11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 $fb = ${$package . "::()"}; # preserve old fallback value RT#68196
13 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
15 if ($_ eq 'fallback') {
19 if (not ref $sub and $sub !~ /::/) {
20 $ {$package . "::(" . $_} = $sub;
23 #print STDERR "Setting '$ {'package'}::\cO$_' to \\&'$sub'.\n";
24 *{$package . "::(" . $_} = \&{ $sub };
27 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
31 $package = (caller())[0];
32 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
34 $package->overload::OVERLOAD(@_);
38 $package = (caller())[0];
39 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
42 if ($_ eq 'fallback') {
43 undef $ {$package . "::()"};
45 delete $ {$package . "::"}{"(" . $_};
52 $package = ref $package if ref $package;
58 return undef unless $globref;
59 my $sub = \&{*$globref};
62 if Scalar::Util::refaddr($sub) != Scalar::Util::refaddr(\&nil);
63 return shift->can($ {*$globref});
66 sub OverloadedStringify {
68 $package = ref $package if ref $package;
70 ov_method mycan($package, '(""'), $package
71 or ov_method mycan($package, '(0+'), $package
72 or ov_method mycan($package, '(bool'), $package
73 or ov_method mycan($package, '(nomethod'), $package;
82 $package = Scalar::Util::blessed($package);
83 return undef if !defined $package;
85 #my $meth = $package->can('(' . shift);
86 ov_method mycan($package, '(' . shift), $package;
87 #return $meth if $meth ne \&nil;
92 my $package = ref $_[0];
93 return "$_[0]" unless $package;
98 my $class = Scalar::Util::blessed($_[0]);
99 my $class_prefix = defined($class) ? "$class=" : "";
100 my $type = Scalar::Util::reftype($_[0]);
101 my $addr = Scalar::Util::refaddr($_[0]);
102 return sprintf("%s%s(0x%x)", $class_prefix, $type, $addr);
107 sub mycan { # Real can would leave stubs.
108 my ($package, $meth) = @_;
114 my $mro = mro::get_linear_isa($package);
115 foreach my $p (@$mro) {
116 my $fqmeth = $p . q{::} . $meth;
117 return \*{$fqmeth} if defined &{$fqmeth};
124 'integer' => 0x1000, # HINT_NEW_INTEGER
125 'float' => 0x2000, # HINT_NEW_FLOAT
126 'binary' => 0x4000, # HINT_NEW_BINARY
127 'q' => 0x8000, # HINT_NEW_STRING
128 'qr' => 0x10000, # HINT_NEW_RE
131 %ops = ( with_assign => "+ - * / % ** << >> x .",
132 assign => "+= -= *= /= %= **= <<= >>= x= .=",
133 num_comparison => "< <= > >= == !=",
134 '3way_comparison'=> "<=> cmp",
135 str_comparison => "lt le gt ge eq ne",
136 binary => '& &= | |= ^ ^=',
139 func => "atan2 cos sin exp abs log sqrt int",
140 conversion => 'bool "" 0+ qr',
143 dereferencing => '${} @{} %{} &{} *{}',
145 special => 'nomethod fallback =');
147 use warnings::register;
149 # Arguments: what, sub
152 warnings::warnif ("Odd number of arguments for overload::constant");
155 elsif (!exists $constants {$_ [0]}) {
156 warnings::warnif ("'$_[0]' is not an overloadable type");
158 elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
159 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
160 # blessed, and C<ref> would return the package the ref is blessed into.
161 if (warnings::enabled) {
162 $_ [1] = "undef" unless defined $_ [1];
163 warnings::warn ("'$_[1]' is not a code reference");
168 $^H |= $constants{$_[0]};
174 sub remove_constant {
175 # Arguments: what, sub
178 $^H &= ~ $constants{$_[0]};
189 overload - Package for overloading Perl operations
202 $a = SomeThing->new( 57 );
205 if (overload::Overloaded $b) {...}
207 $strval = overload::StrVal $b;
211 This pragma allows overloading of Perl's operators for a class.
212 To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
218 Arguments of the C<use overload> directive are (key, value) pairs.
219 For the full set of legal keys, see L<Overloadable Operations> below.
221 Operator implementations (the values) can be subroutines,
222 references to subroutines, or anonymous subroutines
223 - in other words, anything legal inside a C<&{ ... }> call.
