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1package overload;
2
3sub OVERLOAD {
4 $package = shift;
5 my %arg = @_;
6 my $hash = \%{$package . "::OVERLOAD"};
7 for (keys %arg) {
8 $hash->{$_} = $arg{$_};
9 }
10}
11
12sub import {
13 $package = (caller())[0];
14 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
15 shift;
16 $package->overload::OVERLOAD(@_);
17}
18
19sub unimport {
20 $package = (caller())[0];
21 my $hash = \%{$package . "::OVERLOAD"};
22 shift;
23 for (@_) {
24 delete $hash->{$_};
25 }
26}
27
28sub Overloaded {
29 defined ($package = ref $_[0]) and defined %{$package . "::OVERLOAD"};
30}
31
32sub OverloadedStringify {
33 defined ($package = ref $_[0]) and
34 defined %{$package . "::OVERLOAD"} and
35 exists $ {$package . "::OVERLOAD"}{'""'} and
36 defined &{$ {$package . "::OVERLOAD"}{'""'}};
37}
38
39sub Method {
40 defined ($package = ref $_[0]) and
41 defined %{$package . "::OVERLOAD"} and
42 $ {$package . "::OVERLOAD"}{$_[1]};
43}
44
45sub AddrRef {
46 $package = ref $_[0];
47 bless $_[0], Overload::Fake; # Non-overloaded package
48 my $str = "$_[0]";
49 bless $_[0], $package; # Back
50 $str;
51}
52
53sub StrVal {
54 (OverloadedStringify) ?
55 (AddrRef) :
56 "$_[0]";
57}
58
591;
60
61__END__
62
63=head1 NAME
64
65C<overload.pm> - Package for overloading perl operations
66
67=head1 SYNOPSIS
68
69 package SomeThing;
70
71 use overload
72 '+' => \&myadd,
73 '-' => \&mysub;
74 # etc
75 ...
76
77 package main;
78 $a = new SomeThing 57;
79 $b=5+$a;
80 ...
81 if (overload::Overloaded $b) {...}
82 ...
83 $strval = overload::StrVal $b;
84
85=head1 CAVEAT SCRIPTOR
86
87Overloading of operators is a subject not to be taken lightly.
88Neither its precise implementation, syntax, nor semantics are
89100% endorsed by Larry Wall. So any of these may be changed
90at some point in the future.
91
92=head1 DESCRIPTION
93
94=head2 Declaration of overloaded functions
95
96The compilation directive
97
98 package Number;
99 use overload
100 "+" => \&add,
101 "*=" => "muas";
102
103declares function Number::add() for addition, and method muas() in
104the "class" C<Number> (or one of its base classes)
105for the assignment form C<*=> of multiplication.
106
107Arguments of this directive come in (key, value) pairs. Legal values
108are values legal inside a C<&{ ... }> call, so the name of a subroutine,
109a reference to a subroutine, or an anonymous subroutine will all work.
110Legal keys are listed below.
111
112The subroutine C<add> will be called to execute C<$a+$b> if $a
113is a reference to an object blessed into the package C<Number>, or if $a is
114not an object from a package with defined mathemagic addition, but $b is a
115reference to a C<Number>. It can also be called in other situations, like
116C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
117methods refer to methods triggered by an overloaded mathematical
118operator.)
119
120=head2 Calling Conventions for Binary Operations
121
122The functions specified in the C<use overload ...> directive are called
123with three (in one particular case with four, see L<Last Resort>)
124arguments. If the corresponding operation is binary, then the first
125two arguments are the two arguments of the operation. However, due to
126general object calling conventions, the first argument should always be
127an object in the package, so in the situation of C<7+$a>, the
128order of the arguments is interchanged. It probably does not matter
129when implementing the addition method, but whether the arguments
130are reversed is vital to the subtraction method. The method can
131query this information by examining the third argument, which can take
132three different values:
133
134=over 7
135
136=item FALSE
137
138the order of arguments is as in the current operation.
139
140=item TRUE
141
142the arguments are reversed.
143
144=item C<undef>
145
146the current operation is an assignment variant (as in
147C<$a+=7>), but the usual function is called instead. This additional
148information can be used to generate some optimizations.
149
150=back
151
152=head2 Calling Conventions for Unary Operations
153
154Unary operation are considered binary operations with the second
155argument being C<undef>. Thus the functions that overloads C<{"++"}>
156is called with arguments C<($a,undef,'')> when $a++ is executed.
157
158=head2 Overloadable Operations
159
160The following symbols can be specified in C<use overload>:
161
162=over 5
163
164=item * I<Arithmetic operations>
165
166 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
167 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
168
169For these operations a substituted non-assignment variant can be called if
170the assignment variant is not available. Methods for operations "C<+>",
171"C<->", "C<+=>", and "C<-=>" can be called to automatically generate
172increment and decrement methods. The operation "C<->" can be used to
173autogenerate missing methods for unary minus or C<abs>.
