Commit | Line | Data |
---|---|---|
4633a7c4 LW |
1 | package overload; |
2 | ||
479c6c1f | 3 | our $VERSION = '1.12'; |
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 | $sub = \&nil; | |
a6006777 | 22 | } |
23 | #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; | |
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]}) { | |
4498a751 | 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 $_ [1]; |
4498a751 | 163 | warnings::warn ("`$_[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 ); |
4633a7c4 LW |
203 | $b=5+$a; |
204 | ... | |
205 | if (overload::Overloaded $b) {...} | |
206 | ... | |
207 | $strval = overload::StrVal $b; | |
208 | ||
4633a7c4 LW |
209 | =head1 DESCRIPTION |
210 | ||
caf82a01 YST |
211 | This pragma allows overloading of Perl's operators for a class. |
212 | To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead. | |
213 | ||
4633a7c4 LW |
214 | =head2 Declaration of overloaded functions |
215 | ||
216 | The compilation directive | |
217 | ||
218 | package Number; | |
219 | use overload | |
b267980d | 220 | "+" => \&add, |
4633a7c4 LW |
221 | "*=" => "muas"; |
222 | ||
223 | declares function Number::add() for addition, and method muas() in | |
224 | the "class" C<Number> (or one of its base classes) | |
b267980d | 225 | for the assignment form C<*=> of multiplication. |
4633a7c4 LW |
226 | |
227 | Arguments of this directive come in (key, value) pairs. Legal values | |
e7ea3e70 IZ |
228 | are values legal inside a C<&{ ... }> call, so the name of a |
229 | subroutine, a reference to a subroutine, or an anonymous subroutine | |
230 | will all work. Note that values specified as strings are | |
231 | interpreted as methods, not subroutines. Legal keys are listed below. | |
4633a7c4 LW |
232 | |
233 | The subroutine C<add> will be called to execute C<$a+$b> if $a | |
234 | is a reference to an object blessed into the package C<Number>, or if $a is | |
235 | not an object from a package with defined mathemagic addition, but $b is a | |
236 | reference to a C<Number>. It can also be called in other situations, like | |
237 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical | |
238 | methods refer to methods triggered by an overloaded mathematical | |
239 | operator.) | |
240 | ||
774d564b | 241 | Since overloading respects inheritance via the @ISA hierarchy, the |
242 | above declaration would also trigger overloading of C<+> and C<*=> in | |
243 | all the packages which inherit from C<Number>. | |
e7ea3e70 | 244 | |
4633a7c4 LW |
245 | =head2 Calling Conventions for Binary Operations |
246 | ||
247 | The functions specified in the C<use overload ...> directive are called | |
248 | with three (in one particular case with four, see L<Last Resort>) | |
249 | arguments. If the corresponding operation is binary, then the first | |
250 | two arguments are the two arguments of the operation. However, due to | |
251 | general object calling conventions, the first argument should always be | |
252 | an object in the package, so in the situation of C<7+$a>, the | |
253 | order of the arguments is interchanged. It probably does not matter | |
254 | when implementing the addition method, but whether the arguments | |
255 | are reversed is vital to the subtraction method. The method can | |
256 | query this information by examining the third argument, which can take | |
257 | three different values: | |
258 | ||
259 | =over 7 | |
260 | ||
261 | =item FALSE | |
262 | ||
263 | the order of arguments is as in the current operation. | |
264 | ||
265 | =item TRUE | |
266 | ||
267 | the arguments are reversed. | |
268 | ||
269 | =item C<undef> | |
270 | ||
271 | the current operation is an assignment variant (as in | |
272 | C<$a+=7>), but the usual function is called instead. This additional | |
ee239bfe IZ |
273 | information can be used to generate some optimizations. Compare |
274 | L<Calling Conventions for Mutators>. | |
4633a7c4 LW |
275 | |
276 | =back | |
277 | ||
278 | =head2 Calling Conventions for Unary Operations | |
279 | ||
280 | Unary operation are considered binary operations with the second | |
281 | argument being C<undef>. Thus the functions that overloads C<{"++"}> | |
282 | is called with arguments C<($a,undef,'')> when $a++ is executed. | |
283 | ||
ee239bfe IZ |
284 | =head2 Calling Conventions for Mutators |
285 | ||
286 | Two types of mutators have different calling conventions: | |
287 | ||
88c28ceb | 288 | =over |
ee239bfe IZ |
289 | |
290 | =item C<++> and C<--> | |
291 | ||
292 | The routines which implement these operators are expected to actually | |
293 | I<mutate> their arguments. So, assuming that $obj is a reference to a | |
294 | number, | |
295 | ||
296 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} | |
297 | ||
298 | is an appropriate implementation of overloaded C<++>. Note that | |
299 | ||
300 | sub incr { ++$ {$_[0]} ; shift } | |
301 | ||
302 | is OK if used with preincrement and with postincrement. (In the case | |
303 | of postincrement a copying will be performed, see L<Copy Constructor>.) | |
304 | ||
305 | =item C<x=> and other assignment versions | |
306 | ||
307 | There is nothing special about these methods. They may change the | |
308 | value of their arguments, and may leave it as is. The result is going | |
309 | to be assigned to the value in the left-hand-side if different from | |
310 | this value. | |
311 | ||
f610777f | 312 | This allows for the same method to be used as overloaded C<+=> and |
ee239bfe IZ |
313 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
314 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, | |
315 | if C<+=> is not overloaded. | |
316 | ||
317 | =back | |
318 | ||
d1be9408 | 319 | B<Warning.> Due to the presence of assignment versions of operations, |
b267980d NIS |
320 | routines which may be called in assignment context may create |
321 | self-referential structures. Currently Perl will not free self-referential | |
ee239bfe IZ |
322 | structures until cycles are C<explicitly> broken. You may get problems |
323 | when traversing your structures too. | |
324 | ||
b267980d | 325 | Say, |
ee239bfe IZ |
326 | |
327 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; | |
328 | ||
329 | is asking for trouble, since for code C<$obj += $foo> the subroutine | |
b267980d | 330 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
ee239bfe IZ |
331 | \$foo]>. If using such a subroutine is an important optimization, one |
332 | can overload C<+=> explicitly by a non-"optimized" version, or switch | |
b267980d | 333 | to non-optimized version if C<not defined $_[2]> (see |
ee239bfe IZ |
334 | L<Calling Conventions for Binary Operations>). |
335 | ||
336 | Even if no I<explicit> assignment-variants of operators are present in | |
337 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or | |
338 | C<',' . $obj . ','> may be both optimized to | |
339 | ||
340 | my $tmp = ',' . $obj; $tmp .= ','; | |
341 | ||
4633a7c4 LW |
342 | =head2 Overloadable Operations |
343 | ||
ee239bfe | 344 | The following symbols can be specified in C<use overload> directive: |
4633a7c4 LW |
345 | |
346 | =over 5 | |
347 | ||
348 | =item * I<Arithmetic operations> | |
349 | ||
350 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", | |
351 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", | |
352 | ||
353 | For these operations a substituted non-assignment variant can be called if | |
fa8a6580 MS |
