1package Class::Std::Utils;
2
3use version; $VERSION = qv('0.0.2');
4
5use warnings;
6use strict;
7use Carp;
8use Scalar::Util qw( refaddr );
9
10sub import {
11    my $caller = caller;
12
13    no strict qw( refs );
14    *{ $caller . '::anon_scalar' }               = \&anon_scalar;
15    *{ $caller . '::ident' }                     = \&refaddr;
16    *{ $caller . '::extract_initializers_from' } = \&extract_initializers_from;
17}
18
19sub anon_scalar { return \my $scalar; }
20
21use List::Util qw( first );
22
23sub extract_initializers_from {
24    my ($arg_ref) = @_;
25
26    my $class_name = caller;
27
28    # Find the class-specific sub-hash (if any)...
29    my $specific_inits_ref
30        = first {defined $_} $arg_ref->{$class_name}, {};
31    croak "$class_name initializer must be a nested hash"        if ref $specific_inits_ref ne 'HASH';
32    # Return initializers, overriding general initializers from the top level
33    # with any second-level initializers that are specific to the class....
34    return ( %{$arg_ref}, %{$specific_inits_ref} );
35}
36
37
381; # Magic true value required at end of module
39__END__
40
41=head1 NAME
42
43Class::Std::Utils - Utility subroutines for building "inside-out" objects
44
45
46=head1 VERSION
47
48This document describes Class::Std::Utils version 0.0.1
49
50
51=head1 SYNOPSIS
52
53    use Class::Std::Utils;
54
55    # Constructor for anonymous scalars...
56    my $new_object = bless anon_scalar(), $class;
57
58    # Convert an object reference into a unique ID number...
59    my $ID_num = ident $new_object;
60
61    # Extract class-specific arguments from a hash reference...
62    my %args = extract_initializers_from($arg_ref);
63
64
65=head1 DESCRIPTION
66
67This module provides three utility subroutines that simplify the creation of
68"inside-out" classes. See Chapters 15 and 16 of "Perl Best Practices"
69(O'Reilly, 2005) for details.
70
71=head1 INTERFACE
72
73=over
74
75=item C<anon_scalar()>
76
77This subroutine is always exported. It takes no arguments and returns a
78reference to an anonymous scalar, suitable for blessing as an object.
79
80=item C<ident()>
81
82This subroutine is always exported. It takes one argument--a reference--
83and acts exactly like the C<Scalar::Util::refaddr()>, returning a unique
84integer value suitable for identifying the referent.
85
86=item C<extract_initializers_from()>
87
88This subroutine is always exported. It takes one argument--a hash reference--
89and returns a "flattened" set of key/value pairs extracted from that hash.
90
91The typical usage is:
92
93    my %class_specific_args = extract_initializers_from($args_ref);
94
95The argument hash is flattened as described in Chapter 16 of "Perl Best
96Practices":
97
98=over
99
100I<The subroutine is always called with the original multi-level argument
101hash from the constructor. It then looks up the class's own name (i.e.
102its C<caller> package) in the argument hash, to see if an initializer
103with that key has been defined. Finally, C<extract_initializers_for()>
104returns the flattened set of key/value pairs for the class's initializer
105set, by appending the class-specific initializer subhash to the end of
106the original generic initializer hash. Appending the specific
107initializers after the generic ones means that any key in the class-
108specific set will override any key in the generic set, thereby ensuring
109that the most relevant initializers are always selected, but that
110generic initializers are still available where no class-specific value
111has been passed in.>
112
113=back
114
115In other words, given:
116
117    my $arg_ref = {
118        key_1 => 'generic value 1',
119        key_2 => 'generic value 2',
120
121        'Base::Class' => {
122            key_1 => 'base value 1'
123        },
124
125        'Der::Class' => {
126            key_1 => 'der value 1'
127            key_2 => 'der value 2'
128        },
129    };
130
131    package Base::Class;
132    use Class::Std::Utils;
133
134    my %base_args = extract_initializers_from($arg_ref);
135
136    package Der::Class;
137    use Class::Std::Utils;
138
139    my %der_args = extract_initializers_from($arg_ref);
140
141then C<%base_args> would be initialized to:
142
143    (
144        key_1 => 'base value 1',
145        key_2 => 'generic value 2',
146
147        'Base::Class' => {
148            key_1 => 'base value 1',
149        },
150
151        'Der::Class' => {
152            key_1 => 'der value 1',
153            key_2 => 'der value 2',
154        },
155    )
156
157whilst C<%der_args> would be initialized to:
158
159    (
160        key_1 => 'der value 1',
161        key_2 => 'der value 2',
162
163        'Base::Class' => {
164            key_1 => 'base value 1',
165        },
166
167        'Der::Class' => {
168            key_1 => 'der value 1',
169            key_2 => 'der value 2',
170        },
171    )
172
173That is, the top-level entries would be replaced by any second-level
174entries with the same key that appear in a top-level entry of the same name as
175the calling package.
176
177This means that each class can just refer to C<$args{key_1}> and
178C<$args{key_2}> and be confident that the resulting values will be the
179most specific available for that class.
180
181=back
182
183=head1 DIAGNOSTICS
184
185=over
186
187=item C<< %s initializer must be a nested hash >>
188
189Thrown by C<extract_initializers_from()>. You specified a top-level key
190that has the same name of the current class, but the value of that key
191wasn't a hash reference.
192
193=back
194
195
196=head1 CONFIGURATION AND ENVIRONMENT
197
198Class::Std::Utils requires no configuration files or environment variables.
199
200
201=head1 DEPENDENCIES
202
203Thsi module requires both the C<Scalar::Util> and C<List::Util> modules,
204which are standard in Perl 5.8 and available from the CPAN for earlier
205versions of Perl.
206
207
208=head1 INCOMPATIBILITIES
209
210None reported.
211
212
213=head1 SEE ALSO
214
215The C<Class::Std> module
216
217"Perl Best Practices", O'Reilly, 2005.
218
219
220=head1 BUGS AND LIMITATIONS
221
222No bugs have been reported.
223
224Please report any bugs or feature requests to
225C<bug-class-std-utils@rt.cpan.org>, or through the web interface at
226L<http://rt.cpan.org>.
227
228
229=head1 AUTHOR
230
231Damian Conway  C<< <DCONWAY@cpan.org> >>
232
233
234=head1 LICENCE AND COPYRIGHT
235
236Copyright (c) 2005, Damian Conway C<< <DCONWAY@cpan.org> >>. All rights reserved.
237
238This module is free software; you can redistribute it and/or
239modify it under the same terms as Perl itself.
240
241
242=head1 DISCLAIMER OF WARRANTY
243
244BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
245FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
246OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
247PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
248EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
249WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
250ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
251YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
252NECESSARY SERVICING, REPAIR, OR CORRECTION.
253
254IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
255WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
256REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE
257LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL,
258OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE
259THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
260RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
261FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
262SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
263SUCH DAMAGES.
264