174
175=item * I<Comparison operations>
176
177 "<", "<=", ">", ">=", "==", "!=", "<=>",
178 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
179
180If the corresponding "spaceship" variant is available, it can be
181used to substitute for the missing operation. During C<sort>ing
182arrays, C<cmp> is used to compare values subject to C<use overload>.
183
184=item * I<Bit operations>
185
186 "&", "^", "|", "neg", "!", "~",
187
188"C<neg>" stands for unary minus. If the method for C<neg> is not
189specified, it can be autogenerated using the method for subtraction.
190
191=item * I<Increment and decrement>
192
193 "++", "--",
194
195If undefined, addition and subtraction methods can be
196used instead. These operations are called both in prefix and
197postfix form.
198
199=item * I<Transcendental functions>
200
201 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
202
203If C<abs> is unavailable, it can be autogenerated using methods
204for "<" or "<=>" combined with either unary minus or subtraction.
205
206=item * I<Boolean, string and numeric conversion>
207
208 "bool", "\"\"", "0+",
209
210If one or two of these operations are unavailable, the remaining ones can
211be used instead. C<bool> is used in the flow control operators
212(like C<while>) and for the ternary "C<?:>" operation. These functions can
213return any arbitrary Perl value. If the corresponding operation for this value
214is overloaded too, that operation will be called again with this value.
215
216=item * I<Special>
217
218 "nomethod", "fallback", "=",
219
220see L<SPECIAL SYMBOLS FOR C<use overload>>.
221
222=back
223
224See L<"Fallback"> for an explanation of when a missing method can be autogenerated.
225
226=head1 SPECIAL SYMBOLS FOR C<use overload>
227
228Three keys are recognized by Perl that are not covered by the above
229description.
230
231=head2 Last Resort
232
233C<"nomethod"> should be followed by a reference to a function of four
234parameters. If defined, it is called when the overloading mechanism
235cannot find a method for some operation. The first three arguments of
236this function coincide with the arguments for the corresponding method if
237it were found, the fourth argument is the symbol
238corresponding to the missing method. If several methods are tried,
239the last one is used. Say, C<1-$a> can be equivalent to
240
241 &nomethodMethod($a,1,1,"-")
242
243if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
244C<use overload> directive.
245
246If some operation cannot be resolved, and there is no function
247assigned to C<"nomethod">, then an exception will be raised via die()--
248unless C<"fallback"> was specified as a key in C<use overload> directive.
249
250=head2 Fallback
251
252The key C<"fallback"> governs what to do if a method for a particular
253operation is not found. Three different cases are possible depending on
254the value of C<"fallback">:
255
256=over 16
257
258=item * C<undef>
259
260Perl tries to use a
261substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
262then tries to calls C<"nomethod"> value; if missing, an exception
263will be raised.
264
265=item * TRUE
266
267The same as for the C<undef> value, but no exception is raised. Instead,
268it silently reverts to what it would have done were there no C<use overload>
269present.
270
271=item * defined, but FALSE
272
273No autogeneration is tried. Perl tries to call
274C<"nomethod"> value, and if this is missing, raises an exception.
275
276=back
277
278=head2 Copy Constructor
279
280The value for C<"="> is a reference to a function with three
281arguments, i.e., it looks like the other values in C<use
282overload>. However, it does not overload the Perl assignment
283operator. This would go against Camel hair.
284
285This operation is called in the situations when a mutator is applied
286to a reference that shares its object with some other reference, such
287as
288
289 $a=$b;
290 $a++;
291
292To make this change $a and not change $b, a copy of C<$$a> is made,
293and $a is assigned a reference to this new object. This operation is
294done during execution of the C<$a++>, and not during the assignment,
295(so before the increment C<$$a> coincides with C<$$b>). This is only
296done if C<++> is expressed via a method for C<'++'> or C<'+='>. Note
297that if this operation is expressed via C<'+'> a nonmutator, i.e., as
298in
299
300 $a=$b;
301 $a=$a+1;
302
303then C<$a> does not reference a new copy of C<$$a>, since $$a does not
304appear as lvalue when the above code is executed.
305
306If the copy constructor is required during the execution of some mutator,
307but a method for C<'='> was not specified, it can be autogenerated as a
308string copy if the object is a plain scalar.
309
310=over 5
311
312=item B<Example>
313
314The actually executed code for
315
316 $a=$b;
317 Something else which does not modify $a or $b....
318 ++$a;
319
320may be
321
322 $a=$b;
323 Something else which does not modify $a or $b....
324 $a = $a->clone(undef,"");
325 $a->incr(undef,"");
326
327if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
328C<'='> was overloaded with C<\&clone>.