354 | the assignment variant is not available. Methods for operations C<+>, |
355 | C<->, C<+=>, and C<-=> can be called to automatically generate | |
356 | increment and decrement methods. The operation C<-> can be used to | |
4633a7c4 LW |
357 | autogenerate missing methods for unary minus or C<abs>. |
358 | ||
ee239bfe IZ |
359 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
360 | L<"Calling Conventions for Binary Operations">) for details of these | |
361 | substitutions. | |
362 | ||
4633a7c4 LW |
363 | =item * I<Comparison operations> |
364 | ||
365 | "<", "<=", ">", ">=", "==", "!=", "<=>", | |
366 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", | |
367 | ||
368 | If the corresponding "spaceship" variant is available, it can be | |
369 | used to substitute for the missing operation. During C<sort>ing | |
370 | arrays, C<cmp> is used to compare values subject to C<use overload>. | |
371 | ||
372 | =item * I<Bit operations> | |
373 | ||
6dd85743 | 374 | "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~", |
4633a7c4 | 375 | |
fa8a6580 | 376 | C<neg> stands for unary minus. If the method for C<neg> is not |
3bc6ec80 | 377 | specified, it can be autogenerated using the method for |
fa8a6580 MS |
378 | subtraction. If the method for C<!> is not specified, it can be |
379 | autogenerated using the methods for C<bool>, or C<"">, or C<0+>. | |
4633a7c4 | 380 | |
6dd85743 AF |
381 | The same remarks in L<"Arithmetic operations"> about |
382 | assignment-variants and autogeneration apply for | |
383 | bit operations C<"&">, C<"^">, and C<"|"> as well. | |
384 | ||
4633a7c4 LW |
385 | =item * I<Increment and decrement> |
386 | ||
387 | "++", "--", | |
388 | ||
389 | If undefined, addition and subtraction methods can be | |
390 | used instead. These operations are called both in prefix and | |
391 | postfix form. | |
392 | ||
393 | =item * I<Transcendental functions> | |
394 | ||
f216259d | 395 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int" |
4633a7c4 LW |
396 | |
397 | If C<abs> is unavailable, it can be autogenerated using methods | |
1fef88e7 | 398 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
4633a7c4 | 399 | |
f216259d IZ |
400 | Note that traditionally the Perl function L<int> rounds to 0, thus for |
401 | floating-point-like types one should follow the same semantic. If | |
402 | C<int> is unavailable, it can be autogenerated using the overloading of | |
403 | C<0+>. | |
404 | ||
206be27c | 405 | =item * I<Boolean, string, numeric and regexp conversions> |
4633a7c4 | 406 | |
7cb0cfe6 | 407 | 'bool', '""', '0+', 'qr' |
4633a7c4 | 408 | |
7cb0cfe6 BM |
409 | If one or two of these operations are not overloaded, the remaining ones |
410 | can be used instead. C<bool> is used in the flow control operators | |
411 | (like C<while>) and for the ternary C<?:> operation; C<qr> is used for | |
412 | the RHS of C<=~> and when an object is interpolated into a regexp. | |
413 | ||
414 | C<bool>, C<"">, and C<0+> can return any arbitrary Perl value. If the | |
415 | corresponding operation for this value is overloaded too, that operation | |
416 | will be called again with this value. C<qr> must return a compiled | |
417 | regexp, or a ref to a compiled regexp (such as C<qr//> returns), and any | |
418 | further overloading on the return value will be ignored. | |
4633a7c4 | 419 | |
1554e226 DC |
420 | As a special case if the overload returns the object itself then it will |
421 | be used directly. An overloaded conversion returning the object is | |
422 | probably a bug, because you're likely to get something that looks like | |
423 | C<YourPackage=HASH(0x8172b34)>. | |
424 | ||
f5284f61 IZ |
425 | =item * I<Iteration> |
426 | ||
427 | "<>" | |
428 | ||
429 | If not overloaded, the argument will be converted to a filehandle or | |
430 | glob (which may require a stringification). The same overloading | |
431 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and | |
432 | I<globbing> syntax C<E<lt>${var}E<gt>>. | |
433 | ||
54f8c773 YST |
434 | B<BUGS> Even in list context, the iterator is currently called only |
435 | once and with scalar context. | |
436 | ||
e2210791 BM |
437 | =item * I<File tests> |
438 | ||
439 | "-X" | |
440 | ||
441 | This overload is used for all the filetest operators (C<-f>, C<-x> and | |
442 | so on: see L<perlfunc/-X> for the full list). Even though these are | |
443 | unary operators, the method will be called with a second argument which | |
444 | is a single letter indicating which test was performed. Note that the | |
445 | overload key is the literal string C<"-X">: you can't provide separate | |
446 | overloads for the different tests. | |
447 | ||
448 | Calling an overloaded filetest operator does not affect the stat value | |
449 | associated with the special filehandle C<_>. It still refers to the | |
450 | result of the last C<stat>, C<lstat> or unoverloaded filetest. | |
451 | ||
452 | If not overloaded, these operators will fall back to the default | |
453 | behaviour even without C<< fallback => 1 >>. This means that if the | |
454 | object is a blessed glob or blessed IO ref it will be treated as a | |
455 | filehandle, otherwise string overloading will be invoked and the result | |
456 | treated as a filename. | |
457 | ||
458 | This overload was introduced in perl 5.12. | |
459 | ||
ae20c3aa RGS |
460 | =item * I<Matching> |
461 | ||
b3ed409d CS |
462 | The key C<"~~"> allows you to override the smart matching logic used by |
463 | the C<~~> operator and the switch construct (C<given>/C<when>). See | |
464 | L<perlsyn/switch> and L<feature>. | |
ae20c3aa | 465 | |
0de1c906 DM |
466 | Unusually, overloading of the smart match operator does not automatically |
467 | take precedence over normal smart match behaviour. In particular, in the | |
468 | following code: | |
469 | ||
470 | package Foo; | |
471 | use overload '~~' => 'match'; | |
472 | ||
473 | my $obj = Foo->new(); | |
474 | $obj ~~ [ 1,2,3 ]; | |
475 | ||
476 | the smart match does I<not> invoke the method call like this: | |
477 | ||
478 | $obj->match([1,2,3],0); | |
479 | ||
480 | rather, the smart match distributive rule takes precedence, so $obj is | |
481 | smart matched against each array element in turn until a match is found, | |
482 | so you may see between one and three of these calls instead: | |
483 | ||
484 | $obj->match(1,0); | |
485 | $obj->match(2,0); | |
486 | $obj->match(3,0); | |
487 | ||
488 | Consult the match table in L<perlsyn/"Smart matching in detail"> for | |
489 | details of when overloading is invoked. | |
490 | ||
f5284f61 IZ |
491 | =item * I<Dereferencing> |
492 | ||
493 | '${}', '@{}', '%{}', '&{}', '*{}'. | |
494 | ||
495 | If not overloaded, the argument will be dereferenced I<as is>, thus | |
496 | should be of correct type. These functions should return a reference | |
497 | of correct type, or another object with overloaded dereferencing. | |
498 | ||
b267980d NIS |
499 | As a special case if the overload returns the object itself then it |
500 | will be used directly (provided it is the correct type). | |
501 | ||
502 | The dereference operators must be specified explicitly they will not be passed to | |
503 | "nomethod". | |
504 | ||
4633a7c4 LW |
505 | =item * I<Special> |
506 | ||
ae20c3aa | 507 | "nomethod", "fallback", "=". |
4633a7c4 LW |
508 | |
509 | see L<SPECIAL SYMBOLS FOR C<use overload>>. | |
510 | ||
511 | =back | |
512 | ||
