NAME¶
Object::InsideOut - Comprehensive inside-out object support module
VERSION¶
This document describes Object::InsideOut version 3.87
SYNOPSIS¶
package My::Class; {
use Object::InsideOut;
# Numeric field
# With combined get+set accessor
my @data
:Field
:Type(numeric)
:Accessor(data);
# Takes 'INPUT' (or 'input', etc.) as a mandatory parameter to ->new()
my %init_args :InitArgs = (
'INPUT' => {
'Regex' => qr/^input$/i,
'Mandatory' => 1,
'Type' => 'numeric',
},
);
# Handle class-specific args as part of ->new()
sub init :Init
{
my ($self, $args) = @_;
# Put 'input' parameter into 'data' field
$self->set(\@data, $args->{'INPUT'});
}
}
package My::Class::Sub; {
use Object::InsideOut qw(My::Class);
# List field
# With standard 'get_X' and 'set_X' accessors
# Takes 'INFO' as an optional list parameter to ->new()
# Value automatically added to @info array
# Defaults to [ 'empty' ]
my @info
:Field
:Type(list)
:Standard(info)
:Arg('Name' => 'INFO', 'Default' => 'empty');
}
package Foo; {
use Object::InsideOut;
# Field containing My::Class objects
# With combined accessor
# Plus automatic parameter processing on object creation
my @foo
:Field
:Type(My::Class)
:All(foo);
}
package main;
my $obj = My::Class::Sub->new('Input' => 69);
my $info = $obj->get_info(); # [ 'empty' ]
my $data = $obj->data(); # 69
$obj->data(42);
$data = $obj->data(); # 42
$obj = My::Class::Sub->new('INFO' => 'help', 'INPUT' => 86);
$data = $obj->data(); # 86
$info = $obj->get_info(); # [ 'help' ]
$obj->set_info(qw(foo bar baz));
$info = $obj->get_info(); # [ 'foo', 'bar', 'baz' ]
my $foo_obj = Foo->new('foo' => $obj);
$foo_obj->foo()->data(); # 86
DESCRIPTION¶
This module provides comprehensive support for implementing classes using the
inside-out object model.
Object::InsideOut implements inside-out objects as anonymous scalar references
that are blessed into a class with the scalar containing the ID for the object
(usually a sequence number). For Perl 5.8.3 and later, the scalar reference is
set as
read-only to prevent
accidental modifications to the ID.
Object data (i.e., fields) are stored within the class's package in either
arrays indexed by the object's ID, or hashes keyed to the object's ID.
The virtues of the inside-out object model over the
blessed hash object
model have been extolled in detail elsewhere. See the informational links
under "SEE ALSO". Briefly, inside-out objects offer the following
advantages over
blessed hash objects:
- •
- Encapsulation
Object data is enclosed within the class's code and is accessible only
through the class-defined interface.
- •
- Field Name Collision Avoidance
Inheritance using blessed hash classes can lead to conflicts if any
classes use the same name for a field (i.e., hash key). Inside-out objects
are immune to this problem because object data is stored inside each
class's package, and not in the object itself.
- •
- Compile-time Name Checking
A common error with blessed hash classes is the misspelling of field
names:
$obj->{'coment'} = 'Say what?'; # Should be 'comment' not 'coment'
As there is no compile-time checking on hash keys, such errors do not
usually manifest themselves until runtime.
With inside-out objects, text hash keys are not used for accessing
field data. Field names and the data index (i.e., $$self) are checked by
the Perl compiler such that any typos are easily caught using
"perl -c".
$coment[$$self] = $value; # Causes a compile-time error
# or with hash-based fields
$comment{$$self} = $value; # Also causes a compile-time error
Object::InsideOut offers all the capabilities of other inside-out object modules
with the following additional key advantages:
- •
- Speed
When using arrays to store object data, Object::InsideOut objects are as
much as 40% faster than blessed hash objects for fetching and
setting data, and even with hashes they are still several percent faster
than blessed hash objects.
- •
- Threads
Object::InsideOut is thread safe, and thoroughly supports sharing objects
between threads using threads::shared.
- •
- Flexibility
Allows control over object ID specification, accessor naming, parameter name
matching, and much more.
- •
- Runtime Support
Supports classes that may be loaded at runtime (i.e., using
"eval { require ...; };"). This makes it
usable from within mod_perl, as well. Also supports additions to class
hierarchies, and dynamic creation of object fields during runtime.
- •
- Exception Objects
Object::InsideOut uses Exception::Class for handling errors in an
OO-compatible manner.
- •
- Object Serialization
Object::InsideOut has built-in support for object dumping and reloading that
can be accomplished in either an automated fashion or through the use of
class-supplied subroutines. Serialization using Storable is also
supported.
- •
- Foreign Class Inheritance
Object::InsideOut allows classes to inherit from foreign (i.e.,
non-Object::InsideOut) classes, thus allowing you to sub-class other Perl
class, and access their methods from your own objects.
- •
- Introspection
Obtain constructor parameters and method metadata for Object::InsideOut
classes.
CLASSES¶
To use this module, each of your classes will start with
"use Object::InsideOut;":
package My::Class; {
use Object::InsideOut;
...
}
Sub-classes (child classes) inherit from base classes (parent classes) by
telling Object::InsideOut what the parent class is:
package My::Sub; {
use Object::InsideOut qw(My::Parent);
...
}
Multiple inheritance is also supported:
package My::Project; {
use Object::InsideOut qw(My::Class Another::Class);
...
}
Object::InsideOut acts as a replacement for the "base" pragma: It
loads the parent module(s), calls their "->import()" methods, and
sets up the sub-class's @ISA array. Therefore, you should not
"use base ..." yourself, nor try to set up @ISA arrays.
Further, you should not use a class's @ISA array to determine a class's
hierarchy: See "INTROSPECTION" for details on how to do this.
If a parent class takes parameters (e.g., symbols to be exported via Exporter),
enclose them in an array ref (mandatory) following the name of the parent
class:
package My::Project; {
use Object::InsideOut 'My::Class' => [ 'param1', 'param2' ],
'Another::Class' => [ 'param' ];
...
}
OBJECTS¶
Object Creation¶
Objects are created using the "->new()" method which is exported by
Object::InsideOut to each class, and is invoked in the following manner:
my $obj = My::Class->new();
Object::InsideOut then handles all the messy details of initializing the object
in each of the classes in the invoking class's hierarchy. As such, classes do
not (normally) implement their own "->new()" method.
Usually, object fields are initially populated with data as part of the object
creation process by passing parameters to the "->new()" method.
Parameters are passed in as combinations of
"key => value" pairs and/or hash refs:
my $obj = My::Class->new('param1' => 'value1');
# or
my $obj = My::Class->new({'param1' => 'value1'});
# or even
my $obj = My::Class->new(
'param_X' => 'value_X',
'param_Y' => 'value_Y',
{
'param_A' => 'value_A',
'param_B' => 'value_B',
},
{
'param_Q' => 'value_Q',
},
);
Additionally, parameters can be segregated in hash refs for specific classes:
my $obj = My::Class->new(
'foo' => 'bar',
'My::Class' => { 'param' => 'value' },
'Parent::Class' => { 'data' => 'info' },
);
The initialization methods for both classes in the above will get
'foo' => 'bar', "My::Class" will also get
'param' => 'value', and "Parent::Class" will also get
'data' => 'info'. In this scheme, class-specific parameters will
override general parameters specified at a higher level:
my $obj = My::Class->new(
'default' => 'bar',
'Parent::Class' => { 'default' => 'baz' },
);
"My::Class" will get 'default' => 'bar', and
"Parent::Class" will get 'default' => 'baz'.
Calling "->new()" on an object works, too, and operates the same as
calling "->new()" for the class of the object (i.e.,
"$obj->new()" is the same as "ref($obj)->new()").
How the parameters passed to the "->new()" method are used to
initialize the object is discussed later under "OBJECT
INITIALIZATION".
NOTE: You cannot create objects from Object::InsideOut itself:
# This is an error
# my $obj = Object::InsideOut->new();
In this way, Object::InsideOut is not an object class, but functions more like a
pragma.
Object IDs¶
As stated earlier, this module implements inside-out objects as anonymous,
read-only scalar references that are blessed into a class with the scalar
containing the ID for the object.
Within methods, the object is passed in as the first argument:
sub my_method
{
my $self = shift;
...
}
The object's ID is then obtained by dereferencing the object: $$self. Normally,
this is only needed when accessing the object's field data:
my @my_field :Field;
sub my_method
{
my $self = shift;
...
my $data = $my_field[$$self];
...
}
At all other times, and especially in application code, the object should be
treated as an
opaque entity.
ATTRIBUTES¶
Much of the power of Object::InsideOut comes from the use of
attributes:
Tags on variables and subroutines that the attributes module sends to
Object::InsideOut at compile time. Object::InsideOut then makes use of the
information in these tags to handle such operations as object construction,
automatic accessor generation, and so on.
(Note: The use of attributes is not the same thing as source filtering.)
An attribute consists of an identifier preceded by a colon, and optionally
followed by a set of parameters in parentheses. For example, the attributes on
the following array declare it as an object field, and specify the generation
of an accessor method for that field:
my @level :Field :Accessor(level);
When multiple attributes are assigned to a single entity, they may all appear on
the same line (as shown above), or on separate lines:
my @level
:Field
:Accessor(level);
However, due to limitations in the Perl parser, the entirety of any one
attribute must be on a single line:
# This doesn't work
# my @level
# :Field
# :Accessor('Name' => 'level',
# 'Return' => 'Old');
# Each attribute must be all on one line
my @level
:Field
:Accessor('Name' => 'level', 'Return' => 'Old');
For Object::InsideOut's purposes, the case of an attribute's name does not
matter:
my @data :Field;
# or
my @data :FIELD;
However, by convention (as denoted in the attributes module), an attribute's
name should not be all lowercase.
FIELDS¶
Field Declarations¶
Object data fields consist of arrays within a class's package into which data
are stored using the object's ID as the array index. An array is declared as
being an object field by following its declaration with the ":Field"
attribute:
my @info :Field;
Object data fields may also be hashes:
my %data :Field;
However, as array access is as much as 40% faster than hash access, you should
stick to using arrays. See "HASH ONLY CLASSES" for more information
on when hashes may be required.
Getting Data¶
In class code, data can be fetched directly from an object's field array (hash)
using the object's ID:
$data = $field[$$self];
# or
$data = $field{$$self};
Setting Data¶
Analogous to the above, data can be put directly into an object's field array
(hash) using the object's ID:
$field[$$self] = $data;
# or
$field{$$self} = $data;
However, in threaded applications that use data sharing (i.e., use
"threads::shared"), the above will not work when the object is
shared between threads and the data being stored is either an array, hash or
scalar reference (this includes other objects). This is because the $data must
first be converted into shared data before it can be put into the field.
