| 1 | package overload; |
| 2 | |
| 3 | our $VERSION = '1.22'; |
| 4 | |
| 5 | %ops = ( |
| 6 | with_assign => "+ - * / % ** << >> x .", |
| 7 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
| 8 | num_comparison => "< <= > >= == !=", |
| 9 | '3way_comparison' => "<=> cmp", |
| 10 | str_comparison => "lt le gt ge eq ne", |
| 11 | binary => '& &= | |= ^ ^=', |
| 12 | unary => "neg ! ~", |
| 13 | mutators => '++ --', |
| 14 | func => "atan2 cos sin exp abs log sqrt int", |
| 15 | conversion => 'bool "" 0+ qr', |
| 16 | iterators => '<>', |
| 17 | filetest => "-X", |
| 18 | dereferencing => '${} @{} %{} &{} *{}', |
| 19 | matching => '~~', |
| 20 | special => 'nomethod fallback =', |
| 21 | ); |
| 22 | |
| 23 | my %ops_seen; |
| 24 | for $category (keys %ops) { |
| 25 | $ops_seen{$_}++ for (split /\s+/, $ops{$category}); |
| 26 | } |
| 27 | |
| 28 | sub nil {} |
| 29 | |
| 30 | sub OVERLOAD { |
| 31 | $package = shift; |
| 32 | my %arg = @_; |
| 33 | my ($sub, $fb); |
| 34 | *{$package . "::(("} = \&nil; # Make it findable via fetchmethod. |
| 35 | for (keys %arg) { |
| 36 | if ($_ eq 'fallback') { |
| 37 | for my $sym (*{$package . "::()"}) { |
| 38 | *$sym = \&nil; # Make it findable via fetchmethod. |
| 39 | $$sym = $arg{$_}; |
| 40 | } |
| 41 | } else { |
| 42 | warnings::warnif("overload arg '$_' is invalid") |
| 43 | unless $ops_seen{$_}; |
| 44 | $sub = $arg{$_}; |
| 45 | if (not ref $sub) { |
| 46 | $ {$package . "::(" . $_} = $sub; |
| 47 | $sub = \&nil; |
| 48 | } |
| 49 | #print STDERR "Setting '$ {'package'}::\cO$_' to \\&'$sub'.\n"; |
| 50 | *{$package . "::(" . $_} = \&{ $sub }; |
| 51 | } |
| 52 | } |
| 53 | } |
| 54 | |
| 55 | sub import { |
| 56 | $package = (caller())[0]; |
| 57 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; |
| 58 | shift; |
| 59 | $package->overload::OVERLOAD(@_); |
| 60 | } |
| 61 | |
| 62 | sub unimport { |
| 63 | $package = (caller())[0]; |
| 64 | shift; |
| 65 | *{$package . "::(("} = \&nil; |
| 66 | for (@_) { |
| 67 | warnings::warnif("overload arg '$_' is invalid") |
| 68 | unless $ops_seen{$_}; |
| 69 | delete $ {$package . "::"}{$_ eq 'fallback' ? '()' : "(" .$_}; |
| 70 | } |
| 71 | } |
| 72 | |
| 73 | sub Overloaded { |
| 74 | my $package = shift; |
| 75 | $package = ref $package if ref $package; |
| 76 | mycan ($package, '()') || mycan ($package, '(('); |
| 77 | } |
| 78 | |
| 79 | sub ov_method { |
| 80 | my $globref = shift; |
| 81 | return undef unless $globref; |
| 82 | my $sub = \&{*$globref}; |
| 83 | no overloading; |
| 84 | return $sub if !ref $sub or $sub != \&nil; |
| 85 | return shift->can($ {*$globref}); |
| 86 | } |
| 87 | |
| 88 | sub OverloadedStringify { |
| 89 | my $package = shift; |
| 90 | $package = ref $package if ref $package; |
| 91 | #$package->can('(""') |
| 92 | ov_method mycan($package, '(""'), $package |
| 93 | or ov_method mycan($package, '(0+'), $package |
| 94 | or ov_method mycan($package, '(bool'), $package |
| 95 | or ov_method mycan($package, '(nomethod'), $package; |
| 96 | } |
| 97 | |
| 98 | sub Method { |
| 99 | my $package = shift; |
| 100 | if(ref $package) { |
| 101 | local $@; |
| 102 | local $!; |
| 103 | require Scalar::Util; |
| 104 | $package = Scalar::Util::blessed($package); |
| 105 | return undef if !defined $package; |
| 106 | } |
| 107 | #my $meth = $package->can('(' . shift); |
| 108 | ov_method mycan($package, '(' . shift), $package; |
| 109 | #return $meth if $meth ne \&nil; |
| 110 | #return $ {*{$meth}}; |
| 111 | } |
| 112 | |
| 113 | sub AddrRef { |
| 114 | no overloading; |
| 115 | "$_[0]"; |
| 116 | } |
| 117 | |
| 118 | *StrVal = *AddrRef; |
| 119 | |
| 120 | sub mycan { # Real can would leave stubs. |
| 121 | my ($package, $meth) = @_; |
| 122 | |
| 123 | local $@; |
| 124 | local $!; |
| 125 | require mro; |
| 126 | |
| 127 | my $mro = mro::get_linear_isa($package); |
| 128 | foreach my $p (@$mro) { |
| 129 | my $fqmeth = $p . q{::} . $meth; |
| 130 | return \*{$fqmeth} if defined &{$fqmeth}; |
| 131 | } |
| 132 | |
| 133 | return undef; |
| 134 | } |
| 135 | |
| 136 | %constants = ( |
| 137 | 'integer' => 0x1000, # HINT_NEW_INTEGER |
| 138 | 'float' => 0x2000, # HINT_NEW_FLOAT |
| 139 | 'binary' => 0x4000, # HINT_NEW_BINARY |
| 140 | 'q' => 0x8000, # HINT_NEW_STRING |
| 141 | 'qr' => 0x10000, # HINT_NEW_RE |
| 142 | ); |
| 143 | |
| 144 | use warnings::register; |
| 145 | sub constant { |
| 146 | # Arguments: what, sub |
| 147 | while (@_) { |
| 148 | if (@_ == 1) { |
| 149 | warnings::warnif ("Odd number of arguments for overload::constant"); |
| 150 | last; |
| 151 | } |
| 152 | elsif (!exists $constants {$_ [0]}) { |
| 153 | warnings::warnif ("'$_[0]' is not an overloadable type"); |
| 154 | } |
| 155 | elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) { |
| 156 | # Can't use C<ref $_[1] eq "CODE"> above as code references can be |
| 157 | # blessed, and C<ref> would return the package the ref is blessed into. |
| 158 | if (warnings::enabled) { |
| 159 | $_ [1] = "undef" unless defined $_ [1]; |
| 160 | warnings::warn ("'$_[1]' is not a code reference"); |
| 161 | } |
| 162 | } |
| 163 | else { |
| 164 | $^H{$_[0]} = $_[1]; |
| 165 | $^H |= $constants{$_[0]}; |
| 166 | } |
| 167 | shift, shift; |
| 168 | } |
| 169 | } |
| 170 | |
| 171 | sub remove_constant { |
| 172 | # Arguments: what, sub |
| 173 | while (@_) { |
| 174 | delete $^H{$_[0]}; |
| 175 | $^H &= ~ $constants{$_[0]}; |
| 176 | shift, shift; |
| 177 | } |
| 178 | } |
| 179 | |
| 180 | 1; |
| 181 | |
| 182 | __END__ |
| 183 | |
| 184 | =head1 NAME |
| 185 | |
| 186 | overload - Package for overloading Perl operations |
| 187 | |
| 188 | =head1 SYNOPSIS |
| 189 | |
| 190 | package SomeThing; |
| 191 | |
| 192 | use overload |
| 193 | '+' => \&myadd, |
| 194 | '-' => \&mysub; |
| 195 | # etc |
| 196 | ... |
| 197 | |
| 198 | package main; |
| 199 | $a = SomeThing->new( 57 ); |
| 200 | $b = 5 + $a; |
| 201 | ... |
| 202 | if (overload::Overloaded $b) {...} |
| 203 | ... |
| 204 | $strval = overload::StrVal $b; |
| 205 | |
| 206 | =head1 DESCRIPTION |
| 207 | |
| 208 | This pragma allows overloading of Perl's operators for a class. |
| 209 | To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead. |
| 210 | |
| 211 | =head2 Fundamentals |
| 212 | |
| 213 | =head3 Declaration |
| 214 | |
| 215 | Arguments of the C<use overload> directive are (key, value) pairs. |
| 216 | For the full set of legal keys, see L<Overloadable Operations> below. |
| 217 | |
| 218 | Operator implementations (the values) can be subroutines, |
| 219 | references to subroutines, or anonymous subroutines |
| 220 | - in other words, anything legal inside a C<&{ ... }> call. |
| 221 | Values specified as strings are interpreted as method names. |
| 222 | Thus |
| 223 | |
| 224 | package Number; |
| 225 | use overload |
| 226 | "-" => "minus", |
| 227 | "*=" => \&muas, |
| 228 | '""' => sub { ...; }; |
| 229 | |
| 230 | declares that subtraction is to be implemented by method C<minus()> |
| 231 | in the class C<Number> (or one of its base classes), |
| 232 | and that the function C<Number::muas()> is to be used for the |
| 233 | assignment form of multiplication, C<*=>. |
| 234 | It also defines an anonymous subroutine to implement stringification: |
| 235 | this is called whenever an object blessed into the package C<Number> |
| 236 | is used in a string context (this subroutine might, for example, |
| 237 | return the number as a Roman numeral). |
| 238 | |
| 239 | =head3 Calling Conventions and Magic Autogeneration |
| 240 | |
| 241 | The following sample implementation of C<minus()> (which assumes |
| 242 | that C<Number> objects are simply blessed references to scalars) |
| 243 | illustrates the calling conventions: |
| 244 | |
| 245 | package Number; |
| 246 | sub minus { |
| 247 | my ($self, $other, $swap) = @_; |