ee239bfe IZ |
513 | See L<"Fallback"> for an explanation of when a missing method can be |
514 | autogenerated. | |
515 | ||
516 | A computer-readable form of the above table is available in the hash | |
517 | %overload::ops, with values being space-separated lists of names: | |
518 | ||
519 | with_assign => '+ - * / % ** << >> x .', | |
520 | assign => '+= -= *= /= %= **= <<= >>= x= .=', | |
2877bd81 | 521 | num_comparison => '< <= > >= == !=', |
ee239bfe | 522 | '3way_comparison'=> '<=> cmp', |
2877bd81 | 523 | str_comparison => 'lt le gt ge eq ne', |
6dd85743 | 524 | binary => '& &= | |= ^ ^=', |
ee239bfe IZ |
525 | unary => 'neg ! ~', |
526 | mutators => '++ --', | |
527 | func => 'atan2 cos sin exp abs log sqrt', | |
7cb0cfe6 | 528 | conversion => 'bool "" 0+ qr', |
f5284f61 | 529 | iterators => '<>', |
e2210791 | 530 | filetest => '-X', |
f5284f61 | 531 | dereferencing => '${} @{} %{} &{} *{}', |
ae20c3aa | 532 | matching => '~~', |
ee239bfe | 533 | special => 'nomethod fallback =' |
4633a7c4 | 534 | |
e7ea3e70 IZ |
535 | =head2 Inheritance and overloading |
536 | ||
774d564b | 537 | Inheritance interacts with overloading in two ways. |
e7ea3e70 | 538 | |
88c28ceb | 539 | =over |
e7ea3e70 IZ |
540 | |
541 | =item Strings as values of C<use overload> directive | |
542 | ||
774d564b | 543 | If C<value> in |
e7ea3e70 IZ |
544 | |
545 | use overload key => value; | |
546 | ||
774d564b | 547 | is a string, it is interpreted as a method name. |
e7ea3e70 IZ |
548 | |
549 | =item Overloading of an operation is inherited by derived classes | |
550 | ||
774d564b | 551 | Any class derived from an overloaded class is also overloaded. The |
552 | set of overloaded methods is the union of overloaded methods of all | |
553 | the ancestors. If some method is overloaded in several ancestor, then | |
e7ea3e70 | 554 | which description will be used is decided by the usual inheritance |
774d564b | 555 | rules: |
e7ea3e70 | 556 | |
774d564b | 557 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
558 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, | |
559 | then the subroutine C<D::plus_sub> will be called to implement | |
560 | operation C<+> for an object in package C<A>. | |
e7ea3e70 IZ |
561 | |
562 | =back | |
563 | ||
774d564b | 564 | Note that since the value of the C<fallback> key is not a subroutine, |
565 | its inheritance is not governed by the above rules. In the current | |
566 | implementation, the value of C<fallback> in the first overloaded | |
567 | ancestor is used, but this is accidental and subject to change. | |
e7ea3e70 | 568 | |
4633a7c4 LW |
569 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
570 | ||
571 | Three keys are recognized by Perl that are not covered by the above | |
572 | description. | |
573 | ||
774d564b | 574 | =head2 Last Resort |
4633a7c4 LW |
575 | |
576 | C<"nomethod"> should be followed by a reference to a function of four | |
577 | parameters. If defined, it is called when the overloading mechanism | |
578 | cannot find a method for some operation. The first three arguments of | |
579 | this function coincide with the arguments for the corresponding method if | |
580 | it were found, the fourth argument is the symbol | |
581 | corresponding to the missing method. If several methods are tried, | |
582 | the last one is used. Say, C<1-$a> can be equivalent to | |
583 | ||
584 | &nomethodMethod($a,1,1,"-") | |
585 | ||
586 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the | |
587 | C<use overload> directive. | |
588 | ||
b267980d NIS |
589 | The C<"nomethod"> mechanism is I<not> used for the dereference operators |
590 | ( ${} @{} %{} &{} *{} ). | |
591 | ||
592 | ||
4633a7c4 LW |
593 | If some operation cannot be resolved, and there is no function |
594 | assigned to C<"nomethod">, then an exception will be raised via die()-- | |
595 | unless C<"fallback"> was specified as a key in C<use overload> directive. | |
596 | ||
b267980d NIS |
597 | |
598 | =head2 Fallback | |
4633a7c4 LW |
599 | |
600 | The key C<"fallback"> governs what to do if a method for a particular | |
601 | operation is not found. Three different cases are possible depending on | |
602 | the value of C<"fallback">: | |
603 | ||
604 | =over 16 | |
605 | ||
606 | =item * C<undef> | |
607 | ||
608 | Perl tries to use a | |
609 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it | |
610 | then tries to calls C<"nomethod"> value; if missing, an exception | |
611 | will be raised. | |
612 | ||
613 | =item * TRUE | |
614 | ||
615 | The same as for the C<undef> value, but no exception is raised. Instead, | |
616 | it silently reverts to what it would have done were there no C<use overload> | |
617 | present. | |
618 | ||
619 | =item * defined, but FALSE | |
620 | ||
621 | No autogeneration is tried. Perl tries to call | |
b267980d | 622 | C<"nomethod"> value, and if this is missing, raises an exception. |
4633a7c4 LW |
623 | |
624 | =back | |
625 | ||
e7ea3e70 IZ |
626 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
627 | yet, see L<"Inheritance and overloading">. | |
628 | ||
4633a7c4 LW |
629 | =head2 Copy Constructor |
630 | ||
631 | The value for C<"="> is a reference to a function with three | |
632 | arguments, i.e., it looks like the other values in C<use | |
633 | overload>. However, it does not overload the Perl assignment | |
634 | operator. This would go against Camel hair. | |
635 | ||
636 | This operation is called in the situations when a mutator is applied | |
637 | to a reference that shares its object with some other reference, such | |
638 | as | |
639 | ||
b267980d | 640 | $a=$b; |
ee239bfe | 641 | ++$a; |
4633a7c4 LW |
642 | |
643 | To make this change $a and not change $b, a copy of C<$$a> is made, | |
644 | and $a is assigned a reference to this new object. This operation is | |
ee239bfe | 645 | done during execution of the C<++$a>, and not during the assignment, |
4633a7c4 | 646 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
ee239bfe IZ |
647 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
648 | C<nomethod>). Note that if this operation is expressed via C<'+'> | |
649 | a nonmutator, i.e., as in | |
4633a7c4 | 650 | |
b267980d | 651 | $a=$b; |
4633a7c4 LW |
652 | $a=$a+1; |
653 | ||
654 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not | |
655 | appear as lvalue when the above code is executed. | |
656 | ||
657 | If the copy constructor is required during the execution of some mutator, | |
658 | but a method for C<'='> was not specified, it can be autogenerated as a | |
49c95d58 RD |
659 | string copy if the object is a plain scalar or a simple assignment if it |
660 | is not. | |
4633a7c4 LW |
661 | |
662 | =over 5 | |
663 | ||
664 | =item B<Example> | |
665 | ||
b267980d | 666 | The actually executed code for |
4633a7c4 | 667 | |
b267980d | 668 | $a=$b; |
4633a7c4 LW |
669 | Something else which does not modify $a or $b.... |
670 | ++$a; | |
671 | ||
672 | may be | |
673 | ||
b267980d | 674 | $a=$b; |
4633a7c4 LW |
675 | Something else which does not modify $a or $b.... |
676 | $a = $a->clone(undef,""); | |
677 | $a->incr(undef,""); | |
678 | ||
679 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, | |
680 | C<'='> was overloaded with C<\&clone>. | |
681 | ||
682 | =back | |
683 | ||
f610777f | 684 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
ee239bfe IZ |
685 | C<$b = $a; ++$a>. |
686 | ||
4633a7c4 LW |
687 | =head1 MAGIC AUTOGENERATION |
