Class::Tiny - Minimalist class construction
version 1.008
In
Person.pm:
package Person;
use Class::Tiny qw( name );
1;
In
Employee.pm:
package Employee;
use parent 'Person';
use Class::Tiny qw( ssn ), {
timestamp => sub { time } # attribute with default
};
1;
In
example.pl:
use Employee;
my $obj = Employee->new( name => "Larry", ssn => "111-22-3333" );
# unknown attributes are ignored
my $obj = Employee->new( name => "Larry", OS => "Linux" );
# $obj->{OS} does not exist
This module offers a minimalist class construction kit in around 120 lines of
code. Here is a list of features:
- •
- defines attributes via import arguments
- •
- generates read-write accessors
- •
- supports lazy attribute defaults
- •
- supports custom accessors
- •
- superclass provides a standard "new"
constructor
- •
- "new" takes a hash reference or list of key/value
pairs
- •
- "new" supports providing "BUILDARGS" to
customize constructor options
- •
- "new" calls "BUILD" for each class from
parent to child
- •
- superclass provides a "DESTROY" method
- •
- "DESTROY" calls "DEMOLISH" for each
class from child to parent
Multiple-inheritance is possible, with superclass order determined via
mro::get_linear_isa.
It uses no non-core modules for any recent Perl. On Perls older than v5.10 it
requires MRO::Compat. On Perls older than v5.14, it requires
Devel::GlobalDestruction.
Define attributes as a list of import arguments:
package Foo::Bar;
use Class::Tiny qw(
name
id
height
weight
);
For each attribute, a read-write accessor is created unless a subroutine of that
name already exists:
$obj->name; # getter
$obj->name( "John Doe" ); # setter
Attribute names must be valid subroutine identifiers or an exception will be
thrown.
You can specify lazy defaults by defining attributes with a hash reference. Keys
define attribute names and values are constants or code references that will
be evaluated when the attribute is first accessed if no value has been set.
The object is passed as an argument to a code reference.
package Foo::WithDefaults;
use Class::Tiny qw/name id/, {
title => 'Peon',
skills => sub { [] },
hire_date => sub { $_[0]->_build_hire_date },
};
When subclassing, if multiple accessors of the same name exist in different
classes, any default (or lack of default) is determined by standard method
resolution order.
To make your own custom accessors, just pre-declare the method name before
loading Class::Tiny:
package Foo::Bar;
use subs 'id';
use Class::Tiny qw( name id );
sub id { ... }
Even if you pre-declare a method name, you must include it in the attribute list
for Class::Tiny to register it as a valid attribute.
If you set a default for a custom accessor, your accessor will need to retrieve
the default and do something with it:
package Foo::Bar;
use subs 'id';
use Class::Tiny qw( name ), { id => sub { int(rand(2*31)) } };
sub id {
my $self = shift;
if (@_) {
return $self->{id} = shift;
}
elsif ( exists $self->{id} ) {
return $self->{id};
}
else {
my $defaults =
Class::Tiny->get_all_attribute_defaults_for( ref $self );
return $self->{id} = $defaults->{id}->();
}
}
If your class
does not already inherit from some class, then
Class::Tiny::Object will be added to your @ISA to provide "new" and
"DESTROY".
If your class
does inherit from something, then no additional inheritance
is set up. If the parent subclasses Class::Tiny::Object, then all is well. If
not, then you'll get accessors set up but no constructor or destructor. Don't
do that unless you really have a special need for it.
Define subclasses as normal. It's best to define them with base, parent or
superclass before defining attributes with Class::Tiny so the @ISA array is
already populated at compile-time:
package Foo::Bar::More;
use parent 'Foo::Bar';
use Class::Tiny qw( shoe_size );
If your class inherits from Class::Tiny::Object (as it should if you followed
the advice above), it provides the "new" constructor for you.
Objects can be created with attributes given as a hash reference or as a list of
key/value pairs:
$obj = Foo::Bar->new( name => "David" );
$obj = Foo::Bar->new( { name => "David" } );
If a reference is passed as a single argument, it must be able to be
dereferenced as a hash or an exception is thrown.
Unknown attributes in the constructor arguments will be ignored. Prior to
version 1.000, unknown attributes were an error, but this made it harder for
people to cleanly subclass Class::Tiny classes so this feature was removed.
You can define a "BUILDARGS" method to change how arguments to new are
handled. It will receive the constructor arguments as they were provided and
must return a hash reference of key/value pairs (or else throw an exception).
sub BUILDARGS {
my $class = shift;
my $name = shift || "John Doe";
return { name => $name };
};
Foo::Bar->new( "David" );
Foo::Bar->new(); # "John Doe"
Unknown attributes returned from "BUILDARGS" will be ignored.
