Commit | Line | Data |
---|---|---|
4633a7c4 LW |
1 | package overload; |
2 | ||
e3ec0a15 | 3 | our $VERSION = '1.16'; |
b75c8c73 | 4 | |
a6006777 | 5 | sub nil {} |
6 | ||
4633a7c4 LW |
7 | sub OVERLOAD { |
8 | $package = shift; | |
9 | my %arg = @_; | |
a6006777 | 10 | my ($sub, $fb); |
11 | $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. | |
c989e6a3 | 12 | $fb = ${$package . "::()"}; # preserve old fallback value RT#68196 |
a6006777 | 13 | *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. |
4633a7c4 | 14 | for (keys %arg) { |
a6006777 | 15 | if ($_ eq 'fallback') { |
16 | $fb = $arg{$_}; | |
17 | } else { | |
18 | $sub = $arg{$_}; | |
19 | if (not ref $sub and $sub !~ /::/) { | |
44a8e56a | 20 | $ {$package . "::(" . $_} = $sub; |
21 | $sub = \&nil; | |
a6006777 | 22 | } |
1f874cb6 | 23 | #print STDERR "Setting '$ {'package'}::\cO$_' to \\&'$sub'.\n"; |
a6006777 | 24 | *{$package . "::(" . $_} = \&{ $sub }; |
25 | } | |
4633a7c4 | 26 | } |
a6006777 | 27 | ${$package . "::()"} = $fb; # Make it findable too (fallback only). |
4633a7c4 LW |
28 | } |
29 | ||
30 | sub import { | |
31 | $package = (caller())[0]; | |
32 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; | |
33 | shift; | |
34 | $package->overload::OVERLOAD(@_); | |
35 | } | |
36 | ||
37 | sub unimport { | |
38 | $package = (caller())[0]; | |
a6006777 | 39 | ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table |
4633a7c4 LW |
40 | shift; |
41 | for (@_) { | |
a6006777 | 42 | if ($_ eq 'fallback') { |
43 | undef $ {$package . "::()"}; | |
44 | } else { | |
45 | delete $ {$package . "::"}{"(" . $_}; | |
46 | } | |
4633a7c4 LW |
47 | } |
48 | } | |
49 | ||
50 | sub Overloaded { | |
a6006777 | 51 | my $package = shift; |
52 | $package = ref $package if ref $package; | |
53 | $package->can('()'); | |
4633a7c4 LW |
54 | } |
55 | ||
44a8e56a | 56 | sub ov_method { |
57 | my $globref = shift; | |
58 | return undef unless $globref; | |
59 | my $sub = \&{*$globref}; | |
56f08af2 FC |
60 | require Scalar::Util; |
61 | return $sub | |
62 | if Scalar::Util::refaddr($sub) != Scalar::Util::refaddr(\&nil); | |
44a8e56a | 63 | return shift->can($ {*$globref}); |
64 | } | |
65 | ||
4633a7c4 | 66 | sub OverloadedStringify { |
a6006777 | 67 | my $package = shift; |
68 | $package = ref $package if ref $package; | |
44a8e56a | 69 | #$package->can('(""') |
ee239bfe IZ |
70 | ov_method mycan($package, '(""'), $package |
71 | or ov_method mycan($package, '(0+'), $package | |
72 | or ov_method mycan($package, '(bool'), $package | |
73 | or ov_method mycan($package, '(nomethod'), $package; | |
4633a7c4 LW |
74 | } |
75 | ||
76 | sub Method { | |
a6006777 | 77 | my $package = shift; |
05a4b9b1 | 78 | if(ref $package) { |
4de05ceb RGS |
79 | local $@; |
80 | local $!; | |
81 | require Scalar::Util; | |
05a4b9b1 BB |
82 | $package = Scalar::Util::blessed($package); |
83 | return undef if !defined $package; | |
84 | } | |
44a8e56a | 85 | #my $meth = $package->can('(' . shift); |
86 | ov_method mycan($package, '(' . shift), $package; | |
87 | #return $meth if $meth ne \&nil; | |
88 | #return $ {*{$meth}}; | |
4633a7c4 LW |
89 | } |
90 | ||
91 | sub AddrRef { | |
a6006777 | 92 | my $package = ref $_[0]; |
93 | return "$_[0]" unless $package; | |
b3c0ec7c | 94 | |
4de05ceb RGS |
95 | local $@; |
96 | local $!; | |
97 | require Scalar::Util; | |
295812e0 RGS |
98 | my $class = Scalar::Util::blessed($_[0]); |
99 | my $class_prefix = defined($class) ? "$class=" : ""; | |
100 | my $type = Scalar::Util::reftype($_[0]); | |
101 | my $addr = Scalar::Util::refaddr($_[0]); | |
10b78b9b | 102 | return sprintf("%s%s(0x%x)", $class_prefix, $type, $addr); |
4633a7c4 LW |
103 | } |
104 | ||
1b1d102f | 105 | *StrVal = *AddrRef; |
4633a7c4 | 106 | |
44a8e56a | 107 | sub mycan { # Real can would leave stubs. |
108 | my ($package, $meth) = @_; | |
e1a479c5 | 109 | |
1e9bd118 NC |
110 | local $@; |
111 | local $!; | |
112 | require mro; | |
113 | ||
e1a479c5 BB |
114 | my $mro = mro::get_linear_isa($package); |
115 | foreach my $p (@$mro) { | |
116 | my $fqmeth = $p . q{::} . $meth; | |
117 | return \*{$fqmeth} if defined &{$fqmeth}; | |
44a8e56a | 118 | } |
e1a479c5 | 119 | |
44a8e56a | 120 | return undef; |
121 | } | |
122 | ||
b3ac6de7 | 123 | %constants = ( |
9cfe5470 RGS |
124 | 'integer' => 0x1000, # HINT_NEW_INTEGER |
125 | 'float' => 0x2000, # HINT_NEW_FLOAT | |
126 | 'binary' => 0x4000, # HINT_NEW_BINARY | |
127 | 'q' => 0x8000, # HINT_NEW_STRING | |
128 | 'qr' => 0x10000, # HINT_NEW_RE | |
b3ac6de7 IZ |
129 | ); |
130 | ||
ee239bfe IZ |
131 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
132 | assign => "+= -= *= /= %= **= <<= >>= x= .=", | |
2877bd81 | 133 | num_comparison => "< <= > >= == !=", |
ee239bfe | 134 | '3way_comparison'=> "<=> cmp", |
2877bd81 | 135 | str_comparison => "lt le gt ge eq ne", |
6dd85743 | 136 | binary => '& &= | |= ^ ^=', |
ee239bfe IZ |
137 | unary => "neg ! ~", |
138 | mutators => '++ --', | |
f216259d | 139 | func => "atan2 cos sin exp abs log sqrt int", |
7cb0cfe6 | 140 | conversion => 'bool "" 0+ qr', |
f5284f61 | 141 | iterators => '<>', |
e2210791 | 142 | filetest => "-X", |
f5284f61 | 143 | dereferencing => '${} @{} %{} &{} *{}', |
ae20c3aa | 144 | matching => '~~', |
ee239bfe IZ |
145 | special => 'nomethod fallback ='); |
146 | ||
6b82e2f5 | 147 | use warnings::register; |
b3ac6de7 IZ |
148 | sub constant { |
149 | # Arguments: what, sub | |
150 | while (@_) { | |
6b82e2f5 | 151 | if (@_ == 1) { |
4498a751 | 152 | warnings::warnif ("Odd number of arguments for overload::constant"); |
6b82e2f5 A |
153 | last; |
154 | } | |
155 | elsif (!exists $constants {$_ [0]}) { | |
1f874cb6 | 156 | warnings::warnif ("'$_[0]' is not an overloadable type"); |
6b82e2f5 | 157 | } |
1e70e886 | 158 | elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) { |
6b82e2f5 A |
159 | # Can't use C<ref $_[1] eq "CODE"> above as code references can be |
160 | # blessed, and C<ref> would return the package the ref is blessed into. | |
161 | if (warnings::enabled) { | |
6b82e2f5 | 162 | $_ [1] = "undef" unless defined $_ [1]; |
1f874cb6 | 163 | warnings::warn ("'$_[1]' is not a code reference"); |
6b82e2f5 A |
164 | } |
165 | } | |
166 | else { | |
167 | $^H{$_[0]} = $_[1]; | |
f22a2069 | 168 | $^H |= $constants{$_[0]}; |
6b82e2f5 | 169 | } |
b3ac6de7 IZ |
170 | shift, shift; |
171 | } | |
172 | } | |
173 | ||
174 | sub remove_constant { | |
175 | # Arguments: what, sub | |
176 | while (@_) { | |
177 | delete $^H{$_[0]}; | |
178 | $^H &= ~ $constants{$_[0]}; | |
179 | shift, shift; | |
180 | } | |
181 | } | |
182 | ||
4633a7c4 LW |
183 | 1; |
184 | ||
185 | __END__ | |
186 | ||
b267980d | 187 | =head1 NAME |
4633a7c4 | 188 | |
7adf7a02 | 189 | overload - Package for overloading Perl operations |
4633a7c4 LW |
190 | |
191 | =head1 SYNOPSIS | |
192 | ||
193 | package SomeThing; | |
194 | ||
b267980d | 195 | use overload |
4633a7c4 LW |
196 | '+' => \&myadd, |
197 | '-' => \&mysub; | |
198 | # etc | |
199 | ... | |
200 | ||
201 | package main; | |
2b393bf4 | 202 | $a = SomeThing->new( 57 ); |
0e0f0835 | 203 | $b = 5 + $a; |
4633a7c4 LW |
204 | ... |
205 | if (overload::Overloaded $b) {...} | |
206 | ... | |
207 | $strval = overload::StrVal $b; | |
208 | ||
4633a7c4 LW |
209 | =head1 DESCRIPTION |
210 | ||
caf82a01 YST |
211 | This pragma allows overloading of Perl's operators for a class. |
212 | To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead. | |
213 | ||
0e0f0835 | 214 | =head2 Fundamentals |
4633a7c4 | 215 | |
0e0f0835 | 216 | =head3 Declaration |
4633a7c4 | 217 | |
0e0f0835 MB |
218 | Arguments of the C<use overload> directive are (key, value) pairs. |
219 | For the full set of legal keys, see L<Overloadable Operations> below. | |
ee239bfe | 220 | |
0e0f0835 MB |
221 | Operator implementations (the values) can be subroutines, |
222 | references to subroutines, or anonymous subroutines | |
223 | - in other words, anything legal inside a C<&{ ... }> call. | |
224 | Values specified as strings are interpreted as method names. | |
225 | Thus | |
ee239bfe | 226 | |
0e0f0835 MB |
227 | package Number; |
228 | use overload | |
229 | "-" => "minus", | |
230 | "*=" => \&muas, | |
231 | '""' => sub { ...; }; | |
ee239bfe | 232 | |
0e0f0835 MB |
233 | declares that subtraction is to be implemented by method C<minus()> |
234 | in the class C<Number> (or one of its base classes), | |
235 | and that the function C<Number::muas()> is to be used for the | |
236 | assignment form of multiplication, C<*=>. | |
237 | It also defines an anonymous subroutine to implement stringification: | |
238 | this is called whenever an object blessed into the package C<Number> | |
239 | is used in a string context (this subroutine might, for example, | |
240 | return the number as a Roman numeral). | |
ee239bfe | 241 | |
0e0f0835 | 242 | =head3 Calling Conventions and Magic Autogeneration |
ee239bfe | 243 | |
0e0f0835 MB |
244 | The following sample implementation of C<minus()> (which assumes |
245 | that C<Number> objects are simply blessed references to scalars) | |
246 | illustrates the calling conventions: | |
