in this manner:
if (! defined($obj)) {
# The object is undefined
....
} elsif (! $obj) {
# The object returned a false value
...
}
The following coercion attributes are supported:
:Stringify
:Numerify
:Boolify
:Arrayify
:Hashify
:Globify
:Codify
Coercing an object to a scalar (":Scalarify") is not supported as $$obj is the ID of the
object and cannot be overridden.
CLONING
Object Cloning
Copies of objects can be created using the "->clone()" method which is exported by
Object::InsideOut to each class:
my $obj2 = $obj->clone();
When called without arguments, "->clone()" creates a shallow copy of the object, meaning
that any complex data structures (i.e., array, hash or scalar refs) stored in the object
will be shared with its clone.
Calling "->clone()" with a true argument:
my $obj2 = $obj->clone(1);
creates a deep copy of the object such that internally held array, hash or scalar refs are
replicated and stored in the newly created clone.
Deep cloning can also be controlled at the field level, and is covered in the next
section.
Note that cloning does not clone internally held objects. For example, if $foo contains a
reference to $bar, a clone of $foo will also contain a reference to $bar; not a clone of
$bar. If such behavior is needed, it must be provided using a :Replicate subroutine.
Field Cloning
Object cloning can be controlled at the field level such that specified fields are deeply
copied when "->clone()" is called without any arguments. This is done by adding the
":Deep" attribute to the field:
my @data :Field :Deep;
WEAK FIELDS
Frequently, it is useful to store weakened references to data or objects in a field. Such
a field can be declared as ":Weak" so that data (i.e., references) set via
Object::InsideOut generated accessors, parameter processing using ":Arg", the "->set()"
method, etc., will automatically be weakened after being stored in the field array/hash.
my @data :Field :Weak;
NOTE: If data in a weak field is set directly (i.e., the "->set()" method is not used),
then weaken() must be invoked on the stored reference afterwards:
$self->set(\@field, $data);
Scalar::Util::weaken($field[$$self]);
(This is another reason why the "->set()" method is recommended for setting field data
within class code.)
DYNAMIC FIELD CREATION
Normally, object fields are declared as part of the class code. However, some classes may
need the capability to create object fields on-the-fly, for example, as part of an
":Automethod". Object::InsideOut provides a class method for this:
# Dynamically create a hash field with standard accessors
My::Class->create_field('%'.$fld, ":Std($fld)");
The first argument is the class into which the field will be added. The second argument
is a string containing the name of the field preceded by either a "@" or "%" to declare an
array field or hash field, respectively. The remaining string arguments should be
attributes declaring accessors and the like. The ":Field" attribute is assumed, and does
not need to be added to the attribute list. For example:
My::Class->create_field('@data', ":Type(numeric)",
":Acc(data)");
My::Class->create_field('@obj', ":Type(Some::Class)",
":Acc(obj)",
":Weak");
Field creation will fail if you try to create an array field within a class whose
hierarchy has been declared :hash_only.
Here's an example of an ":Automethod" subroutine that uses dynamic field creation:
package My::Class; {
use Object::InsideOut;
sub _automethod :Automethod
{
my $self = $_[0];
my $class = ref($self) || $self;
my $method = $_;
# Extract desired field name from get_/set_ method name
my ($fld_name) = $method =~ /^[gs]et_(.*)$/;
if (! $fld_name) {
return; # Not a recognized method
}
# Create the field and its standard accessors
$class->create_field('@'.$fld_name, ":Std($fld_name)");
# Return code ref for newly created accessor
no strict 'refs';
return *{$class.'::'.$method}{'CODE'};
}
}
RUNTIME INHERITANCE
The class method "->add_class()" provides the capability to dynamically add classes to a
class hierarchy at runtime.
For example, suppose you had a simple state class:
package Trait::State; {
use Object::InsideOut;
my %state :Field :Set(state);
}
This could be added to another class at runtime using:
My::Class->add_class('Trait::State');
This permits, for example, application code to dynamically modify a class without having
it create an actual sub-class.
PREPROCESSING
Parameter Preprocessing
You can specify a code ref (either in the form of an anonymous subroutine, or a subroutine
name) for an object initialization parameter that will be called on that parameter prior
to taking any of the other parameter actions described above. Here's an example:
package My::Class; {
use Object::InsideOut;
# The parameter preprocessing subroutine
sub preproc
{
my ($class, $param, $spec, $obj, $value) = @_;
# Preform parameter preprocessing
...
