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1 package overload;
3 $overload::hint_bits = 0x20000;
5 sub nil {}
7 sub OVERLOAD {
8 $package = shift;
9 my %arg = @_;
10 my ($sub, $fb);
11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
13 for (keys %arg) {
14 if ($_ eq 'fallback') {
15 $fb = $arg{$_};
16 } else {
17 $sub = $arg{$_};
18 if (not ref $sub and $sub !~ /::/) {
19 $ {$package . "::(" . $_} = $sub;
20 $sub = \&nil;
22 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
23 *{$package . "::(" . $_} = \&{ $sub };
26 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
29 sub import {
30 $package = (caller())[0];
31 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
32 shift;
33 $package->overload::OVERLOAD(@_);
36 sub unimport {
37 $package = (caller())[0];
38 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
39 shift;
40 for (@_) {
41 if ($_ eq 'fallback') {
42 undef $ {$package . "::()"};
43 } else {
44 delete $ {$package . "::"}{"(" . $_};
49 sub Overloaded {
50 my $package = shift;
51 $package = ref $package if ref $package;
52 $package->can('()');
55 sub ov_method {
56 my $globref = shift;
57 return undef unless $globref;
58 my $sub = \&{*$globref};
59 return $sub if $sub ne \&nil;
60 return shift->can($ {*$globref});
63 sub OverloadedStringify {
64 my $package = shift;
65 $package = ref $package if ref $package;
66 #$package->can('(""')
67 ov_method mycan($package, '(""'), $package
68 or ov_method mycan($package, '(0+'), $package
69 or ov_method mycan($package, '(bool'), $package
70 or ov_method mycan($package, '(nomethod'), $package;
73 sub Method {
74 my $package = shift;
75 $package = ref $package if ref $package;
76 #my $meth = $package->can('(' . shift);
77 ov_method mycan($package, '(' . shift), $package;
78 #return $meth if $meth ne \&nil;
79 #return $ {*{$meth}};
82 sub AddrRef {
83 my $package = ref $_[0];
84 return "$_[0]" unless $package;
85 bless $_[0], overload::Fake; # Non-overloaded package
86 my $str = "$_[0]";
87 bless $_[0], $package; # Back
88 $package . substr $str, index $str, '=';
91 sub StrVal {
92 (OverloadedStringify($_[0]) or ref($_[0]) eq 'Regexp') ?
93 (AddrRef(shift)) :
94 "$_[0]";
97 sub mycan { # Real can would leave stubs.
98 my ($package, $meth) = @_;
99 return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
100 my $p;
101 foreach $p (@{$package . "::ISA"}) {
102 my $out = mycan($p, $meth);
103 return $out if $out;
105 return undef;
108 %constants = (
109 'integer' => 0x1000,
110 'float' => 0x2000,
111 'binary' => 0x4000,
112 'q' => 0x8000,
113 'qr' => 0x10000,
116 %ops = ( with_assign => "+ - * / % ** << >> x .",
117 assign => "+= -= *= /= %= **= <<= >>= x= .=",
118 num_comparison => "< <= > >= == !=",
119 '3way_comparison'=> "<=> cmp",
120 str_comparison => "lt le gt ge eq ne",
121 binary => "& | ^",
122 unary => "neg ! ~",
123 mutators => '++ --',
124 func => "atan2 cos sin exp abs log sqrt",
125 conversion => 'bool "" 0+',
126 iterators => '<>',
127 dereferencing => '${} @{} %{} &{} *{}',
128 special => 'nomethod fallback =');
130 use warnings::register;
131 sub constant {
132 # Arguments: what, sub
133 while (@_) {
134 if (@_ == 1) {
135 warnings::warnif ("Odd number of arguments for overload::constant");
136 last;
138 elsif (!exists $constants {$_ [0]}) {
139 warnings::warnif ("`$_[0]' is not an overloadable type");
141 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
142 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
143 # blessed, and C<ref> would return the package the ref is blessed into.
144 if (warnings::enabled) {
145 $_ [1] = "undef" unless defined $_ [1];
146 warnings::warn ("`$_[1]' is not a code reference");
149 else {
150 $^H{$_[0]} = $_[1];
151 $^H |= $constants{$_[0]} | $overload::hint_bits;
153 shift, shift;
157 sub remove_constant {
158 # Arguments: what, sub
159 while (@_) {
160 delete $^H{$_[0]};
161 $^H &= ~ $constants{$_[0]};
162 shift, shift;
168 __END__
170 =head1 NAME
172 overload - Package for overloading perl operations
174 =head1 SYNOPSIS
176 package SomeThing;
178 use overload
179 '+' => \&myadd,
180 '-' => \&mysub;
181 # etc
184 package main;
185 $a = new SomeThing 57;
186 $b=5+$a;
188 if (overload::Overloaded $b) {...}
190 $strval = overload::StrVal $b;
192 =head1 DESCRIPTION
194 =head2 Declaration of overloaded functions
196 The compilation directive
198 package Number;
199 use overload
200 "+" => \&add,
201 "*=" => "muas";
203 declares function Number::add() for addition, and method muas() in
204 the "class" C<Number> (or one of its base classes)
205 for the assignment form C<*=> of multiplication.
207 Arguments of this directive come in (key, value) pairs. Legal values
208 are values legal inside a C<&{ ... }> call, so the name of a
209 subroutine, a reference to a subroutine, or an anonymous subroutine
210 will all work. Note that values specified as strings are
211 interpreted as methods, not subroutines. Legal keys are listed below.