ee239bfe | 247 | |
0e0f0835 MB |
248 | package Number; |
249 | sub minus { | |
250 | my ($self, $other, $swap) = @_; | |
251 | my $result = $$self - $other; # * | |
252 | $result = -$result if $swap; | |
253 | ref $result ? $result : bless \$result; | |
254 | } | |
255 | # * may recurse once - see table below | |
256 | ||
257 | Three arguments are passed to all subroutines specified in the | |
258 | C<use overload> directive (with one exception - see L</nomethod>). | |
259 | The first of these is the operand providing the overloaded | |
260 | operator implementation - | |
261 | in this case, the object whose C<minus()> method is being called. | |
262 | ||
263 | The second argument is the other operand, or C<undef> in the | |
264 | case of a unary operator. | |
265 | ||
266 | The third argument is set to TRUE if (and only if) the two | |
267 | operands have been swapped. Perl may do this to ensure that the | |
268 | first argument (C<$self>) is an object implementing the overloaded | |
269 | operation, in line with general object calling conventions. | |
270 | For example, if C<$x> and C<$y> are C<Number>s: | |
271 | ||
272 | operation | generates a call to | |
273 | ============|====================== | |
274 | $x - $y | minus($x, $y, '') | |
275 | $x - 7 | minus($x, 7, '') | |
276 | 7 - $x | minus($x, 7, 1) | |
277 | ||
278 | Perl may also use C<minus()> to implement other operators which | |
279 | have not been specified in the C<use overload> directive, | |
280 | according to the rules for L<Magic Autogeneration> described later. | |
281 | For example, the C<use overload> above declared no subroutine | |
282 | for any of the operators C<-->, C<neg> (the overload key for | |
283 | unary minus), or C<-=>. Thus | |
284 | ||
285 | operation | generates a call to | |
286 | ============|====================== | |
287 | -$x | minus($x, 0, 1) | |
288 | $x-- | minus($x, 1, undef) | |
289 | $x -= 3 | minus($x, 3, undef) | |
290 | ||
291 | Note the C<undef>s: | |
292 | where autogeneration results in the method for a standard | |
293 | operator which does not change either of its operands, such | |
294 | as C<->, being used to implement an operator which changes | |
295 | the operand ("mutators": here, C<--> and C<-=>), | |
296 | Perl passes undef as the third argument. | |
297 | This still evaluates as FALSE, consistent with the fact that | |
298 | the operands have not been swapped, but gives the subroutine | |
299 | a chance to alter its behaviour in these cases. | |
300 | ||
301 | In all the above examples, C<minus()> is required | |
302 | only to return the result of the subtraction: | |
303 | Perl takes care of the assignment to $x. | |
304 | In fact, such methods should I<not> modify their operands, | |
305 | even if C<undef> is passed as the third argument | |
306 | (see L<Overloadable Operations>). | |
307 | ||
308 | The same is not true of implementations of C<++> and C<-->: | |
309 | these are expected to modify their operand. | |
310 | An appropriate implementation of C<--> might look like | |
311 | ||
312 | use overload '--' => "decr", | |
313 | # ... | |
314 | sub decr { --${$_[0]}; } | |
315 | ||
316 | =head3 Mathemagic, Mutators, and Copy Constructors | |
317 | ||
318 | The term 'mathemagic' describes the overloaded implementation | |
319 | of mathematical operators. | |
320 | Mathemagical operations raise an issue. | |
321 | Consider the code: | |
322 | ||
323 | $a = $b; | |
324 | --$a; | |
325 | ||
326 | If C<$a> and C<$b> are scalars then after these statements | |
327 | ||
328 | $a == $b - 1 | |
329 | ||
330 | An object, however, is a reference to blessed data, so if | |
331 | C<$a> and C<$b> are objects then the assignment C<$a = $b> | |
332 | copies only the reference, leaving C<$a> and C<$b> referring | |
333 | to the same object data. | |
334 | One might therefore expect the operation C<--$a> to decrement | |
335 | C<$b> as well as C<$a>. | |
336 | However, this would not be consistent with how we expect the | |
337 | mathematical operators to work. | |
338 | ||
339 | Perl resolves this dilemma by transparently calling a copy | |
340 | constructor before calling a method defined to implement | |
341 | a mutator (C<-->, C<+=>, and so on.). | |
342 | In the above example, when Perl reaches the decrement | |
343 | statement, it makes a copy of the object data in C<$a> and | |
344 | assigns to C<$a> a reference to the copied data. | |
345 | Only then does it call C<decr()>, which alters the copied | |
346 | data, leaving C<$b> unchanged. | |
347 | Thus the object metaphor is preserved as far as possible, | |
348 | while mathemagical operations still work according to the | |
349 | arithmetic metaphor. | |
350 | ||
351 | Note: the preceding paragraph describes what happens when | |
352 | Perl autogenerates the copy constructor for an object based | |
353 | on a scalar. | |
354 | For other cases, see L<Copy Constructor>. | |
ee239bfe | 355 | |
0e0f0835 | 356 | =head2 Overloadable Operations |
ee239bfe | 357 | |
0e0f0835 MB |
358 | The complete list of keys that can be specified in the C<use overload> |
359 | directive are given, separated by spaces, in the values of the | |
360 | hash C<%overload::ops>: | |
ee239bfe | 361 | |
0e0f0835 MB |
362 | with_assign => '+ - * / % ** << >> x .', |
363 | assign => '+= -= *= /= %= **= <<= >>= x= .=', | |
364 | num_comparison => '< <= > >= == !=', | |
365 | '3way_comparison'=> '<=> cmp', | |
366 | str_comparison => 'lt le gt ge eq ne', | |
367 | binary => '& &= | |= ^ ^=', | |
368 | unary => 'neg ! ~', | |
369 | mutators => '++ --', | |
370 | func => 'atan2 cos sin exp abs log sqrt int', | |
371 | conversion => 'bool "" 0+ qr', | |
372 | iterators => '<>', | |
373 | filetest => '-X', | |
374 | dereferencing => '${} @{} %{} &{} *{}', | |
375 | matching => '~~', | |
376 | special => 'nomethod fallback =' | |
ee239bfe | 377 | |
0e0f0835 MB |
378 | Most of the overloadable operators map one-to-one to these keys. |
379 | Exceptions, including additional overloadable operations not | |
380 | apparent from this hash, are included in the notes which follow. | |
ee239bfe | 381 | |
0e0f0835 | 382 | =over 5 |
ee239bfe | 383 | |
0e0f0835 | 384 | =item * C<not> |
4633a7c4 | 385 | |
0e0f0835 MB |
386 | The operator C<not> is not a valid key for C<use overload>. |
387 | However, if the operator C<!> is overloaded then the same | |
388 | implementation will be used for C<not> | |
389 | (since the two operators differ only in precedence). | |
4633a7c4 | 390 | |
0e0f0835 | 391 | =item * C<neg> |
4633a7c4 | 392 | |
0e0f0835 MB |
393 | The key C<neg> is used for unary minus to disambiguate it from |
394 | binary C<->. | |
4633a7c4 | 395 | |
0e0f0835 | 396 | =item * C<++>, C<--> |
4633a7c4 | 397 | |
0e0f0835 MB |
398 | Assuming they are to behave analogously to Perl's C<++> and C<-->, |
399 | overloaded implementations of these operators are required to | |
400 | mutate their operands. | |
4633a7c4 | 401 | |
0e0f0835 MB |
402 | No distinction is made between prefix and postfix forms of the |
403 | increment and decrement operators: these differ only in the | |
404 | point at which Perl calls the associated subroutine when | |
405 | evaluating an expression. | |
ee239bfe | 406 | |
0e0f0835 | 407 | =item * I<Assignments> |
4633a7c4 | 408 | |
0e0f0835 MB |
409 | += -= *= /= %= **= <<= >>= x= .= |
410 | &= |= ^= | |
4633a7c4 | 411 | |
0e0f0835 MB |
412 | Simple assignment is not overloadable (the C<'='> key is used |
413 | for the L<Copy Constructor>). | |
414 | Perl does have a way to make assignments to an object do whatever | |
415 | you want, but this involves using tie(), not overload - | |
416 | see L<perlfunc/tie> and the L</COOKBOOK> examples below. | |
4633a7c4 | 417 | |
0e0f0835 MB |
418 | The subroutine for the assignment variant of an operator is |
419 | required only to return the result of the operation. | |
420 | It is permitted to change the value of its operand | |
421 | (this is safe because Perl calls the copy constructor first), | |
422 | but this is optional since Perl assigns the returned value to | |
423 | the left-hand operand anyway. | |
4633a7c4 | 424 | |
0e0f0835 MB |
425 | An object that overloads an assignment operator does so only in |
426 | respect of assignments to that object. | |
427 | In other words, Perl never calls the corresponding methods with | |
428 | the third argument (the "swap" argument) set to TRUE. | |
429 | For example, the operation | |
4633a7c4 | 430 | |
0e0f0835 | 431 | $a *= $b |
4633a7c4 | 432 | |
0e0f0835 MB |
433 | cannot lead to C<$b>'s implementation of C<*=> being called, |
434 | even if C<$a> is a scalar. | |
435 | (It can, however, generate a call to C<$b>'s method for C<*>). | |
6dd85743 | 436 | |
0e0f0835 | 437 | =item * I<Non-mutators with a mutator variant> |
4633a7c4 | 438 | |
0e0f0835 MB |
439 | + - * / % ** << >> x . |
440 | & | ^ | |
4633a7c4 | 441 | |
0e0f0835 MB |
442 | As described L<above|"Calling Conventions and Magic Autogeneration">, |
443 | Perl may call methods for operators like C<+> and C<&> in the course | |
