Commit | Line | Data |
4633a7c4 |
1 | package overload; |
2 | |
1e9bd118 |
3 | our $VERSION = '1.07'; |
b75c8c73 |
4 | |
a6006777 |
5 | sub nil {} |
6 | |
4633a7c4 |
7 | sub OVERLOAD { |
8 | $package = shift; |
9 | my %arg = @_; |
a6006777 |
10 | my ($sub, $fb); |
11 | $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. |
12 | *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. |
4633a7c4 |
13 | for (keys %arg) { |
a6006777 |
14 | if ($_ eq 'fallback') { |
15 | $fb = $arg{$_}; |
16 | } else { |
17 | $sub = $arg{$_}; |
18 | if (not ref $sub and $sub !~ /::/) { |
44a8e56a |
19 | $ {$package . "::(" . $_} = $sub; |
20 | $sub = \&nil; |
a6006777 |
21 | } |
22 | #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; |
23 | *{$package . "::(" . $_} = \&{ $sub }; |
24 | } |
4633a7c4 |
25 | } |
a6006777 |
26 | ${$package . "::()"} = $fb; # Make it findable too (fallback only). |
4633a7c4 |
27 | } |
28 | |
29 | sub import { |
30 | $package = (caller())[0]; |
31 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; |
32 | shift; |
33 | $package->overload::OVERLOAD(@_); |
34 | } |
35 | |
36 | sub unimport { |
37 | $package = (caller())[0]; |
a6006777 |
38 | ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table |
4633a7c4 |
39 | shift; |
40 | for (@_) { |
a6006777 |
41 | if ($_ eq 'fallback') { |
42 | undef $ {$package . "::()"}; |
43 | } else { |
44 | delete $ {$package . "::"}{"(" . $_}; |
45 | } |
4633a7c4 |
46 | } |
47 | } |
48 | |
49 | sub Overloaded { |
a6006777 |
50 | my $package = shift; |
51 | $package = ref $package if ref $package; |
52 | $package->can('()'); |
4633a7c4 |
53 | } |
54 | |
44a8e56a |
55 | sub ov_method { |
56 | my $globref = shift; |
57 | return undef unless $globref; |
58 | my $sub = \&{*$globref}; |
59 | return $sub if $sub ne \&nil; |
60 | return shift->can($ {*$globref}); |
61 | } |
62 | |
4633a7c4 |
63 | sub OverloadedStringify { |
a6006777 |
64 | my $package = shift; |
65 | $package = ref $package if ref $package; |
44a8e56a |
66 | #$package->can('(""') |
ee239bfe |
67 | ov_method mycan($package, '(""'), $package |
68 | or ov_method mycan($package, '(0+'), $package |
69 | or ov_method mycan($package, '(bool'), $package |
70 | or ov_method mycan($package, '(nomethod'), $package; |
4633a7c4 |
71 | } |
72 | |
73 | sub Method { |
a6006777 |
74 | my $package = shift; |
05a4b9b1 |
75 | if(ref $package) { |
4de05ceb |
76 | local $@; |
77 | local $!; |
78 | require Scalar::Util; |
05a4b9b1 |
79 | $package = Scalar::Util::blessed($package); |
80 | return undef if !defined $package; |
81 | } |
44a8e56a |
82 | #my $meth = $package->can('(' . shift); |
83 | ov_method mycan($package, '(' . shift), $package; |
84 | #return $meth if $meth ne \&nil; |
85 | #return $ {*{$meth}}; |
4633a7c4 |
86 | } |
87 | |
88 | sub AddrRef { |
a6006777 |
89 | my $package = ref $_[0]; |
90 | return "$_[0]" unless $package; |
b3c0ec7c |
91 | |
4de05ceb |
92 | local $@; |
93 | local $!; |
94 | require Scalar::Util; |
295812e0 |
95 | my $class = Scalar::Util::blessed($_[0]); |
96 | my $class_prefix = defined($class) ? "$class=" : ""; |
97 | my $type = Scalar::Util::reftype($_[0]); |
98 | my $addr = Scalar::Util::refaddr($_[0]); |
99 | return sprintf("$class_prefix$type(0x%x)", $addr); |
4633a7c4 |
100 | } |
101 | |
1b1d102f |
102 | *StrVal = *AddrRef; |
4633a7c4 |
103 | |
44a8e56a |
104 | sub mycan { # Real can would leave stubs. |
105 | my ($package, $meth) = @_; |
e1a479c5 |
106 | |
1e9bd118 |
107 | local $@; |
108 | local $!; |
109 | require mro; |
110 | |
e1a479c5 |
111 | my $mro = mro::get_linear_isa($package); |
112 | foreach my $p (@$mro) { |
113 | my $fqmeth = $p . q{::} . $meth; |
114 | return \*{$fqmeth} if defined &{$fqmeth}; |
44a8e56a |
115 | } |
e1a479c5 |
116 | |
44a8e56a |
117 | return undef; |
118 | } |
119 | |
b3ac6de7 |
120 | %constants = ( |
9cfe5470 |
121 | 'integer' => 0x1000, # HINT_NEW_INTEGER |
122 | 'float' => 0x2000, # HINT_NEW_FLOAT |
123 | 'binary' => 0x4000, # HINT_NEW_BINARY |
124 | 'q' => 0x8000, # HINT_NEW_STRING |
125 | 'qr' => 0x10000, # HINT_NEW_RE |
b3ac6de7 |
126 | ); |
127 | |
ee239bfe |
128 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
129 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
2877bd81 |
130 | num_comparison => "< <= > >= == !=", |
ee239bfe |
131 | '3way_comparison'=> "<=> cmp", |
2877bd81 |
132 | str_comparison => "lt le gt ge eq ne", |
6dd85743 |
133 | binary => '& &= | |= ^ ^=', |
ee239bfe |
134 | unary => "neg ! ~", |
135 | mutators => '++ --', |
f216259d |
136 | func => "atan2 cos sin exp abs log sqrt int", |
ee239bfe |
137 | conversion => 'bool "" 0+', |
f5284f61 |
138 | iterators => '<>', |
e2210791 |
139 | filetest => "-X", |
f5284f61 |
140 | dereferencing => '${} @{} %{} &{} *{}', |
ae20c3aa |
141 | matching => '~~', |
ee239bfe |
142 | special => 'nomethod fallback ='); |
143 | |
6b82e2f5 |
144 | use warnings::register; |
b3ac6de7 |
145 | sub constant { |
146 | # Arguments: what, sub |
147 | while (@_) { |
6b82e2f5 |
148 | if (@_ == 1) { |
4498a751 |
149 | warnings::warnif ("Odd number of arguments for overload::constant"); |
6b82e2f5 |
150 | last; |
151 | } |
152 | elsif (!exists $constants {$_ [0]}) { |
4498a751 |
153 | warnings::warnif ("`$_[0]' is not an overloadable type"); |
6b82e2f5 |
154 | } |
1e70e886 |
155 | elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) { |
6b82e2f5 |
156 | # Can't use C<ref $_[1] eq "CODE"> above as code references can be |
157 | # blessed, and C<ref> would return the package the ref is blessed into. |
158 | if (warnings::enabled) { |
6b82e2f5 |
159 | $_ [1] = "undef" unless defined $_ [1]; |
4498a751 |
160 | warnings::warn ("`$_[1]' is not a code reference"); |
6b82e2f5 |
161 | } |
162 | } |
163 | else { |
164 | $^H{$_[0]} = $_[1]; |
f22a2069 |
165 | $^H |= $constants{$_[0]}; |
6b82e2f5 |
166 | } |
b3ac6de7 |
167 | shift, shift; |
168 | } |
169 | } |
170 | |
171 | sub remove_constant { |
172 | # Arguments: what, sub |
173 | while (@_) { |
174 | delete $^H{$_[0]}; |
175 | $^H &= ~ $constants{$_[0]}; |
176 | shift, shift; |
177 | } |
178 | } |
179 | |
4633a7c4 |
180 | 1; |
181 | |
182 | __END__ |
183 | |
b267980d |
184 | =head1 NAME |
4633a7c4 |
185 | |
7adf7a02 |
186 | overload - Package for overloading Perl operations |
4633a7c4 |
187 | |
188 | =head1 SYNOPSIS |
189 | |
190 | package SomeThing; |
191 | |
b267980d |
192 | use overload |
4633a7c4 |
193 | '+' => \&myadd, |
194 | '-' => \&mysub; |
195 | # etc |
196 | ... |
197 | |
198 | package main; |
2b393bf4 |
199 | $a = SomeThing->new( 57 ); |
4633a7c4 |
200 | $b=5+$a; |
201 | ... |
202 | if (overload::Overloaded $b) {...} |
203 | ... |
204 | $strval = overload::StrVal $b; |
205 | |
4633a7c4 |
206 | =head1 DESCRIPTION |
207 | |
caf82a01 |
208 | This pragma allows overloading of Perl's operators for a class. |
209 | To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead. |
210 | |
4633a7c4 |
211 | =head2 Declaration of overloaded functions |
212 | |
213 | The compilation directive |
214 | |
215 | package Number; |
216 | use overload |
b267980d |
217 | "+" => \&add, |
4633a7c4 |
218 | "*=" => "muas"; |
219 | |
220 | declares function Number::add() for addition, and method muas() in |
221 | the "class" C<Number> (or one of its base classes) |
b267980d |
222 | for the assignment form C<*=> of multiplication. |
4633a7c4 |
223 | |
224 | Arguments of this directive come in (key, value) pairs. Legal values |
e7ea3e70 |
225 | are values legal inside a C<&{ ... }> call, so the name of a |
226 | subroutine, a reference to a subroutine, or an anonymous subroutine |
227 | will all work. Note that values specified as strings are |
228 | interpreted as methods, not subroutines. Legal keys are listed below. |
4633a7c4 |