329
330=back
331
332=head1 MAGIC AUTOGENERATION
333
334If a method for an operation is not found, and the value for C<"fallback"> is
335TRUE or undefined, Perl tries to autogenerate a substitute method for
336the missing operation based on the defined operations. Autogenerated method
337substitutions are possible for the following operations:
338
339=over 16
340
341=item I<Assignment forms of arithmetic operations>
342
343C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
344is not defined.
345
346=item I<Conversion operations>
347
348String, numeric, and boolean conversion are calculated in terms of one
349another if not all of them are defined.
350
351=item I<Increment and decrement>
352
353The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
354and C<$a--> in terms of C<$a-=1> and C<$a-1>.
355
356=item C<abs($a)>
357
358can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
359
360=item I<Unary minus>
361
362can be expressed in terms of subtraction.
363
364=item I<Concatenation>
365
366can be expressed in terms of string conversion.
367
368=item I<Comparison operations>
369
370can be expressed in terms of its "spaceship" counterpart: either
371C<E<lt>=E<gt>> or C<cmp>:
372
373 <, >, <=, >=, ==, != in terms of <=>
374 lt, gt, le, ge, eq, ne in terms of cmp
375
376=item I<Copy operator>
377
378can be expressed in terms of an assignment to the dereferenced value, if this
379value is a scalar and not a reference.
380
381=back
382
383=head1 WARNING
384
385The restriction for the comparison operation is that even if, for example,
386`C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
387function will produce only a standard logical value based on the
388numerical value of the result of `C<cmp>'. In particular, a working
389numeric conversion is needed in this case (possibly expressed in terms of
390other conversions).
391
392Similarly, C<.=> and C<x=> operators lose their mathemagical properties
393if the string conversion substitution is applied.
394
395When you chop() a mathemagical object it is promoted to a string and its
396mathemagical properties are lost. The same can happen with other
397operations as well.
398
399=head1 Run-time Overloading
400
401Since all C<use> directives are executed at compile-time, the only way to
402change overloading during run-time is to
403
404 eval 'use overload "+" => \&addmethod';
405
406You can also use
407
408 eval 'no overload "+", "--", "<="';
409
410though the use of these constructs during run-time is questionable.
411
412=head1 Public functions
413
414Package C<overload.pm> provides the following public functions:
415
416=over 5
417
418=item overload::StrVal(arg)
419
420Gives string value of C<arg> as in absence of stringify overloading.
421
422=item overload::Overloaded(arg)
423
424Returns true if C<arg> is subject to overloading of some operations.
425
426=item overload::Method(obj,op)
427
428Returns C<undef> or a reference to the method that implements C<op>.
429
430=back
431
432=head1 IMPLEMENTATION
433
434What follows is subject to change RSN.
435
436The table of methods for all operations is cached as magic in the
437symbol table hash for the package. The table is rechecked for changes due to
438C<use overload>, C<no overload>, and @ISA only during
439C<bless>ing; so if they are changed dynamically, you'll need an
440additional fake C<bless>ing to update the table.
441
442(Every SVish thing has a magic queue, and magic is an entry in that queue.
443This is how a single variable may participate in multiple forms of magic
444simultaneously. For instance, environment variables regularly have two
445forms at once: their %ENV magic and their taint magic.)
446
447If an object belongs to a package using overload, it carries a special
448flag. Thus the only speed penalty during arithmetic operations without
449overloading is the checking of this flag.
450
451In fact, if C<use overload> is not present, there is almost no overhead for
452overloadable operations, so most programs should not suffer measurable
453performance penalties. A considerable effort was made to minimize the overhead
454when overload is used and the current operation is overloadable but
455the arguments in question do not belong to packages using overload. When
456in doubt, test your speed with C<use overload> and without it. So far there
457have been no reports of substantial speed degradation if Perl is compiled
458with optimization turned on.
459
460There is no size penalty for data if overload is not used.
461
462Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
463carried out before any operation that can imply an assignment to the
464object $a (or $b) refers to, like C<$a++>. You can override this
465behavior by defining your own copy constructor (see L<"Copy Constructor">).
466
467It is expected that arguments to methods that are not explicitly supposed
468to be changed are constant (but this is not enforced).
469
470=head1 AUTHOR
471
472Ilya Zakharevich <F<ilya@math.mps.ohio-state.edu>>.
473
474=head1 DIAGNOSTICS
475
476When Perl is run with the B<-Do> switch or its equivalent, overloading
477induces diagnostic messages.
478
479=head1 BUGS
480
481Because it is used for overloading, the per-package associative array
482%OVERLOAD now has a special meaning in Perl.
483
484As shipped, mathemagical properties are not inherited via the @ISA tree.
485
486This document is confusing.
487
488=cut
489