444 | of implementing missing operations like C<++>, C<+=>, and C<&=>. | |
445 | While these methods may detect this usage by testing the definedness | |
446 | of the third argument, they should in all cases avoid changing their | |
447 | operands. | |
448 | This is because Perl does not call the copy constructor before | |
449 | invoking these methods. | |
4633a7c4 | 450 | |
0e0f0835 | 451 | =item * C<int> |
4633a7c4 | 452 | |
0e0f0835 MB |
453 | Traditionally, the Perl function C<int> rounds to 0 |
454 | (see L<perlfunc/int>), and so for floating-point-like types one | |
455 | should follow the same semantic. | |
4633a7c4 | 456 | |
0e0f0835 | 457 | =item * I<String, numeric, boolean, and regexp conversions> |
4633a7c4 | 458 | |
0e0f0835 | 459 | "" 0+ bool |
f216259d | 460 | |
0e0f0835 MB |
461 | These conversions are invoked according to context as necessary. |
462 | For example, the subroutine for C<'""'> (stringify) may be used | |
463 | where the overloaded object is passed as an argument to C<print>, | |
464 | and that for C<'bool'> where it is tested in the condition of a flow | |
465 | control statement (like C<while>) or the ternary C<?:> operation. | |
4633a7c4 | 466 | |
0e0f0835 MB |
467 | Of course, in contexts like, for example, C<$obj + 1>, Perl will |
468 | invoke C<$obj>'s implementation of C<+> rather than (in this | |
469 | example) converting C<$obj> to a number using the numify method | |
470 | C<'0+'> (an exception to this is when no method has been provided | |
471 | for C<'+'> and L</fallback> is set to TRUE). | |
4633a7c4 | 472 | |
0e0f0835 MB |
473 | The subroutines for C<'""'>, C<'0+'>, and C<'bool'> can return |
474 | any arbitrary Perl value. | |
475 | If the corresponding operation for this value is overloaded too, | |
476 | the operation will be called again with this value. | |
4633a7c4 | 477 | |
1554e226 DC |
478 | As a special case if the overload returns the object itself then it will |
479 | be used directly. An overloaded conversion returning the object is | |
480 | probably a bug, because you're likely to get something that looks like | |
481 | C<YourPackage=HASH(0x8172b34)>. | |
482 | ||
0e0f0835 | 483 | qr |
f5284f61 | 484 | |
0e0f0835 MB |
485 | The subroutine for C<'qr'> is used wherever the object is |
486 | interpolated into or used as a regexp, including when it | |
487 | appears on the RHS of a C<=~> or C<!~> operator. | |
f5284f61 | 488 | |
0e0f0835 MB |
489 | C<qr> must return a compiled regexp, or a ref to a compiled regexp |
490 | (such as C<qr//> returns), and any further overloading on the return | |
491 | value will be ignored. | |
492 | ||
493 | =item * I<Iteration> | |
494 | ||
495 | If C<E<lt>E<gt>> is overloaded then the same implementation is used | |
496 | for both the I<read-filehandle> syntax C<E<lt>$varE<gt>> and | |
f5284f61 IZ |
497 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
498 | ||
54f8c773 YST |
499 | B<BUGS> Even in list context, the iterator is currently called only |
500 | once and with scalar context. | |
501 | ||
e2210791 BM |
502 | =item * I<File tests> |
503 | ||
0e0f0835 MB |
504 | The key C<'-X'> is used to specify a subroutine to handle all the |
505 | filetest operators (C<-f>, C<-x>, and so on: see L<perlfunc/-X> for | |
506 | the full list); | |
507 | it is not possible to overload any filetest operator individually. | |
508 | To distinguish them, the letter following the '-' is passed as the | |
509 | second argument (that is, in the slot that for binary operators | |
510 | is used to pass the second operand). | |
e2210791 BM |
511 | |
512 | Calling an overloaded filetest operator does not affect the stat value | |
513 | associated with the special filehandle C<_>. It still refers to the | |
514 | result of the last C<stat>, C<lstat> or unoverloaded filetest. | |
515 | ||
0e0f0835 | 516 | This overload was introduced in Perl 5.12. |
e2210791 | 517 | |
ae20c3aa RGS |
518 | =item * I<Matching> |
519 | ||
b3ed409d CS |
520 | The key C<"~~"> allows you to override the smart matching logic used by |
521 | the C<~~> operator and the switch construct (C<given>/C<when>). See | |
8d00ae59 | 522 | L<perlsyn/Switch statements> and L<feature>. |
ae20c3aa | 523 | |
0e0f0835 MB |
524 | Unusually, the overloaded implementation of the smart match operator |
525 | does not get full control of the smart match behaviour. | |
526 | In particular, in the following code: | |
0de1c906 DM |
527 | |
528 | package Foo; | |
529 | use overload '~~' => 'match'; | |
530 | ||
531 | my $obj = Foo->new(); | |
532 | $obj ~~ [ 1,2,3 ]; | |
533 | ||
534 | the smart match does I<not> invoke the method call like this: | |
535 | ||
536 | $obj->match([1,2,3],0); | |
537 | ||
538 | rather, the smart match distributive rule takes precedence, so $obj is | |
539 | smart matched against each array element in turn until a match is found, | |
540 | so you may see between one and three of these calls instead: | |
541 | ||
542 | $obj->match(1,0); | |
543 | $obj->match(2,0); | |
544 | $obj->match(3,0); | |
545 | ||
546 | Consult the match table in L<perlsyn/"Smart matching in detail"> for | |
547 | details of when overloading is invoked. | |
548 | ||
f5284f61 IZ |
549 | =item * I<Dereferencing> |
550 | ||
0e0f0835 MB |
551 | ${} @{} %{} &{} *{} |
552 | ||
553 | If these operators are not explicitly overloaded then they | |
554 | work in the normal way, yielding the underlying scalar, | |
555 | array, or whatever stores the object data (or the appropriate | |
556 | error message if the dereference operator doesn't match it). | |
557 | Defining a catch-all C<'nomethod'> (see L<below|/nomethod>) | |
558 | makes no difference to this as the catch-all function will | |
559 | not be called to implement a missing dereference operator. | |
560 | ||
561 | If a dereference operator is overloaded then it must return a | |
562 | I<reference> of the appropriate type (for example, the | |
563 | subroutine for key C<'${}'> should return a reference to a | |
564 | scalar, not a scalar), or another object which overloads the | |
565 | operator: that is, the subroutine only determines what is | |
566 | dereferenced and the actual dereferencing is left to Perl. | |
567 | As a special case, if the subroutine returns the object itself | |
568 | then it will not be called again - avoiding infinite recursion. | |
b267980d | 569 | |
4633a7c4 LW |
570 | =item * I<Special> |
571 | ||
0e0f0835 | 572 | nomethod fallback = |
4633a7c4 | 573 | |
0e0f0835 | 574 | See L<Special Keys for C<use overload>>. |
4633a7c4 LW |
575 | |
576 | =back | |
577 | ||
0e0f0835 MB |
578 | =head2 Magic Autogeneration |
579 | ||
580 | If a method for an operation is not found then Perl tries to | |
581 | autogenerate a substitute implementation from the operations | |
582 | that have been defined. | |
583 | ||
584 | Note: the behaviour described in this section can be disabled | |
585 | by setting C<fallback> to FALSE (see L</fallback>). | |
586 | ||
587 | In the following tables, numbers indicate priority. | |
588 | For example, the table below states that, | |
589 | if no implementation for C<'!'> has been defined then Perl will | |
590 | implement it using C<'bool'> (that is, by inverting the value | |
591 | returned by the method for C<'bool'>); | |
592 | if boolean conversion is also unimplemented then Perl will | |
593 | use C<'0+'> or, failing that, C<'""'>. | |
594 | ||
595 | operator | can be autogenerated from | |
596 | | | |
597 | | 0+ "" bool . x | |
598 | =========|========================== | |
599 | 0+ | 1 2 | |
600 | "" | 1 2 | |
601 | bool | 1 2 | |
602 | int | 1 2 3 | |
603 | ! | 2 3 1 | |
604 | qr | 2 1 3 | |
605 | . | 2 1 3 | |
606 | x | 2 1 3 | |
607 | .= | 3 2 4 1 | |
608 | x= | 3 2 4 1 | |
609 | <> | 2 1 3 | |
610 | -X | 2 1 3 | |
611 | ||
612 | Note: The iterator (C<'E<lt>E<gt>'>) and file test (C<'-X'>) | |
613 | operators work as normal: if the operand is not a blessed glob or | |
614 | IO reference then it is converted to a string (using the method | |
615 | for C<'""'>, C<'0+'>, or C<'bool'>) to be interpreted as a glob | |
616 | or filename. | |
617 | ||
618 | operator | can be autogenerated from | |
619 | | | |
620 | | < <=> neg -= - | |
621 | =========|========================== | |
622 | neg | 1 | |
623 | -= | 1 | |
624 | -- | 1 2 | |
625 | abs | a1 a2 b1 b2 [*] | |
626 | < | 1 | |
627 | <= | 1 | |
628 | > | 1 | |
629 | >= | 1 | |
630 | == | 1 | |
631 | != | 1 | |
632 | ||
633 | * one from [a1, a2] and one from [b1, b2] | |
634 | ||
635 | Just as numeric comparisons can be autogenerated from the method | |
636 | for C<< '<=>' >>, string comparisons can be autogenerated from | |
637 | that for C<'cmp'>: | |
638 | ||
639 | operators | can be autogenerated from | |
640 | ====================|=========================== | |
641 | lt gt le ge eq ne | cmp | |
642 | ||
643 | Similarly, autogeneration for keys C<'+='> and C<'++'> is analogous | |
644 | to C<'-='> and C<'--'> above: | |
645 | ||
646 | operator | can be autogenerated from | |
647 | | | |
648 | | += + | |
649 | =========|========================== | |