229 | |
230 | The subroutine C<add> will be called to execute C<$a+$b> if $a |
231 | is a reference to an object blessed into the package C<Number>, or if $a is |
232 | not an object from a package with defined mathemagic addition, but $b is a |
233 | reference to a C<Number>. It can also be called in other situations, like |
234 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical |
235 | methods refer to methods triggered by an overloaded mathematical |
236 | operator.) |
237 | |
774d564b |
238 | Since overloading respects inheritance via the @ISA hierarchy, the |
239 | above declaration would also trigger overloading of C<+> and C<*=> in |
240 | all the packages which inherit from C<Number>. |
e7ea3e70 |
241 | |
4633a7c4 |
242 | =head2 Calling Conventions for Binary Operations |
243 | |
244 | The functions specified in the C<use overload ...> directive are called |
245 | with three (in one particular case with four, see L<Last Resort>) |
246 | arguments. If the corresponding operation is binary, then the first |
247 | two arguments are the two arguments of the operation. However, due to |
248 | general object calling conventions, the first argument should always be |
249 | an object in the package, so in the situation of C<7+$a>, the |
250 | order of the arguments is interchanged. It probably does not matter |
251 | when implementing the addition method, but whether the arguments |
252 | are reversed is vital to the subtraction method. The method can |
253 | query this information by examining the third argument, which can take |
254 | three different values: |
255 | |
256 | =over 7 |
257 | |
258 | =item FALSE |
259 | |
260 | the order of arguments is as in the current operation. |
261 | |
262 | =item TRUE |
263 | |
264 | the arguments are reversed. |
265 | |
266 | =item C<undef> |
267 | |
268 | the current operation is an assignment variant (as in |
269 | C<$a+=7>), but the usual function is called instead. This additional |
ee239bfe |
270 | information can be used to generate some optimizations. Compare |
271 | L<Calling Conventions for Mutators>. |
4633a7c4 |
272 | |
273 | =back |
274 | |
275 | =head2 Calling Conventions for Unary Operations |
276 | |
277 | Unary operation are considered binary operations with the second |
278 | argument being C<undef>. Thus the functions that overloads C<{"++"}> |
279 | is called with arguments C<($a,undef,'')> when $a++ is executed. |
280 | |
ee239bfe |
281 | =head2 Calling Conventions for Mutators |
282 | |
283 | Two types of mutators have different calling conventions: |
284 | |
88c28ceb |
285 | =over |
ee239bfe |
286 | |
287 | =item C<++> and C<--> |
288 | |
289 | The routines which implement these operators are expected to actually |
290 | I<mutate> their arguments. So, assuming that $obj is a reference to a |
291 | number, |
292 | |
293 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} |
294 | |
295 | is an appropriate implementation of overloaded C<++>. Note that |
296 | |
297 | sub incr { ++$ {$_[0]} ; shift } |
298 | |
299 | is OK if used with preincrement and with postincrement. (In the case |
300 | of postincrement a copying will be performed, see L<Copy Constructor>.) |
301 | |
302 | =item C<x=> and other assignment versions |
303 | |
304 | There is nothing special about these methods. They may change the |
305 | value of their arguments, and may leave it as is. The result is going |
306 | to be assigned to the value in the left-hand-side if different from |
307 | this value. |
308 | |
f610777f |
309 | This allows for the same method to be used as overloaded C<+=> and |
ee239bfe |
310 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
311 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, |
312 | if C<+=> is not overloaded. |
313 | |
314 | =back |
315 | |
d1be9408 |
316 | B<Warning.> Due to the presence of assignment versions of operations, |
b267980d |
317 | routines which may be called in assignment context may create |
318 | self-referential structures. Currently Perl will not free self-referential |
ee239bfe |
319 | structures until cycles are C<explicitly> broken. You may get problems |
320 | when traversing your structures too. |
321 | |
b267980d |
322 | Say, |
ee239bfe |
323 | |
324 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; |
325 | |
326 | is asking for trouble, since for code C<$obj += $foo> the subroutine |
b267980d |
327 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
ee239bfe |
328 | \$foo]>. If using such a subroutine is an important optimization, one |
329 | can overload C<+=> explicitly by a non-"optimized" version, or switch |
b267980d |
330 | to non-optimized version if C<not defined $_[2]> (see |
ee239bfe |
331 | L<Calling Conventions for Binary Operations>). |
332 | |
333 | Even if no I<explicit> assignment-variants of operators are present in |
334 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or |
335 | C<',' . $obj . ','> may be both optimized to |
336 | |
337 | my $tmp = ',' . $obj; $tmp .= ','; |
338 | |
4633a7c4 |
339 | =head2 Overloadable Operations |
340 | |
ee239bfe |
341 | The following symbols can be specified in C<use overload> directive: |
4633a7c4 |
342 | |
343 | =over 5 |
344 | |
345 | =item * I<Arithmetic operations> |
346 | |
347 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", |
348 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", |
349 | |
350 | For these operations a substituted non-assignment variant can be called if |
fa8a6580 |
351 | the assignment variant is not available. Methods for operations C<+>, |
352 | C<->, C<+=>, and C<-=> can be called to automatically generate |
353 | increment and decrement methods. The operation C<-> can be used to |
4633a7c4 |
354 | autogenerate missing methods for unary minus or C<abs>. |
355 | |
ee239bfe |
356 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
357 | L<"Calling Conventions for Binary Operations">) for details of these |
358 | substitutions. |
359 | |
4633a7c4 |
360 | =item * I<Comparison operations> |
361 | |
362 | "<", "<=", ">", ">=", "==", "!=", "<=>", |
363 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", |
364 | |
365 | If the corresponding "spaceship" variant is available, it can be |
366 | used to substitute for the missing operation. During C<sort>ing |
367 | arrays, C<cmp> is used to compare values subject to C<use overload>. |
368 | |
369 | =item * I<Bit operations> |
370 | |
6dd85743 |
371 | "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~", |
4633a7c4 |
372 | |
fa8a6580 |
373 | C<neg> stands for unary minus. If the method for C<neg> is not |
3bc6ec80 |
374 | specified, it can be autogenerated using the method for |
fa8a6580 |
375 | subtraction. If the method for C<!> is not specified, it can be |
376 | autogenerated using the methods for C<bool>, or C<"">, or C<0+>. |
4633a7c4 |
377 | |
6dd85743 |
378 | The same remarks in L<"Arithmetic operations"> about |
379 | assignment-variants and autogeneration apply for |
380 | bit operations C<"&">, C<"^">, and C<"|"> as well. |
381 | |
4633a7c4 |
382 | =item * I<Increment and decrement> |
383 | |
384 | "++", "--", |
385 | |
386 | If undefined, addition and subtraction methods can be |
387 | used instead. These operations are called both in prefix and |
388 | postfix form. |
389 | |
390 | =item * I<Transcendental functions> |
391 | |
f216259d |
392 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int" |
4633a7c4 |
393 | |
394 | If C<abs> is unavailable, it can be autogenerated using methods |
1fef88e7 |
395 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
4633a7c4 |
396 | |
f216259d |
397 | Note that traditionally the Perl function L<int> rounds to 0, thus for |