| 248 | my $result = $$self - $other; # * |
| 249 | $result = -$result if $swap; |
| 250 | ref $result ? $result : bless \$result; |
| 251 | } |
| 252 | # * may recurse once - see table below |
| 253 | |
| 254 | Three arguments are passed to all subroutines specified in the |
| 255 | C<use overload> directive (with one exception - see L</nomethod>). |
| 256 | The first of these is the operand providing the overloaded |
| 257 | operator implementation - |
| 258 | in this case, the object whose C<minus()> method is being called. |
| 259 | |
| 260 | The second argument is the other operand, or C<undef> in the |
| 261 | case of a unary operator. |
| 262 | |
| 263 | The third argument is set to TRUE if (and only if) the two |
| 264 | operands have been swapped. Perl may do this to ensure that the |
| 265 | first argument (C<$self>) is an object implementing the overloaded |
| 266 | operation, in line with general object calling conventions. |
| 267 | For example, if C<$x> and C<$y> are C<Number>s: |
| 268 | |
| 269 | operation | generates a call to |
| 270 | ============|====================== |
| 271 | $x - $y | minus($x, $y, '') |
| 272 | $x - 7 | minus($x, 7, '') |
| 273 | 7 - $x | minus($x, 7, 1) |
| 274 | |
| 275 | Perl may also use C<minus()> to implement other operators which |
| 276 | have not been specified in the C<use overload> directive, |
| 277 | according to the rules for L<Magic Autogeneration> described later. |
| 278 | For example, the C<use overload> above declared no subroutine |
| 279 | for any of the operators C<-->, C<neg> (the overload key for |
| 280 | unary minus), or C<-=>. Thus |
| 281 | |
| 282 | operation | generates a call to |
| 283 | ============|====================== |
| 284 | -$x | minus($x, 0, 1) |
| 285 | $x-- | minus($x, 1, undef) |
| 286 | $x -= 3 | minus($x, 3, undef) |
| 287 | |
| 288 | Note the C<undef>s: |
| 289 | where autogeneration results in the method for a standard |
| 290 | operator which does not change either of its operands, such |
| 291 | as C<->, being used to implement an operator which changes |
| 292 | the operand ("mutators": here, C<--> and C<-=>), |
| 293 | Perl passes undef as the third argument. |
| 294 | This still evaluates as FALSE, consistent with the fact that |
| 295 | the operands have not been swapped, but gives the subroutine |
| 296 | a chance to alter its behaviour in these cases. |
| 297 | |
| 298 | In all the above examples, C<minus()> is required |
| 299 | only to return the result of the subtraction: |
| 300 | Perl takes care of the assignment to $x. |
| 301 | In fact, such methods should I<not> modify their operands, |
| 302 | even if C<undef> is passed as the third argument |
| 303 | (see L<Overloadable Operations>). |
| 304 | |
| 305 | The same is not true of implementations of C<++> and C<-->: |
| 306 | these are expected to modify their operand. |
| 307 | An appropriate implementation of C<--> might look like |
| 308 | |
| 309 | use overload '--' => "decr", |
| 310 | # ... |
| 311 | sub decr { --${$_[0]}; } |
| 312 | |
| 313 | =head3 Mathemagic, Mutators, and Copy Constructors |
| 314 | |
| 315 | The term 'mathemagic' describes the overloaded implementation |
| 316 | of mathematical operators. |
| 317 | Mathemagical operations raise an issue. |
| 318 | Consider the code: |
| 319 | |
| 320 | $a = $b; |
| 321 | --$a; |
| 322 | |
| 323 | If C<$a> and C<$b> are scalars then after these statements |
| 324 | |
| 325 | $a == $b - 1 |
| 326 | |
| 327 | An object, however, is a reference to blessed data, so if |
| 328 | C<$a> and C<$b> are objects then the assignment C<$a = $b> |
| 329 | copies only the reference, leaving C<$a> and C<$b> referring |
| 330 | to the same object data. |
| 331 | One might therefore expect the operation C<--$a> to decrement |
| 332 | C<$b> as well as C<$a>. |
| 333 | However, this would not be consistent with how we expect the |
| 334 | mathematical operators to work. |
| 335 | |
| 336 | Perl resolves this dilemma by transparently calling a copy |
| 337 | constructor before calling a method defined to implement |
| 338 | a mutator (C<-->, C<+=>, and so on.). |
| 339 | In the above example, when Perl reaches the decrement |
| 340 | statement, it makes a copy of the object data in C<$a> and |
| 341 | assigns to C<$a> a reference to the copied data. |
| 342 | Only then does it call C<decr()>, which alters the copied |
| 343 | data, leaving C<$b> unchanged. |
| 344 | Thus the object metaphor is preserved as far as possible, |
| 345 | while mathemagical operations still work according to the |
| 346 | arithmetic metaphor. |
| 347 | |
| 348 | Note: the preceding paragraph describes what happens when |
| 349 | Perl autogenerates the copy constructor for an object based |
| 350 | on a scalar. |
| 351 | For other cases, see L<Copy Constructor>. |
| 352 | |
| 353 | =head2 Overloadable Operations |
| 354 | |
| 355 | The complete list of keys that can be specified in the C<use overload> |
| 356 | directive are given, separated by spaces, in the values of the |
| 357 | hash C<%overload::ops>: |
| 358 | |
| 359 | with_assign => '+ - * / % ** << >> x .', |
| 360 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
| 361 | num_comparison => '< <= > >= == !=', |
| 362 | '3way_comparison'=> '<=> cmp', |
| 363 | str_comparison => 'lt le gt ge eq ne', |
| 364 | binary => '& &= | |= ^ ^=', |
| 365 | unary => 'neg ! ~', |
| 366 | mutators => '++ --', |
| 367 | func => 'atan2 cos sin exp abs log sqrt int', |
| 368 | conversion => 'bool "" 0+ qr', |
| 369 | iterators => '<>', |
| 370 | filetest => '-X', |
| 371 | dereferencing => '${} @{} %{} &{} *{}', |
| 372 | matching => '~~', |
| 373 | special => 'nomethod fallback =' |
| 374 | |
| 375 | Most of the overloadable operators map one-to-one to these keys. |
| 376 | Exceptions, including additional overloadable operations not |
| 377 | apparent from this hash, are included in the notes which follow. |
| 378 | |
| 379 | A warning is issued if an attempt is made to register an operator not found |
| 380 | above. |
| 381 | |
| 382 | =over 5 |
| 383 | |
| 384 | =item * C<not> |
| 385 | |
| 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). |
| 390 | |
| 391 | =item * C<neg> |
| 392 | |
| 393 | The key C<neg> is used for unary minus to disambiguate it from |
| 394 | binary C<->. |
| 395 | |
| 396 | =item * C<++>, C<--> |
| 397 | |
| 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. |
| 401 | |
| 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. |
| 406 | |
| 407 | =item * I<Assignments> |
| 408 | |
| 409 | += -= *= /= %= **= <<= >>= x= .= |
| 410 | &= |= ^= |
| 411 | |
| 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. |
| 417 | |
| 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. |
| 424 | |
| 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 |
| 430 | |
| 431 | $a *= $b |
| 432 | |
| 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<*>). |
| 436 | |
| 437 | =item * I<Non-mutators with a mutator variant> |
| 438 | |
| 439 | + - * / % ** << >> x . |
| 440 | & | ^ |
| 441 | |
| 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. |
| 450 | |
| 451 | =item * C<int> |
| 452 | |
| 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. |
| 456 | |
| 457 | =item * I<String, numeric, boolean, and regexp conversions> |
| 458 | |
| 459 | "" 0+ bool |
| 460 | |
| 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. |
| 466 | |
| 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). |
| 472 | |
| 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. |
| 477 | |
| 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 | |
| 483 | qr |
| 484 | |
| 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. |
| 488 | |
| 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 |
| 497 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
| 498 | |
| 499 | =item * I<File tests> |