650 | += | 1 | |
651 | ++ | 1 2 | |
652 | ||
653 | And other assignment variations are analogous to | |
654 | C<'+='> and C<'-='> (and similar to C<'.='> and C<'x='> above): | |
655 | ||
656 | operator || *= /= %= **= <<= >>= &= ^= |= | |
657 | -------------------||-------------------------------- | |
658 | autogenerated from || * / % ** << >> & ^ | | |
659 | ||
660 | Note also that the copy constructor (key C<'='>) may be | |
661 | autogenerated, but only for objects based on scalars. | |
662 | See L<Copy Constructor>. | |
663 | ||
664 | =head3 Minimal Set of Overloaded Operations | |
4633a7c4 | 665 | |
0e0f0835 MB |
666 | Since some operations can be automatically generated from others, there is |
667 | a minimal set of operations that need to be overloaded in order to have | |
668 | the complete set of overloaded operations at one's disposal. | |
669 | Of course, the autogenerated operations may not do exactly what the user | |
670 | expects. The minimal set is: | |
e7ea3e70 | 671 | |
0e0f0835 MB |
672 | + - * / % ** << >> x |
673 | <=> cmp | |
674 | & | ^ ~ | |
675 | atan2 cos sin exp log sqrt int | |
676 | "" 0+ bool | |
677 | ~~ | |
e7ea3e70 | 678 | |
0e0f0835 MB |
679 | Of the conversions, only one of string, boolean or numeric is |
680 | needed because each can be generated from either of the other two. | |
e7ea3e70 | 681 | |
0e0f0835 | 682 | =head2 Special Keys for C<use overload> |
e7ea3e70 | 683 | |
0e0f0835 | 684 | =head3 C<nomethod> |
e7ea3e70 | 685 | |
0e0f0835 MB |
686 | The C<'nomethod'> key is used to specify a catch-all function to |
687 | be called for any operator that is not individually overloaded. | |
688 | The specified function will be passed four parameters. | |
689 | The first three arguments coincide with those that would have been | |
690 | passed to the corresponding method if it had been defined. | |
691 | The fourth argument is the C<use overload> key for that missing | |
692 | method. | |
e7ea3e70 | 693 | |
0e0f0835 | 694 | For example, if C<$a> is an object blessed into a package declaring |
4633a7c4 | 695 | |
0e0f0835 | 696 | use overload 'nomethod' => 'catch_all', # ... |
4633a7c4 | 697 | |
0e0f0835 | 698 | then the operation |
4633a7c4 | 699 | |
0e0f0835 | 700 | 3 + $a |
4633a7c4 | 701 | |
0e0f0835 MB |
702 | could (unless a method is specifically declared for the key |
703 | C<'+'>) result in a call | |
4633a7c4 | 704 | |
0e0f0835 | 705 | catch_all($a, 3, 1, '+') |
4633a7c4 | 706 | |
0e0f0835 | 707 | See L<How Perl Chooses an Operator Implementation>. |
b267980d | 708 | |
0e0f0835 | 709 | =head3 C<fallback> |
b267980d | 710 | |
0e0f0835 MB |
711 | The value assigned to the key C<'fallback'> tells Perl how hard |
712 | it should try to find an alternative way to implement a missing | |
713 | operator. | |
4633a7c4 | 714 | |
0e0f0835 | 715 | =over |
b267980d | 716 | |
0e0f0835 | 717 | =item * defined, but FALSE |
4633a7c4 | 718 | |
0e0f0835 | 719 | use overload "fallback" => 0, # ... ; |
4633a7c4 | 720 | |
0e0f0835 | 721 | This disables L<Magic Autogeneration>. |
4633a7c4 LW |
722 | |
723 | =item * C<undef> | |
724 | ||
0e0f0835 MB |
725 | In the default case where no value is explicitly assigned to |
726 | C<fallback>, magic autogeneration is enabled. | |
4633a7c4 LW |
727 | |
728 | =item * TRUE | |
729 | ||
0e0f0835 MB |
730 | The same as for C<undef>, but if a missing operator cannot be |
731 | autogenerated then, instead of issuing an error message, Perl | |
732 | is allowed to revert to what it would have done for that | |
733 | operator if there had been no C<use overload> directive. | |
4633a7c4 | 734 | |
0e0f0835 MB |
735 | Note: in most cases, particularly the L<Copy Constructor>, |
736 | this is unlikely to be appropriate behaviour. | |
4633a7c4 LW |
737 | |
738 | =back | |
739 | ||
0e0f0835 | 740 | See L<How Perl Chooses an Operator Implementation>. |
4633a7c4 | 741 | |
0e0f0835 | 742 | =head3 Copy Constructor |
4633a7c4 | 743 | |
0e0f0835 MB |
744 | As mentioned L<above|"Mathemagic, Mutators, and Copy Constructors">, |
745 | this operation is called when a mutator is applied to a reference | |
746 | that shares its object with some other reference. | |
747 | For example, if C<$b> is mathemagical, and C<'++'> is overloaded | |
748 | with C<'incr'>, and C<'='> is overloaded with C<'clone'>, then the | |
749 | code | |
4633a7c4 | 750 | |
0e0f0835 MB |
751 | $a = $b; |
752 | # ... (other code which does not modify $a or $b) ... | |
753 | ++$b; | |
4633a7c4 | 754 | |
0e0f0835 | 755 | would be executed in a manner equivalent to |
4633a7c4 | 756 | |
0e0f0835 MB |
757 | $a = $b; |
758 | # ... | |
759 | $b = $b->clone(undef, ""); | |
760 | $b->incr(undef, ""); | |
4633a7c4 | 761 | |
0e0f0835 | 762 | Note: |
4633a7c4 | 763 | |
0e0f0835 | 764 | =over |
4633a7c4 | 765 | |
0e0f0835 | 766 | =item * |
4633a7c4 | 767 | |
0e0f0835 MB |
768 | The subroutine for C<'='> does not overload the Perl assignment |
769 | operator: it is used only to allow mutators to work as described | |
770 | here. (See L</Assignments> above.) | |
4633a7c4 | 771 | |
0e0f0835 | 772 | =item * |
4633a7c4 | 773 | |
0e0f0835 MB |
774 | As for other operations, the subroutine implementing '=' is passed |
775 | three arguments, though the last two are always C<undef> and C<''>. | |
4633a7c4 | 776 | |
0e0f0835 | 777 | =item * |
4633a7c4 | 778 | |
0e0f0835 MB |
779 | The copy constructor is called only before a call to a function |
780 | declared to implement a mutator, for example, if C<++$b;> in the | |
781 | code above is effected via a method declared for key C<'++'> | |
782 | (or 'nomethod', passed C<'++'> as the fourth argument) or, by | |
783 | autogeneration, C<'+='>. | |
784 | It is not called if the increment operation is effected by a call | |
785 | to the method for C<'+'> since, in the equivalent code, | |
4633a7c4 | 786 | |
0e0f0835 MB |
787 | $a = $b; |
788 | $b = $b + 1; | |
4633a7c4 | 789 | |
0e0f0835 MB |
790 | the data referred to by C<$a> is unchanged by the assignment to |
791 | C<$b> of a reference to new object data. | |
4633a7c4 | 792 | |
0e0f0835 | 793 | =item * |
ee239bfe | 794 | |
0e0f0835 MB |
795 | The copy constructor is not called if Perl determines that it is |
796 | unnecessary because there is no other reference to the data being | |
797 | modified. | |
4633a7c4 | 798 | |
0e0f0835 | 799 | =item * |
4633a7c4 | 800 | |
0e0f0835 MB |
801 | If C<'fallback'> is undefined or TRUE then a copy constructor |
802 | can be autogenerated, but only for objects based on scalars. | |
803 | In other cases it needs to be defined explicitly. | |
804 | Where an object's data is stored as, for example, an array of | |
805 | scalars, the following might be appropriate: | |
4633a7c4 | 806 | |
0e0f0835 | 807 | use overload '=' => sub { bless [ @{$_[0]} ] }, # ... |
4633a7c4 | 808 | |
0e0f0835 | 809 | =item * |
4633a7c4 | 810 | |
0e0f0835 MB |
811 | If C<'fallback'> is TRUE and no copy constructor is defined then, |
812 | for objects not based on scalars, Perl may silently fall back on | |
813 | simple assignment - that is, assignment of the object reference. | |
814 | In effect, this disables the copy constructor mechanism since | |
815 | no new copy of the object data is created. | |
816 | This is almost certainly not what you want. | |
817 | (It is, however, consistent: for example, Perl's fallback for the | |
818 | C<++> operator is to increment the reference itself.) | |
4633a7c4 | 819 | |
0e0f0835 | 820 | =back |
4633a7c4 | 821 | |
0e0f0835 | 822 | =head2 How Perl Chooses an Operator Implementation |
4633a7c4 | 823 | |
0e0f0835 MB |
824 | Which is checked first, C<nomethod> or C<fallback>? |
825 | If the two operands of an operator are of different types and | |
826 | both overload the operator, which implementation is used? | |
827 | The following are the precedence rules: | |
4633a7c4 | 828 | |
0e0f0835 | 829 | =over |
4633a7c4 | 830 | |
0e0f0835 | 831 | =item 1. |
4633a7c4 | 832 | |
0e0f0835 MB |
833 | If the first operand has declared a subroutine to overload the |
834 | operator then use that implementation. | |
4633a7c4 | 835 | |
0e0f0835 | 836 | =item 2. |
4633a7c4 | 837 | |
0e0f0835 MB |
838 | Otherwise, if fallback is TRUE or undefined for the |
839 | first operand then see if the | |
840 | L<rules for autogeneration|"Magic Autogeneration"> | |
841 | allows another of its operators to be used instead. | |
3bc6ec80 | 842 | |
0e0f0835 | 843 | =item 3. |
3bc6ec80 | 844 | |
0e0f0835 MB |
845 | Unless the operator is an assignment (C<+=>, C<-=>, etc.), |
846 | repeat step (1) in respect of the second operand. | |
4633a7c4 | 847 | |
0e0f0835 | 848 | =item 4. |
4633a7c4 | 849 | |
0e0f0835 | 850 | Repeat Step (2) in respect of the second operand. |
4633a7c4 | 851 | |
0e0f0835 | 852 | =item 5. |
1fef88e7 | 853 | |
0e0f0835 | 854 | If the first operand has a "nomethod" method then use that. |
4633a7c4 | 855 | |
0e0f0835 | 856 | =item 6. |
f5284f61 | 857 | |
0e0f0835 | 858 | If the second operand has a "nomethod" method then use that. |
f5284f61 | 859 | |
0e0f0835 | 860 | =item 7. |
f5284f61 | 861 | |
0e0f0835 MB |