398 | floating-point-like types one should follow the same semantic. If |
399 | C<int> is unavailable, it can be autogenerated using the overloading of |
400 | C<0+>. |
401 | |
4633a7c4 |
402 | =item * I<Boolean, string and numeric conversion> |
403 | |
fa8a6580 |
404 | 'bool', '""', '0+', |
4633a7c4 |
405 | |
f5284f61 |
406 | If one or two of these operations are not overloaded, the remaining ones can |
4633a7c4 |
407 | be used instead. C<bool> is used in the flow control operators |
fa8a6580 |
408 | (like C<while>) and for the ternary C<?:> operation. These functions can |
4633a7c4 |
409 | return any arbitrary Perl value. If the corresponding operation for this value |
410 | is overloaded too, that operation will be called again with this value. |
411 | |
1554e226 |
412 | As a special case if the overload returns the object itself then it will |
413 | be used directly. An overloaded conversion returning the object is |
414 | probably a bug, because you're likely to get something that looks like |
415 | C<YourPackage=HASH(0x8172b34)>. |
416 | |
f5284f61 |
417 | =item * I<Iteration> |
418 | |
419 | "<>" |
420 | |
421 | If not overloaded, the argument will be converted to a filehandle or |
422 | glob (which may require a stringification). The same overloading |
423 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and |
424 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
425 | |
54f8c773 |
426 | B<BUGS> Even in list context, the iterator is currently called only |
427 | once and with scalar context. |
428 | |
e2210791 |
429 | =item * I<File tests> |
430 | |
431 | "-X" |
432 | |
433 | This overload is used for all the filetest operators (C<-f>, C<-x> and |
434 | so on: see L<perlfunc/-X> for the full list). Even though these are |
435 | unary operators, the method will be called with a second argument which |
436 | is a single letter indicating which test was performed. Note that the |
437 | overload key is the literal string C<"-X">: you can't provide separate |
438 | overloads for the different tests. |
439 | |
440 | Calling an overloaded filetest operator does not affect the stat value |
441 | associated with the special filehandle C<_>. It still refers to the |
442 | result of the last C<stat>, C<lstat> or unoverloaded filetest. |
443 | |
444 | If not overloaded, these operators will fall back to the default |
445 | behaviour even without C<< fallback => 1 >>. This means that if the |
446 | object is a blessed glob or blessed IO ref it will be treated as a |
447 | filehandle, otherwise string overloading will be invoked and the result |
448 | treated as a filename. |
449 | |
450 | This overload was introduced in perl 5.12. |
451 | |
ae20c3aa |
452 | =item * I<Matching> |
453 | |
454 | The key C<"~~"> allows you to override the smart matching used by |
455 | the switch construct. See L<feature>. |
456 | |
f5284f61 |
457 | =item * I<Dereferencing> |
458 | |
459 | '${}', '@{}', '%{}', '&{}', '*{}'. |
460 | |
461 | If not overloaded, the argument will be dereferenced I<as is>, thus |
462 | should be of correct type. These functions should return a reference |
463 | of correct type, or another object with overloaded dereferencing. |
464 | |
b267980d |
465 | As a special case if the overload returns the object itself then it |
466 | will be used directly (provided it is the correct type). |
467 | |
468 | The dereference operators must be specified explicitly they will not be passed to |
469 | "nomethod". |
470 | |
4633a7c4 |
471 | =item * I<Special> |
472 | |
ae20c3aa |
473 | "nomethod", "fallback", "=". |
4633a7c4 |
474 | |
475 | see L<SPECIAL SYMBOLS FOR C<use overload>>. |
476 | |
477 | =back |
478 | |
ee239bfe |
479 | See L<"Fallback"> for an explanation of when a missing method can be |
480 | autogenerated. |
481 | |
482 | A computer-readable form of the above table is available in the hash |
483 | %overload::ops, with values being space-separated lists of names: |
484 | |
485 | with_assign => '+ - * / % ** << >> x .', |
486 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
2877bd81 |
487 | num_comparison => '< <= > >= == !=', |
ee239bfe |
488 | '3way_comparison'=> '<=> cmp', |
2877bd81 |
489 | str_comparison => 'lt le gt ge eq ne', |
6dd85743 |
490 | binary => '& &= | |= ^ ^=', |
ee239bfe |
491 | unary => 'neg ! ~', |
492 | mutators => '++ --', |
493 | func => 'atan2 cos sin exp abs log sqrt', |
494 | conversion => 'bool "" 0+', |
f5284f61 |
495 | iterators => '<>', |
e2210791 |
496 | filetest => '-X', |
f5284f61 |
497 | dereferencing => '${} @{} %{} &{} *{}', |
ae20c3aa |
498 | matching => '~~', |
ee239bfe |
499 | special => 'nomethod fallback =' |
4633a7c4 |
500 | |
e7ea3e70 |
501 | =head2 Inheritance and overloading |
502 | |
774d564b |
503 | Inheritance interacts with overloading in two ways. |
e7ea3e70 |
504 | |
88c28ceb |
505 | =over |
e7ea3e70 |
506 | |
507 | =item Strings as values of C<use overload> directive |
508 | |
774d564b |
509 | If C<value> in |
e7ea3e70 |
510 | |
511 | use overload key => value; |
512 | |
774d564b |
513 | is a string, it is interpreted as a method name. |
e7ea3e70 |
514 | |
515 | =item Overloading of an operation is inherited by derived classes |
516 | |
774d564b |
517 | Any class derived from an overloaded class is also overloaded. The |
518 | set of overloaded methods is the union of overloaded methods of all |
519 | the ancestors. If some method is overloaded in several ancestor, then |
e7ea3e70 |
520 | which description will be used is decided by the usual inheritance |
774d564b |
521 | rules: |
e7ea3e70 |
522 | |
774d564b |
523 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
524 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, |
525 | then the subroutine C<D::plus_sub> will be called to implement |
526 | operation C<+> for an object in package C<A>. |
e7ea3e70 |
527 | |
528 | =back |
529 | |
774d564b |
530 | Note that since the value of the C<fallback> key is not a subroutine, |
531 | its inheritance is not governed by the above rules. In the current |
532 | implementation, the value of C<fallback> in the first overloaded |
533 | ancestor is used, but this is accidental and subject to change. |
e7ea3e70 |
534 | |
4633a7c4 |
535 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
536 | |
537 | Three keys are recognized by Perl that are not covered by the above |
538 | description. |
539 | |
774d564b |
540 | =head2 Last Resort |
4633a7c4 |
541 | |
542 | C<"nomethod"> should be followed by a reference to a function of four |
543 | parameters. If defined, it is called when the overloading mechanism |
544 | cannot find a method for some operation. The first three arguments of |
545 | this function coincide with the arguments for the corresponding method if |
546 | it were found, the fourth argument is the symbol |
547 | corresponding to the missing method. If several methods are tried, |
548 | the last one is used. Say, C<1-$a> can be equivalent to |
549 | |
550 | &nomethodMethod($a,1,1,"-") |
551 | |
552 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the |
553 | C<use overload> directive. |
554 | |
b267980d |
555 | The C<"nomethod"> mechanism is I<not> used for the dereference operators |
556 | ( ${} @{} %{} &{} *{} ). |
557 | |
558 | |
4633a7c4 |
559 | If some operation cannot be resolved, and there is no function |
560 | assigned to C<"nomethod">, then an exception will be raised via die()-- |
561 | unless C<"fallback"> was specified as a key in C<use overload> directive. |
562 | |
b267980d |
563 | |
564 | =head2 Fallback |
4633a7c4 |
565 | |
566 | The key C<"fallback"> governs what to do if a method for a particular |
567 | operation is not found. Three different cases are possible depending on |
568 | the value of C<"fallback">: |