224 Values specified as strings are interpreted as method names.
231 '""' => sub { ...; };
233 declares that subtraction is to be implemented by method C<minus()>
234 in the class C<Number> (or one of its base classes),
235 and that the function C<Number::muas()> is to be used for the
236 assignment form of multiplication, C<*=>.
237 It also defines an anonymous subroutine to implement stringification:
238 this is called whenever an object blessed into the package C<Number>
239 is used in a string context (this subroutine might, for example,
240 return the number as a Roman numeral).
242 =head3 Calling Conventions and Magic Autogeneration
244 The following sample implementation of C<minus()> (which assumes
245 that C<Number> objects are simply blessed references to scalars)
246 illustrates the calling conventions:
250 my ($self, $other, $swap) = @_;
251 my $result = $$self - $other; # *
252 $result = -$result if $swap;
253 ref $result ? $result : bless \$result;
255 # * may recurse once - see table below
257 Three arguments are passed to all subroutines specified in the
258 C<use overload> directive (with one exception - see L</nomethod>).
259 The first of these is the operand providing the overloaded
260 operator implementation -
261 in this case, the object whose C<minus()> method is being called.
263 The second argument is the other operand, or C<undef> in the
264 case of a unary operator.
266 The third argument is set to TRUE if (and only if) the two
267 operands have been swapped. Perl may do this to ensure that the
268 first argument (C<$self>) is an object implementing the overloaded
269 operation, in line with general object calling conventions.
270 For example, if C<$x> and C<$y> are C<Number>s:
272 operation | generates a call to
273 ============|======================
274 $x - $y | minus($x, $y, '')
275 $x - 7 | minus($x, 7, '')
276 7 - $x | minus($x, 7, 1)
278 Perl may also use C<minus()> to implement other operators which
279 have not been specified in the C<use overload> directive,
280 according to the rules for L<Magic Autogeneration> described later.
281 For example, the C<use overload> above declared no subroutine
282 for any of the operators C<-->, C<neg> (the overload key for
283 unary minus), or C<-=>. Thus
285 operation | generates a call to
286 ============|======================
287 -$x | minus($x, 0, 1)
288 $x-- | minus($x, 1, undef)
289 $x -= 3 | minus($x, 3, undef)
292 where autogeneration results in the method for a standard
293 operator which does not change either of its operands, such
294 as C<->, being used to implement an operator which changes
295 the operand ("mutators": here, C<--> and C<-=>),
296 Perl passes undef as the third argument.
297 This still evaluates as FALSE, consistent with the fact that
298 the operands have not been swapped, but gives the subroutine
299 a chance to alter its behaviour in these cases.
301 In all the above examples, C<minus()> is required
302 only to return the result of the subtraction:
303 Perl takes care of the assignment to $x.
304 In fact, such methods should I<not> modify their operands,
305 even if C<undef> is passed as the third argument
306 (see L<Overloadable Operations>).
308 The same is not true of implementations of C<++> and C<-->:
309 these are expected to modify their operand.
310 An appropriate implementation of C<--> might look like
312 use overload '--' => "decr",
314 sub decr { --${$_[0]}; }
316 =head3 Mathemagic, Mutators, and Copy Constructors
318 The term 'mathemagic' describes the overloaded implementation
319 of mathematical operators.
320 Mathemagical operations raise an issue.
326 If C<$a> and C<$b> are scalars then after these statements
330 An object, however, is a reference to blessed data, so if
331 C<$a> and C<$b> are objects then the assignment C<$a = $b>
332 copies only the reference, leaving C<$a> and C<$b> referring
333 to the same object data.