862 | If C<fallback> is TRUE for both operands |
863 | then perform the usual operation for the operator, | |
864 | treating the operands as numbers, strings, or booleans | |
865 | as appropriate for the operator (see note). | |
f5284f61 | 866 | |
0e0f0835 | 867 | =item 8. |
4633a7c4 | 868 | |
0e0f0835 | 869 | Nothing worked - die. |
4633a7c4 LW |
870 | |
871 | =back | |
872 | ||
0e0f0835 MB |
873 | Where there is only one operand (or only one operand with |
874 | overloading) the checks in respect of the other operand above are | |
875 | skipped. | |
84fc275b | 876 | |
0e0f0835 MB |
877 | There are exceptions to the above rules for dereference operations |
878 | (which, if Step 1 fails, always fall back to the normal, built-in | |
879 | implementations - see Dereferencing), and for C<~~> (which has its | |
e846f1dc KW |
880 | own set of rules - see C<Matching> under L</Overloadable Operations> |
881 | above). | |
84fc275b | 882 | |
0e0f0835 MB |
883 | Note on Step 7: some operators have a different semantic depending |
884 | on the type of their operands. | |
885 | As there is no way to instruct Perl to treat the operands as, e.g., | |
886 | numbers instead of strings, the result here may not be what you | |
887 | expect. | |
888 | See L<BUGS AND PITFALLS>. | |
84fc275b | 889 | |
0e0f0835 | 890 | =head2 Losing Overloading |
4633a7c4 LW |
891 | |
892 | The restriction for the comparison operation is that even if, for example, | |
1f874cb6 | 893 | C<cmp> should return a blessed reference, the autogenerated C<lt> |
4633a7c4 | 894 | function will produce only a standard logical value based on the |
1f874cb6 | 895 | numerical value of the result of C<cmp>. In particular, a working |
4633a7c4 LW |
896 | numeric conversion is needed in this case (possibly expressed in terms of |
897 | other conversions). | |
898 | ||
899 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties | |
900 | if the string conversion substitution is applied. | |
901 | ||
902 | When you chop() a mathemagical object it is promoted to a string and its | |
903 | mathemagical properties are lost. The same can happen with other | |
904 | operations as well. | |
905 | ||
0e0f0835 MB |
906 | =head2 Inheritance and Overloading |
907 | ||
908 | Overloading respects inheritance via the @ISA hierarchy. | |
909 | Inheritance interacts with overloading in two ways. | |
910 | ||
911 | =over | |
912 | ||
913 | =item Method names in the C<use overload> directive | |
914 | ||
915 | If C<value> in | |
916 | ||
917 | use overload key => value; | |
918 | ||
919 | is a string, it is interpreted as a method name - which may | |
920 | (in the usual way) be inherited from another class. | |
921 | ||
922 | =item Overloading of an operation is inherited by derived classes | |
923 | ||
924 | Any class derived from an overloaded class is also overloaded | |
925 | and inherits its operator implementations. | |
926 | If the same operator is overloaded in more than one ancestor | |
927 | then the implementation is determined by the usual inheritance | |
928 | rules. | |
929 | ||
930 | For example, if C<A> inherits from C<B> and C<C> (in that order), | |
931 | C<B> overloads C<+> with C<\&D::plus_sub>, and C<C> overloads | |
932 | C<+> by C<"plus_meth">, then the subroutine C<D::plus_sub> will | |
933 | be called to implement operation C<+> for an object in package C<A>. | |
934 | ||
935 | =back | |
936 | ||
937 | Note that since the value of the C<fallback> key is not a subroutine, | |
938 | its inheritance is not governed by the above rules. In the current | |
939 | implementation, the value of C<fallback> in the first overloaded | |
940 | ancestor is used, but this is accidental and subject to change. | |
941 | ||
942 | =head2 Run-time Overloading | |
4633a7c4 LW |
943 | |
944 | Since all C<use> directives are executed at compile-time, the only way to | |
945 | change overloading during run-time is to | |
946 | ||
947 | eval 'use overload "+" => \&addmethod'; | |
948 | ||
949 | You can also use | |
950 | ||
951 | eval 'no overload "+", "--", "<="'; | |
952 | ||
953 | though the use of these constructs during run-time is questionable. | |
954 | ||
0e0f0835 | 955 | =head2 Public Functions |
4633a7c4 LW |
956 | |
957 | Package C<overload.pm> provides the following public functions: | |
958 | ||
959 | =over 5 | |
960 | ||
961 | =item overload::StrVal(arg) | |
962 | ||
6a0e9e72 FD |
963 | Gives string value of C<arg> as in absence of stringify overloading. If you |
964 | are using this to get the address of a reference (useful for checking if two | |
965 | references point to the same thing) then you may be better off using | |
966 | C<Scalar::Util::refaddr()>, which is faster. | |
4633a7c4 LW |
967 | |
968 | =item overload::Overloaded(arg) | |
969 | ||
970 | Returns true if C<arg> is subject to overloading of some operations. | |
971 | ||
972 | =item overload::Method(obj,op) | |
973 | ||
974 | Returns C<undef> or a reference to the method that implements C<op>. | |
975 | ||
976 | =back | |
977 | ||
0e0f0835 | 978 | =head2 Overloading Constants |
b3ac6de7 | 979 | |
7adf7a02 | 980 | For some applications, the Perl parser mangles constants too much. |
bfce84ec | 981 | It is possible to hook into this process via C<overload::constant()> |
7adf7a02 | 982 | and C<overload::remove_constant()> functions. |
b3ac6de7 IZ |
983 | |
984 | These functions take a hash as an argument. The recognized keys of this hash | |
7adf7a02 | 985 | are: |
b3ac6de7 IZ |
986 | |
987 | =over 8 | |
988 | ||
989 | =item integer | |
990 | ||
991 | to overload integer constants, | |
992 | ||
993 | =item float | |
994 | ||
995 | to overload floating point constants, | |
996 | ||
997 | =item binary | |
998 | ||
999 | to overload octal and hexadecimal constants, | |
1000 | ||
1001 | =item q | |
1002 | ||
1003 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted | |
1004 | strings and here-documents, | |
1005 | ||
1006 | =item qr | |
1007 | ||
1008 | to overload constant pieces of regular expressions. | |
1009 | ||
1010 | =back | |
1011 | ||
1012 | The corresponding values are references to functions which take three arguments: | |
1013 | the first one is the I<initial> string form of the constant, the second one | |
b267980d | 1014 | is how Perl interprets this constant, the third one is how the constant is used. |
b3ac6de7 | 1015 | Note that the initial string form does not |
b267980d | 1016 | contain string delimiters, and has backslashes in backslash-delimiter |
b3ac6de7 | 1017 | combinations stripped (thus the value of delimiter is not relevant for |
b267980d | 1018 | processing of this string). The return value of this function is how this |
b3ac6de7 IZ |
1019 | constant is going to be interpreted by Perl. The third argument is undefined |
1020 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote | |
1021 | context (comes from strings, regular expressions, and single-quote HERE | |
b267980d | 1022 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
b3ac6de7 IZ |
1023 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
1024 | ||
1025 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, | |
1026 | it is expected that overloaded constant strings are equipped with reasonable | |
b267980d | 1027 | overloaded catenation operator, otherwise absurd results will result. |
b3ac6de7 IZ |
1028 | Similarly, negative numbers are considered as negations of positive constants. |
1029 | ||
1030 | Note that it is probably meaningless to call the functions overload::constant() | |
1031 | and overload::remove_constant() from anywhere but import() and unimport() methods. | |
1032 | From these methods they may be called as | |
1033 | ||
1034 | sub import { | |
1035 | shift; | |
1036 | return unless @_; | |
1037 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; | |
1038 | overload::constant integer => sub {Math::BigInt->new(shift)}; | |
1039 | } | |
1040 | ||
4633a7c4 LW |
1041 | =head1 IMPLEMENTATION |
1042 | ||
1043 | What follows is subject to change RSN. | |
1044 | ||
e7ea3e70 IZ |
1045 | The table of methods for all operations is cached in magic for the |
1046 | symbol table hash for the package. The cache is invalidated during | |
1047 | processing of C<use overload>, C<no overload>, new function | |
1048 | definitions, and changes in @ISA. However, this invalidation remains | |
1049 | unprocessed until the next C<bless>ing into the package. Hence if you | |
1050 | want to change overloading structure dynamically, you'll need an | |
1051 | additional (fake) C<bless>ing to update the table. | |
1052 | ||
1053 | (Every SVish thing has a magic queue, and magic is an entry in that | |
1054 | queue. This is how a single variable may participate in multiple | |
1055 | forms of magic simultaneously. For instance, environment variables | |
1056 | regularly have two forms at once: their %ENV magic and their taint | |
1057 | magic. However, the magic which implements overloading is applied to | |
1058 | the stashes, which are rarely used directly, thus should not slow down | |
1059 | Perl.) | |
4633a7c4 LW |
1060 | |