569 | |
570 | =over 16 |
571 | |
572 | =item * C<undef> |
573 | |
574 | Perl tries to use a |
575 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it |
576 | then tries to calls C<"nomethod"> value; if missing, an exception |
577 | will be raised. |
578 | |
579 | =item * TRUE |
580 | |
581 | The same as for the C<undef> value, but no exception is raised. Instead, |
582 | it silently reverts to what it would have done were there no C<use overload> |
583 | present. |
584 | |
585 | =item * defined, but FALSE |
586 | |
587 | No autogeneration is tried. Perl tries to call |
b267980d |
588 | C<"nomethod"> value, and if this is missing, raises an exception. |
4633a7c4 |
589 | |
590 | =back |
591 | |
e7ea3e70 |
592 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
593 | yet, see L<"Inheritance and overloading">. |
594 | |
4633a7c4 |
595 | =head2 Copy Constructor |
596 | |
597 | The value for C<"="> is a reference to a function with three |
598 | arguments, i.e., it looks like the other values in C<use |
599 | overload>. However, it does not overload the Perl assignment |
600 | operator. This would go against Camel hair. |
601 | |
602 | This operation is called in the situations when a mutator is applied |
603 | to a reference that shares its object with some other reference, such |
604 | as |
605 | |
b267980d |
606 | $a=$b; |
ee239bfe |
607 | ++$a; |
4633a7c4 |
608 | |
609 | To make this change $a and not change $b, a copy of C<$$a> is made, |
610 | and $a is assigned a reference to this new object. This operation is |
ee239bfe |
611 | done during execution of the C<++$a>, and not during the assignment, |
4633a7c4 |
612 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
ee239bfe |
613 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
614 | C<nomethod>). Note that if this operation is expressed via C<'+'> |
615 | a nonmutator, i.e., as in |
4633a7c4 |
616 | |
b267980d |
617 | $a=$b; |
4633a7c4 |
618 | $a=$a+1; |
619 | |
620 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not |
621 | appear as lvalue when the above code is executed. |
622 | |
623 | If the copy constructor is required during the execution of some mutator, |
624 | but a method for C<'='> was not specified, it can be autogenerated as a |
49c95d58 |
625 | string copy if the object is a plain scalar or a simple assignment if it |
626 | is not. |
4633a7c4 |
627 | |
628 | =over 5 |
629 | |
630 | =item B<Example> |
631 | |
b267980d |
632 | The actually executed code for |
4633a7c4 |
633 | |
b267980d |
634 | $a=$b; |
4633a7c4 |
635 | Something else which does not modify $a or $b.... |
636 | ++$a; |
637 | |
638 | may be |
639 | |
b267980d |
640 | $a=$b; |
4633a7c4 |
641 | Something else which does not modify $a or $b.... |
642 | $a = $a->clone(undef,""); |
643 | $a->incr(undef,""); |
644 | |
645 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, |
646 | C<'='> was overloaded with C<\&clone>. |
647 | |
648 | =back |
649 | |
f610777f |
650 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
ee239bfe |
651 | C<$b = $a; ++$a>. |
652 | |
4633a7c4 |
653 | =head1 MAGIC AUTOGENERATION |
654 | |
655 | If a method for an operation is not found, and the value for C<"fallback"> is |
656 | TRUE or undefined, Perl tries to autogenerate a substitute method for |
657 | the missing operation based on the defined operations. Autogenerated method |
658 | substitutions are possible for the following operations: |
659 | |
660 | =over 16 |
661 | |
662 | =item I<Assignment forms of arithmetic operations> |
663 | |
664 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> |
665 | is not defined. |
666 | |
b267980d |
667 | =item I<Conversion operations> |
4633a7c4 |
668 | |
669 | String, numeric, and boolean conversion are calculated in terms of one |
670 | another if not all of them are defined. |
671 | |
672 | =item I<Increment and decrement> |
673 | |
674 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, |
675 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. |
676 | |
677 | =item C<abs($a)> |
678 | |
679 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). |
680 | |
681 | =item I<Unary minus> |
682 | |
683 | can be expressed in terms of subtraction. |
684 | |
3bc6ec80 |
685 | =item I<Negation> |
686 | |
687 | C<!> and C<not> can be expressed in terms of boolean conversion, or |
688 | string or numerical conversion. |
689 | |
4633a7c4 |
690 | =item I<Concatenation> |
691 | |
692 | can be expressed in terms of string conversion. |
693 | |
b267980d |
694 | =item I<Comparison operations> |
4633a7c4 |
695 | |
696 | can be expressed in terms of its "spaceship" counterpart: either |
697 | C<E<lt>=E<gt>> or C<cmp>: |
1fef88e7 |
698 | |
4633a7c4 |
699 | <, >, <=, >=, ==, != in terms of <=> |
700 | lt, gt, le, ge, eq, ne in terms of cmp |
701 | |
f5284f61 |
702 | =item I<Iterator> |
703 | |
704 | <> in terms of builtin operations |
705 | |
706 | =item I<Dereferencing> |
707 | |
708 | ${} @{} %{} &{} *{} in terms of builtin operations |
709 | |
4633a7c4 |
710 | =item I<Copy operator> |
711 | |
712 | can be expressed in terms of an assignment to the dereferenced value, if this |
49c95d58 |
713 | value is a scalar and not a reference, or simply a reference assignment |
714 | otherwise. |
4633a7c4 |
715 | |
716 | =back |
717 | |
84fc275b |
718 | =head1 Minimal set of overloaded operations |
719 | |
720 | Since some operations can be automatically generated from others, there is |
721 | a minimal set of operations that need to be overloaded in order to have |
299476e0 |
722 | the complete set of overloaded operations at one's disposal. |
723 | Of course, the autogenerated operations may not do exactly what the user |
724 | expects. See L<MAGIC AUTOGENERATION> above. The minimal set is: |
84fc275b |
725 | |
726 | + - * / % ** << >> x |
727 | <=> cmp |
728 | & | ^ ~ |
729 | atan2 cos sin exp log sqrt int |
730 | |
731 | Additionally, you need to define at least one of string, boolean or |
299476e0 |
732 | numeric conversions because any one can be used to emulate the others. |
733 | The string conversion can also be used to emulate concatenation. |
84fc275b |
734 | |
ee239bfe |
735 | =head1 Losing overloading |
4633a7c4 |
736 | |
737 | The restriction for the comparison operation is that even if, for example, |
738 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' |
739 | function will produce only a standard logical value based on the |
740 | numerical value of the result of `C<cmp>'. In particular, a working |
741 | numeric conversion is needed in this case (possibly expressed in terms of |
742 | other conversions). |
743 | |
744 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
745 | if the string conversion substitution is applied. |
746 | |
747 | When you chop() a mathemagical object it is promoted to a string and its |
748 | mathemagical properties are lost. The same can happen with other |
749 | operations as well. |
750 | |
751 | =head1 Run-time Overloading |
752 | |
753 | Since all C<use> directives are executed at compile-time, the only way to |
754 | change overloading during run-time is to |
755 | |
756 | eval 'use overload "+" => \&addmethod'; |
757 | |
758 | You can also use |
759 | |
760 | eval 'no overload "+", "--", "<="'; |
761 | |
762 | though the use of these constructs during run-time is questionable. |
763 | |
764 | =head1 Public functions |
765 | |
766 | Package C<overload.pm> provides the following public functions: |
767 | |
768 | =over 5 |
769 | |
770 | =item overload::StrVal(arg) |
771 | |
6a0e9e72 |
772 | Gives string value of C<arg> as in absence of stringify overloading. If you |
773 | are using this to get the address of a reference (useful for checking if two |