688 | ||
689 | If a method for an operation is not found, and the value for C<"fallback"> is | |
690 | TRUE or undefined, Perl tries to autogenerate a substitute method for | |
691 | the missing operation based on the defined operations. Autogenerated method | |
692 | substitutions are possible for the following operations: | |
693 | ||
694 | =over 16 | |
695 | ||
696 | =item I<Assignment forms of arithmetic operations> | |
697 | ||
698 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> | |
699 | is not defined. | |
700 | ||
b267980d | 701 | =item I<Conversion operations> |
4633a7c4 | 702 | |
206be27c | 703 | String, numeric, boolean and regexp conversions are calculated in terms |
7cb0cfe6 | 704 | of one another if not all of them are defined. |
4633a7c4 LW |
705 | |
706 | =item I<Increment and decrement> | |
707 | ||
708 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, | |
709 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. | |
710 | ||
711 | =item C<abs($a)> | |
712 | ||
713 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). | |
714 | ||
715 | =item I<Unary minus> | |
716 | ||
717 | can be expressed in terms of subtraction. | |
718 | ||
3bc6ec80 | 719 | =item I<Negation> |
720 | ||
721 | C<!> and C<not> can be expressed in terms of boolean conversion, or | |
722 | string or numerical conversion. | |
723 | ||
4633a7c4 LW |
724 | =item I<Concatenation> |
725 | ||
726 | can be expressed in terms of string conversion. | |
727 | ||
b267980d | 728 | =item I<Comparison operations> |
4633a7c4 LW |
729 | |
730 | can be expressed in terms of its "spaceship" counterpart: either | |
731 | C<E<lt>=E<gt>> or C<cmp>: | |
1fef88e7 | 732 | |
4633a7c4 LW |
733 | <, >, <=, >=, ==, != in terms of <=> |
734 | lt, gt, le, ge, eq, ne in terms of cmp | |
735 | ||
f5284f61 IZ |
736 | =item I<Iterator> |
737 | ||
738 | <> in terms of builtin operations | |
739 | ||
740 | =item I<Dereferencing> | |
741 | ||
742 | ${} @{} %{} &{} *{} in terms of builtin operations | |
743 | ||
4633a7c4 LW |
744 | =item I<Copy operator> |
745 | ||
746 | can be expressed in terms of an assignment to the dereferenced value, if this | |
49c95d58 RD |
747 | value is a scalar and not a reference, or simply a reference assignment |
748 | otherwise. | |
4633a7c4 LW |
749 | |
750 | =back | |
751 | ||
84fc275b S |
752 | =head1 Minimal set of overloaded operations |
753 | ||
754 | Since some operations can be automatically generated from others, there is | |
755 | a minimal set of operations that need to be overloaded in order to have | |
299476e0 S |
756 | the complete set of overloaded operations at one's disposal. |
757 | Of course, the autogenerated operations may not do exactly what the user | |
758 | expects. See L<MAGIC AUTOGENERATION> above. The minimal set is: | |
84fc275b S |
759 | |
760 | + - * / % ** << >> x | |
761 | <=> cmp | |
762 | & | ^ ~ | |
763 | atan2 cos sin exp log sqrt int | |
764 | ||
765 | Additionally, you need to define at least one of string, boolean or | |
299476e0 S |
766 | numeric conversions because any one can be used to emulate the others. |
767 | The string conversion can also be used to emulate concatenation. | |
84fc275b | 768 | |
ee239bfe | 769 | =head1 Losing overloading |
4633a7c4 LW |
770 | |
771 | The restriction for the comparison operation is that even if, for example, | |
772 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' | |
773 | function will produce only a standard logical value based on the | |
774 | numerical value of the result of `C<cmp>'. In particular, a working | |
775 | numeric conversion is needed in this case (possibly expressed in terms of | |
776 | other conversions). | |
777 | ||
778 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties | |
779 | if the string conversion substitution is applied. | |
780 | ||
781 | When you chop() a mathemagical object it is promoted to a string and its | |
782 | mathemagical properties are lost. The same can happen with other | |
783 | operations as well. | |
784 | ||
785 | =head1 Run-time Overloading | |
786 | ||
787 | Since all C<use> directives are executed at compile-time, the only way to | |
788 | change overloading during run-time is to | |
789 | ||
790 | eval 'use overload "+" => \&addmethod'; | |
791 | ||
792 | You can also use | |
793 | ||
794 | eval 'no overload "+", "--", "<="'; | |
795 | ||
796 | though the use of these constructs during run-time is questionable. | |
797 | ||
798 | =head1 Public functions | |
799 | ||
800 | Package C<overload.pm> provides the following public functions: | |
801 | ||
802 | =over 5 | |
803 | ||
804 | =item overload::StrVal(arg) | |
805 | ||
6a0e9e72 FD |
806 | Gives string value of C<arg> as in absence of stringify overloading. If you |
807 | are using this to get the address of a reference (useful for checking if two | |
808 | references point to the same thing) then you may be better off using | |
809 | C<Scalar::Util::refaddr()>, which is faster. | |
4633a7c4 LW |
810 | |
811 | =item overload::Overloaded(arg) | |
812 | ||
813 | Returns true if C<arg> is subject to overloading of some operations. | |
814 | ||
815 | =item overload::Method(obj,op) | |
816 | ||
817 | Returns C<undef> or a reference to the method that implements C<op>. | |
818 | ||
819 | =back | |
820 | ||
b3ac6de7 IZ |
821 | =head1 Overloading constants |
822 | ||
7adf7a02 | 823 | For some applications, the Perl parser mangles constants too much. |
bfce84ec | 824 | It is possible to hook into this process via C<overload::constant()> |
7adf7a02 | 825 | and C<overload::remove_constant()> functions. |
b3ac6de7 IZ |
826 | |
827 | These functions take a hash as an argument. The recognized keys of this hash | |
7adf7a02 | 828 | are: |
b3ac6de7 IZ |
829 | |
830 | =over 8 | |
831 | ||
832 | =item integer | |
833 | ||
834 | to overload integer constants, | |
835 | ||
836 | =item float | |
837 | ||
838 | to overload floating point constants, | |
839 | ||
840 | =item binary | |
841 | ||
842 | to overload octal and hexadecimal constants, | |
843 | ||
844 | =item q | |
845 | ||
846 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted | |
847 | strings and here-documents, | |
848 | ||
849 | =item qr | |
850 | ||
851 | to overload constant pieces of regular expressions. | |
852 | ||
853 | =back | |
854 | ||
855 | The corresponding values are references to functions which take three arguments: | |
856 | the first one is the I<initial> string form of the constant, the second one | |
b267980d | 857 | is how Perl interprets this constant, the third one is how the constant is used. |
b3ac6de7 | 858 | Note that the initial string form does not |
b267980d | 859 | contain string delimiters, and has backslashes in backslash-delimiter |
b3ac6de7 | 860 | combinations stripped (thus the value of delimiter is not relevant for |
b267980d | 861 | processing of this string). The return value of this function is how this |
b3ac6de7 IZ |
862 | constant is going to be interpreted by Perl. The third argument is undefined |
863 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote | |
864 | context (comes from strings, regular expressions, and single-quote HERE | |
b267980d | 865 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
b3ac6de7 IZ |
866 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
867 | ||
868 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, | |
869 | it is expected that overloaded constant strings are equipped with reasonable | |