1061 | If an object belongs to a package using overload, it carries a special | |
1062 | flag. Thus the only speed penalty during arithmetic operations without | |
1063 | overloading is the checking of this flag. | |
1064 | ||
774d564b | 1065 | In fact, if C<use overload> is not present, there is almost no overhead |
1066 | for overloadable operations, so most programs should not suffer | |
1067 | measurable performance penalties. A considerable effort was made to | |
1068 | minimize the overhead when overload is used in some package, but the | |
1069 | arguments in question do not belong to packages using overload. When | |
1070 | in doubt, test your speed with C<use overload> and without it. So far | |
1071 | there have been no reports of substantial speed degradation if Perl is | |
1072 | compiled with optimization turned on. | |
4633a7c4 | 1073 | |
e7ea3e70 IZ |
1074 | There is no size penalty for data if overload is not used. The only |
1075 | size penalty if overload is used in some package is that I<all> the | |
1076 | packages acquire a magic during the next C<bless>ing into the | |
1077 | package. This magic is three-words-long for packages without | |
f610777f | 1078 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 | 1079 | |
4633a7c4 LW |
1080 | It is expected that arguments to methods that are not explicitly supposed |
1081 | to be changed are constant (but this is not enforced). | |
1082 | ||
0e0f0835 | 1083 | =head1 COOKBOOK |
ee239bfe IZ |
1084 | |
1085 | Please add examples to what follows! | |
1086 | ||
0e0f0835 | 1087 | =head2 Two-face Scalars |
ee239bfe IZ |
1088 | |
1089 | Put this in F<two_face.pm> in your Perl library directory: | |
1090 | ||
1091 | package two_face; # Scalars with separate string and | |
1092 | # numeric values. | |
1093 | sub new { my $p = shift; bless [@_], $p } | |
1094 | use overload '""' => \&str, '0+' => \&num, fallback => 1; | |
1095 | sub num {shift->[1]} | |
1096 | sub str {shift->[0]} | |
1097 | ||
1098 | Use it as follows: | |
1099 | ||
1100 | require two_face; | |
2b393bf4 | 1101 | my $seven = two_face->new("vii", 7); |
ee239bfe | 1102 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
1f874cb6 | 1103 | print "seven contains 'i'\n" if $seven =~ /i/; |
ee239bfe IZ |
1104 | |
1105 | (The second line creates a scalar which has both a string value, and a | |
1106 | numeric value.) This prints: | |
1107 | ||
1108 | seven=vii, seven=7, eight=8 | |
1f874cb6 | 1109 | seven contains 'i' |
ee239bfe | 1110 | |
0e0f0835 | 1111 | =head2 Two-face References |
f5284f61 IZ |
1112 | |
1113 | Suppose you want to create an object which is accessible as both an | |
6d822dc4 | 1114 | array reference and a hash reference. |
f5284f61 IZ |
1115 | |
1116 | package two_refs; | |
1117 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; | |
b267980d NIS |
1118 | sub new { |
1119 | my $p = shift; | |
f5284f61 IZ |
1120 | bless \ [@_], $p; |
1121 | } | |
1122 | sub gethash { | |
1123 | my %h; | |
1124 | my $self = shift; | |
1125 | tie %h, ref $self, $self; | |
1126 | \%h; | |
1127 | } | |
1128 | ||
1129 | sub TIEHASH { my $p = shift; bless \ shift, $p } | |
1130 | my %fields; | |
1131 | my $i = 0; | |
1132 | $fields{$_} = $i++ foreach qw{zero one two three}; | |
b267980d | 1133 | sub STORE { |
f5284f61 IZ |
1134 | my $self = ${shift()}; |
1135 | my $key = $fields{shift()}; | |
1136 | defined $key or die "Out of band access"; | |
1137 | $$self->[$key] = shift; | |
1138 | } | |
b267980d | 1139 | sub FETCH { |
f5284f61 IZ |
1140 | my $self = ${shift()}; |
1141 | my $key = $fields{shift()}; | |
1142 | defined $key or die "Out of band access"; | |
1143 | $$self->[$key]; | |
1144 | } | |
1145 | ||
1146 | Now one can access an object using both the array and hash syntax: | |
1147 | ||
2b393bf4 | 1148 | my $bar = two_refs->new(3,4,5,6); |
f5284f61 IZ |
1149 | $bar->[2] = 11; |
1150 | $bar->{two} == 11 or die 'bad hash fetch'; | |
1151 | ||
1152 | Note several important features of this example. First of all, the | |
1153 | I<actual> type of $bar is a scalar reference, and we do not overload | |
1154 | the scalar dereference. Thus we can get the I<actual> non-overloaded | |
1155 | contents of $bar by just using C<$$bar> (what we do in functions which | |
1156 | overload dereference). Similarly, the object returned by the | |
1157 | TIEHASH() method is a scalar reference. | |
1158 | ||
1159 | Second, we create a new tied hash each time the hash syntax is used. | |
1160 | This allows us not to worry about a possibility of a reference loop, | |
d1be9408 | 1161 | which would lead to a memory leak. |
f5284f61 IZ |
1162 | |
1163 | Both these problems can be cured. Say, if we want to overload hash | |
1164 | dereference on a reference to an object which is I<implemented> as a | |
1165 | hash itself, the only problem one has to circumvent is how to access | |
1fd16925 | 1166 | this I<actual> hash (as opposed to the I<virtual> hash exhibited by the |
f5284f61 IZ |
1167 | overloaded dereference operator). Here is one possible fetching routine: |
1168 | ||
1169 | sub access_hash { | |
1170 | my ($self, $key) = (shift, shift); | |
1171 | my $class = ref $self; | |
b267980d | 1172 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
f5284f61 IZ |
1173 | my $out = $self->{$key}; |
1174 | bless $self, $class; # Restore overloading | |
1175 | $out; | |
1176 | } | |
1177 | ||
1fd16925 | 1178 | To remove creation of the tied hash on each access, one may an extra |
f5284f61 IZ |
1179 | level of indirection which allows a non-circular structure of references: |
1180 | ||
1181 | package two_refs1; | |
1182 | use overload '%{}' => sub { ${shift()}->[1] }, | |
1183 | '@{}' => sub { ${shift()}->[0] }; | |
b267980d NIS |
1184 | sub new { |
1185 | my $p = shift; | |
f5284f61 IZ |
1186 | my $a = [@_]; |
1187 | my %h; | |
1188 | tie %h, $p, $a; | |
1189 | bless \ [$a, \%h], $p; | |
1190 | } | |
1191 | sub gethash { | |
1192 | my %h; | |
1193 | my $self = shift; | |
1194 | tie %h, ref $self, $self; | |
1195 | \%h; | |
1196 | } | |
1197 | ||
1198 | sub TIEHASH { my $p = shift; bless \ shift, $p } | |
1199 | my %fields; | |
1200 | my $i = 0; | |
1201 | $fields{$_} = $i++ foreach qw{zero one two three}; | |
b267980d | 1202 | sub STORE { |
f5284f61 IZ |
1203 | my $a = ${shift()}; |
1204 | my $key = $fields{shift()}; | |
1205 | defined $key or die "Out of band access"; | |
1206 | $a->[$key] = shift; | |
1207 | } | |
b267980d | 1208 | sub FETCH { |
f5284f61 IZ |
1209 | my $a = ${shift()}; |
1210 | my $key = $fields{shift()}; | |
1211 | defined $key or die "Out of band access"; | |
1212 | $a->[$key]; | |
1213 | } | |
1214 | ||
1fd16925 | 1215 | Now if $baz is overloaded like this, then C<$baz> is a reference to a |
f5284f61 IZ |
1216 | reference to the intermediate array, which keeps a reference to an |
1217 | actual array, and the access hash. The tie()ing object for the access | |
1fd16925 | 1218 | hash is a reference to a reference to the actual array, so |
f5284f61 | 1219 | |
88c28ceb | 1220 | =over |
f5284f61 IZ |
1221 | |
1222 | =item * | |
1223 | ||
1224 | There are no loops of references. | |
1225 | ||
1226 | =item * | |
1227 | ||
1228 | Both "objects" which are blessed into the class C<two_refs1> are | |
1229 | references to a reference to an array, thus references to a I<scalar>. | |
1230 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no | |
1231 | overloaded operations. | |
1232 | ||
1233 | =back | |
1234 | ||
0e0f0835 | 1235 | =head2 Symbolic Calculator |
ee239bfe IZ |
1236 | |
1237 | Put this in F<symbolic.pm> in your Perl library directory: | |
1238 | ||
1239 | package symbolic; # Primitive symbolic calculator | |
1240 | use overload nomethod => \&wrap; | |
1241 | ||
1242 | sub new { shift; bless ['n', @_] } | |
1243 | sub wrap { | |
1244 | my ($obj, $other, $inv, $meth) = @_; | |
1245 | ($obj, $other) = ($other, $obj) if $inv; | |
1246 | bless [$meth, $obj, $other]; | |
1247 | } | |
1248 | ||
1249 | This module is very unusual as overloaded modules go: it does not | |
0e0f0835 MB |
1250 | provide any usual overloaded operators, instead it provides an |
1251 | implementation for L<C<nomethod>>. In this example the C<nomethod> | |
f610777f | 1252 | subroutine returns an object which encapsulates operations done over |
2b393bf4 RS |
1253 | the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 + |
1254 | symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>. | |
ee239bfe IZ |
1255 | |
1256 | Here is an example of the script which "calculates" the side of | |
1257 | circumscribed octagon using the above package: | |
1258 | ||
1259 | require symbolic; | |
1260 | my $iter = 1; # 2**($iter+2) = 8 | |
2b393bf4 | 1261 | my $side = symbolic->new(1); |
ee239bfe | 1262 | my $cnt = $iter; |
3cb6de81 | 1263 | |
ee239bfe IZ |
1264 | while ($cnt--) { |
1265 | $side = (sqrt(1 + $side**2) - 1)/$side; | |
1266 | } | |
1267 | print "OK\n"; | |
1268 | ||
1269 | The value of $side is | |
1270 | ||
1271 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], | |
1272 | undef], 1], ['n', 1]] | |
1273 | ||
1274 | Note that while we obtained this value using a nice little script, | |