If your class or any superclass defines a "BUILD" method, it will be
called by the constructor from the furthest parent class down to the child
class after the object has been created.
It is passed the constructor arguments as a hash reference. The return value is
ignored. Use "BUILD" for validation, checking required attributes or
setting default values that depend on other attributes.
sub BUILD {
my ($self, $args) = @_;
for my $req ( qw/name age/ ) {
croak "$req attribute required" unless defined $self->$req;
}
croak "Age must be non-negative" if $self->age < 0;
$self->msg( "Hello " . $self->name );
}
The argument reference is a copy, so deleting elements won't affect data in the
original (but changes will be passed to other BUILD methods in @ISA).
Class::Tiny provides a "DESTROY" method. If your class or any
superclass defines a "DEMOLISH" method, they will be called from the
child class to the furthest parent class during object destruction. It is
provided a single boolean argument indicating whether Perl is in global
destruction. Return values are ignored. Errors are caught and rethrown.
sub DEMOLISH {
my ($self, $global_destruct) = @_;
$self->cleanup();
}
You can retrieve an unsorted list of valid attributes known to Class::Tiny for a
class and its superclasses with the "get_all_attributes_for" class
method.
my @attrs = Class::Tiny->get_all_attributes_for("Employee");
# returns qw/name ssn timestamp/
Likewise, a hash reference of all valid attributes and default values (or code
references) may be retrieved with the
"get_all_attribute_defaults_for" class method. Any attributes
without a default will be "undef".
my $def = Class::Tiny->get_all_attribute_defaults_for("Employee");
# returns {
# name => undef,
# ssn => undef
# timestamp => $coderef
# }
The "import" method uses two class methods, "prepare_class"
and "create_attributes" to set up the @ISA array and attributes.
Anyone attempting to extend Class::Tiny itself should use these instead of
mocking up a call to "import".
When the first object is created, linearized @ISA, the valid attribute list and
various subroutine references are cached for speed. Ensure that all
inheritance and methods are in place before creating objects. (You don't want
to be changing that once you create objects anyway, right?)
I wanted something so simple that it could potentially be used by core Perl
modules I help maintain (or hope to write), most of which either use
Class::Struct or roll-their-own OO framework each time.
Object::Tiny and Object::Tiny::RW were close to what I wanted, but lacking some
features I deemed necessary, and their maintainers have an even more strict
philosophy against feature creep than I have.
I also considered Class::Accessor, which has been around a long time and is
heavily used, but it, too, lacked features I wanted and did things in ways I
considered poor design.
I looked for something else on CPAN, but after checking a dozen class creators I
realized I could implement exactly what I wanted faster than I could search
CPAN for something merely sufficient.
In general, compared to most things on CPAN (other than Object::Tiny),
Class::Tiny is smaller in implementation and simpler in API.
Specifically, here is how Class::Tiny ("C::T") compares to
Object::Tiny ("O::T") and Class::Accessor ("C::A"):
FEATURE C::T O::T C::A
--------------------------------------------------------------
attributes defined via import yes yes no
read/write accessors yes no yes
lazy attribute defaults yes no no
provides new yes yes yes
provides DESTROY yes no no
new takes either hashref or list yes no (list) no (hash)
Moo(se)-like BUILD/DEMOLISH yes no no
Moo(se)-like BUILDARGS yes no no
no extraneous methods via @ISA yes yes no
Moose and Moo are both excellent OO frameworks. Moose offers a powerful
meta-object protocol (MOP), but is slow to start up and has about 30 non-core
dependencies including XS modules. Moo is faster to start up and has about 10
pure Perl dependencies but provides no true MOP, relying instead on its
ability to transparently upgrade Moo to Moose when Moose's full feature set is
required.
By contrast, Class::Tiny has no MOP and has
zero non-core dependencies
for Perls in the support window. It has far less code, less complexity and no
learning curve. If you don't need or can't afford what Moo or Moose offer,
this is intended to be a reasonable fallback.
That said, Class::Tiny offers Moose-like conventions for things like
"BUILD" and "DEMOLISH" for some minimal interoperability
and an easier upgrade path.
David Golden <
[email protected]>
- •
- Dagfinn Ilmari Mannsåker
<[email protected]>
- •
- David Golden <[email protected]>
- •
- Gelu Lupas <[email protected]>
- •
- Karen Etheridge <[email protected]>
- •
- Matt S Trout <[email protected]>
- •
- Olivier Mengué <[email protected]>
- •
- Toby Inkster <[email protected]>
This software is Copyright (c) 2013 by David Golden.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004