774 | references point to the same thing) then you may be better off using |
775 | C<Scalar::Util::refaddr()>, which is faster. |
4633a7c4 |
776 | |
777 | =item overload::Overloaded(arg) |
778 | |
779 | Returns true if C<arg> is subject to overloading of some operations. |
780 | |
781 | =item overload::Method(obj,op) |
782 | |
783 | Returns C<undef> or a reference to the method that implements C<op>. |
784 | |
785 | =back |
786 | |
b3ac6de7 |
787 | =head1 Overloading constants |
788 | |
7adf7a02 |
789 | For some applications, the Perl parser mangles constants too much. |
bfce84ec |
790 | It is possible to hook into this process via C<overload::constant()> |
7adf7a02 |
791 | and C<overload::remove_constant()> functions. |
b3ac6de7 |
792 | |
793 | These functions take a hash as an argument. The recognized keys of this hash |
7adf7a02 |
794 | are: |
b3ac6de7 |
795 | |
796 | =over 8 |
797 | |
798 | =item integer |
799 | |
800 | to overload integer constants, |
801 | |
802 | =item float |
803 | |
804 | to overload floating point constants, |
805 | |
806 | =item binary |
807 | |
808 | to overload octal and hexadecimal constants, |
809 | |
810 | =item q |
811 | |
812 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
813 | strings and here-documents, |
814 | |
815 | =item qr |
816 | |
817 | to overload constant pieces of regular expressions. |
818 | |
819 | =back |
820 | |
821 | The corresponding values are references to functions which take three arguments: |
822 | the first one is the I<initial> string form of the constant, the second one |
b267980d |
823 | is how Perl interprets this constant, the third one is how the constant is used. |
b3ac6de7 |
824 | Note that the initial string form does not |
b267980d |
825 | contain string delimiters, and has backslashes in backslash-delimiter |
b3ac6de7 |
826 | combinations stripped (thus the value of delimiter is not relevant for |
b267980d |
827 | processing of this string). The return value of this function is how this |
b3ac6de7 |
828 | constant is going to be interpreted by Perl. The third argument is undefined |
829 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
830 | context (comes from strings, regular expressions, and single-quote HERE |
b267980d |
831 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
b3ac6de7 |
832 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
833 | |
834 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
835 | it is expected that overloaded constant strings are equipped with reasonable |
b267980d |
836 | overloaded catenation operator, otherwise absurd results will result. |
b3ac6de7 |
837 | Similarly, negative numbers are considered as negations of positive constants. |
838 | |
839 | Note that it is probably meaningless to call the functions overload::constant() |
840 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
841 | From these methods they may be called as |
842 | |
843 | sub import { |
844 | shift; |
845 | return unless @_; |
846 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
847 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
848 | } |
849 | |
4633a7c4 |
850 | =head1 IMPLEMENTATION |
851 | |
852 | What follows is subject to change RSN. |
853 | |
e7ea3e70 |
854 | The table of methods for all operations is cached in magic for the |
855 | symbol table hash for the package. The cache is invalidated during |
856 | processing of C<use overload>, C<no overload>, new function |
857 | definitions, and changes in @ISA. However, this invalidation remains |
858 | unprocessed until the next C<bless>ing into the package. Hence if you |
859 | want to change overloading structure dynamically, you'll need an |
860 | additional (fake) C<bless>ing to update the table. |
861 | |
862 | (Every SVish thing has a magic queue, and magic is an entry in that |
863 | queue. This is how a single variable may participate in multiple |
864 | forms of magic simultaneously. For instance, environment variables |
865 | regularly have two forms at once: their %ENV magic and their taint |
866 | magic. However, the magic which implements overloading is applied to |
867 | the stashes, which are rarely used directly, thus should not slow down |
868 | Perl.) |
4633a7c4 |
869 | |
870 | If an object belongs to a package using overload, it carries a special |
871 | flag. Thus the only speed penalty during arithmetic operations without |
872 | overloading is the checking of this flag. |
873 | |
774d564b |
874 | In fact, if C<use overload> is not present, there is almost no overhead |
875 | for overloadable operations, so most programs should not suffer |
876 | measurable performance penalties. A considerable effort was made to |
877 | minimize the overhead when overload is used in some package, but the |
878 | arguments in question do not belong to packages using overload. When |
879 | in doubt, test your speed with C<use overload> and without it. So far |
880 | there have been no reports of substantial speed degradation if Perl is |
881 | compiled with optimization turned on. |
4633a7c4 |
882 | |
e7ea3e70 |
883 | There is no size penalty for data if overload is not used. The only |
884 | size penalty if overload is used in some package is that I<all> the |
885 | packages acquire a magic during the next C<bless>ing into the |
886 | package. This magic is three-words-long for packages without |
f610777f |
887 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 |
888 | |
b267980d |
889 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
4633a7c4 |
890 | carried out before any operation that can imply an assignment to the |
891 | object $a (or $b) refers to, like C<$a++>. You can override this |
892 | behavior by defining your own copy constructor (see L<"Copy Constructor">). |
893 | |
894 | It is expected that arguments to methods that are not explicitly supposed |
895 | to be changed are constant (but this is not enforced). |
896 | |
ee239bfe |
897 | =head1 Metaphor clash |
898 | |
f610777f |
899 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
b267980d |
900 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
901 | clash. |
ee239bfe |
902 | |
903 | Here is a Perl object metaphor: |
904 | |
905 | I< object is a reference to blessed data> |
906 | |
907 | and an arithmetic metaphor: |
908 | |
909 | I< object is a thing by itself>. |
910 | |
911 | The I<main> problem of overloading C<=> is the fact that these metaphors |
912 | imply different actions on the assignment C<$a = $b> if $a and $b are |
913 | objects. Perl-think implies that $a becomes a reference to whatever |
914 | $b was referencing. Arithmetic-think implies that the value of "object" |
915 | $a is changed to become the value of the object $b, preserving the fact |
916 | that $a and $b are separate entities. |
917 | |
918 | The difference is not relevant in the absence of mutators. After |
919 | a Perl-way assignment an operation which mutates the data referenced by $a |
b267980d |
920 | would change the data referenced by $b too. Effectively, after |
ee239bfe |
921 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
922 | |
b267980d |
923 | On the other hand, anyone who has used algebraic notation knows the |
ee239bfe |
924 | expressive power of the arithmetic metaphor. Overloading works hard |
925 | to enable this metaphor while preserving the Perlian way as far as |
d1be9408 |
926 | possible. Since it is not possible to freely mix two contradicting |
ee239bfe |
927 | metaphors, overloading allows the arithmetic way to write things I<as |
928 | far as all the mutators are called via overloaded access only>. The |
929 | way it is done is described in L<Copy Constructor>. |
930 | |
931 | If some mutator methods are directly applied to the overloaded values, |