# Return result
return ...;
}
my @data :Field
:Arg('Name' => 'DATA', 'Preprocess' => \&My::Class::preproc);
my %init_args :InitArgs = (
'PARAM' => {
'Preprocess' => \&preproc,
},
);
...
}
When used in the ":Arg" attribute, the subroutine name must be fully-qualified, as
illustrated. Further, if not referenced in the ":InitArgs" hash, the preprocessing
subroutine can be given the ":Private" attribute.
As the above illustrates, the parameter preprocessing subroutine is sent five arguments:
• The name of the class associated with the parameter
This would be "My::Class" in the example above.
• The name of the parameter
Either "DATA" or "PARAM" in the example above.
• A hash ref of the parameter's specifiers
This is either a hash ref containing the ":Arg" attribute parameters, or the hash ref
paired to the parameter's key in the ":InitArgs" hash.
• The object being initialized
• The parameter's value
This is the value assigned to the parameter in the "->new()" method's argument list.
If the parameter was not provided to "->new()", then "undef" will be sent.
The return value of the preprocessing subroutine will then be assigned to the parameter.
Be careful about what types of data the preprocessing subroutine tries to make use of
"external" to the arguments supplied. For instance, because the order of parameter
processing is not specified, the preprocessing subroutine cannot rely on whether or not
some other parameter is set. Such processing would need to be done in the ":Init"
subroutine. It can, however, make use of object data set by classes higher up in the
class hierarchy. (That is why the object is provided as one of the arguments.)
Possible uses for parameter preprocessing include:
• Overriding the supplied value (or even deleting it by returning "undef")
• Providing a dynamically-determined default value
Preprocess may be abbreviated to Preproc or Pre.
Set Accessor Preprocessing
You can specify a code ref (either in the form of an anonymous subroutine, or a fully-
qualified subroutine name) for a set/combined accessor that will be called on the
arguments supplied to the accessor prior to its taking the usual actions of type checking
and adding the data to the field. Here's an example:
package My::Class; {
use Object::InsideOut;
my @data :Field
:Acc('Name' => 'data', 'Preprocess' => \&My::Class::preproc);
# The set accessor preprocessing subroutine may be made 'Private'
sub preproc :Private
{
my ($self, $field, @args) = @_;
# Preform preprocessing on the accessor's arguments
...
# Return result
return ...;
}
}
As the above illustrates, the accessor preprocessing subroutine is sent the following
arguments:
• The object used to invoke the accessor
• A reference to the field associated with the accessor
• The argument(s) sent to the accessor
There will always be at least one argument.
Usually, the preprocessing subroutine would return just a single value. For fields
declared as type "List", multiple values may be returned.
Following preprocessing, the set accessor will operate on whatever value(s) are returned
by the preprocessing subroutine.
SPECIAL PROCESSING
Object ID
By default, the ID of an object is derived from a sequence counter for the object's class
hierarchy. This should suffice for nearly all cases of class development. If there is a
special need for the module code to control the object ID (see Math::Random::MT::Auto as
an example), then a subroutine labelled with the ":ID" attribute can be specified:
sub _id :ID
{
my $class = $_[0];
# Generate/determine a unique object ID
...
return ($id);
}
The ID returned by your subroutine can be any kind of regular scalar (e.g., a string or a
number). However, if the ID is something other than a low-valued integer, then you will
have to architect all your classes using hashes for the object fields. See "HASH ONLY
CLASSES" for details.
Within any class hierarchy, only one class may specify an ":ID" subroutine.
Object Replication
Object replication occurs explicitly when the "->clone()" method is called on an object,
and implicitly when threads are created in a threaded application. In nearly all cases,
Object::InsideOut will take care of all the details for you.
In rare cases, a class may require special handling for object replication. It must then
provide a subroutine labeled with the ":Replicate" attribute. This subroutine will be
sent three arguments: The parent and the cloned objects, and a flag:
sub _replicate :Replicate
{
my ($parent, $clone, $flag) = @_;
# Special object replication processing
if ($clone eq 'CLONE') {
# Handling for thread cloning
...
} elsif ($clone eq 'deep') {
# Deep copy of the parent
...
} else {
# Shallow copying
...
}
}
In the case of thread cloning, $flag will be set to the 'CLONE', and the $parent object is
just a non-blessed anonymous scalar reference that contains the ID for the object in the
parent thread.
When invoked via the "->clone()" method, $flag will be either an empty string which
denotes that a shallow copy is being produced for the clone, or $flag will be set to
'deep' indicating a deep copy is being produced.