213 The subroutine C<add> will be called to execute C<$a+$b> if $a
214 is a reference to an object blessed into the package C<Number>, or if $a is
215 not an object from a package with defined mathemagic addition, but $b is a
216 reference to a C<Number>. It can also be called in other situations, like
217 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
218 methods refer to methods triggered by an overloaded mathematical
219 operator.)
221 Since overloading respects inheritance via the @ISA hierarchy, the
222 above declaration would also trigger overloading of C<+> and C<*=> in
223 all the packages which inherit from C<Number>.
225 =head2 Calling Conventions for Binary Operations
227 The functions specified in the C<use overload ...> directive are called
228 with three (in one particular case with four, see L<Last Resort>)
229 arguments. If the corresponding operation is binary, then the first
230 two arguments are the two arguments of the operation. However, due to
231 general object calling conventions, the first argument should always be
232 an object in the package, so in the situation of C<7+$a>, the
233 order of the arguments is interchanged. It probably does not matter
234 when implementing the addition method, but whether the arguments
235 are reversed is vital to the subtraction method. The method can
236 query this information by examining the third argument, which can take
237 three different values:
239 =over 7
241 =item FALSE
243 the order of arguments is as in the current operation.
245 =item TRUE
247 the arguments are reversed.
249 =item C<undef>
251 the current operation is an assignment variant (as in
252 C<$a+=7>), but the usual function is called instead. This additional
253 information can be used to generate some optimizations. Compare
254 L<Calling Conventions for Mutators>.
256 =back
258 =head2 Calling Conventions for Unary Operations
260 Unary operation are considered binary operations with the second
261 argument being C<undef>. Thus the functions that overloads C<{"++"}>
262 is called with arguments C<($a,undef,'')> when $a++ is executed.
264 =head2 Calling Conventions for Mutators
266 Two types of mutators have different calling conventions:
268 =over
270 =item C<++> and C<-->
272 The routines which implement these operators are expected to actually
273 I<mutate> their arguments. So, assuming that $obj is a reference to a
274 number,
276 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
278 is an appropriate implementation of overloaded C<++>. Note that
280 sub incr { ++$ {$_[0]} ; shift }
282 is OK if used with preincrement and with postincrement. (In the case
283 of postincrement a copying will be performed, see L<Copy Constructor>.)
285 =item C<x=> and other assignment versions
287 There is nothing special about these methods. They may change the
288 value of their arguments, and may leave it as is. The result is going
289 to be assigned to the value in the left-hand-side if different from
290 this value.
292 This allows for the same method to be used as overloaded C<+=> and
293 C<+>. Note that this is I<allowed>, but not recommended, since by the
294 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
295 if C<+=> is not overloaded.
297 =back
299 B<Warning.> Due to the presense of assignment versions of operations,
300 routines which may be called in assignment context may create
301 self-referential structures. Currently Perl will not free self-referential
302 structures until cycles are C<explicitly> broken. You may get problems
303 when traversing your structures too.
305 Say,
307 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
309 is asking for trouble, since for code C<$obj += $foo> the subroutine
310 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
311 \$foo]>. If using such a subroutine is an important optimization, one
312 can overload C<+=> explicitly by a non-"optimized" version, or switch
313 to non-optimized version if C<not defined $_[2]> (see
314 L<Calling Conventions for Binary Operations>).
316 Even if no I<explicit> assignment-variants of operators are present in
317 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
318 C<',' . $obj . ','> may be both optimized to
320 my $tmp = ',' . $obj; $tmp .= ',';
322 =head2 Overloadable Operations
324 The following symbols can be specified in C<use overload> directive:
326 =over 5
328 =item * I<Arithmetic operations>
330 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
331 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
333 For these operations a substituted non-assignment variant can be called if
334 the assignment variant is not available. Methods for operations "C<+>",
335 "C<->", "C<+=>", and "C<-=>" can be called to automatically generate
336 increment and decrement methods. The operation "C<->" can be used to
337 autogenerate missing methods for unary minus or C<abs>.
339 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
340 L<"Calling Conventions for Binary Operations">) for details of these
341 substitutions.
343 =item * I<Comparison operations>
345 "<", "<=", ">", ">=", "==", "!=", "<=>",
346 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
348 If the corresponding "spaceship" variant is available, it can be
349 used to substitute for the missing operation. During C<sort>ing
350 arrays, C<cmp> is used to compare values subject to C<use overload>.
352 =item * I<Bit operations>
354 "&", "^", "|", "neg", "!", "~",
356 "C<neg>" stands for unary minus. If the method for C<neg> is not
357 specified, it can be autogenerated using the method for
358 subtraction. If the method for "C<!>" is not specified, it can be
359 autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>".
361 =item * I<Increment and decrement>
363 "++", "--",
365 If undefined, addition and subtraction methods can be
366 used instead. These operations are called both in prefix and
367 postfix form.
369 =item * I<Transcendental functions>
371 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
373 If C<abs> is unavailable, it can be autogenerated using methods
374 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
376 =item * I<Boolean, string and numeric conversion>
378 "bool", "\"\"", "0+",
380 If one or two of these operations are not overloaded, the remaining ones can
381 be used instead. C<bool> is used in the flow control operators
382 (like C<while>) and for the ternary "C<?:>" operation. These functions can
383 return any arbitrary Perl value. If the corresponding operation for this value
384 is overloaded too, that operation will be called again with this value.