| 500 | |
| 501 | The key C<'-X'> is used to specify a subroutine to handle all the |
| 502 | filetest operators (C<-f>, C<-x>, and so on: see L<perlfunc/-X> for |
| 503 | the full list); |
| 504 | it is not possible to overload any filetest operator individually. |
| 505 | To distinguish them, the letter following the '-' is passed as the |
| 506 | second argument (that is, in the slot that for binary operators |
| 507 | is used to pass the second operand). |
| 508 | |
| 509 | Calling an overloaded filetest operator does not affect the stat value |
| 510 | associated with the special filehandle C<_>. It still refers to the |
| 511 | result of the last C<stat>, C<lstat> or unoverloaded filetest. |
| 512 | |
| 513 | This overload was introduced in Perl 5.12. |
| 514 | |
| 515 | =item * I<Matching> |
| 516 | |
| 517 | The key C<"~~"> allows you to override the smart matching logic used by |
| 518 | the C<~~> operator and the switch construct (C<given>/C<when>). See |
| 519 | L<perlsyn/Switch Statements> and L<feature>. |
| 520 | |
| 521 | Unusually, the overloaded implementation of the smart match operator |
| 522 | does not get full control of the smart match behaviour. |
| 523 | In particular, in the following code: |
| 524 | |
| 525 | package Foo; |
| 526 | use overload '~~' => 'match'; |
| 527 | |
| 528 | my $obj = Foo->new(); |
| 529 | $obj ~~ [ 1,2,3 ]; |
| 530 | |
| 531 | the smart match does I<not> invoke the method call like this: |
| 532 | |
| 533 | $obj->match([1,2,3],0); |
| 534 | |
| 535 | rather, the smart match distributive rule takes precedence, so $obj is |
| 536 | smart matched against each array element in turn until a match is found, |
| 537 | so you may see between one and three of these calls instead: |
| 538 | |
| 539 | $obj->match(1,0); |
| 540 | $obj->match(2,0); |
| 541 | $obj->match(3,0); |
| 542 | |
| 543 | Consult the match table in L<perlop/"Smartmatch Operator"> for |
| 544 | details of when overloading is invoked. |
| 545 | |
| 546 | =item * I<Dereferencing> |
| 547 | |
| 548 | ${} @{} %{} &{} *{} |
| 549 | |
| 550 | If these operators are not explicitly overloaded then they |
| 551 | work in the normal way, yielding the underlying scalar, |
| 552 | array, or whatever stores the object data (or the appropriate |
| 553 | error message if the dereference operator doesn't match it). |
| 554 | Defining a catch-all C<'nomethod'> (see L<below|/nomethod>) |
| 555 | makes no difference to this as the catch-all function will |
| 556 | not be called to implement a missing dereference operator. |
| 557 | |
| 558 | If a dereference operator is overloaded then it must return a |
| 559 | I<reference> of the appropriate type (for example, the |
| 560 | subroutine for key C<'${}'> should return a reference to a |
| 561 | scalar, not a scalar), or another object which overloads the |
| 562 | operator: that is, the subroutine only determines what is |
| 563 | dereferenced and the actual dereferencing is left to Perl. |
| 564 | As a special case, if the subroutine returns the object itself |
| 565 | then it will not be called again - avoiding infinite recursion. |
| 566 | |
| 567 | =item * I<Special> |
| 568 | |
| 569 | nomethod fallback = |
| 570 | |
| 571 | See L<Special Keys for C<use overload>>. |
| 572 | |
| 573 | =back |
| 574 | |
| 575 | =head2 Magic Autogeneration |
| 576 | |
| 577 | If a method for an operation is not found then Perl tries to |
| 578 | autogenerate a substitute implementation from the operations |
| 579 | that have been defined. |
| 580 | |
| 581 | Note: the behaviour described in this section can be disabled |
| 582 | by setting C<fallback> to FALSE (see L</fallback>). |
| 583 | |
| 584 | In the following tables, numbers indicate priority. |
| 585 | For example, the table below states that, |
| 586 | if no implementation for C<'!'> has been defined then Perl will |
| 587 | implement it using C<'bool'> (that is, by inverting the value |
| 588 | returned by the method for C<'bool'>); |
| 589 | if boolean conversion is also unimplemented then Perl will |
| 590 | use C<'0+'> or, failing that, C<'""'>. |
| 591 | |
| 592 | operator | can be autogenerated from |
| 593 | | |
| 594 | | 0+ "" bool . x |
| 595 | =========|========================== |
| 596 | 0+ | 1 2 |
| 597 | "" | 1 2 |
| 598 | bool | 1 2 |
| 599 | int | 1 2 3 |
| 600 | ! | 2 3 1 |
| 601 | qr | 2 1 3 |
| 602 | . | 2 1 3 |
| 603 | x | 2 1 3 |
| 604 | .= | 3 2 4 1 |
| 605 | x= | 3 2 4 1 |
| 606 | <> | 2 1 3 |
| 607 | -X | 2 1 3 |
| 608 | |
| 609 | Note: The iterator (C<'E<lt>E<gt>'>) and file test (C<'-X'>) |
| 610 | operators work as normal: if the operand is not a blessed glob or |
| 611 | IO reference then it is converted to a string (using the method |
| 612 | for C<'""'>, C<'0+'>, or C<'bool'>) to be interpreted as a glob |
| 613 | or filename. |
| 614 | |
| 615 | operator | can be autogenerated from |
| 616 | | |
| 617 | | < <=> neg -= - |
| 618 | =========|========================== |
| 619 | neg | 1 |
| 620 | -= | 1 |
| 621 | -- | 1 2 |
| 622 | abs | a1 a2 b1 b2 [*] |
| 623 | < | 1 |
| 624 | <= | 1 |
| 625 | > | 1 |
| 626 | >= | 1 |
| 627 | == | 1 |
| 628 | != | 1 |
| 629 | |
| 630 | * one from [a1, a2] and one from [b1, b2] |
| 631 | |
| 632 | Just as numeric comparisons can be autogenerated from the method |
| 633 | for C<< '<=>' >>, string comparisons can be autogenerated from |
| 634 | that for C<'cmp'>: |
| 635 | |
| 636 | operators | can be autogenerated from |
| 637 | ====================|=========================== |
| 638 | lt gt le ge eq ne | cmp |
| 639 | |
| 640 | Similarly, autogeneration for keys C<'+='> and C<'++'> is analogous |
| 641 | to C<'-='> and C<'--'> above: |
| 642 | |
| 643 | operator | can be autogenerated from |
| 644 | | |
| 645 | | += + |
| 646 | =========|========================== |
| 647 | += | 1 |
| 648 | ++ | 1 2 |
| 649 | |
| 650 | And other assignment variations are analogous to |
| 651 | C<'+='> and C<'-='> (and similar to C<'.='> and C<'x='> above): |
| 652 | |
| 653 | operator || *= /= %= **= <<= >>= &= ^= |= |
| 654 | -------------------||-------------------------------- |
| 655 | autogenerated from || * / % ** << >> & ^ | |
| 656 | |
| 657 | Note also that the copy constructor (key C<'='>) may be |
| 658 | autogenerated, but only for objects based on scalars. |
| 659 | See L<Copy Constructor>. |
| 660 | |
| 661 | =head3 Minimal Set of Overloaded Operations |
| 662 | |
| 663 | Since some operations can be automatically generated from others, there is |
| 664 | a minimal set of operations that need to be overloaded in order to have |
| 665 | the complete set of overloaded operations at one's disposal. |
| 666 | Of course, the autogenerated operations may not do exactly what the user |
| 667 | expects. The minimal set is: |
| 668 | |
| 669 | + - * / % ** << >> x |
| 670 | <=> cmp |
| 671 | & | ^ ~ |
| 672 | atan2 cos sin exp log sqrt int |
| 673 | "" 0+ bool |
| 674 | ~~ |
| 675 | |
| 676 | Of the conversions, only one of string, boolean or numeric is |
| 677 | needed because each can be generated from either of the other two. |
| 678 | |
| 679 | =head2 Special Keys for C<use overload> |
| 680 | |
| 681 | =head3 C<nomethod> |
| 682 | |
| 683 | The C<'nomethod'> key is used to specify a catch-all function to |
| 684 | be called for any operator that is not individually overloaded. |
| 685 | The specified function will be passed four parameters. |
| 686 | The first three arguments coincide with those that would have been |
| 687 | passed to the corresponding method if it had been defined. |
| 688 | The fourth argument is the C<use overload> key for that missing |
| 689 | method. |
| 690 | |
| 691 | For example, if C<$a> is an object blessed into a package declaring |
| 692 | |
| 693 | use overload 'nomethod' => 'catch_all', # ... |
| 694 | |
| 695 | then the operation |