The ":Replicate" subroutine only needs to deal with the special replication processing
needed by the object: Object::InsideOut will handle all the other details.
Object Destruction
Object::InsideOut exports a "DESTROY" method to each class that deletes an object's data
from the object field arrays (hashes). If a class requires additional destruction
processing (e.g., closing filehandles), then it must provide a subroutine labeled with the
":Destroy" attribute. This subroutine will be sent the object that is being destroyed:
sub _destroy :Destroy
{
my $obj = $_[0];
# Special object destruction processing
}
The ":Destroy" subroutine only needs to deal with the special destruction processing: The
"DESTROY" method will handle all the other details of object destruction.
FOREIGN CLASS INHERITANCE
Object::InsideOut supports inheritance from foreign (i.e., non-Object::InsideOut) classes.
This means that your classes can inherit from other Perl class, and access their methods
from your own objects.
One method of declaring foreign class inheritance is to add the class name to the
Object::InsideOut declaration inside your package:
package My::Class; {
use Object::InsideOut qw(Foreign::Class);
...
}
This allows you to access the foreign class's static (i.e., class) methods from your own
class. For example, suppose "Foreign::Class" has a class method called "foo". With the
above, you can access that method using "My::Class->foo()" instead.
Multiple foreign inheritance is supported, as well:
package My::Class; {
use Object::InsideOut qw(Foreign::Class Other::Foreign::Class);
...
}
$self->inherit($obj, ...);
To use object methods from foreign classes, an object must inherit from an object of
that class. This would normally be done inside a class's ":Init" subroutine:
package My::Class; {
use Object::InsideOut qw(Foreign::Class);
sub init :Init
{
my ($self, $args) = @_;
my $foreign_obj = Foreign::Class->new(...);
$self->inherit($foreign_obj);
}
}
Thus, with the above, if "Foreign::Class" has an object method called "bar", you can
call that method from your own objects:
package main;
my $obj = My::Class->new();
$obj->bar();
Object::InsideOut's "AUTOLOAD" subroutine handles the dispatching of the "->bar()"
method call using the internally held inherited object (in this case, $foreign_obj).
Multiple inheritance is supported, as well: You can call the "->inherit()" method
multiple times, or make just one call with all the objects to be inherited from.
"->inherit()" is a restricted method. In other words, you cannot use it on an object
outside of code belonging to the object's class tree (e.g., you can't call it from
application code).
In the event of a method naming conflict, the "->inherit()" method can be called using
its fully-qualified name:
$self->Object::InsideOut::inherit($obj);
my @objs = $self->heritage();
my $obj = $self->heritage($class);
my @objs = $self->heritage($class1, $class2, ...);
Your class code can retrieve any inherited objects using the "->heritage()" method.
When called without any arguments, it returns a list of any objects that were stored
by the calling class using the calling object. In other words, if class "My::Class"
uses object $obj to store foreign objects $fobj1 and $fobj2, then later on in class
"My::Class", "$obj->heritage()" will return $fobj1 and $fobj2.
"->heritage()" can also be called with one or more class name arguments. In this
case, only objects of the specified class(es) are returned.
In the event of a method naming conflict, the "->heritage()" method can be called
using its fully-qualified name:
my @objs = $self->Object::InsideOut::heritage();
$self->disinherit($class [, ...])
$self->disinherit($obj [, ...])
The "->disinherit()" method disassociates (i.e., deletes) the inheritance of foreign
object(s) from an object. The foreign objects may be specified by class, or using the
actual inherited object (retrieved via "->heritage()", for example).
The call is only effective when called inside the class code that established the
initial inheritance. In other words, if an inheritance is set up inside a class, then
disinheritance can only be done from inside that class.
In the event of a method naming conflict, the "->disinherit()" method can be called
using its fully-qualified name:
$self->Object::InsideOut::disinherit($obj [, ...])
NOTE: With foreign inheritance, you only have access to class and object methods. The
encapsulation of the inherited objects is strong, meaning that only the class where the
inheritance takes place has direct access to the inherited object. If access to the
inherited objects themselves, or their internal hash fields (in the case of blessed hash
objects), is needed outside the class, then you'll need to write your own accessors for
that.
LIMITATION: You cannot use fully-qualified method names to access foreign methods (when
encapsulated foreign objects are involved). Thus, the following will not work:
my $obj = My::Class->new();
$obj->Foreign::Class::bar();
Normally, you shouldn't ever need to do the above: "$obj->bar()" would suffice.