334 One might therefore expect the operation C<--$a> to decrement
335 C<$b> as well as C<$a>.
336 However, this would not be consistent with how we expect the
337 mathematical operators to work.
339 Perl resolves this dilemma by transparently calling a copy
340 constructor before calling a method defined to implement
341 a mutator (C<-->, C<+=>, and so on.).
342 In the above example, when Perl reaches the decrement
343 statement, it makes a copy of the object data in C<$a> and
344 assigns to C<$a> a reference to the copied data.
345 Only then does it call C<decr()>, which alters the copied
346 data, leaving C<$b> unchanged.
347 Thus the object metaphor is preserved as far as possible,
348 while mathemagical operations still work according to the
351 Note: the preceding paragraph describes what happens when
352 Perl autogenerates the copy constructor for an object based
354 For other cases, see L<Copy Constructor>.
356 =head2 Overloadable Operations
358 The complete list of keys that can be specified in the C<use overload>
359 directive are given, separated by spaces, in the values of the
360 hash C<%overload::ops>:
362 with_assign => '+ - * / % ** << >> x .',
363 assign => '+= -= *= /= %= **= <<= >>= x= .=',
364 num_comparison => '< <= > >= == !=',
365 '3way_comparison'=> '<=> cmp',
366 str_comparison => 'lt le gt ge eq ne',
367 binary => '& &= | |= ^ ^=',
370 func => 'atan2 cos sin exp abs log sqrt int',
371 conversion => 'bool "" 0+ qr',
374 dereferencing => '${} @{} %{} &{} *{}',
376 special => 'nomethod fallback ='
378 Most of the overloadable operators map one-to-one to these keys.
379 Exceptions, including additional overloadable operations not
380 apparent from this hash, are included in the notes which follow.
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).
393 The key C<neg> is used for unary minus to disambiguate it from
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.
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.
407 =item * I<Assignments>
409 += -= *= /= %= **= <<= >>= x= .=
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.
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.
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
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<*>).
437 =item * I<Non-mutators with a mutator variant>
439 + - * / % ** << >> x .
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
448 This is because Perl does not call the copy constructor before
449 invoking these methods.
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.
457 =item * I<String, numeric, boolean, and regexp conversions>
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.
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).
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.
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)>.
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.
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.
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>>.
499 B<BUGS> Even in list context, the iterator is currently called only
500 once and with scalar context.
502 =item * I<File tests>
504 The key C<'-X'> is used to specify a subroutine to handle all the
505 filetest operators (C<-f>, C<-x>, and so on: see L<perlfunc/-X> for
507 it is not possible to overload any filetest operator individually.
508 To distinguish them, the letter following the '-' is passed as the
509 second argument (that is, in the slot that for binary operators
510 is used to pass the second operand).
512 Calling an overloaded filetest operator does not affect the stat value
513 associated with the special filehandle C<_>. It still refers to the
514 result of the last C<stat>, C<lstat> or unoverloaded filetest.
516 This overload was introduced in Perl 5.12.
520 The key C<"~~"> allows you to override the smart matching logic used by
521 the C<~~> operator and the switch construct (C<given>/C<when>). See
522 L<perlsyn/Switch statements> and L<feature>.
524 Unusually, the overloaded implementation of the smart match operator
525 does not get full control of the smart match behaviour.
526 In particular, in the following code:
529 use overload '~~' => 'match';
531 my $obj = Foo->new();
534 the smart match does I<not> invoke the method call like this:
536 $obj->match([1,2,3],0);
538 rather, the smart match distributive rule takes precedence, so $obj is
539 smart matched against each array element in turn until a match is found,
540 so you may see between one and three of these calls instead:
546 Consult the match table in L<perlsyn/"Smart matching in detail"> for
547 details of when overloading is invoked.
