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NAME

Object::InsideOut - Comprehensive inside-out object support module

VERSION

This document describes Object::InsideOut version 4.02

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:

Object::InsideOut offers all the capabilities of other inside-out object modules with the following additional key advantages:

CLASSES

To use this module, each of your classes will start with pppppppppppppppppppppppppppppppppppppppppp:

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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp, My::Class will also get pppppppppppppppppppppppppppppppppppppppppp, and Parent::Class will also get pppppppppppppppppppppppppppppppppppppppppp. 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 pppppppppppppppppppppppppppppppppppppppppp, and Parent::Class will get pppppppppppppppppppppppppppppppppppppppppp.

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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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.

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., pppppppppppppppppppppppppppppppppppppppppp).

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 pppppppppppppppppppppppppppppppppppppppppp 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:
  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 pppppppppppppppppppppppppppppppppppppppppp):

$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 substituion 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):

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 pppppppppppppppppppppppppppppppppppppppppp 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:

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 pppppppppppppppppppppppppppppppppppppppppp), and propagate from the top down through the class hierarchy (i.e., from the parent classes down through the child classes). If tagged with pppppppppppppppppppppppppppppppppppppppppp, 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 pppppppppppppppppppppppppppppppppppppppppp to make this more explicit.

If you label the method with the pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp works.

ARGUMENT MERGING

As mentioned under "Object Creation", the ->new() method can take parameters that are passed in as combinations of pppppppppppppppppppppppppppppppppppppppppp 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 . 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

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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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:

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 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:

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:

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);
    ...
}

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., pppppppppppppppppppppppppppppppppppppppppp), 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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp for consistency, or use pppppppppppppppppppppppppppppppppppppppppp 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., pppppppppppppppppppppppppppppppppppppppppp). 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 pppppppppppppppppppppppppppppppppppppppppp.

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.

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 pppppppppppppppppppppppppppppppppppppppppp at the beginning of the application. (The use of pppppppppppppppppppppppppppppppppppppppppp after the program is running is not supported.) If object sharing is to be utilized, then pppppppppppppppppppppppppppppppppppppppppp should follow.

If you just pppppppppppppppppppppppppppppppppppppppppp, then objects from one thread will be copied and made available in a child thread.

The addition of pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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 pppppppppppppppppppppppppppppppppppppppppp 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:

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 pppppppppppppppppppppppppppppppppppppppppp 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:

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 pppppppppppppppppppppppppppppppppppppppppp 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

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 Discussion Forum on CPAN: http://www.cpanforum.com/dist/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

ACKNOWLEDGEMENTS

Abigail pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp for inside-out objects in general.

Damian Conway ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp for Class::Std, and for delegator methods.

David A. Golden pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp for thread handling for inside-out objects.

Dan Kubb ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp for :Chained methods.

AUTHOR

Jerry D. Hedden, pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp

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 pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp is available at http://perldoc.jp/docs/modules/.