1Class::Struct(3pm)     Perl Programmers Reference Guide     Class::Struct(3pm)
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NAME

6       Class::Struct - declare struct-like datatypes as Perl classes
7

SYNOPSIS

9           use Class::Struct;
10                   # declare struct, based on array:
11           struct( CLASS_NAME => [ ELEMENT_NAME => ELEMENT_TYPE, ... ]);
12                   # declare struct, based on hash:
13           struct( CLASS_NAME => { ELEMENT_NAME => ELEMENT_TYPE, ... });
14
15           package CLASS_NAME;
16           use Class::Struct;
17                   # declare struct, based on array, implicit class name:
18           struct( ELEMENT_NAME => ELEMENT_TYPE, ... );
19
20           # Declare struct at compile time
21           use Class::Struct CLASS_NAME => [ELEMENT_NAME => ELEMENT_TYPE, ...];
22           use Class::Struct CLASS_NAME => {ELEMENT_NAME => ELEMENT_TYPE, ...};
23
24           # declare struct at compile time, based on array, implicit
25           # class name:
26           package CLASS_NAME;
27           use Class::Struct ELEMENT_NAME => ELEMENT_TYPE, ... ;
28
29           package Myobj;
30           use Class::Struct;
31                   # declare struct with four types of elements:
32           struct( s => '$', a => '@', h => '%', c => 'My_Other_Class' );
33
34           $obj = new Myobj;               # constructor
35
36                                           # scalar type accessor:
37           $element_value = $obj->s;           # element value
38           $obj->s('new value');               # assign to element
39
40                                           # array type accessor:
41           $ary_ref = $obj->a;                 # reference to whole array
42           $ary_element_value = $obj->a(2);    # array element value
43           $obj->a(2, 'new value');            # assign to array element
44
45                                           # hash type accessor:
46           $hash_ref = $obj->h;                # reference to whole hash
47           $hash_element_value = $obj->h('x'); # hash element value
48           $obj->h('x', 'new value');          # assign to hash element
49
50                                           # class type accessor:
51           $element_value = $obj->c;           # object reference
52           $obj->c->method(...);               # call method of object
53           $obj->c(new My_Other_Class);        # assign a new object
54