| 696 | |
| 697 | 3 + $a |
| 698 | |
| 699 | could (unless a method is specifically declared for the key |
| 700 | C<'+'>) result in a call |
| 701 | |
| 702 | catch_all($a, 3, 1, '+') |
| 703 | |
| 704 | See L<How Perl Chooses an Operator Implementation>. |
| 705 | |
| 706 | =head3 C<fallback> |
| 707 | |
| 708 | The value assigned to the key C<'fallback'> tells Perl how hard |
| 709 | it should try to find an alternative way to implement a missing |
| 710 | operator. |
| 711 | |
| 712 | =over |
| 713 | |
| 714 | =item * defined, but FALSE |
| 715 | |
| 716 | use overload "fallback" => 0, # ... ; |
| 717 | |
| 718 | This disables L<Magic Autogeneration>. |
| 719 | |
| 720 | =item * C<undef> |
| 721 | |
| 722 | In the default case where no value is explicitly assigned to |
| 723 | C<fallback>, magic autogeneration is enabled. |
| 724 | |
| 725 | =item * TRUE |
| 726 | |
| 727 | The same as for C<undef>, but if a missing operator cannot be |
| 728 | autogenerated then, instead of issuing an error message, Perl |
| 729 | is allowed to revert to what it would have done for that |
| 730 | operator if there had been no C<use overload> directive. |
| 731 | |
| 732 | Note: in most cases, particularly the L<Copy Constructor>, |
| 733 | this is unlikely to be appropriate behaviour. |
| 734 | |
| 735 | =back |
| 736 | |
| 737 | See L<How Perl Chooses an Operator Implementation>. |
| 738 | |
| 739 | =head3 Copy Constructor |
| 740 | |
| 741 | As mentioned L<above|"Mathemagic, Mutators, and Copy Constructors">, |
| 742 | this operation is called when a mutator is applied to a reference |
| 743 | that shares its object with some other reference. |
| 744 | For example, if C<$b> is mathemagical, and C<'++'> is overloaded |
| 745 | with C<'incr'>, and C<'='> is overloaded with C<'clone'>, then the |
| 746 | code |
| 747 | |
| 748 | $a = $b; |
| 749 | # ... (other code which does not modify $a or $b) ... |
| 750 | ++$b; |
| 751 | |
| 752 | would be executed in a manner equivalent to |
| 753 | |
| 754 | $a = $b; |
| 755 | # ... |
| 756 | $b = $b->clone(undef, ""); |
| 757 | $b->incr(undef, ""); |
| 758 | |
| 759 | Note: |
| 760 | |
| 761 | =over |
| 762 | |
| 763 | =item * |
| 764 | |
| 765 | The subroutine for C<'='> does not overload the Perl assignment |
| 766 | operator: it is used only to allow mutators to work as described |
| 767 | here. (See L</Assignments> above.) |
| 768 | |
| 769 | =item * |
| 770 | |
| 771 | As for other operations, the subroutine implementing '=' is passed |
| 772 | three arguments, though the last two are always C<undef> and C<''>. |
| 773 | |
| 774 | =item * |
| 775 | |
| 776 | The copy constructor is called only before a call to a function |
| 777 | declared to implement a mutator, for example, if C<++$b;> in the |
| 778 | code above is effected via a method declared for key C<'++'> |
| 779 | (or 'nomethod', passed C<'++'> as the fourth argument) or, by |
| 780 | autogeneration, C<'+='>. |
| 781 | It is not called if the increment operation is effected by a call |
| 782 | to the method for C<'+'> since, in the equivalent code, |
| 783 | |
| 784 | $a = $b; |
| 785 | $b = $b + 1; |
| 786 | |
| 787 | the data referred to by C<$a> is unchanged by the assignment to |
| 788 | C<$b> of a reference to new object data. |
| 789 | |
| 790 | =item * |
| 791 | |
| 792 | The copy constructor is not called if Perl determines that it is |
| 793 | unnecessary because there is no other reference to the data being |
| 794 | modified. |
| 795 | |
| 796 | =item * |
| 797 | |
| 798 | If C<'fallback'> is undefined or TRUE then a copy constructor |
| 799 | can be autogenerated, but only for objects based on scalars. |
| 800 | In other cases it needs to be defined explicitly. |
| 801 | Where an object's data is stored as, for example, an array of |
| 802 | scalars, the following might be appropriate: |
| 803 | |
| 804 | use overload '=' => sub { bless [ @{$_[0]} ] }, # ... |
| 805 | |
| 806 | =item * |
| 807 | |
| 808 | If C<'fallback'> is TRUE and no copy constructor is defined then, |
| 809 | for objects not based on scalars, Perl may silently fall back on |
| 810 | simple assignment - that is, assignment of the object reference. |
| 811 | In effect, this disables the copy constructor mechanism since |
| 812 | no new copy of the object data is created. |
| 813 | This is almost certainly not what you want. |
| 814 | (It is, however, consistent: for example, Perl's fallback for the |
| 815 | C<++> operator is to increment the reference itself.) |
| 816 | |
| 817 | =back |
| 818 | |
| 819 | =head2 How Perl Chooses an Operator Implementation |
| 820 | |
| 821 | Which is checked first, C<nomethod> or C<fallback>? |
| 822 | If the two operands of an operator are of different types and |
| 823 | both overload the operator, which implementation is used? |
| 824 | The following are the precedence rules: |
| 825 | |
| 826 | =over |
| 827 | |
| 828 | =item 1. |
| 829 | |
| 830 | If the first operand has declared a subroutine to overload the |
| 831 | operator then use that implementation. |
| 832 | |
| 833 | =item 2. |
| 834 | |
| 835 | Otherwise, if fallback is TRUE or undefined for the |
| 836 | first operand then see if the |
| 837 | L<rules for autogeneration|"Magic Autogeneration"> |
| 838 | allows another of its operators to be used instead. |
| 839 | |
| 840 | =item 3. |
| 841 | |
| 842 | Unless the operator is an assignment (C<+=>, C<-=>, etc.), |
| 843 | repeat step (1) in respect of the second operand. |
| 844 | |
| 845 | =item 4. |
| 846 | |
| 847 | Repeat Step (2) in respect of the second operand. |
| 848 | |
| 849 | =item 5. |
| 850 | |
| 851 | If the first operand has a "nomethod" method then use that. |
| 852 | |
| 853 | =item 6. |
| 854 | |
| 855 | If the second operand has a "nomethod" method then use that. |
| 856 | |
| 857 | =item 7. |
| 858 | |
| 859 | If C<fallback> is TRUE for both operands |
| 860 | then perform the usual operation for the operator, |
| 861 | treating the operands as numbers, strings, or booleans |
| 862 | as appropriate for the operator (see note). |
| 863 | |
| 864 | =item 8. |
| 865 | |
| 866 | Nothing worked - die. |
| 867 | |
| 868 | =back |
| 869 | |
| 870 | Where there is only one operand (or only one operand with |
| 871 | overloading) the checks in respect of the other operand above are |
| 872 | skipped. |
| 873 | |
| 874 | There are exceptions to the above rules for dereference operations |
| 875 | (which, if Step 1 fails, always fall back to the normal, built-in |
| 876 | implementations - see Dereferencing), and for C<~~> (which has its |
| 877 | own set of rules - see C<Matching> under L</Overloadable Operations> |
| 878 | above). |
| 879 | |
| 880 | Note on Step 7: some operators have a different semantic depending |
| 881 | on the type of their operands. |
| 882 | As there is no way to instruct Perl to treat the operands as, e.g., |
| 883 | numbers instead of strings, the result here may not be what you |
| 884 | expect. |
| 885 | See L<BUGS AND PITFALLS>. |
| 886 | |
| 887 | =head2 Losing Overloading |
| 888 | |
| 889 | The restriction for the comparison operation is that even if, for example, |
| 890 | C<cmp> should return a blessed reference, the autogenerated C<lt> |
| 891 | function will produce only a standard logical value based on the |
| 892 | numerical value of the result of C<cmp>. In particular, a working |
| 893 | numeric conversion is needed in this case (possibly expressed in terms of |
| 894 | other conversions). |
| 895 | |
| 896 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
| 897 | if the string conversion substitution is applied. |
| 898 | |
| 899 | When you chop() a mathemagical object it is promoted to a string and its |
| 900 | mathemagical properties are lost. The same can happen with other |
| 901 | operations as well. |
