mixin(3pm)

1mixin(3)              User Contributed Perl Documentation             mixin(3)
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NAME

6       mixin - Mix-in inheritance, an alternative to multiple inheritance
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SYNOPSIS

9         package Dog;
10         sub speak { print "Bark!\n" }
11         sub new { my $class = shift;  bless {}, $class }
12
13         package Dog::Small;
14         use base 'Dog';
15         sub speak { print "Yip!\n"; }
16
17         package Dog::Retriever;
18         use mixin::with 'Dog';
19         sub fetch { print "Get your own stinking $_[1]\n" }
20
21         package Dog::Small::Retriever;
22         use base 'Dog::Small';
23         use mixin 'Dog::Retriever';
24
25         my $small_retriever = Dog::Small::Retriever->new;
26         $small_retriever->speak;          # Yip!
27         $small_retriever->fetch('ball');  # Get your own stinking ball
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DESCRIPTION

30       NOTE You probably want to look into the similar but superior concept of
31       traits/roles instead.  See "SEE ALSO" for suggested modules.
32
33       Mixin inheritance is an alternative to the usual multiple-inheritance
34       and solves the problem of knowing which parent will be called.  It also
35       solves a number of tricky problems like diamond inheritence.
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37       The idea is to solve the same sets of problems which MI solves without
38       the problems of MI.  For all practical purposes you can think of a
39       mixin as multiple inheritance without the actual inheritance.
40
41       Mixins are a band-aid for the problems of MI.  A better solution is to
42       use traits (called "Roles" in Perl 6), which are like mixins on
43       steroids.  Class::Trait implements this.
44
45   Using a mixin class
46       There are two steps to using a mixin-class.
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48       First, make sure you are inherited from the class with which the mixin-
49       class is to be mixed.
50
51         package Dog::Small::Retriever;
52         use base 'Dog::Small';
53
54       Since Dog::Small isa Dog, that does it.  Then simply mixin the new
55       functionality
56
57         use mixin 'Dog::Retriever';
58
59       and now you can use fetch().
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61   Writing a mixin class
62       See mixin::with.
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64   Mixins, Inheritance and SUPER
65       A class which uses a mixin does not inherit from it.  However, through
66       some clever trickery, "SUPER" continues to work.  Here's an example.
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68           {
69               package Parent;
70               sub foo { "Parent" }
71           }
72
73           {
74               package Middle;
75               use mixin::with "Parent";
76
77               sub foo {
78                   my $self = shift;
79                   return $self->SUPER::foo(), "Middle";
80               }
81           }
82
83           {
84               package Child;
85               use base "Parent";
86               use mixin "Middle";
87
88               sub foo {
89                   my $self = shift;
90                   return $self->SUPER::foo(), "Child";
91               }
92           }
93
94           print join " ", Child->foo;  # Parent Middle Child
95
96       This will print "Parent Middle Child".  You'll note that this is the
97       same result if Child inherited from Middle and Middle from Parent.  Its
98       also the same result if Child multiply inherited from Middle and Parent
99       but NOT if it inherited from Parent then Middle.  The advantage of
100       mixins vs multiple inheritance is such ambiguities do not exist.
101
102       Note that even though both the Child and Middle define foo() the Middle
103       mixin does not overwrite Child's foo().  A mixin does not simply export
104       its methods into the mixer and thus does not blow over existing
105       methods.
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NOTES

108       A mixin will not warn if the mixin and the user define the same method.
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AUTHOR

111       Michael G Schwern <schwern@pobox.com>
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LICENSE

114       Copyright 2002-2015 by Michael G Schwern
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116       This library is free software; you can redistribute it and/or modify it
117       under the same terms as Perl itself.
118
119       <http://dev.perl.org/licenses/>
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