386 As a special case if the overload returns the object itself then it will
387 be used directly. An overloaded conversion returning the object is
388 probably a bug, because you're likely to get something that looks like
389 C<YourPackage=HASH(0x8172b34)>.
391 =item * I<Iteration>
393 "<>"
395 If not overloaded, the argument will be converted to a filehandle or
396 glob (which may require a stringification). The same overloading
397 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
398 I<globbing> syntax C<E<lt>${var}E<gt>>.
400 =item * I<Dereferencing>
402 '${}', '@{}', '%{}', '&{}', '*{}'.
404 If not overloaded, the argument will be dereferenced I<as is>, thus
405 should be of correct type. These functions should return a reference
406 of correct type, or another object with overloaded dereferencing.
408 As a special case if the overload returns the object itself then it
409 will be used directly (provided it is the correct type).
411 The dereference operators must be specified explicitly they will not be passed to
412 "nomethod".
414 =item * I<Special>
416 "nomethod", "fallback", "=",
418 see L<SPECIAL SYMBOLS FOR C<use overload>>.
420 =back
422 See L<"Fallback"> for an explanation of when a missing method can be
423 autogenerated.
425 A computer-readable form of the above table is available in the hash
426 %overload::ops, with values being space-separated lists of names:
428 with_assign => '+ - * / % ** << >> x .',
429 assign => '+= -= *= /= %= **= <<= >>= x= .=',
430 num_comparison => '< <= > >= == !=',
431 '3way_comparison'=> '<=> cmp',
432 str_comparison => 'lt le gt ge eq ne',
433 binary => '& | ^',
434 unary => 'neg ! ~',
435 mutators => '++ --',
436 func => 'atan2 cos sin exp abs log sqrt',
437 conversion => 'bool "" 0+',
438 iterators => '<>',
439 dereferencing => '${} @{} %{} &{} *{}',
440 special => 'nomethod fallback ='
442 =head2 Inheritance and overloading
444 Inheritance interacts with overloading in two ways.
446 =over
448 =item Strings as values of C<use overload> directive
450 If C<value> in
452 use overload key => value;
454 is a string, it is interpreted as a method name.
456 =item Overloading of an operation is inherited by derived classes
458 Any class derived from an overloaded class is also overloaded. The
459 set of overloaded methods is the union of overloaded methods of all
460 the ancestors. If some method is overloaded in several ancestor, then
461 which description will be used is decided by the usual inheritance
462 rules:
464 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
465 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
466 then the subroutine C<D::plus_sub> will be called to implement
467 operation C<+> for an object in package C<A>.
469 =back
471 Note that since the value of the C<fallback> key is not a subroutine,
472 its inheritance is not governed by the above rules. In the current
473 implementation, the value of C<fallback> in the first overloaded
474 ancestor is used, but this is accidental and subject to change.
476 =head1 SPECIAL SYMBOLS FOR C<use overload>
478 Three keys are recognized by Perl that are not covered by the above
479 description.
481 =head2 Last Resort
483 C<"nomethod"> should be followed by a reference to a function of four
484 parameters. If defined, it is called when the overloading mechanism
485 cannot find a method for some operation. The first three arguments of
486 this function coincide with the arguments for the corresponding method if
487 it were found, the fourth argument is the symbol
488 corresponding to the missing method. If several methods are tried,
489 the last one is used. Say, C<1-$a> can be equivalent to
491 &nomethodMethod($a,1,1,"-")
493 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
494 C<use overload> directive.
496 The C<"nomethod"> mechanism is I<not> used for the dereference operators
497 ( ${} @{} %{} &{} *{} ).
500 If some operation cannot be resolved, and there is no function
501 assigned to C<"nomethod">, then an exception will be raised via die()--
502 unless C<"fallback"> was specified as a key in C<use overload> directive.
505 =head2 Fallback
507 The key C<"fallback"> governs what to do if a method for a particular
508 operation is not found. Three different cases are possible depending on
509 the value of C<"fallback">:
511 =over 16
513 =item * C<undef>
515 Perl tries to use a
516 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
517 then tries to calls C<"nomethod"> value; if missing, an exception
518 will be raised.
520 =item * TRUE
522 The same as for the C<undef> value, but no exception is raised. Instead,
523 it silently reverts to what it would have done were there no C<use overload>
524 present.
526 =item * defined, but FALSE
528 No autogeneration is tried. Perl tries to call
529 C<"nomethod"> value, and if this is missing, raises an exception.
531 =back
533 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
534 yet, see L<"Inheritance and overloading">.
536 =head2 Copy Constructor
538 The value for C<"="> is a reference to a function with three
539 arguments, i.e., it looks like the other values in C<use
540 overload>. However, it does not overload the Perl assignment
541 operator. This would go against Camel hair.
543 This operation is called in the situations when a mutator is applied
544 to a reference that shares its object with some other reference, such
547 $a=$b;
548 ++$a;
550 To make this change $a and not change $b, a copy of C<$$a> is made,
551 and $a is assigned a reference to this new object. This operation is
552 done during execution of the C<++$a>, and not during the assignment,
553 (so before the increment C<$$a> coincides with C<$$b>). This is only
554 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
555 C<nomethod>). Note that if this operation is expressed via C<'+'>
556 a nonmutator, i.e., as in
558 $a=$b;
559 $a=$a+1;
561 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
562 appear as lvalue when the above code is executed.