Therefore, Object::InsideOut automatically exports a method called
"->set()" to each class. This method should be used in class code
to put data into object fields whenever there is the possibility that the
class code may be used in an application that uses threads::shared (i.e., to
make your class code
thread-safe). The "->set()" method
handles all details of converting the data to a shared form, and storing it in
the field.
The "->set()" method, requires two arguments: A reference to the
object field array/hash, and the data (as a scalar) to be put in it:
my @my_field :Field;
sub store_data
{
my ($self, $data) = @_;
...
$self->set(\@my_field, $data);
}
To be clear, the "->set()" method is used inside class code; not
application code. Use it inside any object methods that set data in object
field arrays/hashes.
In the event of a method naming conflict, the "->set()" method can
be called using its fully-qualified name:
$self->Object::InsideOut::set(\@field, $data);
OBJECT INITIALIZATION¶
As stated in "Object Creation", object fields are initially populated
with data as part of the object creation process by passing
"key => value" parameters to the "->new()"
method. These parameters can be processed automatically into object fields, or
can be passed to a class-specific object initialization subroutine.
Field-Specific Parameters¶
When an object creation parameter corresponds directly to an object field, you
can specify for Object::InsideOut to automatically place the parameter into
the field by adding the ":Arg" attribute to the field declaration:
my @foo :Field :Arg(foo);
For the above, the following would result in $val being placed in
"My::Class"'s @foo field during object creation:
my $obj = My::Class->new('foo' => $val);
Object Initialization Subroutines¶
Many times, object initialization parameters do not correspond directly to
object fields, or they may require special handling. For these, parameter
processing is accomplished through a combination of an ":InitArgs"
labeled hash, and an ":Init" labeled subroutine.
The ":InitArgs" labeled hash specifies the parameters to be extracted
from the argument list supplied to the "->new()" method. Those
parameters (and only those parameters) which match the keys in the
":InitArgs" hash are then packaged together into a single hash ref.
The newly created object and this parameter hash ref are then sent to the
":Init" subroutine for processing.
Here is an example of a class with an
automatically handled field and an
:Init handled field:
package My::Class; {
use Object::InsideOut;
# Automatically handled field
my @my_data :Field :Acc(data) :Arg(MY_DATA);
# ':Init' handled field
my @my_field :Field;
my %init_args :InitArgs = (
'MY_PARAM' => '',
);
sub _init :Init
{
my ($self, $args) = @_;
if (exists($args->{'MY_PARAM'})) {
$self->set(\@my_field, $args->{'MY_PARAM'});
}
}
...
}
An object for this class would be created as follows:
my $obj = My::Class->new('MY_DATA' => $dat,
'MY_PARAM' => $parm);
This results in, first of all, $dat being placed in the object's @my_data field
because the "MY_DATA" key is specified in the ":Arg"
attribute for that field.
Then, "_init" is invoked with arguments consisting of the object
(i.e., $self) and a hash ref consisting only of
"{ 'MY_PARAM' => $param }" because the key
"MY_PARAM" is specified in the ":InitArgs" hash.
"_init" checks that the parameter "MY_PARAM" exists in the
hash ref, and then (since it does exist) adds $parm to the object's @my_field
field.
- Setting Data
- Data processed by the ":Init" subroutine may be
placed directly into the class's field arrays (hashes) using the object's
ID (i.e., $$self):
$my_field[$$self] = $args->{'MY_PARAM'};
However, as shown in the example above, it is strongly recommended that you
use the -> set() method:
$self->set(\@my_field, $args->{'MY_PARAM'});
which handles converting the data to a shared format when needed for
applications using threads::shared.
- All Parameters
- The ":InitArgs" hash and the ":Arg"
attribute on fields act as filters that constrain which initialization
parameters are and are not sent to the ":Init" subroutine. If,
however, a class does not have an ":InitArgs" hash and
does not use the ":Arg" attribute on any of its fields, then its
":Init" subroutine (if it exists, of course) will get all the
initialization parameters supplied to the "->new()"
method.
Mandatory Parameters¶
Field-specific parameters may be declared mandatory as follows:
my @data :Field
:Arg('Name' => 'data', 'Mandatory' => 1);
If a mandatory parameter is missing from the argument list to
"->new()", an error is generated.
For ":Init" handled parameters, use:
my %init_args :InitArgs = (
'data' => {
'Mandatory' => 1,
},
);
"Mandatory" may be abbreviated to "Mand", and
"Required" or "Req" are synonymous.
Default Values¶
For optional parameters, defaults can be specified for field-specific
parameters:
my @data :Field
:Arg('Name' => 'data', 'Default' => 'foo');
If an optional parameter with a specified default is missing from the argument
list to "->new()", then the default is assigned to the field when
the object is created (before the ":Init" subroutine, if any, is
called).
The format for ":Init" handled parameters is:
my %init_args :InitArgs = (
'data' => {
'Default' => 'foo',
},
);
In this case, if the parameter is missing from the argument list to
"->new()", then the parameter key is paired with the default
value and added to the ":Init" argument hash ref (e.g.,
"{ 'data' => 'foo' }").
"Default" may be abbreviated to "Def".
Fields can also be assigned a default value even if not associated with an
initialization parameter:
my @data :Field
:Default('foo');
Note that when using ":Default", the value must be properly specified
(e.g., strings must be quoted as illustrated above).
Using an array or hash reference as a default will probably not produce the
result that you might expect:
my @foo :Field
:Default({});
This does
not result in a new empty hash reference being created for each
new object. Rather, a single empty hash reference is created when the module
is loaded, and then that reference is assigned to each newly created object.
In other words, it is equivalent to:
my %bar = ();
my @foo :Field
:Default(\%bar);
To get the other result, assign a new empty hash reference using an
":Init" subroutine:
sub _init :Init
{
my ($self, $args) = @_;
if (! exists($foo[$$self])) {
$self->set(\@foo, {});
}
}
":Default" may be abbreviated to ":Def".
Parameter Name Matching¶
Rather than having to rely on exact matches to parameter keys in the
"->new()" argument list, you can specify a regular expressions to
be used to match them to field-specific parameters:
my @param :Field
:Arg('Name' => 'param', 'Regexp' => qr/^PARA?M$/i);
In this case, the parameter's key could be any of the following: PARAM, PARM,
Param, Parm, param, parm, and so on. And the following would result in $data
being placed in "My::Class"'s @param field during object creation:
my $obj = My::Class->new('Parm' => $data);
For ":Init" handled parameters, you would similarly use:
my %init_args :InitArgs = (
'Param' => {
'Regex' => qr/^PARA?M$/i,
},
);
In this case, the match results in
"{ 'Param' => $data }" being sent to the
":Init" subroutine as the argument hash. Note that the
":InitArgs" hash key is substituted for the original argument key.
This eliminates the need for any parameter key pattern matching within the
":Init" subroutine.
"Regexp" may be abbreviated to "Regex" or "Re".
Object Pre-initialization¶
Occasionally, a child class may need to send a parameter to a parent class as
part of object initialization. This can be accomplished by supplying a
":PreInit" labeled subroutine in the child class. These subroutines,
if found, are called in order from the bottom of the class hierarchy upward
(i.e., child classes first).
The subroutine should expect two arguments: The newly created (uninitialized)
object (i.e., $self), and a hash ref of all the parameters from the
"->new()" method call, including any additional parameters added
by other ":PreInit" subroutines.
sub pre_init :PreInit
{
my ($self, $args) = @_;
...
}
The parameter hash ref will not be exactly as supplied to
"->new()", but will be
flattened into a single hash ref.
For example,
my $obj = My::Class->new(
'param_X' => 'value_X',
{
'param_A' => 'value_A',
'param_B' => 'value_B',
},
'My::Class' => { 'param' => 'value' },
);
would produce
{
'param_X' => 'value_X',
'param_A' => 'value_A',
'param_B' => 'value_B',
'My::Class' => { 'param' => 'value' }
}
as the hash ref to the ":PreInit" subroutine.
The ":PreInit" subroutine may then add, modify or even remove any
parameters from the hash ref as needed for its purposes. After all the
":PreInit" subroutines have been executed, object initialization
will then proceed using the resulting parameter hash.
The ":PreInit" subroutine should not try to set data in its class's
fields or in other class's fields (e.g., using
set methods) as such
changes will be overwritten during initialization phase which follows
pre-initialization. The ":PreInit" subroutine is only intended for
modifying initialization parameters prior to initialization.
Initialization Sequence¶
For the most part, object initialization can be conceptualized as proceeding
from parent classes down through child classes. As such, calling child class
methods from a parent class during object initialization may not work because
the object will not have been fully initialized in the child classes.
Knowing the order of events during object initialization may help in determining
when this can be done safely:
- 1. The scalar reference for the object is created,
populated with an "Object ID", and blessed into the appropriate
class.
- 2. :PreInit subroutines are called in order from the bottom
of the class hierarchy upward (i.e., child classes first).
- 3. From the top of the class hierarchy downward (i.e.,
parent classes first), "Default Values" are assigned to fields.
(These may be overwritten by subsequent steps below.)
- 4. From the top of the class hierarchy downward, parameters
to the "->new()" method are processed for ":Arg"
field attributes and entries in the ":InitArgs" hash:
- a. "Parameter Preprocessing" is performed.
- b. Checks for "Mandatory Parameters" are
made.
- c. "Default Values" specified in the
":InitArgs" hash are added for subsequent processing by the
":Init" subroutine.
- d. Type checking is performed.
- e. "Field-Specific Parameters" are assigned to
fields.
- 5. From the top of the class hierarchy downward, :Init
subroutines are called with parameters specified in the
":InitArgs" hash.
- 6. Checks are made for any parameters to
"->new()" that were not handled in the above. (See next
section.)
Unhandled Parameters¶
It is an error to include any parameters to the "->new()" method
that are not handled by at least one class in the hierarchy. The primary
purpose of this is to catch typos in parameter names:
my $obj = Person->new('nane' => 'John'); # Should be 'name'
The only time that checks for unhandled parameters are not made is when at least
one class in the hierarchy does not have an ":InitArgs" hash
and does not use the ":Arg" attribute on any of its fields
and uses an :Init subroutine for processing parameters. In such a case,
it is not possible for Object::InsideOut to determine which if any of the
parameters are not handled by the ":Init" subroutine.
If you add the following construct to the start of your application:
BEGIN {
no warnings 'once';
$OIO::Args::Unhandled::WARN_ONLY = 1;
}
then unhandled parameters will only generate warnings rather than causing
exceptions to be thrown.