| 902 | |
| 903 | =head2 Inheritance and Overloading |
| 904 | |
| 905 | Overloading respects inheritance via the @ISA hierarchy. |
| 906 | Inheritance interacts with overloading in two ways. |
| 907 | |
| 908 | =over |
| 909 | |
| 910 | =item Method names in the C<use overload> directive |
| 911 | |
| 912 | If C<value> in |
| 913 | |
| 914 | use overload key => value; |
| 915 | |
| 916 | is a string, it is interpreted as a method name - which may |
| 917 | (in the usual way) be inherited from another class. |
| 918 | |
| 919 | =item Overloading of an operation is inherited by derived classes |
| 920 | |
| 921 | Any class derived from an overloaded class is also overloaded |
| 922 | and inherits its operator implementations. |
| 923 | If the same operator is overloaded in more than one ancestor |
| 924 | then the implementation is determined by the usual inheritance |
| 925 | rules. |
| 926 | |
| 927 | For example, if C<A> inherits from C<B> and C<C> (in that order), |
| 928 | C<B> overloads C<+> with C<\&D::plus_sub>, and C<C> overloads |
| 929 | C<+> by C<"plus_meth">, then the subroutine C<D::plus_sub> will |
| 930 | be called to implement operation C<+> for an object in package C<A>. |
| 931 | |
| 932 | =back |
| 933 | |
| 934 | Note that in Perl version prior to 5.18 inheritance of the C<fallback> key |
| 935 | was not governed by the above rules. The value of C<fallback> in the first |
| 936 | overloaded ancestor was used. This was fixed in 5.18 to follow the usual |
| 937 | rules of inheritance. |
| 938 | |
| 939 | =head2 Run-time Overloading |
| 940 | |
| 941 | Since all C<use> directives are executed at compile-time, the only way to |
| 942 | change overloading during run-time is to |
| 943 | |
| 944 | eval 'use overload "+" => \&addmethod'; |
| 945 | |
| 946 | You can also use |
| 947 | |
| 948 | eval 'no overload "+", "--", "<="'; |
| 949 | |
| 950 | though the use of these constructs during run-time is questionable. |
| 951 | |
| 952 | =head2 Public Functions |
| 953 | |
| 954 | Package C<overload.pm> provides the following public functions: |
| 955 | |
| 956 | =over 5 |
| 957 | |
| 958 | =item overload::StrVal(arg) |
| 959 | |
| 960 | Gives the string value of C<arg> as in the |
| 961 | absence of stringify overloading. If you |
| 962 | are using this to get the address of a reference (useful for checking if two |
| 963 | references point to the same thing) then you may be better off using |
| 964 | C<Scalar::Util::refaddr()>, which is faster. |
| 965 | |
| 966 | =item overload::Overloaded(arg) |
| 967 | |
| 968 | Returns true if C<arg> is subject to overloading of some operations. |
| 969 | |
| 970 | =item overload::Method(obj,op) |
| 971 | |
| 972 | Returns C<undef> or a reference to the method that implements C<op>. |
| 973 | |
| 974 | =back |
| 975 | |
| 976 | =head2 Overloading Constants |
| 977 | |
| 978 | For some applications, the Perl parser mangles constants too much. |
| 979 | It is possible to hook into this process via C<overload::constant()> |
| 980 | and C<overload::remove_constant()> functions. |
| 981 | |
| 982 | These functions take a hash as an argument. The recognized keys of this hash |
| 983 | are: |
| 984 | |
| 985 | =over 8 |
| 986 | |
| 987 | =item integer |
| 988 | |
| 989 | to overload integer constants, |
| 990 | |
| 991 | =item float |
| 992 | |
| 993 | to overload floating point constants, |
| 994 | |
| 995 | =item binary |
| 996 | |
| 997 | to overload octal and hexadecimal constants, |
| 998 | |
| 999 | =item q |
| 1000 | |
| 1001 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
| 1002 | strings and here-documents, |
| 1003 | |
| 1004 | =item qr |
| 1005 | |
| 1006 | to overload constant pieces of regular expressions. |
| 1007 | |
| 1008 | =back |
| 1009 | |
| 1010 | The corresponding values are references to functions which take three arguments: |
| 1011 | the first one is the I<initial> string form of the constant, the second one |
| 1012 | is how Perl interprets this constant, the third one is how the constant is used. |
| 1013 | Note that the initial string form does not |
| 1014 | contain string delimiters, and has backslashes in backslash-delimiter |
| 1015 | combinations stripped (thus the value of delimiter is not relevant for |
| 1016 | processing of this string). The return value of this function is how this |
| 1017 | constant is going to be interpreted by Perl. The third argument is undefined |
| 1018 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
| 1019 | context (comes from strings, regular expressions, and single-quote HERE |
| 1020 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
| 1021 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
| 1022 | |
| 1023 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
| 1024 | it is expected that overloaded constant strings are equipped with reasonable |
| 1025 | overloaded catenation operator, otherwise absurd results will result. |
| 1026 | Similarly, negative numbers are considered as negations of positive constants. |
| 1027 | |
| 1028 | Note that it is probably meaningless to call the functions overload::constant() |
| 1029 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
| 1030 | From these methods they may be called as |
| 1031 | |
| 1032 | sub import { |
| 1033 | shift; |
| 1034 | return unless @_; |
| 1035 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
| 1036 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
| 1037 | } |
| 1038 | |
| 1039 | =head1 IMPLEMENTATION |
| 1040 | |
| 1041 | What follows is subject to change RSN. |
| 1042 | |
| 1043 | The table of methods for all operations is cached in magic for the |
| 1044 | symbol table hash for the package. The cache is invalidated during |
| 1045 | processing of C<use overload>, C<no overload>, new function |
| 1046 | definitions, and changes in @ISA. |
| 1047 | |
| 1048 | (Every SVish thing has a magic queue, and magic is an entry in that |
| 1049 | queue. This is how a single variable may participate in multiple |
| 1050 | forms of magic simultaneously. For instance, environment variables |
| 1051 | regularly have two forms at once: their %ENV magic and their taint |
| 1052 | magic. However, the magic which implements overloading is applied to |
| 1053 | the stashes, which are rarely used directly, thus should not slow down |
| 1054 | Perl.) |
| 1055 | |
| 1056 | If a package uses overload, it carries a special flag. This flag is also |
| 1057 | set when new function are defined or @ISA is modified. There will be a |
| 1058 | slight speed penalty on the very first operation thereafter that supports |
| 1059 | overloading, while the overload tables are updated. If there is no |
| 1060 | overloading present, the flag is turned off. Thus the only speed penalty |
| 1061 | thereafter is the checking of this flag. |
| 1062 | |
| 1063 | It is expected that arguments to methods that are not explicitly supposed |
| 1064 | to be changed are constant (but this is not enforced). |
| 1065 | |
| 1066 | =head1 COOKBOOK |
| 1067 | |
| 1068 | Please add examples to what follows! |
| 1069 | |
| 1070 | =head2 Two-face Scalars |
| 1071 | |
| 1072 | Put this in F<two_face.pm> in your Perl library directory: |
| 1073 | |
| 1074 | package two_face; # Scalars with separate string and |
| 1075 | # numeric values. |
| 1076 | sub new { my $p = shift; bless [@_], $p } |
| 1077 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
| 1078 | sub num {shift->[1]} |
| 1079 | sub str {shift->[0]} |
| 1080 | |
| 1081 | Use it as follows: |
| 1082 | |
| 1083 | require two_face; |
| 1084 | my $seven = two_face->new("vii", 7); |
| 1085 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
| 1086 | print "seven contains 'i'\n" if $seven =~ /i/; |
| 1087 | |
| 1088 | (The second line creates a scalar which has both a string value, and a |