564 If the copy constructor is required during the execution of some mutator,
565 but a method for C<'='> was not specified, it can be autogenerated as a
566 string copy if the object is a plain scalar.
568 =over 5
570 =item B<Example>
572 The actually executed code for
574 $a=$b;
575 Something else which does not modify $a or $b....
576 ++$a;
578 may be
580 $a=$b;
581 Something else which does not modify $a or $b....
582 $a = $a->clone(undef,"");
583 $a->incr(undef,"");
585 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
586 C<'='> was overloaded with C<\&clone>.
588 =back
590 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
591 C<$b = $a; ++$a>.
593 =head1 MAGIC AUTOGENERATION
595 If a method for an operation is not found, and the value for C<"fallback"> is
596 TRUE or undefined, Perl tries to autogenerate a substitute method for
597 the missing operation based on the defined operations. Autogenerated method
598 substitutions are possible for the following operations:
600 =over 16
602 =item I<Assignment forms of arithmetic operations>
604 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
605 is not defined.
607 =item I<Conversion operations>
609 String, numeric, and boolean conversion are calculated in terms of one
610 another if not all of them are defined.
612 =item I<Increment and decrement>
614 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
615 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
617 =item C<abs($a)>
619 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
621 =item I<Unary minus>
623 can be expressed in terms of subtraction.
625 =item I<Negation>
627 C<!> and C<not> can be expressed in terms of boolean conversion, or
628 string or numerical conversion.
630 =item I<Concatenation>
632 can be expressed in terms of string conversion.
634 =item I<Comparison operations>
636 can be expressed in terms of its "spaceship" counterpart: either
637 C<E<lt>=E<gt>> or C<cmp>:
639 <, >, <=, >=, ==, != in terms of <=>
640 lt, gt, le, ge, eq, ne in terms of cmp
642 =item I<Iterator>
644 <> in terms of builtin operations
646 =item I<Dereferencing>
648 ${} @{} %{} &{} *{} in terms of builtin operations
650 =item I<Copy operator>
652 can be expressed in terms of an assignment to the dereferenced value, if this
653 value is a scalar and not a reference.
655 =back
657 =head1 Losing overloading
659 The restriction for the comparison operation is that even if, for example,
660 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
661 function will produce only a standard logical value based on the
662 numerical value of the result of `C<cmp>'. In particular, a working
663 numeric conversion is needed in this case (possibly expressed in terms of
664 other conversions).
666 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
667 if the string conversion substitution is applied.
669 When you chop() a mathemagical object it is promoted to a string and its
670 mathemagical properties are lost. The same can happen with other
671 operations as well.
673 =head1 Run-time Overloading
675 Since all C<use> directives are executed at compile-time, the only way to
676 change overloading during run-time is to
678 eval 'use overload "+" => \&addmethod';
680 You can also use
682 eval 'no overload "+", "--", "<="';
684 though the use of these constructs during run-time is questionable.
686 =head1 Public functions
688 Package C<overload.pm> provides the following public functions:
690 =over 5
692 =item overload::StrVal(arg)
694 Gives string value of C<arg> as in absence of stringify overloading.
696 =item overload::Overloaded(arg)
698 Returns true if C<arg> is subject to overloading of some operations.
700 =item overload::Method(obj,op)
702 Returns C<undef> or a reference to the method that implements C<op>.
704 =back
706 =head1 Overloading constants
708 For some application Perl parser mangles constants too much. It is possible
709 to hook into this process via overload::constant() and overload::remove_constant()
710 functions.
712 These functions take a hash as an argument. The recognized keys of this hash
715 =over 8
717 =item integer
719 to overload integer constants,
721 =item float
723 to overload floating point constants,
725 =item binary
727 to overload octal and hexadecimal constants,
729 =item q
731 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
732 strings and here-documents,
734 =item qr
736 to overload constant pieces of regular expressions.
738 =back
740 The corresponding values are references to functions which take three arguments:
741 the first one is the I<initial> string form of the constant, the second one
742 is how Perl interprets this constant, the third one is how the constant is used.
743 Note that the initial string form does not
744 contain string delimiters, and has backslashes in backslash-delimiter
745 combinations stripped (thus the value of delimiter is not relevant for
746 processing of this string). The return value of this function is how this
747 constant is going to be interpreted by Perl. The third argument is undefined
748 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
749 context (comes from strings, regular expressions, and single-quote HERE
750 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
751 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
753 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
754 it is expected that overloaded constant strings are equipped with reasonable
755 overloaded catenation operator, otherwise absurd results will result.
756 Similarly, negative numbers are considered as negations of positive constants.
758 Note that it is probably meaningless to call the functions overload::constant()
759 and overload::remove_constant() from anywhere but import() and unimport() methods.
760 From these methods they may be called as
762 sub import {
763 shift;
764 return unless @_;
765 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
766 overload::constant integer => sub {Math::BigInt->new(shift)};
769 B<BUGS> Currently overloaded-ness of constants does not propagate
770 into C<eval '...'>.