Modifying ":InitArgs"¶
For performance purposes, Object::InsideOut
normalizes each class's
":InitArgs" hash by creating keys in the form of '_X' for the
various options it handles (e.g., '_R' for 'Regexp').
If a class has the unusual requirement to modify its ":InitArgs" hash
during runtime, then it must renormalize the hash after making such changes by
invoking "Object::InsideOut::normalize()" on it so that
Object::InsideOut will pick up the changes:
Object::InsideOut::normalize(\%init_args);
ACCESSOR GENERATION¶
Accessors are object methods used to get data out of and put data into an
object. You can, of course, write your own accessor code, but this can get a
bit tedious, especially if your class has lots of fields. Object::InsideOut
provides the capability to automatically generate accessors for you.
Basic Accessors¶
A
get accessor is vary basic: It just returns the value of an object's
field:
my @data :Field;
sub fetch_data
{
my $self = shift;
return ($data[$$self]);
}
and you would use it as follows:
my $data = $obj->fetch_data();
To have Object::InsideOut generate such a
get accessor for you, add a
":Get" attribute to the field declaration, specifying the name for
the accessor in parentheses:
my @data :Field :Get(fetch_data);
Similarly, a
set accessor puts data in an object's field. The
set
accessors generated by Object::InsideOut check that they are called with at
least one argument. They are specified using the ":Set" attribute:
my @data :Field :Set(store_data);
Some programmers use the convention of naming
get and
set
accessors using
get_ and
set_ prefixes. Such
standard
accessors can be generated using the ":Standard" attribute (which
may be abbreviated to ":Std"):
my @data :Field :Std(data);
which is equivalent to:
my @data :Field :Get(get_data) :Set(set_data);
Other programmers prefer to use a single
combination accessors that
performs both functions: When called with no arguments, it
gets, and
when called with an argument, it
sets. Object::InsideOut will generate
such accessors with the ":Accessor" attribute. (This can be
abbreviated to ":Acc", or you can use ":Get_Set" or
":Combined" or ":Combo" or even "Mutator".) For
example:
my @data :Field :Acc(data);
The generated accessor would be used in this manner:
$obj->data($val); # Puts data into the object's field
my $data = $obj->data(); # Fetches the object's field data
Set Accessor Return Value¶
For any of the automatically generated methods that perform
set
operations, the default for the method's return value is the value being set
(i.e., the
new value).
You can specify the
set accessor's return value using the
"Return" attribute parameter (which may be abbreviated to
"Ret"). For example, to explicitly specify the default behavior use:
my @data :Field :Set('Name' => 'store_data', 'Return' => 'New');
You can specify that the accessor should return the
old (previous) value
(or "undef" if unset):
my @data :Field :Acc('Name' => 'data', 'Ret' => 'Old');
You may use "Previous", "Prev" or "Prior" as
synonyms for "Old".
Finally, you can specify that the accessor should return the object itself:
my @data :Field :Std('Name' => 'data', 'Ret' => 'Object');
"Object" may be abbreviated to "Obj", and is also synonymous
with "Self".
Method Chaining¶
An obvious case where method chaining can be used is when a field is used to
store an object: A method for the stored object can be chained to the
get accessor call that retrieves that object:
$obj->get_stored_object()->stored_object_method()
Chaining can be done off of
set accessors based on their return value
(see above). In this example with a
set accessor that returns the
new value:
$obj->set_stored_object($stored_obj)->stored_object_method()
the
set_stored_object() call stores the new object,
returning it as well, and then the
stored_object_method()
call is invoked via the stored/returned object. The same would work for
set accessors that return the
old value, too, but in that case
the chained method is invoked via the previously stored (and now returned)
object.
If the Want module (version 0.12 or later) is available, then Object::InsideOut
also tries to do
the right thing with method chaining for
set
accessors that don't store/return objects. In this case, the object used to
invoke the
set accessor will also be used to invoke the chained method
(just as though the
set accessor were declared with
'Return' => 'Object'):
$obj->set_data('data')->do_something();
To make use of this feature, just add "use Want;" to the beginning of
your application.
Note, however, that this special handling does not apply to
get
accessors, nor to
combination accessors invoked without an argument
(i.e., when used as a
get accessor). These must return objects in order
for method chaining to succeed.
:lvalue Accessors¶
As documented in "Lvalue subroutines" in perlsub, an
":lvalue" subroutine returns a modifiable value. This modifiable
value can then, for example, be used on the left-hand side (hence
"LVALUE") of an assignment statement, or a substitution regular
expression.
For Perl 5.8.0 and later, Object::InsideOut supports the generation of
":lvalue" accessors such that their use in an "LVALUE"
context will set the value of the object's field. Just add "'lvalue'
=> 1" to the
set accessor's attribute. ('lvalue' may be
abbreviated to 'lv'.)
Additionally, ":Lvalue" (or its abbreviation ":lv") may be
used for a combined
get/set :lvalue accessor. In other words,
the following are equivalent:
:Acc('Name' => 'email', 'lvalue' => 1)
:Lvalue(email)
Here is a detailed example:
package Contact; {
use Object::InsideOut;
# Create separate a get accessor and an :lvalue set accessor
my @name :Field
:Get(name)
:Set('Name' => 'set_name', 'lvalue' => 1);
# Create a standard get_/set_ pair of accessors
# The set_ accessor will be an :lvalue accessor
my @phone :Field
:Std('Name' => 'phone', 'lvalue' => 1);
# Create a combined get/set :lvalue accessor
my @email :Field
:Lvalue(email);
}
package main;
my $obj = Contact->new();
# Use :lvalue accessors in assignment statements
$obj->set_name() = 'Jerry D. Hedden';
$obj->set_phone() = '800-555-1212';
$obj->email() = 'jdhedden AT cpan DOT org';
# Use :lvalue accessor in 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');
DELEGATORS¶
In addition to autogenerating accessors for a given field, you can also
autogenerate
delegators to that field. A delegator is a method 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.
"Handles" may be abbreviated to "Handle" or
"Hand".
NOTES: Failure to add the appropriate object to the delegation field will lead
to errors such as:
Can't call method "bar" on an undefined
value.
Typos in ":Handles" attribute declarations will lead to errors such
as:
Can't locate object method "bat" via package
"Foo". Adding an object of the wrong class to the delegation
field will lead to the same error, but can be avoided by adding a
":Type" attribute for the appropriate class.
PERMISSIONS¶
Restricted and Private Accessors¶
By default, automatically generated accessors, can be called at any time. In
other words, their access permission is
public.
If desired, accessors can be made
restricted - in which case they can
only be called from within the class and any child classes in the hierarchy
that are derived from it - or
private - such that they can only be
called from within the accessors' class. Here are examples of the syntax for
adding permissions:
my @data :Field :Std('Name' => 'data', 'Permission' => 'private');
my @info :Field :Set('Name' => 'set_info', 'Perm' => 'restricted');
my @internal :Field :Acc('Name' => 'internal', 'Private' => 1);
my @state :Field :Get('Name' => 'state', 'Restricted' => 1);
When creating a
standard pair of
get_/set_ accessors, the
permission setting is applied to both accessors. If different permissions are
required on the two accessors, then you'll have to use separate
":Get" and ":Set" attributes on the field.
# Create a private set method
# and a restricted get method on the 'foo' field
my @foo :Field
:Set('Name' => 'set_foo', 'Priv' => 1)
:Get('Name' => 'get_foo', 'Rest' => 1);
# Create a restricted set method
# and a public get method on the 'bar' field
my %bar :Field
:Set('Name' => 'set_bar', 'Perm' => 'restrict')
:Get(get_bar);
"Permission" may be abbreviated to "Perm";
"Private" may be abbreviated to "Priv"; and
"Restricted" may be abbreviated to "Restrict".
Restricted and Private Methods¶
In the same vein as describe above, access to methods can be narrowed by use of
":Restricted" and ":Private" attributes.
sub foo :Restricted
{
my $self = shift;
...
}
Without either of these attributes, most methods have
public access. If
desired, you may explicitly label them with the ":Public" attribute.
Exemptions¶
It is also possible to specify classes that are exempt from the
Restricted and
Private access permissions (i.e., the method may
be called from those classes as well):
my %foo :Field
:Acc('Name' => 'foo', 'Perm' => 'Restrict(Exempt::Class)')
:Get(get_bar);
sub bar :Private(Some::Class, Another::Class)
{
my $self = shift;
...
}
An example of when this might be needed is with delegation mechanisms.
Hidden Methods¶
For subroutines marked with the following attributes (most of which are
discussed later in this document):
- :ID
- :PreInit
- :Init
- :Replicate
- :Destroy
- :Automethod
- :Dumper
- :Pumper
- :MOD_*_ATTRS
- :FETCH_*_ATTRS
Object::InsideOut normally renders them uncallable (hidden) to class and
application code (as they should normally only be needed by Object::InsideOut
itself). If needed, this behavior can be overridden by adding the
"Public", "Restricted" or "Private" attribute
parameters:
sub _init :Init(private) # Callable from within this class
{
my ($self, $args) = @_;
...
}
Restricted and Private Classes¶
Permission for object creation on a class can be narrowed by adding a
":Restricted" or ":Private" flag to its
"use Object::InsideOut ..." declaration. This basically
adds ":Restricted/:Private" permissions on the
"->new()" method for that class. Exemptions are also supported.
package Foo; {
use Object::InsideOut;
...
}
package Bar; {
use Object::InsideOut 'Foo', ':Restricted(Ping, Pong)';
...
}
In the above, class "Bar" inherits from class "Foo", and its
constructor is restricted to itself, classes that inherit from
"Bar", and the classes "Ping" and "Pong".
As constructors are inherited, any class that inherits from "Bar"
would also be a restricted class. To overcome this, any child class would need
to add its own permission declaration:
package Baz; {
use Object::InsideOut qw/Bar :Private(My::Class)/;
...
}
Here, class "Baz" inherits from class "Bar", and its
constructor is restricted to itself (i.e., private) and class
"My::Class".
Inheriting from a ":Private" class is permitted, but objects cannot be
created for that class unless it has a permission declaration of its own:
package Zork; {
use Object::InsideOut qw/:Public Baz/;
...
}
Here, class "Zork" inherits from class "Baz", and its
constructor has unrestricted access. (In general, don't use the
":Public" declaration for a class except to overcome constructor
permissions inherited from parent classes.)
TYPE CHECKING¶
Object::InsideOut can be directed to add type-checking code to the
set/combined accessors it generates, and to perform type checking on
object initialization parameters.
Field Type Checking¶
Type checking for a field can be specified by adding the ":Type"
attribute to the field declaration:
my @count :Field :Type(numeric);
my @objs :Field :Type(list(My::Class));
The ":Type" attribute results in type checking code being added to
set/combined accessors generated by Object::InsideOut, and will perform
type checking on object initialization parameters processed by the
":Arg" attribute.