The only time this may be an issue is when the native class overrides an inherited foreign
class's method (e.g., "My::Class" has its own "->bar()" method). Such overridden methods
are not directly callable. If such overriding is intentional, then this should not be an
issue: No one should be writing code that tries to by-pass the override. However, if the
overriding is accidentally, then either the native method should be renamed, or the native
class should provide a wrapper method so that the functionality of the overridden method
is made available under a different name.
"use base" and Fully-qualified Method Names
The foreign inheritance methodology handled by the above is predicated on
non-Object::InsideOut classes that generate their own objects and expect their object
methods to be invoked via those objects.
There are exceptions to this rule:
1. Foreign object methods that expect to be invoked via the inheriting class's object, or
foreign object methods that don't care how they are invoked (i.e., they don't make
reference to the invoking object).
This is the case where a class provides auxiliary methods for your objects, but from
which you don't actually create any objects (i.e., there is no corresponding foreign
object, and "$obj->inherit($foreign)" is not used.)
In this case, you can either:
a. Declare the foreign class using the standard method (i.e.,
"use Object::InsideOut qw(Foreign::Class);"), and invoke its methods using their full
path (e.g., "$obj->Foreign::Class::method();"); or
b. You can use the base pragma so that you don't have to use the full path for foreign
methods.
package My::Class; {
use Object::InsideOut;
use base 'Foreign::Class';
...
}
The former scheme is faster.
2. Foreign class methods that expect to be invoked via the inheriting class.
As with the above, you can either invoke the class methods using their full path
(e.g., "My::Class->Foreign::Class::method();"), or you can "use base" so that you
don't have to use the full path. Again, using the full path is faster.
Class::Singleton is an example of this type of class.
3. Class methods that don't care how they are invoked (i.e., they don't make reference to
the invoking class).
In this case, you can either use "use Object::InsideOut qw(Foreign::Class);" for
consistency, or use "use base qw(Foreign::Class);" if (slightly) better performance is
needed.
If you're not familiar with the inner workings of the foreign class such that you don't
know if or which of the above exceptions applies, then the formulaic approach would be to
first use the documented method for foreign inheritance (i.e.,
"use Object::InsideOut qw(Foreign::Class);"). If that works, then I strongly recommend
that you just use that approach unless you have a good reason not to. If it doesn't work,
then try "use base".
INTROSPECTION
For Perl 5.8.0 and later, Object::InsideOut provides an introspection API that allow you
to obtain metadata on a class's hierarchy, constructor parameters, and methods.
my $meta = My::Class->meta();
my $meta = $obj->meta();
The "->meta()" method, which is exported by Object::InsideOut to each class, returns
an Object::InsideOut::Metadata object which can then be queried for information about
the invoking class or invoking object's class:
# Get an object's class hierarchy
my @classes = $obj->meta()->get_classes();
# Get info on the args for a class's constructor (i.e., ->new() parameters)
my %args = My::Class->meta()->get_args();
# Get info on the methods that can be called by an object
my %methods = $obj->meta()->get_methods();
My::Class->isa();
$obj->isa();
When called in an array context, calling "->isa()" without any arguments on an
Object::InsideOut class or object returns a list of the classes in the class hierarchy
for that class or object, and is equivalent to:
my @classes = $obj->meta()->get_classes();
When called in a scalar context, it returns an array ref containing the classes.
My::Class->can();
$obj->can();
When called in an array context, calling "->can()" without any arguments on an
Object::InsideOut class or object returns a list of the method names for that class or
object, and is equivalent to:
my %methods = $obj->meta()->get_methods();
my @methods = keys(%methods);
When called in a scalar context, it returns an array ref containing the method names.
See Object::InsideOut::Metadata for more details.
THREAD SUPPORT
For Perl 5.8.1 and later, Object::InsideOut fully supports threads (i.e., is thread safe),
and supports the sharing of Object::InsideOut objects between threads using
threads::shared.
To use Object::InsideOut in a threaded application, you must put "use threads;" at the
beginning of the application. (The use of "require threads;" after the program is running
is not supported.) If object sharing is to be utilized, then "use threads::shared;"
should follow.
If you just "use threads;", then objects from one thread will be copied and made available
in a child thread.
The addition of "use threads::shared;" in and of itself does not alter the behavior of
Object::InsideOut objects. The default behavior is to not share objects between threads
(i.e., they act the same as with "use threads;" alone).