b267980d | 870 | overloaded catenation operator, otherwise absurd results will result. |
b3ac6de7 IZ |
871 | Similarly, negative numbers are considered as negations of positive constants. |
872 | ||
873 | Note that it is probably meaningless to call the functions overload::constant() | |
874 | and overload::remove_constant() from anywhere but import() and unimport() methods. | |
875 | From these methods they may be called as | |
876 | ||
877 | sub import { | |
878 | shift; | |
879 | return unless @_; | |
880 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; | |
881 | overload::constant integer => sub {Math::BigInt->new(shift)}; | |
882 | } | |
883 | ||
4633a7c4 LW |
884 | =head1 IMPLEMENTATION |
885 | ||
886 | What follows is subject to change RSN. | |
887 | ||
e7ea3e70 IZ |
888 | The table of methods for all operations is cached in magic for the |
889 | symbol table hash for the package. The cache is invalidated during | |
890 | processing of C<use overload>, C<no overload>, new function | |
891 | definitions, and changes in @ISA. However, this invalidation remains | |
892 | unprocessed until the next C<bless>ing into the package. Hence if you | |
893 | want to change overloading structure dynamically, you'll need an | |
894 | additional (fake) C<bless>ing to update the table. | |
895 | ||
896 | (Every SVish thing has a magic queue, and magic is an entry in that | |
897 | queue. This is how a single variable may participate in multiple | |
898 | forms of magic simultaneously. For instance, environment variables | |
899 | regularly have two forms at once: their %ENV magic and their taint | |
900 | magic. However, the magic which implements overloading is applied to | |
901 | the stashes, which are rarely used directly, thus should not slow down | |
902 | Perl.) | |
4633a7c4 LW |
903 | |
904 | If an object belongs to a package using overload, it carries a special | |
905 | flag. Thus the only speed penalty during arithmetic operations without | |
906 | overloading is the checking of this flag. | |
907 | ||
774d564b | 908 | In fact, if C<use overload> is not present, there is almost no overhead |
909 | for overloadable operations, so most programs should not suffer | |
910 | measurable performance penalties. A considerable effort was made to | |
911 | minimize the overhead when overload is used in some package, but the | |
912 | arguments in question do not belong to packages using overload. When | |
913 | in doubt, test your speed with C<use overload> and without it. So far | |
914 | there have been no reports of substantial speed degradation if Perl is | |
915 | compiled with optimization turned on. | |
4633a7c4 | 916 | |
e7ea3e70 IZ |
917 | There is no size penalty for data if overload is not used. The only |
918 | size penalty if overload is used in some package is that I<all> the | |
919 | packages acquire a magic during the next C<bless>ing into the | |
920 | package. This magic is three-words-long for packages without | |
f610777f | 921 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 | 922 | |
b267980d | 923 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
4633a7c4 LW |
924 | carried out before any operation that can imply an assignment to the |
925 | object $a (or $b) refers to, like C<$a++>. You can override this | |
926 | behavior by defining your own copy constructor (see L<"Copy Constructor">). | |
927 | ||
928 | It is expected that arguments to methods that are not explicitly supposed | |
929 | to be changed are constant (but this is not enforced). | |
930 | ||
ee239bfe IZ |
931 | =head1 Metaphor clash |
932 | ||
f610777f | 933 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
b267980d NIS |
934 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
935 | clash. | |
ee239bfe IZ |
936 | |
937 | Here is a Perl object metaphor: | |
938 | ||
939 | I< object is a reference to blessed data> | |
940 | ||
941 | and an arithmetic metaphor: | |
942 | ||
943 | I< object is a thing by itself>. | |
944 | ||
945 | The I<main> problem of overloading C<=> is the fact that these metaphors | |
946 | imply different actions on the assignment C<$a = $b> if $a and $b are | |
947 | objects. Perl-think implies that $a becomes a reference to whatever | |
948 | $b was referencing. Arithmetic-think implies that the value of "object" | |
949 | $a is changed to become the value of the object $b, preserving the fact | |
950 | that $a and $b are separate entities. | |
951 | ||
952 | The difference is not relevant in the absence of mutators. After | |
953 | a Perl-way assignment an operation which mutates the data referenced by $a | |
b267980d | 954 | would change the data referenced by $b too. Effectively, after |
ee239bfe IZ |
955 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
956 | ||
b267980d | 957 | On the other hand, anyone who has used algebraic notation knows the |
ee239bfe IZ |
958 | expressive power of the arithmetic metaphor. Overloading works hard |
959 | to enable this metaphor while preserving the Perlian way as far as | |
d1be9408 | 960 | possible. Since it is not possible to freely mix two contradicting |
ee239bfe IZ |
961 | metaphors, overloading allows the arithmetic way to write things I<as |
962 | far as all the mutators are called via overloaded access only>. The | |
963 | way it is done is described in L<Copy Constructor>. | |
964 | ||
965 | If some mutator methods are directly applied to the overloaded values, | |
b267980d | 966 | one may need to I<explicitly unlink> other values which references the |
ee239bfe IZ |
967 | same value: |
968 | ||
2b393bf4 | 969 | $a = Data->new(23); |
ee239bfe IZ |
970 | ... |
971 | $b = $a; # $b is "linked" to $a | |
972 | ... | |
973 | $a = $a->clone; # Unlink $b from $a | |
974 | $a->increment_by(4); | |
975 | ||
976 | Note that overloaded access makes this transparent: | |
977 | ||
2b393bf4 | 978 | $a = Data->new(23); |
ee239bfe IZ |
979 | $b = $a; # $b is "linked" to $a |
980 | $a += 4; # would unlink $b automagically | |
981 | ||
982 | However, it would not make | |
983 | ||
2b393bf4 | 984 | $a = Data->new(23); |
ee239bfe IZ |
985 | $a = 4; # Now $a is a plain 4, not 'Data' |
986 | ||
987 | preserve "objectness" of $a. But Perl I<has> a way to make assignments | |
988 | to an object do whatever you want. It is just not the overload, but | |
989 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method | |
b267980d | 990 | which returns the object itself, and STORE() method which changes the |
ee239bfe IZ |
991 | value of the object, one can reproduce the arithmetic metaphor in its |
992 | completeness, at least for variables which were tie()d from the start. | |
993 | ||
994 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) | |
995 | ||
996 | =head1 Cookbook | |
997 | ||
998 | Please add examples to what follows! | |
999 | ||
1000 | =head2 Two-face scalars | |
1001 | ||
1002 | Put this in F<two_face.pm> in your Perl library directory: | |
1003 | ||
1004 | package two_face; # Scalars with separate string and | |
1005 | # numeric values. | |
1006 | sub new { my $p = shift; bless [@_], $p } | |
1007 | use overload '""' => \&str, '0+' => \&num, fallback => 1; | |
1008 | sub num {shift->[1]} | |
1009 | sub str {shift->[0]} | |
1010 | ||
1011 | Use it as follows: | |
1012 | ||
1013 | require two_face; | |
2b393bf4 | 1014 | my $seven = two_face->new("vii", 7); |
ee239bfe IZ |
1015 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