1275 | there is no simple way to I<use> this value. In fact this value may | |
2d3232d7 | 1276 | be inspected in debugger (see L<perldebug>), but only if |
ee239bfe IZ |
1277 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1278 | ||
1279 | If one attempts to print this value, then the overloaded operator | |
1280 | C<""> will be called, which will call C<nomethod> operator. The | |
1281 | result of this operator will be stringified again, but this result is | |
1282 | again of type C<symbolic>, which will lead to an infinite loop. | |
1283 | ||
1284 | Add a pretty-printer method to the module F<symbolic.pm>: | |
1285 | ||
1286 | sub pretty { | |
1287 | my ($meth, $a, $b) = @{+shift}; | |
1288 | $a = 'u' unless defined $a; | |
1289 | $b = 'u' unless defined $b; | |
1290 | $a = $a->pretty if ref $a; | |
1291 | $b = $b->pretty if ref $b; | |
1292 | "[$meth $a $b]"; | |
b267980d | 1293 | } |
ee239bfe IZ |
1294 | |
1295 | Now one can finish the script by | |
1296 | ||
1297 | print "side = ", $side->pretty, "\n"; | |
1298 | ||
1299 | The method C<pretty> is doing object-to-string conversion, so it | |
1300 | is natural to overload the operator C<""> using this method. However, | |
1301 | inside such a method it is not necessary to pretty-print the | |
1302 | I<components> $a and $b of an object. In the above subroutine | |
1303 | C<"[$meth $a $b]"> is a catenation of some strings and components $a | |
1304 | and $b. If these components use overloading, the catenation operator | |
1fd16925 | 1305 | will look for an overloaded operator C<.>; if not present, it will |
ee239bfe IZ |
1306 | look for an overloaded operator C<"">. Thus it is enough to use |
1307 | ||
1308 | use overload nomethod => \&wrap, '""' => \&str; | |
1309 | sub str { | |
1310 | my ($meth, $a, $b) = @{+shift}; | |
1311 | $a = 'u' unless defined $a; | |
1312 | $b = 'u' unless defined $b; | |
1313 | "[$meth $a $b]"; | |
b267980d | 1314 | } |
ee239bfe IZ |
1315 | |
1316 | Now one can change the last line of the script to | |
1317 | ||
1318 | print "side = $side\n"; | |
1319 | ||
1320 | which outputs | |
1321 | ||
1322 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] | |
1323 | ||
1324 | and one can inspect the value in debugger using all the possible | |
b267980d | 1325 | methods. |
ee239bfe | 1326 | |
d1be9408 | 1327 | Something is still amiss: consider the loop variable $cnt of the |
ee239bfe IZ |
1328 | script. It was a number, not an object. We cannot make this value of |
1329 | type C<symbolic>, since then the loop will not terminate. | |
1330 | ||
1331 | Indeed, to terminate the cycle, the $cnt should become false. | |
1332 | However, the operator C<bool> for checking falsity is overloaded (this | |
1333 | time via overloaded C<"">), and returns a long string, thus any object | |
1334 | of type C<symbolic> is true. To overcome this, we need a way to | |
1335 | compare an object to 0. In fact, it is easier to write a numeric | |
1336 | conversion routine. | |
1337 | ||
1338 | Here is the text of F<symbolic.pm> with such a routine added (and | |
f610777f | 1339 | slightly modified str()): |
ee239bfe IZ |
1340 | |
1341 | package symbolic; # Primitive symbolic calculator | |
1342 | use overload | |
1343 | nomethod => \&wrap, '""' => \&str, '0+' => \# | |
1344 | ||
1345 | sub new { shift; bless ['n', @_] } | |
1346 | sub wrap { | |
1347 | my ($obj, $other, $inv, $meth) = @_; | |
1348 | ($obj, $other) = ($other, $obj) if $inv; | |
1349 | bless [$meth, $obj, $other]; | |
1350 | } | |
1351 | sub str { | |
1352 | my ($meth, $a, $b) = @{+shift}; | |
1353 | $a = 'u' unless defined $a; | |
1354 | if (defined $b) { | |
1355 | "[$meth $a $b]"; | |
1356 | } else { | |
1357 | "[$meth $a]"; | |
1358 | } | |
b267980d NIS |
1359 | } |
1360 | my %subr = ( n => sub {$_[0]}, | |
1361 | sqrt => sub {sqrt $_[0]}, | |
ee239bfe IZ |
1362 | '-' => sub {shift() - shift()}, |
1363 | '+' => sub {shift() + shift()}, | |
1364 | '/' => sub {shift() / shift()}, | |
1365 | '*' => sub {shift() * shift()}, | |
1366 | '**' => sub {shift() ** shift()}, | |
1367 | ); | |
1368 | sub num { | |
1369 | my ($meth, $a, $b) = @{+shift}; | |
b267980d | 1370 | my $subr = $subr{$meth} |
ee239bfe IZ |
1371 | or die "Do not know how to ($meth) in symbolic"; |
1372 | $a = $a->num if ref $a eq __PACKAGE__; | |
1373 | $b = $b->num if ref $b eq __PACKAGE__; | |
1374 | $subr->($a,$b); | |
1375 | } | |
1376 | ||
1377 | All the work of numeric conversion is done in %subr and num(). Of | |
f610777f | 1378 | course, %subr is not complete, it contains only operators used in the |
ee239bfe IZ |
1379 | example below. Here is the extra-credit question: why do we need an |
1380 | explicit recursion in num()? (Answer is at the end of this section.) | |
1381 | ||
1382 | Use this module like this: | |
1383 | ||
1384 | require symbolic; | |
2b393bf4 RS |
1385 | my $iter = symbolic->new(2); # 16-gon |
1386 | my $side = symbolic->new(1); | |
ee239bfe | 1387 | my $cnt = $iter; |
3cb6de81 | 1388 | |
ee239bfe | 1389 | while ($cnt) { |
1f874cb6 | 1390 | $cnt = $cnt - 1; # Mutator '--' not implemented |
ee239bfe IZ |
1391 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1392 | } | |
1393 | printf "%s=%f\n", $side, $side; | |
1394 | printf "pi=%f\n", $side*(2**($iter+2)); | |
1395 | ||
1396 | It prints (without so many line breaks) | |
1397 | ||
1398 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] | |
1399 | [n 1]] 2]]] 1] | |
1400 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 | |
1401 | pi=3.182598 | |
1402 | ||
1403 | The above module is very primitive. It does not implement | |
1404 | mutator methods (C<++>, C<-=> and so on), does not do deep copying | |
1405 | (not required without mutators!), and implements only those arithmetic | |
1406 | operations which are used in the example. | |
1407 | ||
1fd16925 | 1408 | To implement most arithmetic operations is easy; one should just use |
ee239bfe IZ |
1409 | the tables of operations, and change the code which fills %subr to |
1410 | ||
1411 | my %subr = ( 'n' => sub {$_[0]} ); | |
1412 | foreach my $op (split " ", $overload::ops{with_assign}) { | |
1413 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; | |
1414 | } | |
1415 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); | |
1416 | foreach my $op (split " ", "@overload::ops{ @bins }") { | |
1417 | $subr{$op} = eval "sub {shift() $op shift()}"; | |
1418 | } | |
1419 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { | |
1f874cb6 | 1420 | print "defining '$op'\n"; |
ee239bfe IZ |
1421 | $subr{$op} = eval "sub {$op shift()}"; |
1422 | } | |
1423 | ||
0e0f0835 MB |
1424 | Since subroutines implementing assignment operators are not required |
1425 | to modify their operands (see L<Overloadable Operations> above), | |
1426 | we do not need anything special to make C<+=> and friends work, | |
1427 | besides adding these operators to %subr and defining a copy | |
1428 | constructor (needed since Perl has no way to know that the | |
1429 | implementation of C<'+='> does not mutate the argument - | |
1430 | see L<Copy Constructor>). | |
ee239bfe | 1431 | |
1fd16925 | 1432 | To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload> |
ee239bfe IZ |
1433 | line, and code (this code assumes that mutators change things one level |
1434 | deep only, so recursive copying is not needed): | |
1435 | ||
1436 | sub cpy { | |
1437 | my $self = shift; | |
1438 | bless [@$self], ref $self; | |
1439 | } | |
1440 | ||
b267980d | 1441 | To make C<++> and C<--> work, we need to implement actual mutators, |
ee239bfe IZ |
1442 | either directly, or in C<nomethod>. We continue to do things inside |
1443 | C<nomethod>, thus add | |
1444 | ||
1445 | if ($meth eq '++' or $meth eq '--') { | |
1446 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference | |
1447 | return $obj; | |
1448 | } | |
1449 | ||
b267980d | 1450 | after the first line of wrap(). This is not a most effective |
ee239bfe IZ |
1451 | implementation, one may consider |
1452 | ||
1453 | sub inc { $_[0] = bless ['++', shift, 1]; } | |
1454 | ||
1455 | instead. | |
1456 | ||
1457 | As a final remark, note that one can fill %subr by | |
1458 | ||
1459 | my %subr = ( 'n' => sub {$_[0]} ); | |
1460 | foreach my $op (split " ", $overload::ops{with_assign}) { | |
1461 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; | |
1462 | } | |
1463 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); | |
1464 | foreach my $op (split " ", "@overload::ops{ @bins }") { | |
1465 | $subr{$op} = eval "sub {shift() $op shift()}"; | |
1466 | } | |
1467 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { | |
1468 | $subr{$op} = eval "sub {$op shift()}"; | |
1469 | } | |
1470 | $subr{'++'} = $subr{'+'}; | |
1471 | $subr{'--'} = $subr{'-'}; | |
1472 | ||
b267980d NIS |
1473 | This finishes implementation of a primitive symbolic calculator in |
1474 | 50 lines of Perl code. Since the numeric values of subexpressions | |
ee239bfe IZ |
1475 | are not cached, the calculator is very slow. |
1476 | ||
1477 | Here is the answer for the exercise: In the case of str(), we need no | |