To enable the sharing of objects between threads, you must specify which classes will be
involved with thread object sharing. There are two methods for doing this. The first
involves setting a "::shared" variable (inside a "BEGIN" block) for the class prior to its
use:
use threads;
use threads::shared;
BEGIN {
$My::Class::shared = 1;
}
use My::Class;
The other method is for a class to add a ":SHARED" flag to its "use Object::InsideOut ..."
declaration:
package My::Class; {
use Object::InsideOut ':SHARED';
...
}
When either sharing flag is set for one class in an object hierarchy, then all the classes
in the hierarchy are affected.
If a class cannot support thread object sharing (e.g., one of the object fields contains
code refs [which Perl cannot share between threads]), it should specifically declare this
fact:
package My::Class; {
use Object::InsideOut ':NOT_SHARED';
...
}
However, you cannot mix thread object sharing classes with non-sharing classes in the same
class hierarchy:
use threads;
use threads::shared;
package My::Class; {
use Object::InsideOut ':SHARED';
...
}
package Other::Class; {
use Object::InsideOut ':NOT_SHARED';
...
}
package My::Derived; {
use Object::InsideOut qw(My::Class Other::Class); # ERROR!
...
}
Here is a complete example with thread object sharing enabled:
use threads;
use threads::shared;
package My::Class; {
use Object::InsideOut ':SHARED';
# One list-type field
my @data :Field :Type(list) :Acc(data);
}
package main;
# New object
my $obj = My::Class->new();
# Set the object's 'data' field
$obj->data(qw(foo bar baz));
# Print out the object's data
print(join(', ', @{$obj->data()}), "\n"); # "foo, bar, baz"
# Create a thread and manipulate the object's data
my $rc = threads->create(
sub {
# Read the object's data
my $data = $obj->data();
# Print out the object's data
print(join(', ', @{$data}), "\n"); # "foo, bar, baz"
# Change the object's data
$obj->data(@$data[1..2], 'zooks');
# Print out the object's modified data
print(join(', ', @{$obj->data()}), "\n"); # "bar, baz, zooks"
return (1);
}
)->join();
# Show that changes in the object are visible in the parent thread
# I.e., this shows that the object was indeed shared between threads
print(join(', ', @{$obj->data()}), "\n"); # "bar, baz, zooks"
HASH ONLY CLASSES
For performance considerations, it is recommended that arrays be used for class fields
whenever possible. The only time when hash-bases fields are required is when a class must
provide its own object ID, and those IDs are something other than low-valued integers. In
this case, hashes must be used for fields not only in the class that defines the object ID
subroutine, but also in every class in any class hierarchy that include such a class.
The hash only requirement can be enforced by adding the ":HASH_ONLY" flag to a class's
"use Object::InsideOut ..." declaration:
package My::Class; {
use Object::InsideOut ':hash_only';
...
}
This will cause Object::Inside to check every class in any class hierarchy that includes
such flagged classes to make sure their fields are hashes and not arrays. It will also
fail any ->create_field() call that tries to create an array-based field in any such
class.
SECURITY
In the default case where Object::InsideOut provides object IDs that are sequential
integers, it is possible to hack together a fake Object::InsideOut object, and so gain
access to another object's data:
my $fake = bless(\do{my $scalar}, 'Some::Class');
$$fake = 86; # ID of another object
my $stolen = $fake->get_data();
Why anyone would try to do this is unknown. How this could be used for any sort of
malicious exploitation is also unknown. However, if preventing this sort of security
issue is a requirement, it can be accomplished by adding the ":SECURE" flag to a class's
"use Object::InsideOut ..." declaration:
package My::Class; {
use Object::InsideOut ':SECURE';
...
}
This places the module "Object::InsideOut::Secure" in the class hierarchy.
Object::InsideOut::Secure provides an :ID subroutine that generates random integers for
object IDs, thus preventing other code from being able to create fake objects by guessing
at IDs.
Using ":SECURE" mode requires Math::Random::MT::Auto (v5.04 or later).
Because the object IDs used with ":SECURE" mode are large random values, the :HASH_ONLY
flag is forced on all the classes in the hierarchy.
For efficiency, it is recommended that the ":SECURE" flag be added to the topmost
class(es) in a hierarchy.