772 =head1 IMPLEMENTATION
774 What follows is subject to change RSN.
776 The table of methods for all operations is cached in magic for the
777 symbol table hash for the package. The cache is invalidated during
778 processing of C<use overload>, C<no overload>, new function
779 definitions, and changes in @ISA. However, this invalidation remains
780 unprocessed until the next C<bless>ing into the package. Hence if you
781 want to change overloading structure dynamically, you'll need an
782 additional (fake) C<bless>ing to update the table.
784 (Every SVish thing has a magic queue, and magic is an entry in that
785 queue. This is how a single variable may participate in multiple
786 forms of magic simultaneously. For instance, environment variables
787 regularly have two forms at once: their %ENV magic and their taint
788 magic. However, the magic which implements overloading is applied to
789 the stashes, which are rarely used directly, thus should not slow down
790 Perl.)
792 If an object belongs to a package using overload, it carries a special
793 flag. Thus the only speed penalty during arithmetic operations without
794 overloading is the checking of this flag.
796 In fact, if C<use overload> is not present, there is almost no overhead
797 for overloadable operations, so most programs should not suffer
798 measurable performance penalties. A considerable effort was made to
799 minimize the overhead when overload is used in some package, but the
800 arguments in question do not belong to packages using overload. When
801 in doubt, test your speed with C<use overload> and without it. So far
802 there have been no reports of substantial speed degradation if Perl is
803 compiled with optimization turned on.
805 There is no size penalty for data if overload is not used. The only
806 size penalty if overload is used in some package is that I<all> the
807 packages acquire a magic during the next C<bless>ing into the
808 package. This magic is three-words-long for packages without
809 overloading, and carries the cache table if the package is overloaded.
811 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
812 carried out before any operation that can imply an assignment to the
813 object $a (or $b) refers to, like C<$a++>. You can override this
814 behavior by defining your own copy constructor (see L<"Copy Constructor">).
816 It is expected that arguments to methods that are not explicitly supposed
817 to be changed are constant (but this is not enforced).
819 =head1 Metaphor clash
821 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
822 If it I<looks> counter intuitive to you, you are subject to a metaphor
823 clash.
825 Here is a Perl object metaphor:
827 I< object is a reference to blessed data>
829 and an arithmetic metaphor:
831 I< object is a thing by itself>.
833 The I<main> problem of overloading C<=> is the fact that these metaphors
834 imply different actions on the assignment C<$a = $b> if $a and $b are
835 objects. Perl-think implies that $a becomes a reference to whatever
836 $b was referencing. Arithmetic-think implies that the value of "object"
837 $a is changed to become the value of the object $b, preserving the fact
838 that $a and $b are separate entities.
840 The difference is not relevant in the absence of mutators. After
841 a Perl-way assignment an operation which mutates the data referenced by $a
842 would change the data referenced by $b too. Effectively, after
843 C<$a = $b> values of $a and $b become I<indistinguishable>.
845 On the other hand, anyone who has used algebraic notation knows the
846 expressive power of the arithmetic metaphor. Overloading works hard
847 to enable this metaphor while preserving the Perlian way as far as
848 possible. Since it is not not possible to freely mix two contradicting
849 metaphors, overloading allows the arithmetic way to write things I<as
850 far as all the mutators are called via overloaded access only>. The
851 way it is done is described in L<Copy Constructor>.
853 If some mutator methods are directly applied to the overloaded values,
854 one may need to I<explicitly unlink> other values which references the
855 same value:
857 $a = new Data 23;
859 $b = $a; # $b is "linked" to $a
861 $a = $a->clone; # Unlink $b from $a
862 $a->increment_by(4);
864 Note that overloaded access makes this transparent:
866 $a = new Data 23;
867 $b = $a; # $b is "linked" to $a
868 $a += 4; # would unlink $b automagically
870 However, it would not make
872 $a = new Data 23;
873 $a = 4; # Now $a is a plain 4, not 'Data'
875 preserve "objectness" of $a. But Perl I<has> a way to make assignments
876 to an object do whatever you want. It is just not the overload, but
877 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
878 which returns the object itself, and STORE() method which changes the
879 value of the object, one can reproduce the arithmetic metaphor in its
880 completeness, at least for variables which were tie()d from the start.
882 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
884 =head1 Cookbook
886 Please add examples to what follows!
888 =head2 Two-face scalars
890 Put this in F<two_face.pm> in your Perl library directory:
892 package two_face; # Scalars with separate string and
893 # numeric values.
894 sub new { my $p = shift; bless [@_], $p }
895 use overload '""' => \&str, '0+' => \&num, fallback => 1;
896 sub num {shift->[1]}
897 sub str {shift->[0]}
899 Use it as follows:
901 require two_face;
902 my $seven = new two_face ("vii", 7);
903 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
904 print "seven contains `i'\n" if $seven =~ /i/;
906 (The second line creates a scalar which has both a string value, and a
907 numeric value.) This prints:
909 seven=vii, seven=7, eight=8
910 seven contains `i'
912 =head2 Two-face references
914 Suppose you want to create an object which is accessible as both an
915 array reference and a hash reference, similar to the
916 L<pseudo-hash|perlref/"Pseudo-hashes: Using an array as a hash">
917 builtin Perl type. Let's make it better than a pseudo-hash by
918 allowing index 0 to be treated as a normal element.