b267980d |
932 | one may need to I<explicitly unlink> other values which references the |
ee239bfe |
933 | same value: |
934 | |
2b393bf4 |
935 | $a = Data->new(23); |
ee239bfe |
936 | ... |
937 | $b = $a; # $b is "linked" to $a |
938 | ... |
939 | $a = $a->clone; # Unlink $b from $a |
940 | $a->increment_by(4); |
941 | |
942 | Note that overloaded access makes this transparent: |
943 | |
2b393bf4 |
944 | $a = Data->new(23); |
ee239bfe |
945 | $b = $a; # $b is "linked" to $a |
946 | $a += 4; # would unlink $b automagically |
947 | |
948 | However, it would not make |
949 | |
2b393bf4 |
950 | $a = Data->new(23); |
ee239bfe |
951 | $a = 4; # Now $a is a plain 4, not 'Data' |
952 | |
953 | preserve "objectness" of $a. But Perl I<has> a way to make assignments |
954 | to an object do whatever you want. It is just not the overload, but |
955 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method |
b267980d |
956 | which returns the object itself, and STORE() method which changes the |
ee239bfe |
957 | value of the object, one can reproduce the arithmetic metaphor in its |
958 | completeness, at least for variables which were tie()d from the start. |
959 | |
960 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) |
961 | |
962 | =head1 Cookbook |
963 | |
964 | Please add examples to what follows! |
965 | |
966 | =head2 Two-face scalars |
967 | |
968 | Put this in F<two_face.pm> in your Perl library directory: |
969 | |
970 | package two_face; # Scalars with separate string and |
971 | # numeric values. |
972 | sub new { my $p = shift; bless [@_], $p } |
973 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
974 | sub num {shift->[1]} |
975 | sub str {shift->[0]} |
976 | |
977 | Use it as follows: |
978 | |
979 | require two_face; |
2b393bf4 |
980 | my $seven = two_face->new("vii", 7); |
ee239bfe |
981 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
982 | print "seven contains `i'\n" if $seven =~ /i/; |
983 | |
984 | (The second line creates a scalar which has both a string value, and a |
985 | numeric value.) This prints: |
986 | |
987 | seven=vii, seven=7, eight=8 |
988 | seven contains `i' |
989 | |
f5284f61 |
990 | =head2 Two-face references |
991 | |
992 | Suppose you want to create an object which is accessible as both an |
6d822dc4 |
993 | array reference and a hash reference. |
f5284f61 |
994 | |
995 | package two_refs; |
996 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
b267980d |
997 | sub new { |
998 | my $p = shift; |
f5284f61 |
999 | bless \ [@_], $p; |
1000 | } |
1001 | sub gethash { |
1002 | my %h; |
1003 | my $self = shift; |
1004 | tie %h, ref $self, $self; |
1005 | \%h; |
1006 | } |
1007 | |
1008 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
1009 | my %fields; |
1010 | my $i = 0; |
1011 | $fields{$_} = $i++ foreach qw{zero one two three}; |
b267980d |
1012 | sub STORE { |
f5284f61 |
1013 | my $self = ${shift()}; |
1014 | my $key = $fields{shift()}; |
1015 | defined $key or die "Out of band access"; |
1016 | $$self->[$key] = shift; |
1017 | } |
b267980d |
1018 | sub FETCH { |
f5284f61 |
1019 | my $self = ${shift()}; |
1020 | my $key = $fields{shift()}; |
1021 | defined $key or die "Out of band access"; |
1022 | $$self->[$key]; |
1023 | } |
1024 | |
1025 | Now one can access an object using both the array and hash syntax: |
1026 | |
2b393bf4 |
1027 | my $bar = two_refs->new(3,4,5,6); |
f5284f61 |
1028 | $bar->[2] = 11; |
1029 | $bar->{two} == 11 or die 'bad hash fetch'; |
1030 | |
1031 | Note several important features of this example. First of all, the |
1032 | I<actual> type of $bar is a scalar reference, and we do not overload |
1033 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
1034 | contents of $bar by just using C<$$bar> (what we do in functions which |
1035 | overload dereference). Similarly, the object returned by the |
1036 | TIEHASH() method is a scalar reference. |
1037 | |
1038 | Second, we create a new tied hash each time the hash syntax is used. |
1039 | This allows us not to worry about a possibility of a reference loop, |
d1be9408 |
1040 | which would lead to a memory leak. |
f5284f61 |
1041 | |
1042 | Both these problems can be cured. Say, if we want to overload hash |
1043 | dereference on a reference to an object which is I<implemented> as a |
1044 | hash itself, the only problem one has to circumvent is how to access |
1fd16925 |
1045 | this I<actual> hash (as opposed to the I<virtual> hash exhibited by the |
f5284f61 |
1046 | overloaded dereference operator). Here is one possible fetching routine: |
1047 | |
1048 | sub access_hash { |
1049 | my ($self, $key) = (shift, shift); |
1050 | my $class = ref $self; |
b267980d |
1051 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
f5284f61 |
1052 | my $out = $self->{$key}; |
1053 | bless $self, $class; # Restore overloading |
1054 | $out; |
1055 | } |
1056 | |
1fd16925 |
1057 | To remove creation of the tied hash on each access, one may an extra |
f5284f61 |
1058 | level of indirection which allows a non-circular structure of references: |
1059 | |
1060 | package two_refs1; |
1061 | use overload '%{}' => sub { ${shift()}->[1] }, |
1062 | '@{}' => sub { ${shift()}->[0] }; |
b267980d |
1063 | sub new { |
1064 | my $p = shift; |
f5284f61 |
1065 | my $a = [@_]; |
1066 | my %h; |
1067 | tie %h, $p, $a; |
1068 | bless \ [$a, \%h], $p; |
1069 | } |
1070 | sub gethash { |
1071 | my %h; |
1072 | my $self = shift; |
1073 | tie %h, ref $self, $self; |
1074 | \%h; |
1075 | } |
1076 | |
1077 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
1078 | my %fields; |
1079 | my $i = 0; |
1080 | $fields{$_} = $i++ foreach qw{zero one two three}; |
b267980d |
1081 | sub STORE { |
f5284f61 |
1082 | my $a = ${shift()}; |
1083 | my $key = $fields{shift()}; |
1084 | defined $key or die "Out of band access"; |
1085 | $a->[$key] = shift; |
1086 | } |
b267980d |
1087 | sub FETCH { |
f5284f61 |
1088 | my $a = ${shift()}; |
1089 | my $key = $fields{shift()}; |
1090 | defined $key or die "Out of band access"; |
1091 | $a->[$key]; |
1092 | } |
1093 | |
1fd16925 |
1094 | Now if $baz is overloaded like this, then C<$baz> is a reference to a |
f5284f61 |
1095 | reference to the intermediate array, which keeps a reference to an |
1096 | actual array, and the access hash. The tie()ing object for the access |
1fd16925 |
1097 | hash is a reference to a reference to the actual array, so |
f5284f61 |
1098 | |
88c28ceb |
1099 | =over |
f5284f61 |
1100 | |
1101 | =item * |
1102 | |
1103 | There are no loops of references. |
1104 | |
1105 | =item * |
1106 | |
1107 | Both "objects" which are blessed into the class C<two_refs1> are |
1108 | references to a reference to an array, thus references to a I<scalar>. |
1109 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
1110 | overloaded operations. |
1111 | |
1112 | =back |
1113 | |
ee239bfe |
1114 | =head2 Symbolic calculator |
1115 | |
1116 | Put this in F<symbolic.pm> in your Perl library directory: |
1117 | |
1118 | package symbolic; # Primitive symbolic calculator |
1119 | use overload nomethod => \&wrap; |
1120 | |
1121 | sub new { shift; bless ['n', @_] } |
1122 | sub wrap { |
1123 | my ($obj, $other, $inv, $meth) = @_; |
1124 | ($obj, $other) = ($other, $obj) if $inv; |
1125 | bless [$meth, $obj, $other]; |
1126 | } |
1127 | |
1128 | This module is very unusual as overloaded modules go: it does not |
88c28ceb |
1129 | provide any usual overloaded operators, instead it provides the L<Last |
1130 | Resort> operator C<nomethod>. In this example the corresponding |
f610777f |
1131 | subroutine returns an object which encapsulates operations done over |
2b393bf4 |
1132 | the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 + |