1016 | print "seven contains `i'\n" if $seven =~ /i/; | |
1017 | ||
1018 | (The second line creates a scalar which has both a string value, and a | |
1019 | numeric value.) This prints: | |
1020 | ||
1021 | seven=vii, seven=7, eight=8 | |
1022 | seven contains `i' | |
1023 | ||
f5284f61 IZ |
1024 | =head2 Two-face references |
1025 | ||
1026 | Suppose you want to create an object which is accessible as both an | |
6d822dc4 | 1027 | array reference and a hash reference. |
f5284f61 IZ |
1028 | |
1029 | package two_refs; | |
1030 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; | |
b267980d NIS |
1031 | sub new { |
1032 | my $p = shift; | |
f5284f61 IZ |
1033 | bless \ [@_], $p; |
1034 | } | |
1035 | sub gethash { | |
1036 | my %h; | |
1037 | my $self = shift; | |
1038 | tie %h, ref $self, $self; | |
1039 | \%h; | |
1040 | } | |
1041 | ||
1042 | sub TIEHASH { my $p = shift; bless \ shift, $p } | |
1043 | my %fields; | |
1044 | my $i = 0; | |
1045 | $fields{$_} = $i++ foreach qw{zero one two three}; | |
b267980d | 1046 | sub STORE { |
f5284f61 IZ |
1047 | my $self = ${shift()}; |
1048 | my $key = $fields{shift()}; | |
1049 | defined $key or die "Out of band access"; | |
1050 | $$self->[$key] = shift; | |
1051 | } | |
b267980d | 1052 | sub FETCH { |
f5284f61 IZ |
1053 | my $self = ${shift()}; |
1054 | my $key = $fields{shift()}; | |
1055 | defined $key or die "Out of band access"; | |
1056 | $$self->[$key]; | |
1057 | } | |
1058 | ||
1059 | Now one can access an object using both the array and hash syntax: | |
1060 | ||
2b393bf4 | 1061 | my $bar = two_refs->new(3,4,5,6); |
f5284f61 IZ |
1062 | $bar->[2] = 11; |
1063 | $bar->{two} == 11 or die 'bad hash fetch'; | |
1064 | ||
1065 | Note several important features of this example. First of all, the | |
1066 | I<actual> type of $bar is a scalar reference, and we do not overload | |
1067 | the scalar dereference. Thus we can get the I<actual> non-overloaded | |
1068 | contents of $bar by just using C<$$bar> (what we do in functions which | |
1069 | overload dereference). Similarly, the object returned by the | |
1070 | TIEHASH() method is a scalar reference. | |
1071 | ||
1072 | Second, we create a new tied hash each time the hash syntax is used. | |
1073 | This allows us not to worry about a possibility of a reference loop, | |
d1be9408 | 1074 | which would lead to a memory leak. |
f5284f61 IZ |
1075 | |
1076 | Both these problems can be cured. Say, if we want to overload hash | |
1077 | dereference on a reference to an object which is I<implemented> as a | |
1078 | hash itself, the only problem one has to circumvent is how to access | |
1fd16925 | 1079 | this I<actual> hash (as opposed to the I<virtual> hash exhibited by the |
f5284f61 IZ |
1080 | overloaded dereference operator). Here is one possible fetching routine: |
1081 | ||
1082 | sub access_hash { | |
1083 | my ($self, $key) = (shift, shift); | |
1084 | my $class = ref $self; | |
b267980d | 1085 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
f5284f61 IZ |
1086 | my $out = $self->{$key}; |
1087 | bless $self, $class; # Restore overloading | |
1088 | $out; | |
1089 | } | |
1090 | ||
1fd16925 | 1091 | To remove creation of the tied hash on each access, one may an extra |
f5284f61 IZ |
1092 | level of indirection which allows a non-circular structure of references: |
1093 | ||
1094 | package two_refs1; | |
1095 | use overload '%{}' => sub { ${shift()}->[1] }, | |
1096 | '@{}' => sub { ${shift()}->[0] }; | |
b267980d NIS |
1097 | sub new { |
1098 | my $p = shift; | |
f5284f61 IZ |
1099 | my $a = [@_]; |
1100 | my %h; | |
1101 | tie %h, $p, $a; | |
1102 | bless \ [$a, \%h], $p; | |
1103 | } | |
1104 | sub gethash { | |
1105 | my %h; | |
1106 | my $self = shift; | |
1107 | tie %h, ref $self, $self; | |
1108 | \%h; | |
1109 | } | |
1110 | ||
1111 | sub TIEHASH { my $p = shift; bless \ shift, $p } | |
1112 | my %fields; | |
1113 | my $i = 0; | |
1114 | $fields{$_} = $i++ foreach qw{zero one two three}; | |
b267980d | 1115 | sub STORE { |
f5284f61 IZ |
1116 | my $a = ${shift()}; |
1117 | my $key = $fields{shift()}; | |
1118 | defined $key or die "Out of band access"; | |
1119 | $a->[$key] = shift; | |
1120 | } | |
b267980d | 1121 | sub FETCH { |
f5284f61 IZ |
1122 | my $a = ${shift()}; |
1123 | my $key = $fields{shift()}; | |
1124 | defined $key or die "Out of band access"; | |
1125 | $a->[$key]; | |
1126 | } | |
1127 | ||
1fd16925 | 1128 | Now if $baz is overloaded like this, then C<$baz> is a reference to a |
f5284f61 IZ |
1129 | reference to the intermediate array, which keeps a reference to an |
1130 | actual array, and the access hash. The tie()ing object for the access | |
1fd16925 | 1131 | hash is a reference to a reference to the actual array, so |
f5284f61 | 1132 | |
88c28ceb | 1133 | =over |
f5284f61 IZ |
1134 | |
1135 | =item * | |
1136 | ||
1137 | There are no loops of references. | |
1138 | ||
1139 | =item * | |
1140 | ||
1141 | Both "objects" which are blessed into the class C<two_refs1> are | |
1142 | references to a reference to an array, thus references to a I<scalar>. | |
1143 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no | |
1144 | overloaded operations. | |
1145 | ||
1146 | =back | |
1147 | ||
ee239bfe IZ |
1148 | =head2 Symbolic calculator |
1149 | ||
1150 | Put this in F<symbolic.pm> in your Perl library directory: | |
1151 | ||
1152 | package symbolic; # Primitive symbolic calculator | |
1153 | use overload nomethod => \&wrap; | |
1154 | ||
1155 | sub new { shift; bless ['n', @_] } | |
1156 | sub wrap { | |
1157 | my ($obj, $other, $inv, $meth) = @_; | |
1158 | ($obj, $other) = ($other, $obj) if $inv; | |
1159 | bless [$meth, $obj, $other]; | |
1160 | } | |
1161 | ||
1162 | This module is very unusual as overloaded modules go: it does not | |
88c28ceb JH |
1163 | provide any usual overloaded operators, instead it provides the L<Last |
1164 | Resort> operator C<nomethod>. In this example the corresponding | |
f610777f | 1165 | subroutine returns an object which encapsulates operations done over |
2b393bf4 RS |
1166 | the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 + |
1167 | symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>. | |
ee239bfe IZ |
1168 | |
1169 | Here is an example of the script which "calculates" the side of | |
1170 | circumscribed octagon using the above package: | |
1171 | ||
1172 | require symbolic; | |
1173 | my $iter = 1; # 2**($iter+2) = 8 | |
2b393bf4 | 1174 | my $side = symbolic->new(1); |
ee239bfe | 1175 | my $cnt = $iter; |
3cb6de81 | 1176 | |
ee239bfe IZ |
1177 | while ($cnt--) { |
1178 | $side = (sqrt(1 + $side**2) - 1)/$side; | |
1179 | } | |
1180 | print "OK\n"; | |
1181 | ||
1182 | The value of $side is | |
1183 | ||
1184 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], | |
1185 | undef], 1], ['n', 1]] | |
1186 | ||
1187 | Note that while we obtained this value using a nice little script, | |
1188 | there is no simple way to I<use> this value. In fact this value may | |
2d3232d7 | 1189 | be inspected in debugger (see L<perldebug>), but only if |
ee239bfe IZ |
1190 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1191 | ||
1192 | If one attempts to print this value, then the overloaded operator | |
1193 | C<""> will be called, which will call C<nomethod> operator. The | |
1194 | result of this operator will be stringified again, but this result is | |
1195 | again of type C<symbolic>, which will lead to an infinite loop. | |