920 package two_refs;
921 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
922 sub new {
923 my $p = shift;
924 bless \ [@_], $p;
926 sub gethash {
927 my %h;
928 my $self = shift;
929 tie %h, ref $self, $self;
930 \%h;
933 sub TIEHASH { my $p = shift; bless \ shift, $p }
934 my %fields;
935 my $i = 0;
936 $fields{$_} = $i++ foreach qw{zero one two three};
937 sub STORE {
938 my $self = ${shift()};
939 my $key = $fields{shift()};
940 defined $key or die "Out of band access";
941 $$self->[$key] = shift;
943 sub FETCH {
944 my $self = ${shift()};
945 my $key = $fields{shift()};
946 defined $key or die "Out of band access";
947 $$self->[$key];
950 Now one can access an object using both the array and hash syntax:
952 my $bar = new two_refs 3,4,5,6;
953 $bar->[2] = 11;
954 $bar->{two} == 11 or die 'bad hash fetch';
956 Note several important features of this example. First of all, the
957 I<actual> type of $bar is a scalar reference, and we do not overload
958 the scalar dereference. Thus we can get the I<actual> non-overloaded
959 contents of $bar by just using C<$$bar> (what we do in functions which
960 overload dereference). Similarly, the object returned by the
961 TIEHASH() method is a scalar reference.
963 Second, we create a new tied hash each time the hash syntax is used.
964 This allows us not to worry about a possibility of a reference loop,
965 would would lead to a memory leak.
967 Both these problems can be cured. Say, if we want to overload hash
968 dereference on a reference to an object which is I<implemented> as a
969 hash itself, the only problem one has to circumvent is how to access
970 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
971 overloaded dereference operator). Here is one possible fetching routine:
973 sub access_hash {
974 my ($self, $key) = (shift, shift);
975 my $class = ref $self;
976 bless $self, 'overload::dummy'; # Disable overloading of %{}
977 my $out = $self->{$key};
978 bless $self, $class; # Restore overloading
979 $out;
982 To remove creation of the tied hash on each access, one may an extra
983 level of indirection which allows a non-circular structure of references:
985 package two_refs1;
986 use overload '%{}' => sub { ${shift()}->[1] },
987 '@{}' => sub { ${shift()}->[0] };
988 sub new {
989 my $p = shift;
990 my $a = [@_];
991 my %h;
992 tie %h, $p, $a;
993 bless \ [$a, \%h], $p;
995 sub gethash {
996 my %h;
997 my $self = shift;
998 tie %h, ref $self, $self;
999 \%h;
1002 sub TIEHASH { my $p = shift; bless \ shift, $p }
1003 my %fields;
1004 my $i = 0;
1005 $fields{$_} = $i++ foreach qw{zero one two three};
1006 sub STORE {
1007 my $a = ${shift()};
1008 my $key = $fields{shift()};
1009 defined $key or die "Out of band access";
1010 $a->[$key] = shift;
1012 sub FETCH {
1013 my $a = ${shift()};
1014 my $key = $fields{shift()};
1015 defined $key or die "Out of band access";
1016 $a->[$key];
1019 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1020 reference to the intermediate array, which keeps a reference to an
1021 actual array, and the access hash. The tie()ing object for the access
1022 hash is a reference to a reference to the actual array, so
1024 =over
1026 =item *
1028 There are no loops of references.
1030 =item *
1032 Both "objects" which are blessed into the class C<two_refs1> are
1033 references to a reference to an array, thus references to a I<scalar>.
1034 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1035 overloaded operations.
1037 =back
1039 =head2 Symbolic calculator
1041 Put this in F<symbolic.pm> in your Perl library directory:
1043 package symbolic; # Primitive symbolic calculator
1044 use overload nomethod => \&wrap;
1046 sub new { shift; bless ['n', @_] }
1047 sub wrap {
1048 my ($obj, $other, $inv, $meth) = @_;
1049 ($obj, $other) = ($other, $obj) if $inv;
1050 bless [$meth, $obj, $other];
1053 This module is very unusual as overloaded modules go: it does not
1054 provide any usual overloaded operators, instead it provides the L<Last
1055 Resort> operator C<nomethod>. In this example the corresponding
1056 subroutine returns an object which encapsulates operations done over
1057 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1058 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1060 Here is an example of the script which "calculates" the side of
1061 circumscribed octagon using the above package:
1063 require symbolic;
1064 my $iter = 1; # 2**($iter+2) = 8
1065 my $side = new symbolic 1;
1066 my $cnt = $iter;
1068 while ($cnt--) {
1069 $side = (sqrt(1 + $side**2) - 1)/$side;
1071 print "OK\n";
1073 The value of $side is
1075 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1076 undef], 1], ['n', 1]]
1078 Note that while we obtained this value using a nice little script,
1079 there is no simple way to I<use> this value. In fact this value may
1080 be inspected in debugger (see L<perldebug>), but ony if
1081 C<bareStringify> B<O>ption is set, and not via C<p> command.
1083 If one attempts to print this value, then the overloaded operator
1084 C<""> will be called, which will call C<nomethod> operator. The
1085 result of this operator will be stringified again, but this result is
1086 again of type C<symbolic>, which will lead to an infinite loop.