| 1089 | numeric value.) This prints: |
| 1090 | |
| 1091 | seven=vii, seven=7, eight=8 |
| 1092 | seven contains 'i' |
| 1093 | |
| 1094 | =head2 Two-face References |
| 1095 | |
| 1096 | Suppose you want to create an object which is accessible as both an |
| 1097 | array reference and a hash reference. |
| 1098 | |
| 1099 | package two_refs; |
| 1100 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
| 1101 | sub new { |
| 1102 | my $p = shift; |
| 1103 | bless \ [@_], $p; |
| 1104 | } |
| 1105 | sub gethash { |
| 1106 | my %h; |
| 1107 | my $self = shift; |
| 1108 | tie %h, ref $self, $self; |
| 1109 | \%h; |
| 1110 | } |
| 1111 | |
| 1112 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
| 1113 | my %fields; |
| 1114 | my $i = 0; |
| 1115 | $fields{$_} = $i++ foreach qw{zero one two three}; |
| 1116 | sub STORE { |
| 1117 | my $self = ${shift()}; |
| 1118 | my $key = $fields{shift()}; |
| 1119 | defined $key or die "Out of band access"; |
| 1120 | $$self->[$key] = shift; |
| 1121 | } |
| 1122 | sub FETCH { |
| 1123 | my $self = ${shift()}; |
| 1124 | my $key = $fields{shift()}; |
| 1125 | defined $key or die "Out of band access"; |
| 1126 | $$self->[$key]; |
| 1127 | } |
| 1128 | |
| 1129 | Now one can access an object using both the array and hash syntax: |
| 1130 | |
| 1131 | my $bar = two_refs->new(3,4,5,6); |
| 1132 | $bar->[2] = 11; |
| 1133 | $bar->{two} == 11 or die 'bad hash fetch'; |
| 1134 | |
| 1135 | Note several important features of this example. First of all, the |
| 1136 | I<actual> type of $bar is a scalar reference, and we do not overload |
| 1137 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
| 1138 | contents of $bar by just using C<$$bar> (what we do in functions which |
| 1139 | overload dereference). Similarly, the object returned by the |
| 1140 | TIEHASH() method is a scalar reference. |
| 1141 | |
| 1142 | Second, we create a new tied hash each time the hash syntax is used. |
| 1143 | This allows us not to worry about a possibility of a reference loop, |
| 1144 | which would lead to a memory leak. |
| 1145 | |
| 1146 | Both these problems can be cured. Say, if we want to overload hash |
| 1147 | dereference on a reference to an object which is I<implemented> as a |
| 1148 | hash itself, the only problem one has to circumvent is how to access |
| 1149 | this I<actual> hash (as opposed to the I<virtual> hash exhibited by the |
| 1150 | overloaded dereference operator). Here is one possible fetching routine: |
| 1151 | |
| 1152 | sub access_hash { |
| 1153 | my ($self, $key) = (shift, shift); |
| 1154 | my $class = ref $self; |
| 1155 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
| 1156 | my $out = $self->{$key}; |
| 1157 | bless $self, $class; # Restore overloading |
| 1158 | $out; |
| 1159 | } |
| 1160 | |
| 1161 | To remove creation of the tied hash on each access, one may an extra |
| 1162 | level of indirection which allows a non-circular structure of references: |
| 1163 | |
| 1164 | package two_refs1; |
| 1165 | use overload '%{}' => sub { ${shift()}->[1] }, |
| 1166 | '@{}' => sub { ${shift()}->[0] }; |
| 1167 | sub new { |
| 1168 | my $p = shift; |
| 1169 | my $a = [@_]; |
| 1170 | my %h; |
| 1171 | tie %h, $p, $a; |
| 1172 | bless \ [$a, \%h], $p; |
| 1173 | } |
| 1174 | sub gethash { |
| 1175 | my %h; |
| 1176 | my $self = shift; |
| 1177 | tie %h, ref $self, $self; |
| 1178 | \%h; |
| 1179 | } |
| 1180 | |
| 1181 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
| 1182 | my %fields; |
| 1183 | my $i = 0; |
| 1184 | $fields{$_} = $i++ foreach qw{zero one two three}; |
| 1185 | sub STORE { |
| 1186 | my $a = ${shift()}; |
| 1187 | my $key = $fields{shift()}; |
| 1188 | defined $key or die "Out of band access"; |
| 1189 | $a->[$key] = shift; |
| 1190 | } |
| 1191 | sub FETCH { |
| 1192 | my $a = ${shift()}; |
| 1193 | my $key = $fields{shift()}; |
| 1194 | defined $key or die "Out of band access"; |
| 1195 | $a->[$key]; |
| 1196 | } |
| 1197 | |
| 1198 | Now if $baz is overloaded like this, then C<$baz> is a reference to a |
| 1199 | reference to the intermediate array, which keeps a reference to an |
| 1200 | actual array, and the access hash. The tie()ing object for the access |
| 1201 | hash is a reference to a reference to the actual array, so |
| 1202 | |
| 1203 | =over |
| 1204 | |
| 1205 | =item * |
| 1206 | |
| 1207 | There are no loops of references. |
| 1208 | |
| 1209 | =item * |
| 1210 | |
| 1211 | Both "objects" which are blessed into the class C<two_refs1> are |
| 1212 | references to a reference to an array, thus references to a I<scalar>. |
| 1213 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
| 1214 | overloaded operations. |
| 1215 | |
| 1216 | =back |
| 1217 | |
| 1218 | =head2 Symbolic Calculator |
| 1219 | |
| 1220 | Put this in F<symbolic.pm> in your Perl library directory: |
| 1221 | |
| 1222 | package symbolic; # Primitive symbolic calculator |
| 1223 | use overload nomethod => \&wrap; |
| 1224 | |
| 1225 | sub new { shift; bless ['n', @_] } |
| 1226 | sub wrap { |
| 1227 | my ($obj, $other, $inv, $meth) = @_; |
| 1228 | ($obj, $other) = ($other, $obj) if $inv; |
| 1229 | bless [$meth, $obj, $other]; |
| 1230 | } |
| 1231 | |
| 1232 | This module is very unusual as overloaded modules go: it does not |
| 1233 | provide any usual overloaded operators, instead it provides an |
| 1234 | implementation for L</C<nomethod>>. In this example the C<nomethod> |
| 1235 | subroutine returns an object which encapsulates operations done over |
| 1236 | the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 + |
| 1237 | symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>. |
| 1238 | |
| 1239 | Here is an example of the script which "calculates" the side of |
| 1240 | circumscribed octagon using the above package: |
| 1241 | |
| 1242 | require symbolic; |
| 1243 | my $iter = 1; # 2**($iter+2) = 8 |
| 1244 | my $side = symbolic->new(1); |
| 1245 | my $cnt = $iter; |
| 1246 | |
| 1247 | while ($cnt--) { |
| 1248 | $side = (sqrt(1 + $side**2) - 1)/$side; |
| 1249 | } |
| 1250 | print "OK\n"; |
| 1251 | |
| 1252 | The value of $side is |
| 1253 | |
| 1254 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
| 1255 | undef], 1], ['n', 1]] |
| 1256 | |
| 1257 | Note that while we obtained this value using a nice little script, |
| 1258 | there is no simple way to I<use> this value. In fact this value may |
| 1259 | be inspected in debugger (see L<perldebug>), but only if |
| 1260 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
| 1261 | |
| 1262 | If one attempts to print this value, then the overloaded operator |
| 1263 | C<""> will be called, which will call C<nomethod> operator. The |
| 1264 | result of this operator will be stringified again, but this result is |
| 1265 | again of type C<symbolic>, which will lead to an infinite loop. |
| 1266 | |
| 1267 | Add a pretty-printer method to the module F<symbolic.pm>: |
| 1268 | |
| 1269 | sub pretty { |
| 1270 | my ($meth, $a, $b) = @{+shift}; |
| 1271 | $a = 'u' unless defined $a; |
| 1272 | $b = 'u' unless defined $b; |
| 1273 | $a = $a->pretty if ref $a; |
| 1274 | $b = $b->pretty if ref $b; |
| 1275 | "[$meth $a $b]"; |
| 1276 | } |
| 1277 | |
| 1278 | Now one can finish the script by |
| 1279 | |
| 1280 | print "side = ", $side->pretty, "\n"; |
| 1281 | |
| 1282 | The method C<pretty> is doing object-to-string conversion, so it |
| 1283 | is natural to overload the operator C<""> using this method. However, |
| 1284 | inside such a method it is not necessary to pretty-print the |
| 1285 | I<components> $a and $b of an object. In the above subroutine |
| 1286 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
| 1287 | and $b. If these components use overloading, the catenation operator |
| 1288 | will look for an overloaded operator C<.>; if not present, it will |