ATTRIBUTE HANDLERS
Object::InsideOut uses attribute 'modify' handlers as described in "Package-specific
Attribute Handling" in attributes, and provides a mechanism for adding attribute handlers
to your own classes. Instead of naming your attribute handler as "MODIFY_*_ATTRIBUTES",
name it something else and then label it with the ":MODIFY_*_ATTRIBUTES" attribute (or
":MOD_*_ATTRS" for short). Your handler should work just as described in "Package-
specific Attribute Handling" in attributes with regard to its input arguments, and must
return a list of the attributes which were not recognized by your handler. Here's an
example:
package My::Class; {
use Object::InsideOut;
sub _scalar_attrs :MOD_SCALAR_ATTRS
{
my ($pkg, $scalar, @attrs) = @_;
my @unused_attrs; # List of any unhandled attributes
while (my $attr = shift(@attrs)) {
if ($attr =~ /.../) {
# Handle attribute
...
} else {
# We don't handle this attribute
push(@unused_attrs, $attr);
}
}
return (@unused_attrs); # Pass along unhandled attributes
}
}
Attribute 'modify' handlers are called upward through the class hierarchy (i.e., bottom
up). This provides child classes with the capability to override the handling of
attributes by parent classes, or to add attributes (via the returned list of unhandled
attributes) for parent classes to process.
Attribute 'modify' handlers should be located at the beginning of a package, or at least
before any use of attributes on the corresponding type of variable or subroutine:
package My::Class; {
use Object::InsideOut;
sub _array_attrs :MOD_ARRAY_ATTRS
{
...
}
my @my_array :MyArrayAttr;
}
For attribute 'fetch' handlers, follow the same procedures: Label the subroutine with the
":FETCH_*_ATTRIBUTES" attribute (or ":FETCH_*_ATTRS" for short). Contrary to the
documentation in "Package-specific Attribute Handling" in attributes, attribute 'fetch'
handlers receive two arguments: The relevant package name, and a reference to a variable
or subroutine for which package-defined attributes are desired.
Attribute handlers are normal rendered hidden.
SPECIAL USAGE
Usage With "Exporter"
It is possible to use Exporter to export functions from one inside-out object class to
another:
use strict;
use warnings;
package Foo; {
use Object::InsideOut 'Exporter';
BEGIN {
our @EXPORT_OK = qw(foo_name);
}
sub foo_name
{
return (__PACKAGE__);
}
}
package Bar; {
use Object::InsideOut 'Foo' => [ qw(foo_name) ];
sub get_foo_name
{
return (foo_name());
}
}
package main;
print("Bar got Foo's name as '", Bar::get_foo_name(), "'\n");
Note that the "BEGIN" block is needed to ensure that the Exporter symbol arrays (in this
case @EXPORT_OK) get populated properly.
Usage With "require" and "mod_perl"
Object::InsideOut usage under mod_perl and with runtime-loaded classes is supported
automatically; no special coding is required.
Caveat: Runtime loading of classes should be performed before any objects are created
within any of the classes in their hierarchies. If Object::InsideOut cannot create a
hierarchy because of previously created objects (even if all those objects have been
destroyed), a runtime error will be generated.
Singleton Classes
A singleton class is a case where you would provide your own "->new()" method that in turn
calls Object::InsideOut's "->new()" method:
package My::Class; {
use Object::InsideOut;
my $singleton;
sub new {
my $thing = shift;
if (! $singleton) {
$singleton = $thing->Object::InsideOut::new(@_);
}
return ($singleton);
}
}
DIAGNOSTICS
Object::InsideOut uses "Exception::Class" for reporting errors. The base error class for
this module is "OIO". Here is an example of the basic manner for trapping and handling
errors:
my $obj;
eval { $obj = My::Class->new(); };
if (my $e = OIO->caught()) {
warn('Failure creating object: '.$e);
...
}
A more comprehensive approach might employ elements of the following:
eval { ... };
if (my $e = OIO->caught()) {
# An error generated by Object::InsideOut
...
} elsif (my $e = Exception::Class::Base->caught()) {
# An error generated by other code that uses Exception::Class
...
} elsif ($@) {
# An unhandled error (i.e., generated by code that doesn't use
# Exception::Class)
...
}
I have tried to make the messages and information returned by the error objects as
informative as possible. Suggested improvements are welcome. Also, please bring to my
attention any conditions that you encounter where an error occurs as a result of
Object::InsideOut code that doesn't generate an Exception::Class object. Here is one such
error:
Invalid ARRAY/HASH attribute
This error indicates you forgot "use Object::InsideOut;" in your class's code.