Available Types are:
- 'scalar'
- Permits anything that is not a reference.
- 'numeric'
- Can also be specified as "Num" or
"Number". This uses Scalar::Util::looks_like_number() to
test the input value.
- 'list' or 'array'
- 'list(_subtype_)' or 'array(_subtype_)'
- This type permits an accessor to accept multiple values
(which are then placed in an array ref) or a single array ref.
For object initialization parameters, it permits a single value (which is
then placed in an array ref) or an array ref.
When specified, the contents of the resulting array ref are checked against
the specified subtype:
- 'scalar'
- Same as for the basic type above.
- 'numeric'
- Same as for the basic type above.
- A class name
- Same as for the basic type below.
- A reference type
- Any reference type (in all caps) as returned by
ref()).
- 'ARRAY_ref'
- 'ARRAY_ref(_subtype_)'
- This specifies that only a single array reference is
permitted. Can also be specified as "ARRAYref".
When specified, the contents of the array ref are checked against the
specified subtype as per the above.
- 'HASH'
- This type permits an accessor to accept multiple
"key => value" pairs (which are then placed in a
hash ref) or a single hash ref.
For object initialization parameters, only a single ref is permitted.
- 'HASH_ref'
- This specifies that only a single hash reference is
permitted. Can also be specified as "HASHref".
- 'SCALAR_ref'
- This type permits an accessor to accept a single scalar
reference. Can also be specified as "SCALARref".
- A class name
- This permits only an object of the specified class, or one
of its sub-classes (i.e., type checking is done using
"->isa()"). For example, "My::Class". The class
name "UNIVERSAL" permits any object. The class name
"Object::InsideOut" permits any object generated by an
Object::InsideOut class.
- Other reference type
- This permits only a reference of the specified type (as
returned by ref()). The type must be specified in all caps. For
example, "CODE".
The ":Type" attribute can also be supplied with a code reference to
provide custom type checking. The code ref may either be in the form of an
anonymous subroutine, or a fully-qualified subroutine name. The result of
executing the code ref on the input argument should be a boolean value. Here's
some examples:
package My::Class; {
use Object::InsideOut;
# Type checking using an anonymous subroutine
# (This checks that the argument is an object)
my @data :Field :Type(sub { Scalar::Util::blessed($_[0]) })
:Acc(data);
# Type checking using a fully-qualified subroutine name
my @num :Field :Type(\&My::Class::positive)
:Acc(num);
# The type checking subroutine may be made 'Private'
sub positive :Private
{
return (Scalar::Util::looks_like_number($_[0]) &&
($_[0] > 0));
}
}
Type Checking on ":Init" Parameters¶
For object initialization parameters that are sent to the ":Init"
subroutine during object initialization, the parameter's type can be specified
in the ":InitArgs" hash for that parameter using the same types as
specified in the previous section. For example:
my %init_args :InitArgs = (
'COUNT' => {
'Type' => 'numeric',
},
'OBJS' => {
'Type' => 'list(My::Class)',
},
);
One exception involves custom type checking: If referenced in an
":InitArgs" hash, the type checking subroutine cannot be made
":Private":
package My::Class; {
use Object::InsideOut;
sub check_type # Cannot be :Private
{
...
}
my %init_args :InitArgs = (
'ARG' => {
'Type' => \&check_type,
},
);
...
}
Also, as shown, it also doesn't have to be a fully-qualified name.
CUMULATIVE METHODS¶
Normally, methods with the same name in a class hierarchy are masked (i.e.,
overridden) by inheritance - only the method in the most-derived class is
called. With cumulative methods, this masking is removed, and the same-named
method is called in each of the classes within the hierarchy. The return
results from each call (if any) are then gathered together into the return
value for the original method call. For example,
package My::Class; {
use Object::InsideOut;
sub what_am_i :Cumulative
{
my $self = shift;
my $ima = (ref($self) eq __PACKAGE__)
? q/I was created as a /
: q/My top class is /;
return ($ima . __PACKAGE__);
}
}
package My::Foo; {
use Object::InsideOut 'My::Class';
sub what_am_i :Cumulative
{
my $self = shift;
my $ima = (ref($self) eq __PACKAGE__)
? q/I was created as a /
: q/I'm also a /;
return ($ima . __PACKAGE__);
}
}
package My::Child; {
use Object::InsideOut 'My::Foo';
sub what_am_i :Cumulative
{
my $self = shift;
my $ima = (ref($self) eq __PACKAGE__)
? q/I was created as a /
: q/I'm in class /;
return ($ima . __PACKAGE__);
}
}
package main;
my $obj = My::Child->new();
my @desc = $obj->what_am_i();
print(join("\n", @desc), "\n");
produces:
My top class is My::Class
I'm also a My::Foo
I was created as a My::Child
When called in a list context (as in the above), the return results of
cumulative methods are accumulated, and returned as a list.
In a scalar context, a results object is returned that segregates the results by
class for each of the cumulative method calls. Through overloading, this
object can then be dereferenced as an array, hash, string, number, or boolean.
For example, the above could be rewritten as:
my $obj = My::Child->new();
my $desc = $obj->what_am_i(); # Results object
print(join("\n", @{$desc}), "\n"); # Dereference as an array
The following uses hash dereferencing:
my $obj = My::Child->new();
my $desc = $obj->what_am_i();
while (my ($class, $value) = each(%{$desc})) {
print("Class $class reports:\n\t$value\n");
}
and produces:
Class My::Class reports:
My top class is My::Class
Class My::Child reports:
I was created as a My::Child
Class My::Foo reports:
I'm also a My::Foo
As illustrated above, cumulative methods are tagged with the
":Cumulative" attribute (or ":Cumulative(top down)"),
and propagate from the
top down through the class hierarchy (i.e., from
the parent classes down through the child classes). If tagged with
":Cumulative(bottom up)", they will propagated from the
object's class upward through the parent classes.
CHAINED METHODS¶
In addition to ":Cumulative", Object::InsideOut provides a way of
creating methods that are chained together so that their return values are
passed as input arguments to other similarly named methods in the same class
hierarchy. In this way, the chained methods act as though they were
piped together.
For example, imagine you had a method called "format_name" that
formats some text for display:
package Subscriber; {
use Object::InsideOut;
sub format_name {
my ($self, $name) = @_;
# Strip leading and trailing whitespace
$name =~ s/^\s+//;
$name =~ s/\s+$//;
return ($name);
}
}
And elsewhere you have a second class that formats the case of names:
package Person; {
use Lingua::EN::NameCase qw(nc);
use Object::InsideOut;
sub format_name
{
my ($self, $name) = @_;
# Attempt to properly case names
return (nc($name));
}
}
And you decide that you'd like to perform some formatting of your own, and then
have all the parent methods apply their own formatting. Normally, if you have
a single parent class, you'd just call the method directly with
"$self->SUPER::format_name($name)", but if you have more than one
parent class you'd have to explicitly call each method directly:
package Customer; {
use Object::InsideOut qw(Person Subscriber);
sub format_name
{
my ($self, $name) = @_;
# Compress all whitespace into a single space
$name =~ s/\s+/ /g;
$name = $self->Subscriber::format_name($name);
$name = $self->Person::format_name($name);
return $name;
}
}
With Object::InsideOut, you'd add the ":Chained" attribute to each
class's "format_name" method, and the methods will be chained
together automatically:
package Subscriber; {
use Object::InsideOut;
sub format_name :Chained
{
my ($self, $name) = @_;
# Strip leading and trailing whitespace
$name =~ s/^\s+//;
$name =~ s/\s+$//;
return ($name);
}
}
package Person; {
use Lingua::EN::NameCase qw(nc);
use Object::InsideOut;
sub format_name :Chained
{
my ($self, $name) = @_;
# Attempt to properly case names
return (nc($name));
}
}
package Customer; {
use Object::InsideOut qw(Person Subscriber);
sub format_name :Chained
{
my ($self, $name) = @_;
# Compress all whitespace into a single space
$name =~ s/\s+/ /g;
return ($name);
}
}
So passing in someone's name to "format_name" in "Customer"
would cause leading and trailing whitespace to be removed, then the name to be
properly cased, and finally whitespace to be compressed to a single space. The
resulting $name would be returned to the caller:
my ($name) = $obj->format_name($name_raw);
Unlike ":Cumulative" methods, ":Chained" methods
always returns an array - even if there is only one value returned.
Therefore, ":Chained" methods should always be called in an array
context, as illustrated above.
The default direction is to chain methods from the parent classes at the top of
the class hierarchy down through the child classes. You may use the attribute
":Chained(top down)" to make this more explicit.
If you label the method with the ":Chained(bottom up)" attribute,
then the chained methods are called starting with the object's class and
working upward through the parent classes in the class hierarchy, similar to
how ":Cumulative(bottom up)" works.
ARGUMENT MERGING¶
As mentioned under "Object Creation", the "->new()"
method can take parameters that are passed in as combinations of
"key => value" pairs and/or hash refs:
my $obj = My::Class->new(
'param_X' => 'value_X',
'param_Y' => 'value_Y',
{
'param_A' => 'value_A',
'param_B' => 'value_B',
},
{
'param_Q' => 'value_Q',
},
);
The parameters are
merged into a single hash ref before they are
processed.
Adding the ":MergeArgs" attribute to your methods gives them a similar
capability. Your method will then get two arguments: The object and a single
hash ref of the
merged arguments. For example:
package Foo; {
use Object::InsideOut;
...
sub my_method :MergeArgs {
my ($self, $args) = @_;
my $param = $args->{'param'};
my $data = $args->{'data'};
my $flag = $args->{'flag'};
...
}
}
package main;
my $obj = Foo->new(...);
$obj->my_method( { 'data' => 42,
'flag' => 'true' },
'param' => 'foo' );
ARGUMENT VALIDATION¶
A number of users have asked about argument validation for methods:
<
http://www.cpanforum.com/threads/3204>. For this, I recommend using
Params::Validate:
package Foo; {
use Object::InsideOut;
use Params::Validate ':all';
sub foo
{
my $self = shift;
my %args = validate(@_, { bar => 1 });
my $bar = $args{bar};
...
}
}
Using Attribute::Params::Validate, attributes are used for argument validation
specifications:
package Foo; {
use Object::InsideOut;
use Attribute::Params::Validate;
sub foo :method :Validate(bar => 1)
{
my $self = shift;
my %args = @_;
my $bar = $args{bar};
...
}
}
Note that in the above, Perl's ":method" attribute (in all lowercase)
is needed.