DESCRIPTION

56       "Class::Struct" exports a single function, "struct".  Given a list of
57       element names and types, and optionally a class name, "struct" creates
58       a Perl 5 class that implements a "struct-like" data structure.
59
60       The new class is given a constructor method, "new", for creating struct
61       objects.
62
63       Each element in the struct data has an accessor method, which is used
64       to assign to the element and to fetch its value.  The default accessor
65       can be overridden by declaring a "sub" of the same name in the package.
66       (See Example 2.)
67
68       Each element's type can be scalar, array, hash, or class.
69
70   The "struct()" function
71       The "struct" function has three forms of parameter-list.
72
73           struct( CLASS_NAME => [ ELEMENT_LIST ]);
74           struct( CLASS_NAME => { ELEMENT_LIST });
75           struct( ELEMENT_LIST );
76
77       The first and second forms explicitly identify the name of the class
78       being created.  The third form assumes the current package name as the
79       class name.
80
81       An object of a class created by the first and third forms is based on
82       an array, whereas an object of a class created by the second form is
83       based on a hash. The array-based forms will be somewhat faster and
84       smaller; the hash-based forms are more flexible.
85
86       The class created by "struct" must not be a subclass of another class
87       other than "UNIVERSAL".
88
89       It can, however, be used as a superclass for other classes. To
90       facilitate this, the generated constructor method uses a two-argument
91       blessing.  Furthermore, if the class is hash-based, the key of each
92       element is prefixed with the class name (see Perl Cookbook, Recipe
93       13.12).
94
95       A function named "new" must not be explicitly defined in a class
96       created by "struct".
97
98       The ELEMENT_LIST has the form
99
100           NAME => TYPE, ...
101
102       Each name-type pair declares one element of the struct. Each element
103       name will be defined as an accessor method unless a method by that name
104       is explicitly defined; in the latter case, a warning is issued if the
105       warning flag (-w) is set.
106
107   Class Creation at Compile Time
108       "Class::Struct" can create your class at compile time.  The main reason
109       for doing this is obvious, so your class acts like every other class in
110       Perl.  Creating your class at compile time will make the order of
111       events similar to using any other class ( or Perl module ).
112
113       There is no significant speed gain between compile time and run time
114       class creation, there is just a new, more standard order of events.
115
116   Element Types and Accessor Methods
117       The four element types -- scalar, array, hash, and class -- are
118       represented by strings -- '$', '@', '%', and a class name -- optionally
119       preceded by a '*'.
120
121       The accessor method provided by "struct" for an element depends on the
122       declared type of the element.
123
124       Scalar ('$' or '*$')
125           The element is a scalar, and by default is initialized to "undef"
126           (but see "Initializing with new").
127
128           The accessor's argument, if any, is assigned to the element.
129
130           If the element type is '$', the value of the element (after
131           assignment) is returned. If the element type is '*$', a reference
132           to the element is returned.
133
134       Array ('@' or '*@')
135           The element is an array, initialized by default to "()".
136
137           With no argument, the accessor returns a reference to the element's
138           whole array (whether or not the element was specified as '@' or
139           '*@').
140
141           With one or two arguments, the first argument is an index
142           specifying one element of the array; the second argument, if
143           present, is assigned to the array element.  If the element type is
144           '@', the accessor returns the array element value.  If the element
145           type is '*@', a reference to the array element is returned.
146
147           As a special case, when the accessor is called with an array
148           reference as the sole argument, this causes an assignment of the
149           whole array element.  The object reference is returned.
150
151       Hash ('%' or '*%')
152           The element is a hash, initialized by default to "()".
153
154           With no argument, the accessor returns a reference to the element's
155           whole hash (whether or not the element was specified as '%' or
156           '*%').
157
158           With one or two arguments, the first argument is a key specifying
159           one element of the hash; the second argument, if present, is
160           assigned to the hash element.  If the element type is '%', the
161           accessor returns the hash element value.  If the element type is
162           '*%', a reference to the hash element is returned.
163
164           As a special case, when the accessor is called with a hash
165           reference as the sole argument, this causes an assignment of the
166           whole hash element.  The object reference is returned.
167
168       Class ('Class_Name' or '*Class_Name')
169           The element's value must be a reference blessed to the named class
170           or to one of its subclasses. The element is not initialized by
171           default.
172
173           The accessor's argument, if any, is assigned to the element. The
174           accessor will "croak" if this is not an appropriate object
175           reference.
176
177           If the element type does not start with a '*', the accessor returns
178           the element value (after assignment). If the element type starts
179           with a '*', a reference to the element itself is returned.
180
181   Initializing with "new"
182       "struct" always creates a constructor called "new". That constructor
183       may take a list of initializers for the various elements of the new
184       struct.
185
186       Each initializer is a pair of values: element name" => "value.  The
187       initializer value for a scalar element is just a scalar value. The
188       initializer for an array element is an array reference. The initializer
189       for a hash is a hash reference.
190
191       The initializer for a class element is an object of the corresponding
192       class, or of one of it's subclasses, or a reference to a hash
193       containing named arguments to be passed to the element's constructor.
194
195       See Example 3 below for an example of initialization.
196