1088 Add a pretty-printer method to the module F<symbolic.pm>:
1090 sub pretty {
1091 my ($meth, $a, $b) = @{+shift};
1092 $a = 'u' unless defined $a;
1093 $b = 'u' unless defined $b;
1094 $a = $a->pretty if ref $a;
1095 $b = $b->pretty if ref $b;
1096 "[$meth $a $b]";
1099 Now one can finish the script by
1101 print "side = ", $side->pretty, "\n";
1103 The method C<pretty> is doing object-to-string conversion, so it
1104 is natural to overload the operator C<""> using this method. However,
1105 inside such a method it is not necessary to pretty-print the
1106 I<components> $a and $b of an object. In the above subroutine
1107 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1108 and $b. If these components use overloading, the catenation operator
1109 will look for an overloaded operator C<.>; if not present, it will
1110 look for an overloaded operator C<"">. Thus it is enough to use
1112 use overload nomethod => \&wrap, '""' => \&str;
1113 sub str {
1114 my ($meth, $a, $b) = @{+shift};
1115 $a = 'u' unless defined $a;
1116 $b = 'u' unless defined $b;
1117 "[$meth $a $b]";
1120 Now one can change the last line of the script to
1122 print "side = $side\n";
1124 which outputs
1126 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1128 and one can inspect the value in debugger using all the possible
1129 methods.
1131 Something is is still amiss: consider the loop variable $cnt of the
1132 script. It was a number, not an object. We cannot make this value of
1133 type C<symbolic>, since then the loop will not terminate.
1135 Indeed, to terminate the cycle, the $cnt should become false.
1136 However, the operator C<bool> for checking falsity is overloaded (this
1137 time via overloaded C<"">), and returns a long string, thus any object
1138 of type C<symbolic> is true. To overcome this, we need a way to
1139 compare an object to 0. In fact, it is easier to write a numeric
1140 conversion routine.
1142 Here is the text of F<symbolic.pm> with such a routine added (and
1143 slightly modified str()):
1145 package symbolic; # Primitive symbolic calculator
1146 use overload
1147 nomethod => \&wrap, '""' => \&str, '0+' => \&num;
1149 sub new { shift; bless ['n', @_] }
1150 sub wrap {
1151 my ($obj, $other, $inv, $meth) = @_;
1152 ($obj, $other) = ($other, $obj) if $inv;
1153 bless [$meth, $obj, $other];
1155 sub str {
1156 my ($meth, $a, $b) = @{+shift};
1157 $a = 'u' unless defined $a;
1158 if (defined $b) {
1159 "[$meth $a $b]";
1160 } else {
1161 "[$meth $a]";
1164 my %subr = ( n => sub {$_[0]},
1165 sqrt => sub {sqrt $_[0]},
1166 '-' => sub {shift() - shift()},
1167 '+' => sub {shift() + shift()},
1168 '/' => sub {shift() / shift()},
1169 '*' => sub {shift() * shift()},
1170 '**' => sub {shift() ** shift()},
1172 sub num {
1173 my ($meth, $a, $b) = @{+shift};
1174 my $subr = $subr{$meth}
1175 or die "Do not know how to ($meth) in symbolic";
1176 $a = $a->num if ref $a eq __PACKAGE__;
1177 $b = $b->num if ref $b eq __PACKAGE__;
1178 $subr->($a,$b);
1181 All the work of numeric conversion is done in %subr and num(). Of
1182 course, %subr is not complete, it contains only operators used in the
1183 example below. Here is the extra-credit question: why do we need an
1184 explicit recursion in num()? (Answer is at the end of this section.)
1186 Use this module like this:
1188 require symbolic;
1189 my $iter = new symbolic 2; # 16-gon
1190 my $side = new symbolic 1;
1191 my $cnt = $iter;
1193 while ($cnt) {
1194 $cnt = $cnt - 1; # Mutator `--' not implemented
1195 $side = (sqrt(1 + $side**2) - 1)/$side;
1197 printf "%s=%f\n", $side, $side;
1198 printf "pi=%f\n", $side*(2**($iter+2));
1200 It prints (without so many line breaks)
1202 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1203 [n 1]] 2]]] 1]
1204 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1205 pi=3.182598
1207 The above module is very primitive. It does not implement
1208 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1209 (not required without mutators!), and implements only those arithmetic
1210 operations which are used in the example.
1212 To implement most arithmetic operations is easy; one should just use
1213 the tables of operations, and change the code which fills %subr to
1215 my %subr = ( 'n' => sub {$_[0]} );
1216 foreach my $op (split " ", $overload::ops{with_assign}) {
1217 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1219 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1220 foreach my $op (split " ", "@overload::ops{ @bins }") {
1221 $subr{$op} = eval "sub {shift() $op shift()}";
1223 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1224 print "defining `$op'\n";
1225 $subr{$op} = eval "sub {$op shift()}";
1228 Due to L<Calling Conventions for Mutators>, we do not need anything
1229 special to make C<+=> and friends work, except filling C<+=> entry of
1230 %subr, and defining a copy constructor (needed since Perl has no
1231 way to know that the implementation of C<'+='> does not mutate
1232 the argument, compare L<Copy Constructor>).
1234 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1235 line, and code (this code assumes that mutators change things one level
1236 deep only, so recursive copying is not needed):
1238 sub cpy {
1239 my $self = shift;
1240 bless [@$self], ref $self;
1243 To make C<++> and C<--> work, we need to implement actual mutators,
1244 either directly, or in C<nomethod>. We continue to do things inside
1245 C<nomethod>, thus add
1247 if ($meth eq '++' or $meth eq '--') {
1248 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1249 return $obj;
1252 after the first line of wrap(). This is not a most effective
1253 implementation, one may consider
1255 sub inc { $_[0] = bless ['++', shift, 1]; }
1257 instead.