There is some incompatibility between Attribute::Params::Validate and some of
Object::InsideOut's attributes. Namely, you cannot use ":Validate"
with ":Private", ":Restricted", ":Cumulative",
":Chained" or ":MergeArgs". In these cases, use the
"validate()" function from Params::Validate instead.
AUTOMETHODS¶
There are significant issues related to Perl's "AUTOLOAD" mechanism
that cause it to be ill-suited for use in a class hierarchy. Therefore,
Object::InsideOut implements its own ":Automethod" mechanism to
overcome these problems.
Classes requiring "AUTOLOAD"-type capabilities must provided a
subroutine labeled with the ":Automethod" attribute. The
":Automethod" subroutine will be called with the object and the
arguments in the original method call (the same as for "AUTOLOAD").
The ":Automethod" subroutine should return either a subroutine
reference that implements the requested method's functionality, or else just
end with "return;" to indicate that it doesn't know how to handle
the request.
Using its own "AUTOLOAD" subroutine (which is exported to every
class), Object::InsideOut walks through the class tree, calling each
":Automethod" subroutine, as needed, to fulfill an unimplemented
method call.
The name of the method being called is passed as $_ instead of $AUTOLOAD, and is
not prefixed with the class name. If the ":Automethod"
subroutine also needs to access the $_ from the caller's scope, it is
available as $CALLER::_.
Automethods can also be made to act as "CUMULATIVE METHODS" or
"CHAINED METHODS". In these cases, the ":Automethod"
subroutine should return two values: The subroutine ref to handle the method
call, and a string designating the type of method. The designator has the same
form as the attributes used to designate ":Cumulative" and
":Chained" methods:
':Cumulative' or ':Cumulative(top down)'
':Cumulative(bottom up)'
':Chained' or ':Chained(top down)'
':Chained(bottom up)'
The following skeletal code illustrates how an ":Automethod"
subroutine could be structured:
sub _automethod :Automethod
{
my $self = shift;
my @args = @_;
my $method_name = $_;
# This class can handle the method directly
if (...) {
my $handler = sub {
my $self = shift;
...
return ...;
};
### OPTIONAL ###
# Install the handler so it gets called directly next time
# no strict refs;
# *{__PACKAGE__.'::'.$method_name} = $handler;
################
return ($handler);
}
# This class can handle the method as part of a chain
if (...) {
my $chained_handler = sub {
my $self = shift;
...
return ...;
};
return ($chained_handler, ':Chained');
}
# This class cannot handle the method request
return;
}
Note: The
OPTIONAL code above for installing the generated handler as a
method should not be used with ":Cumulative" or ":Chained"
automethods.
OBJECT SERIALIZATION¶
Basic Serialization¶
- my $array_ref = $obj->dump();
- my $string = $obj->dump(1);
- Object::InsideOut exports a method called
"->dump()" to each class that returns either a Perl or
a string representation of the object that invokes the method.
The Perl representation is returned when "->dump()" is
called without arguments. It consists of an array ref whose first element
is the name of the object's class, and whose second element is a hash ref
containing the object's data. The object data hash ref contains keys for
each of the classes that make up the object's hierarchy. The values for
those keys are hash refs containing "key => value"
pairs for the object's fields. For example:
[
'My::Class::Sub',
{
'My::Class' => {
'data' => 'value'
},
'My::Class::Sub' => {
'life' => 42
}
}
]
The name for an object field ( data and life in the example
above) can be specified by adding the ":Name" attribute to the
field:
my @life :Field :Name(life);
If the ":Name" attribute is not used, then the name for a field
will be either the name associated with an ":All" or
":Arg" attribute, its get method name, its set
method name, or, failing all that, a string of the form
"ARRAY(0x...)" or "HASH(0x...)".
When called with a true argument, "->dump()" returns a
string version of the Perl representation using Data::Dumper.
Note that using Data::Dumper directly on an inside-out object will not
produce the desired results (it'll just output the contents of the scalar
ref). Also, if inside-out objects are stored inside other structures, a
dump of those structures will not contain the contents of the object's
fields.
In the event of a method naming conflict, the "->dump()" method
can be called using its fully-qualified name:
my $dump = $obj->Object::InsideOut::dump();
- my $obj = Object::InsideOut->pump($data);
- "Object::InsideOut->pump()" takes the output
from the "->dump()" method, and returns an object that is
created using that data. If $data is the array ref returned by using
"$obj->dump()", then the data is inserted directly into the
corresponding fields for each class in the object's class hierarchy. If
$data is the string returned by using "$obj->dump(1)", then
it is "eval"ed to turn it into an array ref, and then processed
as above.
Caveats: If any of an object's fields are dumped to field name keys
of the form "ARRAY(0x...)" or "HASH(0x...)" (see
above), then the data will not be reloadable using
"Object::InsideOut->pump()". To overcome this problem, the
class developer must either add ":Name" attributes to the
":Field" declarations (see above), or provide a
":Dumper"/":Pumper" pair of subroutines as described
below.
Dynamically altering a class (e.g., using -> create_field()) after
objects have been dumped will result in "undef" fields when
pumped back in regardless of whether or not the added fields have
defaults.
Modifying the output from "->dump()", and then feeding it into
"Object::InsideOut->pump()" will work, but is not
specifically supported. If you know what you're doing, fine, but you're on
your own.
- ":Dumper" Subroutine Attribute
- If a class requires special processing to dump its data,
then it can provide a subroutine labeled with the ":Dumper"
attribute. This subroutine will be sent the object that is being dumped.
It may then return any type of scalar the developer deems appropriate.
Usually, this would be a hash ref containing
"key => value" pairs for the object's fields. For
example:
my @data :Field;
sub _dump :Dumper
{
my $obj = $_[0];
my %field_data;
$field_data{'data'} = $data[$$obj];
return (\%field_data);
}
Just be sure not to call your ":Dumper" subroutine
"dump" as that is the name of the dump method exported by
Object::InsideOut as explained above.
- ":Pumper" Subroutine Attribute
- If a class supplies a ":Dumper" subroutine, it
will most likely need to provide a complementary ":Pumper"
labeled subroutine that will be used as part of creating an object from
dumped data using "Object::InsideOut->pump()". The subroutine
will be supplied the new object that is being created, and whatever scalar
was returned by the ":Dumper" subroutine. The corresponding
":Pumper" for the example ":Dumper" above would be:
sub _pump :Pumper
{
my ($obj, $field_data) = @_;
$obj->set(\@data, $field_data->{'data'});
}
Storable¶
Object::InsideOut also supports object serialization using the Storable module.
There are two methods for specifying that a class can be serialized using
Storable. The first method involves adding Storable to the Object::InsideOut
declaration in your package:
package My::Class; {
use Object::InsideOut qw(Storable);
...
}
and adding "use Storable;" in your application. Then you can use
the "->store()" and "->freeze()" methods to
serialize your objects, and the "retrieve()" and "thaw()"
subroutines to de-serialize them.
package main;
use Storable;
use My::Class;
my $obj = My::Class->new(...);
$obj->store('/tmp/object.dat');
...
my $obj2 = retrieve('/tmp/object.dat');
The other method of specifying Storable serialization involves setting a
"::storable" variable inside a "BEGIN" block for the class
prior to its use:
package main;
use Storable;
BEGIN {
$My::Class::storable = 1;
}
use My::Class;
NOTE: The
caveats discussed above for the "->pump()" method
are also applicable when using the Storable module.
OBJECT COERCION¶
Object::InsideOut provides support for various forms of object coercion through
the overload mechanism. For instance, if you want an object to be usable
directly in a string, you would supply a subroutine in your class labeled with
the ":Stringify" attribute:
sub as_string :Stringify
{
my $self = $_[0];
my $string = ...;
return ($string);
}
Then you could do things like:
print("The object says, '$obj'\n");
For a boolean context, you would supply:
sub as_bool :Boolify
{
my $self = $_[0];
my $true_or_false = ...;
return ($true_or_false);
}
and use it in this manner:
if (! defined($obj)) {
# The object is undefined
....
} elsif (! $obj) {
# The object returned a false value
...
}
The following coercion attributes are supported:
- :Stringify
- :Numerify
- :Boolify
- :Arrayify
- :Hashify
- :Globify
- :Codify
Coercing an object to a scalar (":Scalarify") is
not supported
as $$obj is the ID of the object and cannot be overridden.
CLONING¶
Object Cloning¶
Copies of objects can be created using the "->clone()" method which
is exported by Object::InsideOut to each class:
my $obj2 = $obj->clone();
When called without arguments, "->clone()" creates a
shallow
copy of the object, meaning that any complex data structures (i.e., array,
hash or scalar refs) stored in the object will be shared with its clone.
Calling "->clone()" with a
true argument:
my $obj2 = $obj->clone(1);
creates a
deep copy of the object such that internally held array, hash
or scalar refs are
replicated and stored in the newly created clone.
Deep cloning can also be controlled at the field level, and is covered in
the next section.
Note that cloning does not clone internally held objects. For example, if $foo
contains a reference to $bar, a clone of $foo will also contain a reference to
$bar; not a clone of $bar. If such behavior is needed, it must be provided
using a :Replicate subroutine.
Field Cloning¶
Object cloning can be controlled at the field level such that specified fields
are
deeply copied when "->clone()" is called without any
arguments. This is done by adding the ":Deep" attribute to the
field:
my @data :Field :Deep;
WEAK FIELDS¶
Frequently, it is useful to store weakened references to data or objects in a
field. Such a field can be declared as ":Weak" so that data (i.e.,
references) set via Object::InsideOut generated accessors, parameter
processing using ":Arg", the "->set()" method, etc.,
will automatically be weakened after being stored in the field array/hash.
my @data :Field :Weak;
NOTE: If data in a
weak field is set directly (i.e., the
"->set()" method is not used), then
weaken() must be
invoked on the stored reference afterwards:
$self->set(\@field, $data);
Scalar::Util::weaken($field[$$self]);
(This is another reason why the "->set()" method is recommended for
setting field data within class code.)
DYNAMIC FIELD CREATION¶
Normally, object fields are declared as part of the class code. However, some
classes may need the capability to create object fields
on-the-fly, for
example, as part of an ":Automethod". Object::InsideOut provides a
class method for this:
# Dynamically create a hash field with standard accessors
My::Class->create_field('%'.$fld, ":Std($fld)");
The first argument is the class into which the field will be added. The second
argument is a string containing the name of the field preceded by either a
"@" or "%" to declare an array field or hash field,
respectively. The remaining string arguments should be attributes declaring
accessors and the like. The ":Field" attribute is assumed, and does
not need to be added to the attribute list. For example:
My::Class->create_field('@data', ":Type(numeric)",
":Acc(data)");
My::Class->create_field('@obj', ":Type(Some::Class)",
":Acc(obj)",
":Weak");
Field creation will fail if you try to create an array field within a class
whose hierarchy has been declared :hash_only.