Object::InsideOut installs a "__DIE__" handler (see "die LIST" in perlfunc and "eval
BLOCK" in perlfunc) to catch any errant exceptions from class-specific code, namely,
":Init", ":Replicate", ":Destroy", etc. subroutines. When using "eval" blocks inside
these subroutines, you should localize $SIG{'__DIE__'} to keep Object::InsideOut's
"__DIE__" handler from interfering with exceptions generated inside the "eval" blocks.
For example:
sub _init :Init {
...
eval {
local $SIG{'__DIE__'};
...
};
if $@ {
# Handle caught exception
}
...
}
Here's another example, where the "die" function is used as a method of flow control for
leaving an "eval" block:
eval {
local $SIG{'__DIE__'}; # Suppress any existing __DIE__ handler
...
die({'found' => 1}) if $found; # Leave the eval block
...
};
if ($@) {
die unless (ref($@) && $@->{'found'}); # Propagate any 'real' error
# Handle 'found' case
...
}
# Handle 'not found' case
Similarly, if calling code from other modules that use the above flow control mechanism,
but without localizing $SIG{'__DIE__'}, you can workaround this deficiency with your own
"eval" block:
eval {
local $SIG{'__DIE__'}; # Suppress any existing __DIE__ handler
Some::Module::func(); # Call function that fails to localize
};
if ($@) {
# Handle caught exception
}
In addition, you should file a bug report against the offending module along with a patch
that adds the missing "local $SIG{'__DIE__'};" statement.
BUGS AND LIMITATIONS
If you receive an error similar to this:
ERROR: Attempt to DESTROY object ID 1 of class Foo twice
the cause may be that some module used by your application is doing "require threads"
somewhere in the background. DBI is one such module. The workaround is to add "use
threads;" at the start of your application.
Another cause of the above is returning a non-shared object from a thread either
explicitly or implicitly when the result of the last statement in the thread subroutine is
an object. For example:
sub thr_func {
my $obj = MyClass->new();
}
which is equivalent to:
sub thr_func {
return MyClass->new();
}
This can be avoided by ensuring your thread subroutine ends with "return;".
The equality operator (e.g., "if ($obj1 == $obj2) { ...") is overloaded for ":SHARED"
classes when threads::shared is loaded. The overload subroutine compares object classes
and IDs because references to the same thread shared object may have different refaddrs.
You cannot overload an object to a scalar context (i.e., can't ":SCALARIFY").
You cannot use two instances of the same class with mixed thread object sharing in same
application.
Cannot use attributes on subroutine stubs (i.e., forward declaration without later
definition) with ":Automethod":
package My::Class; {
sub method :Private; # Will not work
sub _automethod :Automethod
{
# Code to handle call to 'method' stub
}
}
Due to limitations in the Perl parser, the entirety of any one attribute must be on a
single line. (However, multiple attributes may appear on separate lines.)
If a set accessor accepts scalars, then you can store any inside-out object type in it.
If its "Type" is set to "HASH", then it can store any blessed hash object.
Returning objects from threads does not work:
my $obj = threads->create(sub { return (Foo->new()); })->join(); # BAD
Instead, use thread object sharing, create the object before launching the thread, and
then manipulate the object inside the thread:
my $obj = Foo->new(); # Class 'Foo' is set ':SHARED'
threads->create(sub { $obj->set_data('bar'); })->join();
my $data = $obj->get_data();
Due to a limitation in threads::shared version 1.39 and earlier, if storing shared objects
inside other shared objects, you should use "delete()" to remove them from internal fields
(e.g., "delete($field[$$self]);") when necessary so that the objects' destructor gets
called. Upgrading to version 1.40 or later alleviates most of this issue except during
global destruction. See threads::shared for more.
With Perl 5.8.8 and earlier, there are bugs associated with threads::shared that may
prevent you from storing objects inside of shared objects, or using foreign inheritance
with shared objects. With Perl 5.8.9 (and later) together with threads::shared 1.15 (and
later), you can store shared objects inside of other shared objects, and you can use
foreign inheritance with shared objects (provided the foreign class supports shared
objects as well).
Due to internal complexities, the following actions are not supported in code that uses
threads::shared while there are any threads active:
• Runtime loading of Object::InsideOut classes
• Using ->add_class()
It is recommended that such activities, if needed, be performed in the main application
code before any threads are created (or at least while there are no active threads).