1133 | symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>. |
ee239bfe |
1134 | |
1135 | Here is an example of the script which "calculates" the side of |
1136 | circumscribed octagon using the above package: |
1137 | |
1138 | require symbolic; |
1139 | my $iter = 1; # 2**($iter+2) = 8 |
2b393bf4 |
1140 | my $side = symbolic->new(1); |
ee239bfe |
1141 | my $cnt = $iter; |
3cb6de81 |
1142 | |
ee239bfe |
1143 | while ($cnt--) { |
1144 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1145 | } |
1146 | print "OK\n"; |
1147 | |
1148 | The value of $side is |
1149 | |
1150 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
1151 | undef], 1], ['n', 1]] |
1152 | |
1153 | Note that while we obtained this value using a nice little script, |
1154 | there is no simple way to I<use> this value. In fact this value may |
2d3232d7 |
1155 | be inspected in debugger (see L<perldebug>), but only if |
ee239bfe |
1156 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1157 | |
1158 | If one attempts to print this value, then the overloaded operator |
1159 | C<""> will be called, which will call C<nomethod> operator. The |
1160 | result of this operator will be stringified again, but this result is |
1161 | again of type C<symbolic>, which will lead to an infinite loop. |
1162 | |
1163 | Add a pretty-printer method to the module F<symbolic.pm>: |
1164 | |
1165 | sub pretty { |
1166 | my ($meth, $a, $b) = @{+shift}; |
1167 | $a = 'u' unless defined $a; |
1168 | $b = 'u' unless defined $b; |
1169 | $a = $a->pretty if ref $a; |
1170 | $b = $b->pretty if ref $b; |
1171 | "[$meth $a $b]"; |
b267980d |
1172 | } |
ee239bfe |
1173 | |
1174 | Now one can finish the script by |
1175 | |
1176 | print "side = ", $side->pretty, "\n"; |
1177 | |
1178 | The method C<pretty> is doing object-to-string conversion, so it |
1179 | is natural to overload the operator C<""> using this method. However, |
1180 | inside such a method it is not necessary to pretty-print the |
1181 | I<components> $a and $b of an object. In the above subroutine |
1182 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
1183 | and $b. If these components use overloading, the catenation operator |
1fd16925 |
1184 | will look for an overloaded operator C<.>; if not present, it will |
ee239bfe |
1185 | look for an overloaded operator C<"">. Thus it is enough to use |
1186 | |
1187 | use overload nomethod => \&wrap, '""' => \&str; |
1188 | sub str { |
1189 | my ($meth, $a, $b) = @{+shift}; |
1190 | $a = 'u' unless defined $a; |
1191 | $b = 'u' unless defined $b; |
1192 | "[$meth $a $b]"; |
b267980d |
1193 | } |
ee239bfe |
1194 | |
1195 | Now one can change the last line of the script to |
1196 | |
1197 | print "side = $side\n"; |
1198 | |
1199 | which outputs |
1200 | |
1201 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
1202 | |
1203 | and one can inspect the value in debugger using all the possible |
b267980d |
1204 | methods. |
ee239bfe |
1205 | |
d1be9408 |
1206 | Something is still amiss: consider the loop variable $cnt of the |
ee239bfe |
1207 | script. It was a number, not an object. We cannot make this value of |
1208 | type C<symbolic>, since then the loop will not terminate. |
1209 | |
1210 | Indeed, to terminate the cycle, the $cnt should become false. |
1211 | However, the operator C<bool> for checking falsity is overloaded (this |
1212 | time via overloaded C<"">), and returns a long string, thus any object |
1213 | of type C<symbolic> is true. To overcome this, we need a way to |
1214 | compare an object to 0. In fact, it is easier to write a numeric |
1215 | conversion routine. |
1216 | |
1217 | Here is the text of F<symbolic.pm> with such a routine added (and |
f610777f |
1218 | slightly modified str()): |
ee239bfe |
1219 | |
1220 | package symbolic; # Primitive symbolic calculator |
1221 | use overload |
1222 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
1223 | |
1224 | sub new { shift; bless ['n', @_] } |
1225 | sub wrap { |
1226 | my ($obj, $other, $inv, $meth) = @_; |
1227 | ($obj, $other) = ($other, $obj) if $inv; |
1228 | bless [$meth, $obj, $other]; |
1229 | } |
1230 | sub str { |
1231 | my ($meth, $a, $b) = @{+shift}; |
1232 | $a = 'u' unless defined $a; |
1233 | if (defined $b) { |
1234 | "[$meth $a $b]"; |
1235 | } else { |
1236 | "[$meth $a]"; |
1237 | } |
b267980d |
1238 | } |
1239 | my %subr = ( n => sub {$_[0]}, |
1240 | sqrt => sub {sqrt $_[0]}, |
ee239bfe |
1241 | '-' => sub {shift() - shift()}, |
1242 | '+' => sub {shift() + shift()}, |
1243 | '/' => sub {shift() / shift()}, |
1244 | '*' => sub {shift() * shift()}, |
1245 | '**' => sub {shift() ** shift()}, |
1246 | ); |
1247 | sub num { |
1248 | my ($meth, $a, $b) = @{+shift}; |
b267980d |
1249 | my $subr = $subr{$meth} |
ee239bfe |
1250 | or die "Do not know how to ($meth) in symbolic"; |
1251 | $a = $a->num if ref $a eq __PACKAGE__; |
1252 | $b = $b->num if ref $b eq __PACKAGE__; |
1253 | $subr->($a,$b); |
1254 | } |
1255 | |
1256 | All the work of numeric conversion is done in %subr and num(). Of |
f610777f |
1257 | course, %subr is not complete, it contains only operators used in the |
ee239bfe |
1258 | example below. Here is the extra-credit question: why do we need an |
1259 | explicit recursion in num()? (Answer is at the end of this section.) |
1260 | |
1261 | Use this module like this: |
1262 | |
1263 | require symbolic; |
2b393bf4 |
1264 | my $iter = symbolic->new(2); # 16-gon |
1265 | my $side = symbolic->new(1); |
ee239bfe |
1266 | my $cnt = $iter; |
3cb6de81 |
1267 | |
ee239bfe |
1268 | while ($cnt) { |
1269 | $cnt = $cnt - 1; # Mutator `--' not implemented |
1270 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1271 | } |
1272 | printf "%s=%f\n", $side, $side; |
1273 | printf "pi=%f\n", $side*(2**($iter+2)); |
1274 | |
1275 | It prints (without so many line breaks) |
1276 | |
1277 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
1278 | [n 1]] 2]]] 1] |
1279 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
1280 | pi=3.182598 |
1281 | |
1282 | The above module is very primitive. It does not implement |
1283 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
1284 | (not required without mutators!), and implements only those arithmetic |
1285 | operations which are used in the example. |
1286 | |
1fd16925 |
1287 | To implement most arithmetic operations is easy; one should just use |
ee239bfe |
1288 | the tables of operations, and change the code which fills %subr to |
1289 | |
1290 | my %subr = ( 'n' => sub {$_[0]} ); |
1291 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1292 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1293 | } |
1294 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1295 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1296 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1297 | } |
1298 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1299 | print "defining `$op'\n"; |
1300 | $subr{$op} = eval "sub {$op shift()}"; |
1301 | } |
1302 | |
1303 | Due to L<Calling Conventions for Mutators>, we do not need anything |
1304 | special to make C<+=> and friends work, except filling C<+=> entry of |
1305 | %subr, and defining a copy constructor (needed since Perl has no |
1306 | way to know that the implementation of C<'+='> does not mutate |
1307 | the argument, compare L<Copy Constructor>). |
1308 | |
1fd16925 |
1309 | To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload> |
ee239bfe |
1310 | line, and code (this code assumes that mutators change things one level |
1311 | deep only, so recursive copying is not needed): |
1312 | |
1313 | sub cpy { |
1314 | my $self = shift; |
1315 | bless [@$self], ref $self; |
1316 | } |
1317 | |
b267980d |