Here's an example of an ":Automethod" subroutine that uses dynamic
field creation:
package My::Class; {
use Object::InsideOut;
sub _automethod :Automethod
{
my $self = $_[0];
my $class = ref($self) || $self;
my $method = $_;
# Extract desired field name from get_/set_ method name
my ($fld_name) = $method =~ /^[gs]et_(.*)$/;
if (! $fld_name) {
return; # Not a recognized method
}
# Create the field and its standard accessors
$class->create_field('@'.$fld_name, ":Std($fld_name)");
# Return code ref for newly created accessor
no strict 'refs';
return *{$class.'::'.$method}{'CODE'};
}
}
RUNTIME INHERITANCE¶
The class method "->add_class()" provides the capability to
dynamically add classes to a class hierarchy at runtime.
For example, suppose you had a simple
state class:
package Trait::State; {
use Object::InsideOut;
my %state :Field :Set(state);
}
This could be added to another class at runtime using:
My::Class->add_class('Trait::State');
This permits, for example, application code to dynamically modify a class
without having it create an actual sub-class.
PREPROCESSING¶
Parameter Preprocessing¶
You can specify a code ref (either in the form of an anonymous subroutine, or a
subroutine name) for an object initialization parameter that will be called on
that parameter prior to taking any of the other parameter actions described
above. Here's an example:
package My::Class; {
use Object::InsideOut;
# The parameter preprocessing subroutine
sub preproc
{
my ($class, $param, $spec, $obj, $value) = @_;
# Preform parameter preprocessing
...
# Return result
return ...;
}
my @data :Field
:Arg('Name' => 'DATA', 'Preprocess' => \&My::Class::preproc);
my %init_args :InitArgs = (
'PARAM' => {
'Preprocess' => \&preproc,
},
);
...
}
When used in the ":Arg" attribute, the subroutine name must be
fully-qualified, as illustrated. Further, if not referenced in the
":InitArgs" hash, the preprocessing subroutine can be given the
":Private" attribute.
As the above illustrates, the parameter preprocessing subroutine is sent five
arguments:
- •
- The name of the class associated with the parameter
This would be "My::Class" in the example above.
- •
- The name of the parameter
Either "DATA" or "PARAM" in the example above.
- •
- A hash ref of the parameter's specifiers
This is either a hash ref containing the ":Arg" attribute
parameters, or the hash ref paired to the parameter's key in the
":InitArgs" hash.
- •
- The object being initialized
- •
- The parameter's value
This is the value assigned to the parameter in the "->new()"
method's argument list. If the parameter was not provided to
"->new()", then "undef" will be sent.
The return value of the preprocessing subroutine will then be assigned to the
parameter.
Be careful about what types of data the preprocessing subroutine tries to make
use of "external" to the arguments supplied. For instance, because
the order of parameter processing is not specified, the preprocessing
subroutine cannot rely on whether or not some other parameter is set. Such
processing would need to be done in the ":Init" subroutine. It can,
however, make use of object data set by classes
higher up in the class
hierarchy. (That is why the object is provided as one of the arguments.)
Possible uses for parameter preprocessing include:
- •
- Overriding the supplied value (or even deleting it by
returning "undef")
- •
- Providing a dynamically-determined default value
Preprocess may be abbreviated to
Preproc or
Pre.
Set Accessor Preprocessing¶
You can specify a code ref (either in the form of an anonymous subroutine, or a
fully-qualified subroutine name) for a
set/combined accessor that will
be called on the arguments supplied to the accessor prior to its taking the
usual actions of type checking and adding the data to the field. Here's an
example:
package My::Class; {
use Object::InsideOut;
my @data :Field
:Acc('Name' => 'data', 'Preprocess' => \&My::Class::preproc);
# The set accessor preprocessing subroutine may be made 'Private'
sub preproc :Private
{
my ($self, $field, @args) = @_;
# Preform preprocessing on the accessor's arguments
...
# Return result
return ...;
}
}
As the above illustrates, the accessor preprocessing subroutine is sent the
following arguments:
- •
- The object used to invoke the accessor
- •
- A reference to the field associated with the accessor
- •
- The argument(s) sent to the accessor
There will always be at least one argument.
Usually, the preprocessing subroutine would return just a single value. For
fields declared as type "List", multiple values may be returned.
Following preprocessing, the
set accessor will operate on whatever
value(s) are returned by the preprocessing subroutine.
SPECIAL PROCESSING¶
Object ID¶
By default, the ID of an object is derived from a sequence counter for the
object's class hierarchy. This should suffice for nearly all cases of class
development. If there is a special need for the module code to control the
object ID (see Math::Random::MT::Auto as an example), then a subroutine
labelled with the ":ID" attribute can be specified:
sub _id :ID
{
my $class = $_[0];
# Generate/determine a unique object ID
...
return ($id);
}
The ID returned by your subroutine can be any kind of
regular scalar
(e.g., a string or a number). However, if the ID is something other than a
low-valued integer, then you will have to architect
all your classes
using hashes for the object fields. See "HASH ONLY CLASSES" for
details.
Within any class hierarchy, only one class may specify an ":ID"
subroutine.
Object Replication¶
Object replication occurs explicitly when the "->clone()" method is
called on an object, and implicitly when threads are created in a threaded
application. In nearly all cases, Object::InsideOut will take care of all the
details for you.
In rare cases, a class may require special handling for object replication. It
must then provide a subroutine labeled with the ":Replicate"
attribute. This subroutine will be sent three arguments: The parent and the
cloned objects, and a flag:
sub _replicate :Replicate
{
my ($parent, $clone, $flag) = @_;
# Special object replication processing
if ($clone eq 'CLONE') {
# Handling for thread cloning
...
} elsif ($clone eq 'deep') {
# Deep copy of the parent
...
} else {
# Shallow copying
...
}
}
In the case of thread cloning, $flag will be set to the 'CLONE', and the $parent
object is just a non-blessed anonymous scalar reference that contains the ID
for the object in the parent thread.
When invoked via the "->clone()" method, $flag will be either an
empty string which denotes that a
shallow copy is being produced for
the clone, or $flag will be set to 'deep' indicating a
deep copy is
being produced.
The ":Replicate" subroutine only needs to deal with the special
replication processing needed by the object: Object::InsideOut will handle all
the other details.
Object Destruction¶
Object::InsideOut exports a "DESTROY" method to each class that
deletes an object's data from the object field arrays (hashes). If a class
requires additional destruction processing (e.g., closing filehandles), then
it must provide a subroutine labeled with the ":Destroy" attribute.
This subroutine will be sent the object that is being destroyed:
sub _destroy :Destroy
{
my $obj = $_[0];
# Special object destruction processing
}
The ":Destroy" subroutine only needs to deal with the special
destruction processing: The "DESTROY" method will handle all the
other details of object destruction.
FOREIGN CLASS INHERITANCE¶
Object::InsideOut supports inheritance from foreign (i.e.,
non-Object::InsideOut) classes. This means that your classes can inherit from
other Perl class, and access their methods from your own objects.
One method of declaring foreign class inheritance is to add the class name to
the Object::InsideOut declaration inside your package:
package My::Class; {
use Object::InsideOut qw(Foreign::Class);
...
}
This allows you to access the foreign class's static (i.e., class) methods from
your own class. For example, suppose "Foreign::Class" has a class
method called "foo". With the above, you can access that method
using "My::Class->foo()" instead.
Multiple foreign inheritance is supported, as well:
package My::Class; {
use Object::InsideOut qw(Foreign::Class Other::Foreign::Class);
...
}
- $self->inherit($obj, ...);
- To use object methods from foreign classes, an object must
inherit from an object of that class. This would normally be done
inside a class's ":Init" subroutine:
package My::Class; {
use Object::InsideOut qw(Foreign::Class);
sub init :Init
{
my ($self, $args) = @_;
my $foreign_obj = Foreign::Class->new(...);
$self->inherit($foreign_obj);
}
}
Thus, with the above, if "Foreign::Class" has an object method
called "bar", you can call that method from your own objects:
package main;
my $obj = My::Class->new();
$obj->bar();
Object::InsideOut's "AUTOLOAD" subroutine handles the dispatching
of the "->bar()" method call using the internally held
inherited object (in this case, $foreign_obj).
Multiple inheritance is supported, as well: You can call the
"->inherit()" method multiple times, or make just one call
with all the objects to be inherited from.
"->inherit()" is a restricted method. In other words, you
cannot use it on an object outside of code belonging to the object's class
tree (e.g., you can't call it from application code).
In the event of a method naming conflict, the "->inherit()"
method can be called using its fully-qualified name:
$self->Object::InsideOut::inherit($obj);
- my @objs = $self->heritage();
- my $obj = $self->heritage($class);
- my @objs = $self->heritage($class1, $class2, ...);
- Your class code can retrieve any inherited objects using
the "->heritage()" method. When called without any arguments,
it returns a list of any objects that were stored by the calling class
using the calling object. In other words, if class "My::Class"
uses object $obj to store foreign objects $fobj1 and $fobj2, then later on
in class "My::Class", "$obj->heritage()" will
return $fobj1 and $fobj2.
"->heritage()" can also be called with one or more class name
arguments. In this case, only objects of the specified class(es) are
returned.
In the event of a method naming conflict, the "->heritage()"
method can be called using its fully-qualified name:
my @objs = $self->Object::InsideOut::heritage();
- $self->disinherit($class [, ...])
- $self->disinherit($obj [, ...])
- The "->disinherit()" method disassociates
(i.e., deletes) the inheritance of foreign object(s) from an object. The
foreign objects may be specified by class, or using the actual inherited
object (retrieved via "->heritage()", for example).
The call is only effective when called inside the class code that
established the initial inheritance. In other words, if an inheritance is
set up inside a class, then disinheritance can only be done from inside
that class.
In the event of a method naming conflict, the "->disinherit()"
method can be called using its fully-qualified name:
$self->Object::InsideOut::disinherit($obj [, ...])
NOTE: With foreign inheritance, you only have access to class and object
methods. The encapsulation of the inherited objects is strong, meaning that
only the class where the inheritance takes place has direct access to the
inherited object. If access to the inherited objects themselves, or their
internal hash fields (in the case of
blessed hash objects), is needed
outside the class, then you'll need to write your own accessors for that.
LIMITATION: You cannot use fully-qualified method names to access foreign
methods (when encapsulated foreign objects are involved). Thus, the following
will not work:
my $obj = My::Class->new();
$obj->Foreign::Class::bar();
Normally, you shouldn't ever need to do the above: "$obj->bar()"
would suffice.