For Perl 5.6.0 through 5.8.0, a Perl bug prevents package variables (e.g., object
attribute arrays/hashes) from being referenced properly from subroutine refs returned by
an ":Automethod" subroutine. For Perl 5.8.0 there is no workaround: This bug causes Perl
to core dump. For Perl 5.6.0 through 5.6.2, the workaround is to create a ref to the
required variable inside the ":Automethod" subroutine, and use that inside the subroutine
ref:
package My::Class; {
use Object::InsideOut;
my %data;
sub auto :Automethod
{
my $self = $_[0];
my $name = $_;
my $data = \%data; # Workaround for 5.6.X bug
return sub {
my $self = shift;
if (! @_) {
return ($$data{$name});
}
$$data{$name} = shift;
};
}
}
For Perl 5.8.1 through 5.8.4, a Perl bug produces spurious warning messages when threads
are destroyed. These messages are innocuous, and can be suppressed by adding the
following to your application code:
$SIG{'__WARN__'} = sub {
if ($_[0] !~ /^Attempt to free unreferenced scalar/) {
print(STDERR @_);
}
};
A better solution would be to upgrade threads and threads::shared from CPAN, especially if
you encounter other problems associated with threads.
For Perl 5.8.4 and 5.8.5, the "Storable" feature does not work due to a Perl bug. Use
Object::InsideOut v1.33 if needed.
Due to bugs in the Perl interpreter, using the introspection API (i.e. "->meta()", etc.)
requires Perl 5.8.0 or later.
The version of Want that is available via PPM for ActivePerl is defective, and causes
failures when using ":lvalue" accessors. Remove it, and then download and install the
Want module using CPAN.
Devel::StackTrace (used by Exception::Class) makes use of the DB namespace. As a
consequence, Object::InsideOut thinks that "package DB" is already loaded. Therefore, if
you create a class called DB that is sub-classed by other packages, you may need to
"require" it as follows:
package DB::Sub; {
require DB;
use Object::InsideOut qw(DB);
...
}
View existing bug reports at, and submit any new bugs, problems, patches, etc. to:
<http://rt.cpan.org/Public/Dist/Display.html?Name=Object-InsideOut>
REQUIREMENTS
Perl 5.6.0 or later
Exception::Class v1.22 or later
Scalar::Util v1.10 or later
It is possible to install a pure perl version of Scalar::Util, however, it will be
missing the weaken() function which is needed by Object::InsideOut. You'll need to
upgrade your version of Scalar::Util to one that supports its "XS" code.
Test::More v0.50 or later
Needed for testing during installation.
Want v0.12 or later
Optional. Provides support for ":lvalue Accessors".
Math::Random::MT::Auto v5.04 or later)
Optional. Provides support for :SECURE mode.
To cover all of the above requirements and more, it is recommended that you install
Bundle::Object::InsideOut using CPAN:
perl -MCPAN -e 'install Bundle::Object::InsideOut'
This will install the latest versions of all the required and optional modules needed for
full support of all of the features provided by Object::InsideOut.
SEE ALSO
Object::InsideOut on MetaCPAN: <https://metacpan.org/release/Object-InsideOut>
Code repository: <https://github.com/jdhedden/Object-InsideOut>
Inside-out Object Model:
<http://www.perlfoundation.org/perl5/index.cgi?inside_out_object>,
<http://www.perlmonks.org/?node_id=219378>, <http://www.perlmonks.org/?node_id=483162>,
<http://www.perlmonks.org/?node_id=515650>, Chapters 15 and 16 of Perl Best Practices by
Damian Conway
Object::InsideOut::Metadata
Storable, <Exception:Class>, Want, Math::Random::MT::Auto, attributes, overload
Sample code in the examples directory of this distribution on CPAN.
ACKNOWLEDGEMENTS
Abigail <perl AT abigail DOT nl> for inside-out objects in general.
Damian Conway <dconway AT cpan DOT org> for Class::Std, and for delegator methods.
David A. Golden <dagolden AT cpan DOT org> for thread handling for inside-out objects.
Dan Kubb <dan.kubb-cpan AT autopilotmarketing DOT com> for ":Chained" methods.
AUTHOR
Jerry D. Hedden, <jdhedden AT cpan DOT org>
COPYRIGHT AND LICENSE
Copyright 2005 - 2012 Jerry D. Hedden. All rights reserved.
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
TRANSLATIONS
A Japanese translation of this documentation by TSUJII, Naofumi
<tsun DOT nt AT gmail DOT com> is available at <http://perldoc.jp/docs/modules/>.