1196 | ||
1197 | Add a pretty-printer method to the module F<symbolic.pm>: | |
1198 | ||
1199 | sub pretty { | |
1200 | my ($meth, $a, $b) = @{+shift}; | |
1201 | $a = 'u' unless defined $a; | |
1202 | $b = 'u' unless defined $b; | |
1203 | $a = $a->pretty if ref $a; | |
1204 | $b = $b->pretty if ref $b; | |
1205 | "[$meth $a $b]"; | |
b267980d | 1206 | } |
ee239bfe IZ |
1207 | |
1208 | Now one can finish the script by | |
1209 | ||
1210 | print "side = ", $side->pretty, "\n"; | |
1211 | ||
1212 | The method C<pretty> is doing object-to-string conversion, so it | |
1213 | is natural to overload the operator C<""> using this method. However, | |
1214 | inside such a method it is not necessary to pretty-print the | |
1215 | I<components> $a and $b of an object. In the above subroutine | |
1216 | C<"[$meth $a $b]"> is a catenation of some strings and components $a | |
1217 | and $b. If these components use overloading, the catenation operator | |
1fd16925 | 1218 | will look for an overloaded operator C<.>; if not present, it will |
ee239bfe IZ |
1219 | look for an overloaded operator C<"">. Thus it is enough to use |
1220 | ||
1221 | use overload nomethod => \&wrap, '""' => \&str; | |
1222 | sub str { | |
1223 | my ($meth, $a, $b) = @{+shift}; | |
1224 | $a = 'u' unless defined $a; | |
1225 | $b = 'u' unless defined $b; | |
1226 | "[$meth $a $b]"; | |
b267980d | 1227 | } |
ee239bfe IZ |
1228 | |
1229 | Now one can change the last line of the script to | |
1230 | ||
1231 | print "side = $side\n"; | |
1232 | ||
1233 | which outputs | |
1234 | ||
1235 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] | |
1236 | ||
1237 | and one can inspect the value in debugger using all the possible | |
b267980d | 1238 | methods. |
ee239bfe | 1239 | |
d1be9408 | 1240 | Something is still amiss: consider the loop variable $cnt of the |
ee239bfe IZ |
1241 | script. It was a number, not an object. We cannot make this value of |
1242 | type C<symbolic>, since then the loop will not terminate. | |
1243 | ||
1244 | Indeed, to terminate the cycle, the $cnt should become false. | |
1245 | However, the operator C<bool> for checking falsity is overloaded (this | |
1246 | time via overloaded C<"">), and returns a long string, thus any object | |
1247 | of type C<symbolic> is true. To overcome this, we need a way to | |
1248 | compare an object to 0. In fact, it is easier to write a numeric | |
1249 | conversion routine. | |
1250 | ||
1251 | Here is the text of F<symbolic.pm> with such a routine added (and | |
f610777f | 1252 | slightly modified str()): |
ee239bfe IZ |
1253 | |
1254 | package symbolic; # Primitive symbolic calculator | |
1255 | use overload | |
1256 | nomethod => \&wrap, '""' => \&str, '0+' => \# | |
1257 | ||
1258 | sub new { shift; bless ['n', @_] } | |
1259 | sub wrap { | |
1260 | my ($obj, $other, $inv, $meth) = @_; | |
1261 | ($obj, $other) = ($other, $obj) if $inv; | |
1262 | bless [$meth, $obj, $other]; | |
1263 | } | |
1264 | sub str { | |
1265 | my ($meth, $a, $b) = @{+shift}; | |
1266 | $a = 'u' unless defined $a; | |
1267 | if (defined $b) { | |
1268 | "[$meth $a $b]"; | |
1269 | } else { | |
1270 | "[$meth $a]"; | |
1271 | } | |
b267980d NIS |
1272 | } |
1273 | my %subr = ( n => sub {$_[0]}, | |
1274 | sqrt => sub {sqrt $_[0]}, | |
ee239bfe IZ |
1275 | '-' => sub {shift() - shift()}, |
1276 | '+' => sub {shift() + shift()}, | |
1277 | '/' => sub {shift() / shift()}, | |
1278 | '*' => sub {shift() * shift()}, | |
1279 | '**' => sub {shift() ** shift()}, | |
1280 | ); | |
1281 | sub num { | |
1282 | my ($meth, $a, $b) = @{+shift}; | |
b267980d | 1283 | my $subr = $subr{$meth} |
ee239bfe IZ |
1284 | or die "Do not know how to ($meth) in symbolic"; |
1285 | $a = $a->num if ref $a eq __PACKAGE__; | |
1286 | $b = $b->num if ref $b eq __PACKAGE__; | |
1287 | $subr->($a,$b); | |
1288 | } | |
1289 | ||
1290 | All the work of numeric conversion is done in %subr and num(). Of | |
f610777f | 1291 | course, %subr is not complete, it contains only operators used in the |
ee239bfe IZ |
1292 | example below. Here is the extra-credit question: why do we need an |
1293 | explicit recursion in num()? (Answer is at the end of this section.) | |
1294 | ||
1295 | Use this module like this: | |
1296 | ||
1297 | require symbolic; | |
2b393bf4 RS |
1298 | my $iter = symbolic->new(2); # 16-gon |
1299 | my $side = symbolic->new(1); | |
ee239bfe | 1300 | my $cnt = $iter; |
3cb6de81 | 1301 | |
ee239bfe IZ |
1302 | while ($cnt) { |
1303 | $cnt = $cnt - 1; # Mutator `--' not implemented | |
1304 | $side = (sqrt(1 + $side**2) - 1)/$side; | |
1305 | } | |
1306 | printf "%s=%f\n", $side, $side; | |
1307 | printf "pi=%f\n", $side*(2**($iter+2)); | |
1308 | ||
1309 | It prints (without so many line breaks) | |
1310 | ||
1311 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] | |
1312 | [n 1]] 2]]] 1] | |
1313 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 | |
1314 | pi=3.182598 | |
1315 | ||
1316 | The above module is very primitive. It does not implement | |
1317 | mutator methods (C<++>, C<-=> and so on), does not do deep copying | |
1318 | (not required without mutators!), and implements only those arithmetic | |
1319 | operations which are used in the example. | |
1320 | ||
1fd16925 | 1321 | To implement most arithmetic operations is easy; one should just use |
ee239bfe IZ |
1322 | the tables of operations, and change the code which fills %subr to |
1323 | ||
1324 | my %subr = ( 'n' => sub {$_[0]} ); | |
1325 | foreach my $op (split " ", $overload::ops{with_assign}) { | |
1326 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; | |
1327 | } | |
1328 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); | |
1329 | foreach my $op (split " ", "@overload::ops{ @bins }") { | |
1330 | $subr{$op} = eval "sub {shift() $op shift()}"; | |
1331 | } | |
1332 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { | |
1333 | print "defining `$op'\n"; | |
1334 | $subr{$op} = eval "sub {$op shift()}"; | |
1335 | } | |
1336 | ||
1337 | Due to L<Calling Conventions for Mutators>, we do not need anything | |
1338 | special to make C<+=> and friends work, except filling C<+=> entry of | |
1339 | %subr, and defining a copy constructor (needed since Perl has no | |
1340 | way to know that the implementation of C<'+='> does not mutate | |
1341 | the argument, compare L<Copy Constructor>). | |
1342 | ||
1fd16925 | 1343 | To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload> |
ee239bfe IZ |
1344 | line, and code (this code assumes that mutators change things one level |
1345 | deep only, so recursive copying is not needed): | |
1346 | ||
1347 | sub cpy { | |
1348 | my $self = shift; | |
1349 | bless [@$self], ref $self; | |
1350 | } | |
1351 | ||
b267980d | 1352 | To make C<++> and C<--> work, we need to implement actual mutators, |
ee239bfe IZ |
1353 | either directly, or in C<nomethod>. We continue to do things inside |
1354 | C<nomethod>, thus add | |
1355 | ||
1356 | if ($meth eq '++' or $meth eq '--') { | |
1357 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference | |
1358 | return $obj; | |
1359 | } | |
1360 | ||
b267980d | 1361 | after the first line of wrap(). This is not a most effective |
ee239bfe IZ |
1362 | implementation, one may consider |
1363 | ||
1364 | sub inc { $_[0] = bless ['++', shift, 1]; } | |
1365 | ||
1366 | instead. | |
1367 | ||
1368 | As a final remark, note that one can fill %subr by | |