1478 | explicit recursion since the overloaded C<.>-operator will fall back | |
1479 | to an existing overloaded operator C<"">. Overloaded arithmetic | |
1480 | operators I<do not> fall back to numeric conversion if C<fallback> is | |
1481 | not explicitly requested. Thus without an explicit recursion num() | |
1482 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild | |
1483 | the argument of num(). | |
1484 | ||
1485 | If you wonder why defaults for conversion are different for str() and | |
1486 | num(), note how easy it was to write the symbolic calculator. This | |
1487 | simplicity is due to an appropriate choice of defaults. One extra | |
f610777f A |
1488 | note: due to the explicit recursion num() is more fragile than sym(): |
1489 | we need to explicitly check for the type of $a and $b. If components | |
ee239bfe IZ |
1490 | $a and $b happen to be of some related type, this may lead to problems. |
1491 | ||
0e0f0835 | 1492 | =head2 I<Really> Symbolic Calculator |
ee239bfe IZ |
1493 | |
1494 | One may wonder why we call the above calculator symbolic. The reason | |
1495 | is that the actual calculation of the value of expression is postponed | |
1496 | until the value is I<used>. | |
1497 | ||
1498 | To see it in action, add a method | |
1499 | ||
b267980d NIS |
1500 | sub STORE { |
1501 | my $obj = shift; | |
1502 | $#$obj = 1; | |
ee239bfe IZ |
1503 | @$obj->[0,1] = ('=', shift); |
1504 | } | |
1505 | ||
1506 | to the package C<symbolic>. After this change one can do | |
1507 | ||
2b393bf4 RS |
1508 | my $a = symbolic->new(3); |
1509 | my $b = symbolic->new(4); | |
ee239bfe IZ |
1510 | my $c = sqrt($a**2 + $b**2); |
1511 | ||
1512 | and the numeric value of $c becomes 5. However, after calling | |
1513 | ||
1514 | $a->STORE(12); $b->STORE(5); | |
1515 | ||
1516 | the numeric value of $c becomes 13. There is no doubt now that the module | |
1517 | symbolic provides a I<symbolic> calculator indeed. | |
1518 | ||
1519 | To hide the rough edges under the hood, provide a tie()d interface to the | |
0e0f0835 | 1520 | package C<symbolic>. Add methods |
ee239bfe IZ |
1521 | |
1522 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } | |
1523 | sub FETCH { shift } | |
1524 | sub nop { } # Around a bug | |
1525 | ||
cfa5b373 FC |
1526 | (the bug, fixed in Perl 5.14, is described in L<"BUGS">). One can use this |
1527 | new interface as | |
ee239bfe IZ |
1528 | |
1529 | tie $a, 'symbolic', 3; | |
1530 | tie $b, 'symbolic', 4; | |
1531 | $a->nop; $b->nop; # Around a bug | |
1532 | ||
1533 | my $c = sqrt($a**2 + $b**2); | |
1534 | ||
1535 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value | |
1536 | of $c becomes 13. To insulate the user of the module add a method | |
1537 | ||
1538 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } | |
1539 | ||
1540 | Now | |
1541 | ||
1542 | my ($a, $b); | |
1543 | symbolic->vars($a, $b); | |
1544 | my $c = sqrt($a**2 + $b**2); | |
1545 | ||
1546 | $a = 3; $b = 4; | |
1547 | printf "c5 %s=%f\n", $c, $c; | |
1548 | ||
1549 | $a = 12; $b = 5; | |
1550 | printf "c13 %s=%f\n", $c, $c; | |
1551 | ||
1552 | shows that the numeric value of $c follows changes to the values of $a | |
1553 | and $b. | |
1554 | ||
4633a7c4 LW |
1555 | =head1 AUTHOR |
1556 | ||
1fef88e7 | 1557 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 | 1558 | |
7e494759 PF |
1559 | =head1 SEE ALSO |
1560 | ||
0e0f0835 MB |
1561 | The C<overloading> pragma can be used to enable or disable overloaded |
1562 | operations within a lexical scope - see L<overloading>. | |
7e494759 | 1563 | |
4633a7c4 LW |
1564 | =head1 DIAGNOSTICS |
1565 | ||
1566 | When Perl is run with the B<-Do> switch or its equivalent, overloading | |
1567 | induces diagnostic messages. | |
1568 | ||
e7ea3e70 IZ |
1569 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1570 | deduce which operations are overloaded (and which ancestor triggers | |
1571 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> | |
1572 | is shown by debugger. The method C<()> corresponds to the C<fallback> | |
1573 | key (in fact a presence of this method shows that this package has | |
1574 | overloading enabled, and it is what is used by the C<Overloaded> | |
ee239bfe | 1575 | function of module C<overload>). |
e7ea3e70 | 1576 | |
6ad11d81 | 1577 | The module might issue the following warnings: |
6b82e2f5 A |
1578 | |
1579 | =over 4 | |
1580 | ||
1581 | =item Odd number of arguments for overload::constant | |
1582 | ||
1583 | (W) The call to overload::constant contained an odd number of arguments. | |
1584 | The arguments should come in pairs. | |
1585 | ||
1f874cb6 | 1586 | =item '%s' is not an overloadable type |
6b82e2f5 A |
1587 | |
1588 | (W) You tried to overload a constant type the overload package is unaware of. | |
1589 | ||
1f874cb6 | 1590 | =item '%s' is not a code reference |
6b82e2f5 A |
1591 | |
1592 | (W) The second (fourth, sixth, ...) argument of overload::constant needs | |
1593 | to be a code reference. Either an anonymous subroutine, or a reference | |
1594 | to a subroutine. | |
1595 | ||
1596 | =back | |
1597 | ||
0e0f0835 MB |
1598 | =head1 BUGS AND PITFALLS |
1599 | ||
1600 | =over | |
1601 | ||
1602 | =item * | |
1603 | ||
1604 | No warning is issued for invalid C<use overload> keys. | |
1605 | Such errors are not always obvious: | |
1606 | ||
1607 | use overload "+0" => sub { ...; }, # should be "0+" | |
1608 | "not" => sub { ...; }; # should be "!" | |
1609 | ||
1610 | (Bug #74098) | |
1611 | ||
1612 | =item * | |
1613 | ||
1614 | A pitfall when fallback is TRUE and Perl resorts to a built-in | |
1615 | implementation of an operator is that some operators have more | |
1616 | than one semantic, for example C<|>: | |
1617 | ||
1618 | use overload '0+' => sub { $_[0]->{n}; }, | |
1619 | fallback => 1; | |
1620 | my $x = bless { n => 4 }, "main"; | |
1621 | my $y = bless { n => 8 }, "main"; | |
1622 | print $x | $y, "\n"; | |
1623 | ||
1624 | You might expect this to output "12". | |
1625 | In fact, it prints "<": the ASCII result of treating "|" | |
1626 | as a bitwise string operator - that is, the result of treating | |
1627 | the operands as the strings "4" and "8" rather than numbers. | |
1628 | The fact that numify (C<0+>) is implemented but stringify | |
1629 | (C<"">) isn't makes no difference since the latter is simply | |
1630 | autogenerated from the former. | |
4633a7c4 | 1631 | |
0e0f0835 MB |
1632 | The only way to change this is to provide your own subroutine |
1633 | for C<'|'>. | |
1634 | ||
1635 | =item * | |
1636 | ||
1637 | Magic autogeneration increases the potential for inadvertently | |
1638 | creating self-referential structures. | |
1639 | Currently Perl will not free self-referential | |
1640 | structures until cycles are explicitly broken. | |
1641 | For example, | |
1642 | ||
1643 | use overload '+' => 'add'; | |
1644 | sub add { bless [ \$_[0], \$_[1] ] }; | |
1645 | ||
1646 | is asking for trouble, since | |
1647 | ||
1648 | $obj += $y; | |
1649 | ||
1650 | will effectively become | |
1651 | ||
1652 | $obj = add($obj, $y, undef); | |
1653 | ||
1654 | with the same result as | |
1655 | ||
1656 | $obj = [\$obj, \$foo]; | |
1657 | ||
1658 | Even if no I<explicit> assignment-variants of operators are present in | |
1659 | the script, they may be generated by the optimizer. | |
1660 | For example, | |
1661 | ||
1662 | "obj = $obj\n" | |
1663 | ||
1664 | may be optimized to | |
1665 | ||
1666 | my $tmp = 'obj = ' . $obj; $tmp .= "\n"; | |
1667 | ||
1668 | =item * | |
1669 | ||
1670 | Because it is used for overloading, the per-package hash | |
1671 | C<%OVERLOAD> now has a special meaning in Perl. | |
1672 | The symbol table is filled with names looking like line-noise. | |
1673 | ||
1674 | =item * | |
4633a7c4 | 1675 | |
a6006777 | 1676 | For the purpose of inheritance every overloaded package behaves as if |
1677 | C<fallback> is present (possibly undefined). This may create | |
1678 | interesting effects if some package is not overloaded, but inherits | |
1679 | from two overloaded packages. | |
4633a7c4 | 1680 | |
0e0f0835 MB |
1681 | =item * |
1682 | ||
cfa5b373 FC |
1683 | Before Perl 5.14, the relation between overloading and tie()ing was broken. |
1684 | Overloading is triggered or not basing on the I<previous> class of the | |
1685 | tie()d variable. | |
dc04e1e9 | 1686 | |
cfa5b373 FC |
1687 | This happened because the presence of overloading was checked |
1688 | too early, before any tie()d access was attempted. If the | |
1689 | class of the value FETCH()ed from the tied variable does not | |
1690 | change, a simple workaround for code that is to run on older Perl | |
1691 | versions is to access the value (via C<() = $foo> or some such) | |
dc04e1e9 FC |
1692 | immediately after tie()ing, so that after this call the I<previous> class |
1693 | coincides with the current one. | |
1694 | ||
dc04e1e9 FC |
1695 | =item * |
1696 | ||
b3ac6de7 IZ |
1697 | Barewords are not covered by overloaded string constants. |
1698 | ||
0e0f0835 | 1699 | =back |
4633a7c4 LW |
1700 | |
1701 | =cut | |
1702 |