| 1289 | look for an overloaded operator C<"">. Thus it is enough to use |
| 1290 | |
| 1291 | use overload nomethod => \&wrap, '""' => \&str; |
| 1292 | sub str { |
| 1293 | my ($meth, $a, $b) = @{+shift}; |
| 1294 | $a = 'u' unless defined $a; |
| 1295 | $b = 'u' unless defined $b; |
| 1296 | "[$meth $a $b]"; |
| 1297 | } |
| 1298 | |
| 1299 | Now one can change the last line of the script to |
| 1300 | |
| 1301 | print "side = $side\n"; |
| 1302 | |
| 1303 | which outputs |
| 1304 | |
| 1305 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
| 1306 | |
| 1307 | and one can inspect the value in debugger using all the possible |
| 1308 | methods. |
| 1309 | |
| 1310 | Something is still amiss: consider the loop variable $cnt of the |
| 1311 | script. It was a number, not an object. We cannot make this value of |
| 1312 | type C<symbolic>, since then the loop will not terminate. |
| 1313 | |
| 1314 | Indeed, to terminate the cycle, the $cnt should become false. |
| 1315 | However, the operator C<bool> for checking falsity is overloaded (this |
| 1316 | time via overloaded C<"">), and returns a long string, thus any object |
| 1317 | of type C<symbolic> is true. To overcome this, we need a way to |
| 1318 | compare an object to 0. In fact, it is easier to write a numeric |
| 1319 | conversion routine. |
| 1320 | |
| 1321 | Here is the text of F<symbolic.pm> with such a routine added (and |
| 1322 | slightly modified str()): |
| 1323 | |
| 1324 | package symbolic; # Primitive symbolic calculator |
| 1325 | use overload |
| 1326 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
| 1327 | |
| 1328 | sub new { shift; bless ['n', @_] } |
| 1329 | sub wrap { |
| 1330 | my ($obj, $other, $inv, $meth) = @_; |
| 1331 | ($obj, $other) = ($other, $obj) if $inv; |
| 1332 | bless [$meth, $obj, $other]; |
| 1333 | } |
| 1334 | sub str { |
| 1335 | my ($meth, $a, $b) = @{+shift}; |
| 1336 | $a = 'u' unless defined $a; |
| 1337 | if (defined $b) { |
| 1338 | "[$meth $a $b]"; |
| 1339 | } else { |
| 1340 | "[$meth $a]"; |
| 1341 | } |
| 1342 | } |
| 1343 | my %subr = ( n => sub {$_[0]}, |
| 1344 | sqrt => sub {sqrt $_[0]}, |
| 1345 | '-' => sub {shift() - shift()}, |
| 1346 | '+' => sub {shift() + shift()}, |
| 1347 | '/' => sub {shift() / shift()}, |
| 1348 | '*' => sub {shift() * shift()}, |
| 1349 | '**' => sub {shift() ** shift()}, |
| 1350 | ); |
| 1351 | sub num { |
| 1352 | my ($meth, $a, $b) = @{+shift}; |
| 1353 | my $subr = $subr{$meth} |
| 1354 | or die "Do not know how to ($meth) in symbolic"; |
| 1355 | $a = $a->num if ref $a eq __PACKAGE__; |
| 1356 | $b = $b->num if ref $b eq __PACKAGE__; |
| 1357 | $subr->($a,$b); |
| 1358 | } |
| 1359 | |
| 1360 | All the work of numeric conversion is done in %subr and num(). Of |
| 1361 | course, %subr is not complete, it contains only operators used in the |
| 1362 | example below. Here is the extra-credit question: why do we need an |
| 1363 | explicit recursion in num()? (Answer is at the end of this section.) |
| 1364 | |
| 1365 | Use this module like this: |
| 1366 | |
| 1367 | require symbolic; |
| 1368 | my $iter = symbolic->new(2); # 16-gon |
| 1369 | my $side = symbolic->new(1); |
| 1370 | my $cnt = $iter; |
| 1371 | |
| 1372 | while ($cnt) { |
| 1373 | $cnt = $cnt - 1; # Mutator '--' not implemented |
| 1374 | $side = (sqrt(1 + $side**2) - 1)/$side; |
| 1375 | } |
| 1376 | printf "%s=%f\n", $side, $side; |
| 1377 | printf "pi=%f\n", $side*(2**($iter+2)); |
| 1378 | |
| 1379 | It prints (without so many line breaks) |
| 1380 | |
| 1381 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
| 1382 | [n 1]] 2]]] 1] |
| 1383 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
| 1384 | pi=3.182598 |
| 1385 | |
| 1386 | The above module is very primitive. It does not implement |
| 1387 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
| 1388 | (not required without mutators!), and implements only those arithmetic |
| 1389 | operations which are used in the example. |
| 1390 | |
| 1391 | To implement most arithmetic operations is easy; one should just use |
| 1392 | the tables of operations, and change the code which fills %subr to |
| 1393 | |
| 1394 | my %subr = ( 'n' => sub {$_[0]} ); |
| 1395 | foreach my $op (split " ", $overload::ops{with_assign}) { |
| 1396 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
| 1397 | } |
| 1398 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
| 1399 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
| 1400 | $subr{$op} = eval "sub {shift() $op shift()}"; |
| 1401 | } |
| 1402 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
| 1403 | print "defining '$op'\n"; |
| 1404 | $subr{$op} = eval "sub {$op shift()}"; |
| 1405 | } |
| 1406 | |
| 1407 | Since subroutines implementing assignment operators are not required |
| 1408 | to modify their operands (see L<Overloadable Operations> above), |
| 1409 | we do not need anything special to make C<+=> and friends work, |
| 1410 | besides adding these operators to %subr and defining a copy |
| 1411 | constructor (needed since Perl has no way to know that the |
| 1412 | implementation of C<'+='> does not mutate the argument - |
| 1413 | see L<Copy Constructor>). |
| 1414 | |
| 1415 | To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload> |
| 1416 | line, and code (this code assumes that mutators change things one level |
| 1417 | deep only, so recursive copying is not needed): |
| 1418 | |
| 1419 | sub cpy { |
| 1420 | my $self = shift; |
| 1421 | bless [@$self], ref $self; |
| 1422 | } |
| 1423 | |
| 1424 | To make C<++> and C<--> work, we need to implement actual mutators, |
| 1425 | either directly, or in C<nomethod>. We continue to do things inside |
| 1426 | C<nomethod>, thus add |
| 1427 | |
| 1428 | if ($meth eq '++' or $meth eq '--') { |
| 1429 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
| 1430 | return $obj; |
| 1431 | } |
| 1432 | |
| 1433 | after the first line of wrap(). This is not a most effective |
| 1434 | implementation, one may consider |
| 1435 | |
| 1436 | sub inc { $_[0] = bless ['++', shift, 1]; } |
| 1437 | |
| 1438 | instead. |
| 1439 | |
| 1440 | As a final remark, note that one can fill %subr by |
| 1441 | |
| 1442 | my %subr = ( 'n' => sub {$_[0]} ); |
| 1443 | foreach my $op (split " ", $overload::ops{with_assign}) { |
| 1444 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
| 1445 | } |
| 1446 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
| 1447 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
| 1448 | $subr{$op} = eval "sub {shift() $op shift()}"; |
| 1449 | } |
| 1450 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
| 1451 | $subr{$op} = eval "sub {$op shift()}"; |
| 1452 | } |
| 1453 | $subr{'++'} = $subr{'+'}; |
| 1454 | $subr{'--'} = $subr{'-'}; |
| 1455 | |
| 1456 | This finishes implementation of a primitive symbolic calculator in |
| 1457 | 50 lines of Perl code. Since the numeric values of subexpressions |
| 1458 | are not cached, the calculator is very slow. |
| 1459 | |
| 1460 | Here is the answer for the exercise: In the case of str(), we need no |
| 1461 | explicit recursion since the overloaded C<.>-operator will fall back |
| 1462 | to an existing overloaded operator C<"">. Overloaded arithmetic |
| 1463 | operators I<do not> fall back to numeric conversion if C<fallback> is |
| 1464 | not explicitly requested. Thus without an explicit recursion num() |
| 1465 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
| 1466 | the argument of num(). |
| 1467 | |
| 1468 | If you wonder why defaults for conversion are different for str() and |
| 1469 | num(), note how easy it was to write the symbolic calculator. This |
| 1470 | simplicity is due to an appropriate choice of defaults. One extra |
| 1471 | note: due to the explicit recursion num() is more fragile than sym(): |