1369 | ||
1370 | my %subr = ( 'n' => sub {$_[0]} ); | |
1371 | foreach my $op (split " ", $overload::ops{with_assign}) { | |
1372 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; | |
1373 | } | |
1374 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); | |
1375 | foreach my $op (split " ", "@overload::ops{ @bins }") { | |
1376 | $subr{$op} = eval "sub {shift() $op shift()}"; | |
1377 | } | |
1378 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { | |
1379 | $subr{$op} = eval "sub {$op shift()}"; | |
1380 | } | |
1381 | $subr{'++'} = $subr{'+'}; | |
1382 | $subr{'--'} = $subr{'-'}; | |
1383 | ||
b267980d NIS |
1384 | This finishes implementation of a primitive symbolic calculator in |
1385 | 50 lines of Perl code. Since the numeric values of subexpressions | |
ee239bfe IZ |
1386 | are not cached, the calculator is very slow. |
1387 | ||
1388 | Here is the answer for the exercise: In the case of str(), we need no | |
1389 | explicit recursion since the overloaded C<.>-operator will fall back | |
1390 | to an existing overloaded operator C<"">. Overloaded arithmetic | |
1391 | operators I<do not> fall back to numeric conversion if C<fallback> is | |
1392 | not explicitly requested. Thus without an explicit recursion num() | |
1393 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild | |
1394 | the argument of num(). | |
1395 | ||
1396 | If you wonder why defaults for conversion are different for str() and | |
1397 | num(), note how easy it was to write the symbolic calculator. This | |
1398 | simplicity is due to an appropriate choice of defaults. One extra | |
f610777f A |
1399 | note: due to the explicit recursion num() is more fragile than sym(): |
1400 | we need to explicitly check for the type of $a and $b. If components | |
ee239bfe IZ |
1401 | $a and $b happen to be of some related type, this may lead to problems. |
1402 | ||
1403 | =head2 I<Really> symbolic calculator | |
1404 | ||
1405 | One may wonder why we call the above calculator symbolic. The reason | |
1406 | is that the actual calculation of the value of expression is postponed | |
1407 | until the value is I<used>. | |
1408 | ||
1409 | To see it in action, add a method | |
1410 | ||
b267980d NIS |
1411 | sub STORE { |
1412 | my $obj = shift; | |
1413 | $#$obj = 1; | |
ee239bfe IZ |
1414 | @$obj->[0,1] = ('=', shift); |
1415 | } | |
1416 | ||
1417 | to the package C<symbolic>. After this change one can do | |
1418 | ||
2b393bf4 RS |
1419 | my $a = symbolic->new(3); |
1420 | my $b = symbolic->new(4); | |
ee239bfe IZ |
1421 | my $c = sqrt($a**2 + $b**2); |
1422 | ||
1423 | and the numeric value of $c becomes 5. However, after calling | |
1424 | ||
1425 | $a->STORE(12); $b->STORE(5); | |
1426 | ||
1427 | the numeric value of $c becomes 13. There is no doubt now that the module | |
1428 | symbolic provides a I<symbolic> calculator indeed. | |
1429 | ||
1430 | To hide the rough edges under the hood, provide a tie()d interface to the | |
1431 | package C<symbolic> (compare with L<Metaphor clash>). Add methods | |
1432 | ||
1433 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } | |
1434 | sub FETCH { shift } | |
1435 | sub nop { } # Around a bug | |
1436 | ||
1437 | (the bug is described in L<"BUGS">). One can use this new interface as | |
1438 | ||
1439 | tie $a, 'symbolic', 3; | |
1440 | tie $b, 'symbolic', 4; | |
1441 | $a->nop; $b->nop; # Around a bug | |
1442 | ||
1443 | my $c = sqrt($a**2 + $b**2); | |
1444 | ||
1445 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value | |
1446 | of $c becomes 13. To insulate the user of the module add a method | |
1447 | ||
1448 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } | |
1449 | ||
1450 | Now | |
1451 | ||
1452 | my ($a, $b); | |
1453 | symbolic->vars($a, $b); | |
1454 | my $c = sqrt($a**2 + $b**2); | |
1455 | ||
1456 | $a = 3; $b = 4; | |
1457 | printf "c5 %s=%f\n", $c, $c; | |
1458 | ||
1459 | $a = 12; $b = 5; | |
1460 | printf "c13 %s=%f\n", $c, $c; | |
1461 | ||
1462 | shows that the numeric value of $c follows changes to the values of $a | |
1463 | and $b. | |
1464 | ||
4633a7c4 LW |
1465 | =head1 AUTHOR |
1466 | ||
1fef88e7 | 1467 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 | 1468 | |
7e494759 PF |
1469 | =head1 SEE ALSO |
1470 | ||
1471 | The L<overloading> pragma can be used to enable or disable overloaded | |
1472 | operations within a lexical scope. | |
1473 | ||
4633a7c4 LW |
1474 | =head1 DIAGNOSTICS |
1475 | ||
1476 | When Perl is run with the B<-Do> switch or its equivalent, overloading | |
1477 | induces diagnostic messages. | |
1478 | ||
e7ea3e70 IZ |
1479 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1480 | deduce which operations are overloaded (and which ancestor triggers | |
1481 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> | |
1482 | is shown by debugger. The method C<()> corresponds to the C<fallback> | |
1483 | key (in fact a presence of this method shows that this package has | |
1484 | overloading enabled, and it is what is used by the C<Overloaded> | |
ee239bfe | 1485 | function of module C<overload>). |
e7ea3e70 | 1486 | |
6ad11d81 | 1487 | The module might issue the following warnings: |
6b82e2f5 A |
1488 | |
1489 | =over 4 | |
1490 | ||
1491 | =item Odd number of arguments for overload::constant | |
1492 | ||
1493 | (W) The call to overload::constant contained an odd number of arguments. | |
1494 | The arguments should come in pairs. | |
1495 | ||
1496 | =item `%s' is not an overloadable type | |
1497 | ||
1498 | (W) You tried to overload a constant type the overload package is unaware of. | |
1499 | ||
1500 | =item `%s' is not a code reference | |
1501 | ||
1502 | (W) The second (fourth, sixth, ...) argument of overload::constant needs | |
1503 | to be a code reference. Either an anonymous subroutine, or a reference | |
1504 | to a subroutine. | |
1505 | ||
1506 | =back | |
1507 | ||
4633a7c4 LW |
1508 | =head1 BUGS |
1509 | ||
aa689395 | 1510 | Because it is used for overloading, the per-package hash %OVERLOAD now |
1511 | has a special meaning in Perl. The symbol table is filled with names | |
1512 | looking like line-noise. | |
4633a7c4 | 1513 | |
a6006777 | 1514 | For the purpose of inheritance every overloaded package behaves as if |
1515 | C<fallback> is present (possibly undefined). This may create | |
1516 | interesting effects if some package is not overloaded, but inherits | |
1517 | from two overloaded packages. | |
4633a7c4 | 1518 | |
b267980d | 1519 | Relation between overloading and tie()ing is broken. Overloading is |
ee239bfe IZ |
1520 | triggered or not basing on the I<previous> class of tie()d value. |
1521 | ||
b267980d | 1522 | This happens because the presence of overloading is checked too early, |
ee239bfe | 1523 | before any tie()d access is attempted. If the FETCH()ed class of the |
b267980d | 1524 | tie()d value does not change, a simple workaround is to access the value |
ee239bfe IZ |
1525 | immediately after tie()ing, so that after this call the I<previous> class |
1526 | coincides with the current one. | |
1527 | ||
1528 | B<Needed:> a way to fix this without a speed penalty. | |
1529 | ||
b3ac6de7 IZ |
1530 | Barewords are not covered by overloaded string constants. |
1531 | ||
ee239bfe IZ |
1532 | This document is confusing. There are grammos and misleading language |
1533 | used in places. It would seem a total rewrite is needed. | |
4633a7c4 LW |
1534 | |
1535 | =cut | |
1536 |