The only time this may be an issue is when the
native class
overrides an inherited foreign class's method (e.g.,
"My::Class" has its own "->bar()" method). Such
overridden methods are not directly callable. If such overriding is
intentional, then this should not be an issue: No one should be writing code
that tries to by-pass the override. However, if the overriding is
accidentally, then either the
native method should be renamed, or the
native class should provide a wrapper method so that the functionality
of the overridden method is made available under a different name.
"use base" and Fully-qualified Method Names¶
The foreign inheritance methodology handled by the above is predicated on
non-Object::InsideOut classes that generate their own objects and expect their
object methods to be invoked via those objects.
There are exceptions to this rule:
- 1. Foreign object methods that expect to be invoked via the
inheriting class's object, or foreign object methods that don't care how
they are invoked (i.e., they don't make reference to the invoking
object).
- This is the case where a class provides auxiliary methods
for your objects, but from which you don't actually create any objects
(i.e., there is no corresponding foreign object, and
"$obj->inherit($foreign)" is not used.)
In this case, you can either:
a. Declare the foreign class using the standard method (i.e.,
"use Object::InsideOut qw(Foreign::Class);"), and
invoke its methods using their full path (e.g.,
"$obj->Foreign::Class::method();"); or
b. You can use the base pragma so that you don't have to use the full path
for foreign methods.
package My::Class; {
use Object::InsideOut;
use base 'Foreign::Class';
...
}
The former scheme is faster.
- 2. Foreign class methods that expect to be invoked via the
inheriting class.
- As with the above, you can either invoke the class methods
using their full path (e.g.,
"My::Class->Foreign::Class::method();"), or you can
"use base" so that you don't have to use the full path.
Again, using the full path is faster.
Class::Singleton is an example of this type of class.
- 3. Class methods that don't care how they are invoked
(i.e., they don't make reference to the invoking class).
- In this case, you can either use
"use Object::InsideOut qw(Foreign::Class);" for
consistency, or use "use base qw(Foreign::Class);" if
(slightly) better performance is needed.
If you're not familiar with the inner workings of the foreign class such that
you don't know if or which of the above exceptions applies, then the formulaic
approach would be to first use the documented method for foreign inheritance
(i.e., "use Object::InsideOut qw(Foreign::Class);"). If
that works, then I strongly recommend that you just use that approach unless
you have a good reason not to. If it doesn't work, then try
"use base".
INTROSPECTION¶
For Perl 5.8.0 and later, Object::InsideOut provides an introspection API that
allow you to obtain metadata on a class's hierarchy, constructor parameters,
and methods.
- my $meta = My::Class->meta();
- my $meta = $obj->meta();
- The "->meta()" method, which is exported by
Object::InsideOut to each class, returns an Object::InsideOut::Metadata
object which can then be queried for information about the invoking
class or invoking object's class:
# Get an object's class hierarchy
my @classes = $obj->meta()->get_classes();
# Get info on the args for a class's constructor (i.e., ->new() parameters)
my %args = My::Class->meta()->get_args();
# Get info on the methods that can be called by an object
my %methods = $obj->meta()->get_methods();
- My::Class->isa();
- $obj->isa();
- When called in an array context, calling
"->isa()" without any arguments on an Object::InsideOut class
or object returns a list of the classes in the class hierarchy for that
class or object, and is equivalent to:
my @classes = $obj->meta()->get_classes();
When called in a scalar context, it returns an array ref containing the
classes.
- My::Class->can();
- $obj->can();
- When called in an array context, calling
"->can()" without any arguments on an Object::InsideOut class
or object returns a list of the method names for that class or object, and
is equivalent to:
my %methods = $obj->meta()->get_methods();
my @methods = keys(%methods);
When called in a scalar context, it returns an array ref containing the
method names.
See Object::InsideOut::Metadata for more details.
THREAD SUPPORT¶
For Perl 5.8.1 and later, Object::InsideOut fully supports threads (i.e., is
thread safe), and supports the sharing of Object::InsideOut objects between
threads using threads::shared.
To use Object::InsideOut in a threaded application, you must put
"use threads;" at the beginning of the application. (The use of
"require threads;" after the program is running is not
supported.) If object sharing is to be utilized, then
"use threads::shared;" should follow.
If you just "use threads;", then objects from one thread will be
copied and made available in a child thread.
The addition of "use threads::shared;" in and of itself does not
alter the behavior of Object::InsideOut objects. The default behavior is to
not share objects between threads (i.e., they act the same as with
"use threads;" alone).
To enable the sharing of objects between threads, you must specify which classes
will be involved with thread object sharing. There are two methods for doing
this. The first involves setting a "::shared" variable (inside a
"BEGIN" block) for the class prior to its use:
use threads;
use threads::shared;
BEGIN {
$My::Class::shared = 1;
}
use My::Class;
The other method is for a class to add a ":SHARED" flag to its
"use Object::InsideOut ..." declaration:
package My::Class; {
use Object::InsideOut ':SHARED';
...
}
When either sharing flag is set for one class in an object hierarchy, then all
the classes in the hierarchy are affected.
If a class cannot support thread object sharing (e.g., one of the object fields
contains code refs [which Perl cannot share between threads]), it should
specifically declare this fact:
package My::Class; {
use Object::InsideOut ':NOT_SHARED';
...
}
However, you cannot mix thread object sharing classes with non-sharing classes
in the same class hierarchy:
use threads;
use threads::shared;
package My::Class; {
use Object::InsideOut ':SHARED';
...
}
package Other::Class; {
use Object::InsideOut ':NOT_SHARED';
...
}
package My::Derived; {
use Object::InsideOut qw(My::Class Other::Class); # ERROR!
...
}
Here is a complete example with thread object sharing enabled:
use threads;
use threads::shared;
package My::Class; {
use Object::InsideOut ':SHARED';
# One list-type field
my @data :Field :Type(list) :Acc(data);
}
package main;
# New object
my $obj = My::Class->new();
# Set the object's 'data' field
$obj->data(qw(foo bar baz));
# Print out the object's data
print(join(', ', @{$obj->data()}), "\n"); # "foo, bar, baz"
# Create a thread and manipulate the object's data
my $rc = threads->create(
sub {
# Read the object's data
my $data = $obj->data();
# Print out the object's data
print(join(', ', @{$data}), "\n"); # "foo, bar, baz"
# Change the object's data
$obj->data(@$data[1..2], 'zooks');
# Print out the object's modified data
print(join(', ', @{$obj->data()}), "\n"); # "bar, baz, zooks"
return (1);
}
)->join();
# Show that changes in the object are visible in the parent thread
# I.e., this shows that the object was indeed shared between threads
print(join(', ', @{$obj->data()}), "\n"); # "bar, baz, zooks"
HASH ONLY CLASSES¶
For performance considerations, it is recommended that arrays be used for class
fields whenever possible. The only time when hash-bases fields are required is
when a class must provide its own object ID, and those IDs are something other
than low-valued integers. In this case, hashes must be used for fields not
only in the class that defines the object ID subroutine, but also in every
class in any class hierarchy that include such a class.
The
hash only requirement can be enforced by adding the
":HASH_ONLY" flag to a class's
"use Object::InsideOut ..." declaration:
package My::Class; {
use Object::InsideOut ':hash_only';
...
}
This will cause Object::Inside to check every class in any class hierarchy that
includes such flagged classes to make sure their fields are hashes and not
arrays. It will also fail any ->
create_field() call that tries to
create an array-based field in any such class.
SECURITY¶
In the default case where Object::InsideOut provides object IDs that are
sequential integers, it is possible to hack together a
fake
Object::InsideOut object, and so gain access to another object's data:
my $fake = bless(\do{my $scalar}, 'Some::Class');
$$fake = 86; # ID of another object
my $stolen = $fake->get_data();
Why anyone would try to do this is unknown. How this could be used for any sort
of malicious exploitation is also unknown. However, if preventing this sort of
security issue is a requirement, it can be accomplished by adding the
":SECURE" flag to a class's
"use Object::InsideOut ..." declaration:
package My::Class; {
use Object::InsideOut ':SECURE';
...
}
This places the module "Object::InsideOut::Secure" in the class
hierarchy. Object::InsideOut::Secure provides an :ID subroutine that generates
random integers for object IDs, thus preventing other code from being able to
create fake objects by
guessing at IDs.
Using ":SECURE" mode requires Math::Random::MT::Auto (v5.04 or later).
Because the object IDs used with ":SECURE" mode are large random
values, the :HASH_ONLY flag is forced on all the classes in the hierarchy.
For efficiency, it is recommended that the ":SECURE" flag be added to
the topmost class(es) in a hierarchy.
ATTRIBUTE HANDLERS¶
Object::InsideOut uses
attribute 'modify' handlers as described in
"Package-specific Attribute Handling" in attributes, and provides a
mechanism for adding attribute handlers to your own classes. Instead of naming
your attribute handler as "MODIFY_*_ATTRIBUTES", name it something
else and then label it with the ":MODIFY_*_ATTRIBUTES" attribute (or
":MOD_*_ATTRS" for short). Your handler should work just as
described in "Package-specific Attribute Handling" in attributes
with regard to its input arguments, and must return a list of the attributes
which were not recognized by your handler. Here's an example:
package My::Class; {
use Object::InsideOut;
sub _scalar_attrs :MOD_SCALAR_ATTRS
{
my ($pkg, $scalar, @attrs) = @_;
my @unused_attrs; # List of any unhandled attributes
while (my $attr = shift(@attrs)) {
if ($attr =~ /.../) {
# Handle attribute
...
} else {
# We don't handle this attribute
push(@unused_attrs, $attr);
}
}
return (@unused_attrs); # Pass along unhandled attributes
}
}
Attribute 'modify' handlers are called
upward through the class hierarchy
(i.e.,
bottom up). This provides child classes with the capability to
override the handling of attributes by parent classes, or to add
attributes (via the returned list of unhandled attributes) for parent classes
to process.
Attribute 'modify' handlers should be located at the beginning of a package, or
at least before any use of attributes on the corresponding type of variable or
subroutine:
package My::Class; {
use Object::InsideOut;
sub _array_attrs :MOD_ARRAY_ATTRS
{
...
}
my @my_array :MyArrayAttr;
}
For
attribute 'fetch' handlers, follow the same procedures: Label the
subroutine with the ":FETCH_*_ATTRIBUTES" attribute (or
":FETCH_*_ATTRS" for short). Contrary to the documentation in
"Package-specific Attribute Handling" in attributes,
attribute
'fetch' handlers receive
two arguments: The relevant package name,
and a reference to a variable or subroutine for which package-defined
attributes are desired.
Attribute handlers are normal rendered hidden.
