1mro(3pm) Perl Programmers Reference Guide mro(3pm)
2
6 mro - Method Resolution Order
7
9 use mro; # enables next::method and friends globally
10 use mro 'dfs'; # enable DFS MRO for this class (Perl default)
12 use mro 'c3'; # enable C3 MRO for this class
13
15 The "mro" namespace provides several utilities for dealing with method
16 resolution order and method caching in general.
17 These interfaces are only available in Perl 5.9.5 and higher. See
19 MRO::Compat on CPAN for a mostly forwards compatible implementation for
20 older Perls.
21
23 It's possible to change the MRO of a given class either by using "use
24 mro" as shown in the synopsis, or by using the "mro::set_mro" function
25 below.
26 The special methods "next::method", "next::can", and
28 "maybe::next::method" are not available until this "mro" module has
29 been loaded via "use" or "require".
30
32 In addition to the traditional Perl default MRO (depth first search,
33 called "DFS" here), Perl now offers the C3 MRO as well. Perl's support
34 for C3 is based on the work done in Stevan Little's module Class::C3,
35 and most of the C3-related documentation here is ripped directly from
36 there.
37 What is C3?
39 C3 is the name of an algorithm which aims to provide a sane method
40 resolution order under multiple inheritance. It was first introduced in
41 the language Dylan (see links in the "SEE ALSO" section), and then
42 later adopted as the preferred MRO (Method Resolution Order) for the
43 new-style classes in Python 2.3. Most recently it has been adopted as
44 the "canonical" MRO for Perl 6 classes, and the default MRO for Parrot
45 objects as well.
46 How does C3 work
48 C3 works by always preserving local precedence ordering. This
49 essentially means that no class will appear before any of its
50 subclasses. Take, for instance, the classic diamond inheritance
51 pattern:
52 <A>
54 / \
55 <B> <C>
56 \ /
57 <D>
58 The standard Perl 5 MRO would be (D, B, A, C). The result being that A
60 appears before C, even though C is the subclass of A. The C3 MRO
61 algorithm however, produces the following order: (D, B, C, A), which
62 does not have this issue.
63 This example is fairly trivial; for more complex cases and a deeper
65 explanation, see the links in the "SEE ALSO" section.
66
68 mro::get_linear_isa($classname[, $type])
69 Returns an arrayref which is the linearized MRO of the given class.
70 Uses whichever MRO is currently in effect for that class by default, or
71 the given MRO (either "c3" or "dfs" if specified as $type).
72 The linearized MRO of a class is an ordered array of all of the classes
74 one would search when resolving a method on that class, starting with
75 the class itself.
76 If the requested class doesn't yet exist, this function will still
78 succeed, and return "[ $classname ]"
79 Note that "UNIVERSAL" (and any members of "UNIVERSAL"'s MRO) are not
81 part of the MRO of a class, even though all classes implicitly inherit
82 methods from "UNIVERSAL" and its parents.
83 mro::set_mro ($classname, $type)
85 Sets the MRO of the given class to the $type argument (either "c3" or
86 "dfs").
87 mro::get_mro($classname)
89 Returns the MRO of the given class (either "c3" or "dfs").
90 mro::get_isarev($classname)
92 Gets the "mro_isarev" for this class, returned as an arrayref of class
93 names. These are every class that "isa" the given class name, even if
94 the isa relationship is indirect. This is used internally by the MRO
95 code to keep track of method/MRO cache invalidations.
96 As with "mro::get_linear_isa" above, "UNIVERSAL" is special.
98 "UNIVERSAL" (and parents') isarev lists do not include every class in
99 existence, even though all classes are effectively descendants for
100 method inheritance purposes.
101 mro::is_universal($classname)
103 Returns a boolean status indicating whether or not the given classname
104 is either "UNIVERSAL" itself, or one of "UNIVERSAL"'s parents by @ISA
105 inheritance.
106 Any class for which this function returns true is "universal" in the
108 sense that all classes potentially inherit methods from it.
109 mro::invalidate_all_method_caches()
111 Increments "PL_sub_generation", which invalidates method caching in all
112 packages.