1259 As a final remark, note that one can fill %subr by
1261 my %subr = ( 'n' => sub {$_[0]} );
1262 foreach my $op (split " ", $overload::ops{with_assign}) {
1263 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1265 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1266 foreach my $op (split " ", "@overload::ops{ @bins }") {
1267 $subr{$op} = eval "sub {shift() $op shift()}";
1269 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1270 $subr{$op} = eval "sub {$op shift()}";
1272 $subr{'++'} = $subr{'+'};
1273 $subr{'--'} = $subr{'-'};
1275 This finishes implementation of a primitive symbolic calculator in
1276 50 lines of Perl code. Since the numeric values of subexpressions
1277 are not cached, the calculator is very slow.
1279 Here is the answer for the exercise: In the case of str(), we need no
1280 explicit recursion since the overloaded C<.>-operator will fall back
1281 to an existing overloaded operator C<"">. Overloaded arithmetic
1282 operators I<do not> fall back to numeric conversion if C<fallback> is
1283 not explicitly requested. Thus without an explicit recursion num()
1284 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1285 the argument of num().
1287 If you wonder why defaults for conversion are different for str() and
1288 num(), note how easy it was to write the symbolic calculator. This
1289 simplicity is due to an appropriate choice of defaults. One extra
1290 note: due to the explicit recursion num() is more fragile than sym():
1291 we need to explicitly check for the type of $a and $b. If components
1292 $a and $b happen to be of some related type, this may lead to problems.
1294 =head2 I<Really> symbolic calculator
1296 One may wonder why we call the above calculator symbolic. The reason
1297 is that the actual calculation of the value of expression is postponed
1298 until the value is I<used>.
1300 To see it in action, add a method
1302 sub STORE {
1303 my $obj = shift;
1304 $#$obj = 1;
1305 @$obj->[0,1] = ('=', shift);
1308 to the package C<symbolic>. After this change one can do
1310 my $a = new symbolic 3;
1311 my $b = new symbolic 4;
1312 my $c = sqrt($a**2 + $b**2);
1314 and the numeric value of $c becomes 5. However, after calling
1316 $a->STORE(12); $b->STORE(5);
1318 the numeric value of $c becomes 13. There is no doubt now that the module
1319 symbolic provides a I<symbolic> calculator indeed.
1321 To hide the rough edges under the hood, provide a tie()d interface to the
1322 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1324 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1325 sub FETCH { shift }
1326 sub nop { } # Around a bug
1328 (the bug is described in L<"BUGS">). One can use this new interface as
1330 tie $a, 'symbolic', 3;
1331 tie $b, 'symbolic', 4;
1332 $a->nop; $b->nop; # Around a bug
1334 my $c = sqrt($a**2 + $b**2);
1336 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1337 of $c becomes 13. To insulate the user of the module add a method
1339 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1343 my ($a, $b);
1344 symbolic->vars($a, $b);
1345 my $c = sqrt($a**2 + $b**2);
1347 $a = 3; $b = 4;
1348 printf "c5 %s=%f\n", $c, $c;
1350 $a = 12; $b = 5;
1351 printf "c13 %s=%f\n", $c, $c;
1353 shows that the numeric value of $c follows changes to the values of $a
1354 and $b.
1356 =head1 AUTHOR
1358 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1360 =head1 DIAGNOSTICS
1362 When Perl is run with the B<-Do> switch or its equivalent, overloading
1363 induces diagnostic messages.
1365 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1366 deduce which operations are overloaded (and which ancestor triggers
1367 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1368 is shown by debugger. The method C<()> corresponds to the C<fallback>
1369 key (in fact a presence of this method shows that this package has
1370 overloading enabled, and it is what is used by the C<Overloaded>
1371 function of module C<overload>).
1373 The module might issue the following warnings:
1375 =over 4
1377 =item Odd number of arguments for overload::constant
1379 (W) The call to overload::constant contained an odd number of arguments.
1380 The arguments should come in pairs.
1382 =item `%s' is not an overloadable type
1384 (W) You tried to overload a constant type the overload package is unaware of.
1386 =item `%s' is not a code reference
1388 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1389 to be a code reference. Either an anonymous subroutine, or a reference
1390 to a subroutine.
1392 =back
1394 =head1 BUGS
1396 Because it is used for overloading, the per-package hash %OVERLOAD now
1397 has a special meaning in Perl. The symbol table is filled with names
1398 looking like line-noise.
1400 For the purpose of inheritance every overloaded package behaves as if
1401 C<fallback> is present (possibly undefined). This may create
1402 interesting effects if some package is not overloaded, but inherits
1403 from two overloaded packages.
1405 Relation between overloading and tie()ing is broken. Overloading is
1406 triggered or not basing on the I<previous> class of tie()d value.
1408 This happens because the presence of overloading is checked too early,
1409 before any tie()d access is attempted. If the FETCH()ed class of the
1410 tie()d value does not change, a simple workaround is to access the value
1411 immediately after tie()ing, so that after this call the I<previous> class
1412 coincides with the current one.
1414 B<Needed:> a way to fix this without a speed penalty.
1416 Barewords are not covered by overloaded string constants.
1418 This document is confusing. There are grammos and misleading language
1419 used in places. It would seem a total rewrite is needed.
1421 =cut