EXAMPLES

198       Example 1
199           Giving a struct element a class type that is also a struct is how
200           structs are nested.  Here, "Timeval" represents a time (seconds and
201           microseconds), and "Rusage" has two elements, each of which is of
202           type "Timeval".
203
204               use Class::Struct;
205
206               struct( Rusage => {
207                   ru_utime => 'Timeval',  # user time used
208                   ru_stime => 'Timeval',  # system time used
209               });
210
211               struct( Timeval => [
212                   tv_secs  => '$',        # seconds
213                   tv_usecs => '$',        # microseconds
214               ]);
215
216               # create an object:
217               my $t = Rusage->new(ru_utime=>Timeval->new(),
218                   ru_stime=>Timeval->new());
219
220               # $t->ru_utime and $t->ru_stime are objects of type Timeval.
221               # set $t->ru_utime to 100.0 sec and $t->ru_stime to 5.0 sec.
222               $t->ru_utime->tv_secs(100);
223               $t->ru_utime->tv_usecs(0);
224               $t->ru_stime->tv_secs(5);
225               $t->ru_stime->tv_usecs(0);
226
227       Example 2
228           An accessor function can be redefined in order to provide
229           additional checking of values, etc.  Here, we want the "count"
230           element always to be nonnegative, so we redefine the "count"
231           accessor accordingly.
232
233               package MyObj;
234               use Class::Struct;
235
236               # declare the struct
237               struct ( 'MyObj', { count => '$', stuff => '%' } );
238
239               # override the default accessor method for 'count'
240               sub count {
241                   my $self = shift;
242                   if ( @_ ) {
243                       die 'count must be nonnegative' if $_[0] < 0;
244                       $self->{'MyObj::count'} = shift;
245                       warn "Too many args to count" if @_;
246                   }
247                   return $self->{'MyObj::count'};
248               }
249
250               package main;
251               $x = new MyObj;
252               print "\$x->count(5) = ", $x->count(5), "\n";
253                                       # prints '$x->count(5) = 5'
254
255               print "\$x->count = ", $x->count, "\n";
256                                       # prints '$x->count = 5'
257
258               print "\$x->count(-5) = ", $x->count(-5), "\n";
259                                       # dies due to negative argument!
260
261       Example 3
262           The constructor of a generated class can be passed a list of
263           element=>value pairs, with which to initialize the struct.  If no
264           initializer is specified for a particular element, its default
265           initialization is performed instead. Initializers for non-existent
266           elements are silently ignored.
267
268           Note that the initializer for a nested class may be specified as an
269           object of that class, or as a reference to a hash of initializers
270           that are passed on to the nested struct's constructor.
271
272               use Class::Struct;
273
274               struct Breed =>
275               {
276                   name  => '$',
277                   cross => '$',
278               };
279
280               struct Cat =>
281               [
282                   name     => '$',
283                   kittens  => '@',
284                   markings => '%',
285                   breed    => 'Breed',
286               ];
287
288
289               my $cat = Cat->new( name => 'Socks',
290                          kittens  => ['Monica', 'Kenneth'],
291                          markings => { socks=>1, blaze=>"white" },
292                          breed    => Breed->new(name=>'short-hair', cross=>1),
293                     or:  breed    => {name=>'short-hair', cross=>1},
294                                 );
295
296               print "Once a cat called ", $cat->name, "\n";
297               print "(which was a ", $cat->breed->name, ")\n";
298               print "had 2 kittens: ", join(' and ', @{$cat->kittens}), "\n";
299

Author and Modification History

301       Modified by Damian Conway, 2001-09-10, v0.62.
302
303          Modified implicit construction of nested objects.
304          Now will also take an object ref instead of requiring a hash ref.
305          Also default initializes nested object attributes to undef, rather
306          than calling object constructor without args
307          Original over-helpfulness was fraught with problems:
308              * the class's constructor might not be called 'new'
309              * the class might not have a hash-like-arguments constructor
310              * the class might not have a no-argument constructor
311              * "recursive" data structures didn't work well:
312                        package Person;
313                        struct { mother => 'Person', father => 'Person'};
314
315       Modified by Casey West, 2000-11-08, v0.59.
316
317           Added the ability for compile time class creation.
318
319       Modified by Damian Conway, 1999-03-05, v0.58.
320
321           Added handling of hash-like arg list to class ctor.
322
323           Changed to two-argument blessing in ctor to support
324           derivation from created classes.
325
326           Added classname prefixes to keys in hash-based classes
327           (refer to "Perl Cookbook", Recipe 13.12 for rationale).
328
329           Corrected behaviour of accessors for '*@' and '*%' struct
330           elements.  Package now implements documented behaviour when
331           returning a reference to an entire hash or array element.
332           Previously these were returned as a reference to a reference
333           to the element.
334
335       Renamed to "Class::Struct" and modified by Jim Miner, 1997-04-02.
336
337           members() function removed.
338           Documentation corrected and extended.
339           Use of struct() in a subclass prohibited.
340           User definition of accessor allowed.
341           Treatment of '*' in element types corrected.
342           Treatment of classes as element types corrected.
343           Class name to struct() made optional.
344           Diagnostic checks added.
345
346       Originally "Class::Template" by Dean Roehrich.
347
348           # Template.pm   --- struct/member template builder
349           #   12mar95
350           #   Dean Roehrich
351           #
352           # changes/bugs fixed since 28nov94 version:
353           #  - podified
354           # changes/bugs fixed since 21nov94 version:
355           #  - Fixed examples.
356           # changes/bugs fixed since 02sep94 version:
357           #  - Moved to Class::Template.
358           # changes/bugs fixed since 20feb94 version:
359           #  - Updated to be a more proper module.
360           #  - Added "use strict".
361           #  - Bug in build_methods, was using @var when @$var needed.
362           #  - Now using my() rather than local().
363           #
364           # Uses perl5 classes to create nested data types.
365           # This is offered as one implementation of Tom Christiansen's
366           # "structs.pl" idea.
367
368
369
370perl v5.30.1                      2019-11-29                Class::Struct(3pm)
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