SPECIAL USAGE¶
Usage With "Exporter"¶
It is possible to use Exporter to export functions from one inside-out object
class to another:
use strict;
use warnings;
package Foo; {
use Object::InsideOut 'Exporter';
BEGIN {
our @EXPORT_OK = qw(foo_name);
}
sub foo_name
{
return (__PACKAGE__);
}
}
package Bar; {
use Object::InsideOut 'Foo' => [ qw(foo_name) ];
sub get_foo_name
{
return (foo_name());
}
}
package main;
print("Bar got Foo's name as '", Bar::get_foo_name(), "'\n");
Note that the "BEGIN" block is needed to ensure that the Exporter
symbol arrays (in this case @EXPORT_OK) get populated properly.
Usage With "require" and "mod_perl"¶
Object::InsideOut usage under mod_perl and with runtime-loaded classes is
supported automatically; no special coding is required.
Caveat: Runtime loading of classes should be performed before any objects
are created within any of the classes in their hierarchies. If
Object::InsideOut cannot create a hierarchy because of previously created
objects (even if all those objects have been destroyed), a runtime error will
be generated.
Singleton Classes¶
A singleton class is a case where you would provide your own
"->new()" method that in turn calls Object::InsideOut's
"->new()" method:
package My::Class; {
use Object::InsideOut;
my $singleton;
sub new {
my $thing = shift;
if (! $singleton) {
$singleton = $thing->Object::InsideOut::new(@_);
}
return ($singleton);
}
}
DIAGNOSTICS¶
Object::InsideOut uses "Exception::Class" for reporting errors. The
base error class for this module is "OIO". Here is an example of the
basic manner for trapping and handling errors:
my $obj;
eval { $obj = My::Class->new(); };
if (my $e = OIO->caught()) {
warn('Failure creating object: '.$e);
...
}
A more comprehensive approach might employ elements of the following:
eval { ... };
if (my $e = OIO->caught()) {
# An error generated by Object::InsideOut
...
} elsif (my $e = Exception::Class::Base->caught()) {
# An error generated by other code that uses Exception::Class
...
} elsif ($@) {
# An unhandled error (i.e., generated by code that doesn't use
# Exception::Class)
...
}
I have tried to make the messages and information returned by the error objects
as informative as possible. Suggested improvements are welcome. Also, please
bring to my attention any conditions that you encounter where an error occurs
as a result of Object::InsideOut code that doesn't generate an
Exception::Class object. Here is one such error:
- Invalid ARRAY/HASH attribute
- This error indicates you forgot "use
Object::InsideOut;" in your class's code.
Object::InsideOut installs a "__DIE__" handler (see "die
LIST" in perlfunc and "eval BLOCK" in perlfunc) to catch any
errant exceptions from class-specific code, namely, ":Init",
":Replicate", ":Destroy", etc. subroutines. When using
"eval" blocks inside these subroutines, you should localize
$SIG{'__DIE__'} to keep Object::InsideOut's "__DIE__" handler from
interfering with exceptions generated inside the "eval" blocks. For
example:
sub _init :Init {
...
eval {
local $SIG{'__DIE__'};
...
};
if $@ {
# Handle caught exception
}
...
}
Here's another example, where the "die" function is used as a method
of flow control for leaving an "eval" block:
eval {
local $SIG{'__DIE__'}; # Suppress any existing __DIE__ handler
...
die({'found' => 1}) if $found; # Leave the eval block
...
};
if ($@) {
die unless (ref($@) && $@->{'found'}); # Propagate any 'real' error
# Handle 'found' case
...
}
# Handle 'not found' case
Similarly, if calling code from other modules that use the above flow control
mechanism, but without localizing $SIG{'__DIE__'}, you can workaround this
deficiency with your own "eval" block:
eval {
local $SIG{'__DIE__'}; # Suppress any existing __DIE__ handler
Some::Module::func(); # Call function that fails to localize
};
if ($@) {
# Handle caught exception
}
In addition, you should file a bug report against the offending module along
with a patch that adds the missing "local $SIG{'__DIE__'};"
statement.
BUGS AND LIMITATIONS¶
If you receive an error similar to this:
ERROR: Attempt to DESTROY object ID 1 of class Foo twice
the cause may be that some module used by your application is doing
"require threads" somewhere in the background. DBI is one such
module. The workaround is to add "use threads;" at the start of your
application.
Another cause of the above is returning a non-shared object from a thread either
explicitly or implicitly when the result of the last statement in the thread
subroutine is an object. For example:
sub thr_func {
my $obj = MyClass->new();
}
which is equivalent to:
sub thr_func {
return MyClass->new();
}
This can be avoided by ensuring your thread subroutine ends with
"return;".
The equality operator (e.g., "if ($obj1 == $obj2) { ...") is
overloaded for ":SHARED" classes when threads::shared is loaded. The
overload subroutine compares object classes and IDs because references to the
same thread shared object may have different refaddrs.
You cannot overload an object to a scalar context (i.e., can't
":SCALARIFY").
You cannot use two instances of the same class with mixed thread object sharing
in same application.
Cannot use attributes on
subroutine stubs (i.e., forward declaration
without later definition) with ":Automethod":
package My::Class; {
sub method :Private; # Will not work
sub _automethod :Automethod
{
# Code to handle call to 'method' stub
}
}
Due to limitations in the Perl parser, the entirety of any one attribute must be
on a single line. (However, multiple attributes may appear on separate lines.)
If a
set accessor accepts scalars, then you can store any inside-out
object type in it. If its "Type" is set to "HASH", then it
can store any
blessed hash object.
Returning objects from threads does not work:
my $obj = threads->create(sub { return (Foo->new()); })->join(); # BAD
Instead, use thread object sharing, create the object before launching the
thread, and then manipulate the object inside the thread:
my $obj = Foo->new(); # Class 'Foo' is set ':SHARED'
threads->create(sub { $obj->set_data('bar'); })->join();
my $data = $obj->get_data();
Due to a limitation in threads::shared version 1.39 and earlier, if storing
shared objects inside other shared objects, you should use
"delete()" to remove them from internal fields (e.g.,
"delete($field[$$self]);") when necessary so that the objects'
destructor gets called. Upgrading to version 1.40 or later alleviates most of
this issue except during global destruction. See threads::shared for more.
With Perl 5.8.8 and earlier, there are bugs associated with threads::shared that
may prevent you from storing objects inside of shared objects, or using
foreign inheritance with shared objects. With Perl 5.8.9 (and later) together
with threads::shared 1.15 (and later), you can store shared objects inside of
other shared objects, and you can use foreign inheritance with shared objects
(provided the foreign class supports shared objects as well).
Due to internal complexities, the following actions are not supported in code
that uses threads::shared while there are any threads active:
- •
- Runtime loading of Object::InsideOut classes
- •
- Using ->add_class()
It is recommended that such activities, if needed, be performed in the main
application code before any threads are created (or at least while there are
no active threads).
For Perl 5.6.0 through 5.8.0, a Perl bug prevents package variables (e.g.,
object attribute arrays/hashes) from being referenced properly from subroutine
refs returned by an ":Automethod" subroutine. For Perl 5.8.0 there
is no workaround: This bug causes Perl to core dump. For Perl 5.6.0 through
5.6.2, the workaround is to create a ref to the required variable inside the
":Automethod" subroutine, and use that inside the subroutine ref:
package My::Class; {
use Object::InsideOut;
my %data;
sub auto :Automethod
{
my $self = $_[0];
my $name = $_;
my $data = \%data; # Workaround for 5.6.X bug
return sub {
my $self = shift;
if (! @_) {
return ($$data{$name});
}
$$data{$name} = shift;
};
}
}
For Perl 5.8.1 through 5.8.4, a Perl bug produces spurious warning messages when
threads are destroyed. These messages are innocuous, and can be suppressed by
adding the following to your application code:
$SIG{'__WARN__'} = sub {
if ($_[0] !~ /^Attempt to free unreferenced scalar/) {
print(STDERR @_);
}
};
A better solution would be to upgrade threads and threads::shared from CPAN,
especially if you encounter other problems associated with threads.
For Perl 5.8.4 and 5.8.5, the "Storable" feature does not work due to
a Perl bug. Use Object::InsideOut v1.33 if needed.
Due to bugs in the Perl interpreter, using the introspection API (i.e.
"->meta()", etc.) requires Perl 5.8.0 or later.
The version of Want that is available via PPM for ActivePerl is defective, and
causes failures when using ":lvalue" accessors. Remove it, and then
download and install the Want module using CPAN.
Devel::StackTrace (used by Exception::Class) makes use of the
DB
namespace. As a consequence, Object::InsideOut thinks that
"package DB" is already loaded. Therefore, if you create a
class called
DB that is sub-classed by other packages, you may need to
"require" it as follows:
package DB::Sub; {
require DB;
use Object::InsideOut qw(DB);
...
}
View existing bug reports at, and submit any new bugs, problems, patches, etc.
to:
http://rt.cpan.org/Public/Dist/Display.html?Name=Object-InsideOut
<
http://rt.cpan.org/Public/Dist/Display.html?Name=Object-InsideOut>
REQUIREMENTS¶
- Perl 5.6.0 or later
- Exception::Class v1.22 or later
- Scalar::Util v1.10 or later
- It is possible to install a pure perl version of
Scalar::Util, however, it will be missing the weaken() function
which is needed by Object::InsideOut. You'll need to upgrade your version
of Scalar::Util to one that supports its "XS" code.
- Test::More v0.50 or later
- Needed for testing during installation.
- Want v0.12 or later
- Optional. Provides support for ":lvalue
Accessors".
- Math::Random::MT::Auto v5.04 or later)
- Optional. Provides support for :SECURE mode.
To cover all of the above requirements and more, it is recommended that you
install Bundle::Object::InsideOut using CPAN:
perl -MCPAN -e 'install Bundle::Object::InsideOut'
This will install the latest versions of all the required and optional modules
needed for full support of all of the features provided by Object::InsideOut.
SEE ALSO¶
Object::InsideOut Discussion Forum on CPAN:
http://www.cpanforum.com/dist/Object-InsideOut
<
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 <perl AT abigail DOT nl> for inside-out
objects in general.
Damian Conway <dconway AT cpan DOT org> for
Class::Std, and for delegator methods.
David A. Golden <dagolden AT cpan DOT org> for thread
handling for inside-out objects.
Dan Kubb <dan.kubb-cpan AT autopilotmarketing DOT com>
for ":Chained" methods.
AUTHOR¶
Jerry D. Hedden, <jdhedden AT cpan DOT org>
COPYRIGHT AND LICENSE¶
Copyright 2005 - 2012 Jerry D. Hedden. All rights reserved.
This program is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.
TRANSLATIONS¶
A Japanese translation of this documentation by TSUJII, Naofumi
<tsun DOT nt AT gmail DOT com> is
available at <
http://perldoc.jp/docs/modules/>.