113 mro::method_changed_in($classname)
115 Invalidates the method cache of any classes dependent on the given
116 class. This is not normally necessary. The only known case where pure
117 perl code can confuse the method cache is when you manually install a
118 new constant subroutine by using a readonly scalar value, like the
119 internals of constant do. If you find another case, please report it
120 so we can either fix it or document the exception here.
121 mro::get_pkg_gen($classname)
123 Returns an integer which is incremented every time a real local method
124 in the package $classname changes, or the local @ISA of $classname is
125 modified.
126 This is intended for authors of modules which do lots of class
128 introspection, as it allows them to very quickly check if anything
129 important about the local properties of a given class have changed
130 since the last time they looked. It does not increment on method/@ISA
131 changes in superclasses.
132 It's still up to you to seek out the actual changes, and there might
134 not actually be any. Perhaps all of the changes since you last checked
135 cancelled each other out and left the package in the state it was in
136 before.
137 This integer normally starts off at a value of 1 when a package stash
139 is instantiated. Calling it on packages whose stashes do not exist at
140 all will return 0. If a package stash is completely deleted (not a
141 normal occurrence, but it can happen if someone does something like
142 "undef %PkgName::"), the number will be reset to either 0 or 1,
143 depending on how completely the package was wiped out.
144 next::method
146 This is somewhat like "SUPER", but it uses the C3 method resolution
147 order to get better consistency in multiple inheritance situations.
148 Note that while inheritance in general follows whichever MRO is in
149 effect for the given class, "next::method" only uses the C3 MRO.
150 One generally uses it like so:
152 sub some_method {
154 my $self = shift;
155 my $superclass_answer = $self->next::method(@_);
156 return $superclass_answer + 1;
157 }
158 Note that you don't (re-)specify the method name. It forces you to
160 always use the same method name as the method you started in.
161 It can be called on an object or a class, of course.
163 The way it resolves which actual method to call is:
165 1. First, it determines the linearized C3 MRO of the object or class
167 it is being called on.
168 2. Then, it determines the class and method name of the context it was
170 invoked from.
171 3. Finally, it searches down the C3 MRO list until it reaches the
173 contextually enclosing class, then searches further down the MRO
174 list for the next method with the same name as the contextually
175 enclosing method.
176 Failure to find a next method will result in an exception being thrown
178 (see below for alternatives).
179 This is substantially different than the behavior of "SUPER" under
181 complex multiple inheritance. (This becomes obvious when one realizes
182 that the common superclasses in the C3 linearizations of a given class
183 and one of its parents will not always be ordered the same for both.)
184 Caveat: Calling "next::method" from methods defined outside the class:
186 There is an edge case when using "next::method" from within a
188 subroutine which was created in a different module than the one it is
189 called from. It sounds complicated, but it really isn't. Here is an
190 example which will not work correctly:
191 *Foo::foo = sub { (shift)->next::method(@_) };
193 The problem exists because the anonymous subroutine being assigned to
195 the *Foo::foo glob will show up in the call stack as being called
196 "__ANON__" and not "foo" as you might expect. Since "next::method" uses
197 "caller" to find the name of the method it was called in, it will fail
198 in this case.
199 But fear not, there's a simple solution. The module "Sub::Name" will
201 reach into the perl internals and assign a name to an anonymous
202 subroutine for you. Simply do this:
203 use Sub::Name 'subname';
205 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
206 and things will Just Work.
208 next::can
210 This is similar to "next::method", but just returns either a code
211 reference or "undef" to indicate that no further methods of this name
212 exist.
213 maybe::next::method
215 In simple cases, it is equivalent to:
216 $self->next::method(@_) if $self->next::can;
218 But there are some cases where only this solution works (like "goto
220 &maybe::next::method");
221
223 The original Dylan paper
224 <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.19.3910&rep=rep1&type=pdf>
225 Python 2.3 MRO
227 <https://www.python.org/download/releases/2.3/mro/>
228 Class::C3
230 Class::C3
231
233 Brandon L. Black, <blblack@gmail.com>
234 Based on Stevan Little's Class::C3
236perl v5.32.1 2021-03-31 mro(3pm)