| 1472 | we need to explicitly check for the type of $a and $b. If components |
| 1473 | $a and $b happen to be of some related type, this may lead to problems. |
| 1474 | |
| 1475 | =head2 I<Really> Symbolic Calculator |
| 1476 | |
| 1477 | One may wonder why we call the above calculator symbolic. The reason |
| 1478 | is that the actual calculation of the value of expression is postponed |
| 1479 | until the value is I<used>. |
| 1480 | |
| 1481 | To see it in action, add a method |
| 1482 | |
| 1483 | sub STORE { |
| 1484 | my $obj = shift; |
| 1485 | $#$obj = 1; |
| 1486 | @$obj->[0,1] = ('=', shift); |
| 1487 | } |
| 1488 | |
| 1489 | to the package C<symbolic>. After this change one can do |
| 1490 | |
| 1491 | my $a = symbolic->new(3); |
| 1492 | my $b = symbolic->new(4); |
| 1493 | my $c = sqrt($a**2 + $b**2); |
| 1494 | |
| 1495 | and the numeric value of $c becomes 5. However, after calling |
| 1496 | |
| 1497 | $a->STORE(12); $b->STORE(5); |
| 1498 | |
| 1499 | the numeric value of $c becomes 13. There is no doubt now that the module |
| 1500 | symbolic provides a I<symbolic> calculator indeed. |
| 1501 | |
| 1502 | To hide the rough edges under the hood, provide a tie()d interface to the |
| 1503 | package C<symbolic>. Add methods |
| 1504 | |
| 1505 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
| 1506 | sub FETCH { shift } |
| 1507 | sub nop { } # Around a bug |
| 1508 | |
| 1509 | (the bug, fixed in Perl 5.14, is described in L<"BUGS">). One can use this |
| 1510 | new interface as |
| 1511 | |
| 1512 | tie $a, 'symbolic', 3; |
| 1513 | tie $b, 'symbolic', 4; |
| 1514 | $a->nop; $b->nop; # Around a bug |
| 1515 | |
| 1516 | my $c = sqrt($a**2 + $b**2); |
| 1517 | |
| 1518 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
| 1519 | of $c becomes 13. To insulate the user of the module add a method |
| 1520 | |
| 1521 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
| 1522 | |
| 1523 | Now |
| 1524 | |
| 1525 | my ($a, $b); |
| 1526 | symbolic->vars($a, $b); |
| 1527 | my $c = sqrt($a**2 + $b**2); |
| 1528 | |
| 1529 | $a = 3; $b = 4; |
| 1530 | printf "c5 %s=%f\n", $c, $c; |
| 1531 | |
| 1532 | $a = 12; $b = 5; |
| 1533 | printf "c13 %s=%f\n", $c, $c; |
| 1534 | |
| 1535 | shows that the numeric value of $c follows changes to the values of $a |
| 1536 | and $b. |
| 1537 | |
| 1538 | =head1 AUTHOR |
| 1539 | |
| 1540 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
| 1541 | |
| 1542 | =head1 SEE ALSO |
| 1543 | |
| 1544 | The C<overloading> pragma can be used to enable or disable overloaded |
| 1545 | operations within a lexical scope - see L<overloading>. |
| 1546 | |
| 1547 | =head1 DIAGNOSTICS |
| 1548 | |
| 1549 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
| 1550 | induces diagnostic messages. |
| 1551 | |
| 1552 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
| 1553 | deduce which operations are overloaded (and which ancestor triggers |
| 1554 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
| 1555 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
| 1556 | key (in fact a presence of this method shows that this package has |
| 1557 | overloading enabled, and it is what is used by the C<Overloaded> |
| 1558 | function of module C<overload>). |
| 1559 | |
| 1560 | The module might issue the following warnings: |
| 1561 | |
| 1562 | =over 4 |
| 1563 | |
| 1564 | =item Odd number of arguments for overload::constant |
| 1565 | |
| 1566 | (W) The call to overload::constant contained an odd number of arguments. |
| 1567 | The arguments should come in pairs. |
| 1568 | |
| 1569 | =item '%s' is not an overloadable type |
| 1570 | |
| 1571 | (W) You tried to overload a constant type the overload package is unaware of. |
| 1572 | |
| 1573 | =item '%s' is not a code reference |
| 1574 | |
| 1575 | (W) The second (fourth, sixth, ...) argument of overload::constant needs |
| 1576 | to be a code reference. Either an anonymous subroutine, or a reference |
| 1577 | to a subroutine. |
| 1578 | |
| 1579 | =item overload arg '%s' is invalid |
| 1580 | |
| 1581 | (W) C<use overload> was passed an argument it did not |
| 1582 | recognize. Did you mistype an operator? |
| 1583 | |
| 1584 | =back |
| 1585 | |
| 1586 | =head1 BUGS AND PITFALLS |
| 1587 | |
| 1588 | =over |
| 1589 | |
| 1590 | =item * |
| 1591 | |
| 1592 | A pitfall when fallback is TRUE and Perl resorts to a built-in |
| 1593 | implementation of an operator is that some operators have more |
| 1594 | than one semantic, for example C<|>: |
| 1595 | |
| 1596 | use overload '0+' => sub { $_[0]->{n}; }, |
| 1597 | fallback => 1; |
| 1598 | my $x = bless { n => 4 }, "main"; |
| 1599 | my $y = bless { n => 8 }, "main"; |
| 1600 | print $x | $y, "\n"; |
| 1601 | |
| 1602 | You might expect this to output "12". |
| 1603 | In fact, it prints "<": the ASCII result of treating "|" |
| 1604 | as a bitwise string operator - that is, the result of treating |
| 1605 | the operands as the strings "4" and "8" rather than numbers. |
| 1606 | The fact that numify (C<0+>) is implemented but stringify |
| 1607 | (C<"">) isn't makes no difference since the latter is simply |
| 1608 | autogenerated from the former. |
| 1609 | |
| 1610 | The only way to change this is to provide your own subroutine |
| 1611 | for C<'|'>. |
| 1612 | |
| 1613 | =item * |
| 1614 | |
| 1615 | Magic autogeneration increases the potential for inadvertently |
| 1616 | creating self-referential structures. |
| 1617 | Currently Perl will not free self-referential |
| 1618 | structures until cycles are explicitly broken. |
| 1619 | For example, |
| 1620 | |
| 1621 | use overload '+' => 'add'; |
| 1622 | sub add { bless [ \$_[0], \$_[1] ] }; |
| 1623 | |
| 1624 | is asking for trouble, since |
| 1625 | |
| 1626 | $obj += $y; |
| 1627 | |
| 1628 | will effectively become |
| 1629 | |
| 1630 | $obj = add($obj, $y, undef); |
| 1631 | |
| 1632 | with the same result as |
| 1633 | |
| 1634 | $obj = [\$obj, \$foo]; |
| 1635 | |
| 1636 | Even if no I<explicit> assignment-variants of operators are present in |
| 1637 | the script, they may be generated by the optimizer. |
| 1638 | For example, |
| 1639 | |
| 1640 | "obj = $obj\n" |
| 1641 | |
| 1642 | may be optimized to |
| 1643 | |
| 1644 | my $tmp = 'obj = ' . $obj; $tmp .= "\n"; |
| 1645 | |
| 1646 | =item * |
| 1647 | |
| 1648 | The symbol table is filled with names looking like line-noise. |
| 1649 | |
| 1650 | =item * |
| 1651 | |
| 1652 | This bug was fixed in Perl 5.18, but may still trip you up if you are using |
| 1653 | older versions: |
| 1654 | |
| 1655 | For the purpose of inheritance every overloaded package behaves as if |
| 1656 | C<fallback> is present (possibly undefined). This may create |
| 1657 | interesting effects if some package is not overloaded, but inherits |
| 1658 | from two overloaded packages. |
| 1659 | |
| 1660 | =item * |
| 1661 | |
| 1662 | Before Perl 5.14, the relation between overloading and tie()ing was broken. |
| 1663 | Overloading was triggered or not based on the I<previous> class of the |
| 1664 | tie()d variable. |
| 1665 | |
| 1666 | This happened because the presence of overloading was checked |
| 1667 | too early, before any tie()d access was attempted. If the |
| 1668 | class of the value FETCH()ed from the tied variable does not |
| 1669 | change, a simple workaround for code that is to run on older Perl |
| 1670 | versions is to access the value (via C<() = $foo> or some such) |
| 1671 | immediately after tie()ing, so that after this call the I<previous> class |
| 1672 | coincides with the current one. |
| 1673 | |
| 1674 | =item * |
| 1675 | |
| 1676 | Barewords are not covered by overloaded string constants. |
| 1677 | |
| 1678 | =item * |
| 1679 | |
| 1680 | The range operator C<..> cannot be overloaded. |
| 1681 | |
| 1682 | =back |
| 1683 | |
| 1684 | =cut |
| 1685 | |