1318 | To make C<++> and C<--> work, we need to implement actual mutators, |
ee239bfe |
1319 | either directly, or in C<nomethod>. We continue to do things inside |
1320 | C<nomethod>, thus add |
1321 | |
1322 | if ($meth eq '++' or $meth eq '--') { |
1323 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
1324 | return $obj; |
1325 | } |
1326 | |
b267980d |
1327 | after the first line of wrap(). This is not a most effective |
ee239bfe |
1328 | implementation, one may consider |
1329 | |
1330 | sub inc { $_[0] = bless ['++', shift, 1]; } |
1331 | |
1332 | instead. |
1333 | |
1334 | As a final remark, note that one can fill %subr by |
1335 | |
1336 | my %subr = ( 'n' => sub {$_[0]} ); |
1337 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1338 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1339 | } |
1340 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1341 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1342 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1343 | } |
1344 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1345 | $subr{$op} = eval "sub {$op shift()}"; |
1346 | } |
1347 | $subr{'++'} = $subr{'+'}; |
1348 | $subr{'--'} = $subr{'-'}; |
1349 | |
b267980d |
1350 | This finishes implementation of a primitive symbolic calculator in |
1351 | 50 lines of Perl code. Since the numeric values of subexpressions |
ee239bfe |
1352 | are not cached, the calculator is very slow. |
1353 | |
1354 | Here is the answer for the exercise: In the case of str(), we need no |
1355 | explicit recursion since the overloaded C<.>-operator will fall back |
1356 | to an existing overloaded operator C<"">. Overloaded arithmetic |
1357 | operators I<do not> fall back to numeric conversion if C<fallback> is |
1358 | not explicitly requested. Thus without an explicit recursion num() |
1359 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
1360 | the argument of num(). |
1361 | |
1362 | If you wonder why defaults for conversion are different for str() and |
1363 | num(), note how easy it was to write the symbolic calculator. This |
1364 | simplicity is due to an appropriate choice of defaults. One extra |
f610777f |
1365 | note: due to the explicit recursion num() is more fragile than sym(): |
1366 | we need to explicitly check for the type of $a and $b. If components |
ee239bfe |
1367 | $a and $b happen to be of some related type, this may lead to problems. |
1368 | |
1369 | =head2 I<Really> symbolic calculator |
1370 | |
1371 | One may wonder why we call the above calculator symbolic. The reason |
1372 | is that the actual calculation of the value of expression is postponed |
1373 | until the value is I<used>. |
1374 | |
1375 | To see it in action, add a method |
1376 | |
b267980d |
1377 | sub STORE { |
1378 | my $obj = shift; |
1379 | $#$obj = 1; |
ee239bfe |
1380 | @$obj->[0,1] = ('=', shift); |
1381 | } |
1382 | |
1383 | to the package C<symbolic>. After this change one can do |
1384 | |
2b393bf4 |
1385 | my $a = symbolic->new(3); |
1386 | my $b = symbolic->new(4); |
ee239bfe |
1387 | my $c = sqrt($a**2 + $b**2); |
1388 | |
1389 | and the numeric value of $c becomes 5. However, after calling |
1390 | |
1391 | $a->STORE(12); $b->STORE(5); |
1392 | |
1393 | the numeric value of $c becomes 13. There is no doubt now that the module |
1394 | symbolic provides a I<symbolic> calculator indeed. |
1395 | |
1396 | To hide the rough edges under the hood, provide a tie()d interface to the |
1397 | package C<symbolic> (compare with L<Metaphor clash>). Add methods |
1398 | |
1399 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
1400 | sub FETCH { shift } |
1401 | sub nop { } # Around a bug |
1402 | |
1403 | (the bug is described in L<"BUGS">). One can use this new interface as |
1404 | |
1405 | tie $a, 'symbolic', 3; |
1406 | tie $b, 'symbolic', 4; |
1407 | $a->nop; $b->nop; # Around a bug |
1408 | |
1409 | my $c = sqrt($a**2 + $b**2); |
1410 | |
1411 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
1412 | of $c becomes 13. To insulate the user of the module add a method |
1413 | |
1414 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
1415 | |
1416 | Now |
1417 | |
1418 | my ($a, $b); |
1419 | symbolic->vars($a, $b); |
1420 | my $c = sqrt($a**2 + $b**2); |
1421 | |
1422 | $a = 3; $b = 4; |
1423 | printf "c5 %s=%f\n", $c, $c; |
1424 | |
1425 | $a = 12; $b = 5; |
1426 | printf "c13 %s=%f\n", $c, $c; |
1427 | |
1428 | shows that the numeric value of $c follows changes to the values of $a |
1429 | and $b. |
1430 | |
4633a7c4 |
1431 | =head1 AUTHOR |
1432 | |
1fef88e7 |
1433 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 |
1434 | |
7e494759 |
1435 | =head1 SEE ALSO |
1436 | |
1437 | The L<overloading> pragma can be used to enable or disable overloaded |
1438 | operations within a lexical scope. |
1439 | |
4633a7c4 |
1440 | =head1 DIAGNOSTICS |
1441 | |
1442 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
1443 | induces diagnostic messages. |
1444 | |
e7ea3e70 |
1445 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1446 | deduce which operations are overloaded (and which ancestor triggers |
1447 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
1448 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
1449 | key (in fact a presence of this method shows that this package has |
1450 | overloading enabled, and it is what is used by the C<Overloaded> |
ee239bfe |
1451 | function of module C<overload>). |
e7ea3e70 |
1452 | |
6ad11d81 |
1453 | The module might issue the following warnings: |
6b82e2f5 |
1454 | |
1455 | =over 4 |
1456 | |
1457 | =item Odd number of arguments for overload::constant |
1458 | |
1459 | (W) The call to overload::constant contained an odd number of arguments. |
1460 | The arguments should come in pairs. |
1461 | |
1462 | =item `%s' is not an overloadable type |
1463 | |
1464 | (W) You tried to overload a constant type the overload package is unaware of. |
1465 | |
1466 | =item `%s' is not a code reference |
1467 | |
1468 | (W) The second (fourth, sixth, ...) argument of overload::constant needs |
1469 | to be a code reference. Either an anonymous subroutine, or a reference |
1470 | to a subroutine. |
1471 | |
1472 | =back |
1473 | |
4633a7c4 |
1474 | =head1 BUGS |
1475 | |
aa689395 |
1476 | Because it is used for overloading, the per-package hash %OVERLOAD now |
1477 | has a special meaning in Perl. The symbol table is filled with names |
1478 | looking like line-noise. |
4633a7c4 |
1479 | |
a6006777 |
1480 | For the purpose of inheritance every overloaded package behaves as if |
1481 | C<fallback> is present (possibly undefined). This may create |
1482 | interesting effects if some package is not overloaded, but inherits |
1483 | from two overloaded packages. |
4633a7c4 |
1484 | |
b267980d |
1485 | Relation between overloading and tie()ing is broken. Overloading is |
ee239bfe |
1486 | triggered or not basing on the I<previous> class of tie()d value. |
1487 | |
b267980d |
1488 | This happens because the presence of overloading is checked too early, |
ee239bfe |
1489 | before any tie()d access is attempted. If the FETCH()ed class of the |
b267980d |
1490 | tie()d value does not change, a simple workaround is to access the value |
ee239bfe |
1491 | immediately after tie()ing, so that after this call the I<previous> class |
1492 | coincides with the current one. |
1493 | |
1494 | B<Needed:> a way to fix this without a speed penalty. |
1495 | |
b3ac6de7 |
1496 | Barewords are not covered by overloaded string constants. |
1497 | |
ee239bfe |
1498 | This document is confusing. There are grammos and misleading language |
1499 | used in places. It would seem a total rewrite is needed. |
4633a7c4 |
1500 | |
1501 | =cut |
1502 | |