549 =item * I<Dereferencing>
553 If these operators are not explicitly overloaded then they
554 work in the normal way, yielding the underlying scalar,
555 array, or whatever stores the object data (or the appropriate
556 error message if the dereference operator doesn't match it).
557 Defining a catch-all C<'nomethod'> (see L<below|/nomethod>)
558 makes no difference to this as the catch-all function will
559 not be called to implement a missing dereference operator.
561 If a dereference operator is overloaded then it must return a
562 I<reference> of the appropriate type (for example, the
563 subroutine for key C<'${}'> should return a reference to a
564 scalar, not a scalar), or another object which overloads the
565 operator: that is, the subroutine only determines what is
566 dereferenced and the actual dereferencing is left to Perl.
567 As a special case, if the subroutine returns the object itself
568 then it will not be called again - avoiding infinite recursion.
574 See L<Special Keys for C<use overload>>.
578 =head2 Magic Autogeneration
580 If a method for an operation is not found then Perl tries to
581 autogenerate a substitute implementation from the operations
582 that have been defined.
584 Note: the behaviour described in this section can be disabled
585 by setting C<fallback> to FALSE (see L</fallback>).
587 In the following tables, numbers indicate priority.
588 For example, the table below states that,
589 if no implementation for C<'!'> has been defined then Perl will
590 implement it using C<'bool'> (that is, by inverting the value
591 returned by the method for C<'bool'>);
592 if boolean conversion is also unimplemented then Perl will
593 use C<'0+'> or, failing that, C<'""'>.
595 operator | can be autogenerated from
598 =========|==========================
612 Note: The iterator (C<'E<lt>E<gt>'>) and file test (C<'-X'>)
613 operators work as normal: if the operand is not a blessed glob or
614 IO reference then it is converted to a string (using the method
615 for C<'""'>, C<'0+'>, or C<'bool'>) to be interpreted as a glob
618 operator | can be autogenerated from
621 =========|==========================
625 abs | a1 a2 b1 b2 [*]
633 * one from [a1, a2] and one from [b1, b2]
635 Just as numeric comparisons can be autogenerated from the method
636 for C<< '<=>' >>, string comparisons can be autogenerated from
639 operators | can be autogenerated from
640 ====================|===========================
641 lt gt le ge eq ne | cmp
643 Similarly, autogeneration for keys C<'+='> and C<'++'> is analogous
644 to C<'-='> and C<'--'> above:
646 operator | can be autogenerated from
649 =========|==========================
653 And other assignment variations are analogous to
654 C<'+='> and C<'-='> (and similar to C<'.='> and C<'x='> above):
656 operator || *= /= %= **= <<= >>= &= ^= |=
657 -------------------||--------------------------------
658 autogenerated from || * / % ** << >> & ^ |
660 Note also that the copy constructor (key C<'='>) may be
661 autogenerated, but only for objects based on scalars.
662 See L<Copy Constructor>.
664 =head3 Minimal Set of Overloaded Operations
666 Since some operations can be automatically generated from others, there is
667 a minimal set of operations that need to be overloaded in order to have
668 the complete set of overloaded operations at one's disposal.
669 Of course, the autogenerated operations may not do exactly what the user
670 expects. The minimal set is:
675 atan2 cos sin exp log sqrt int
679 Of the conversions, only one of string, boolean or numeric is
680 needed because each can be generated from either of the other two.
682 =head2 Special Keys for C<use overload>
686 The C<'nomethod'> key is used to specify a catch-all function to
687 be called for any operator that is not individually overloaded.
688 The specified function will be passed four parameters.
689 The first three arguments coincide with those that would have been
690 passed to the corresponding method if it had been defined.
691 The fourth argument is the C<use overload> key for that missing
694 For example, if C<$a> is an object blessed into a package declaring
696 use overload 'nomethod' => 'catch_all', # ...
702 could (unless a method is specifically declared for the key
703 C<'+'>) result in a call
705 catch_all($a, 3, 1, '+')
707 See L<How Perl Chooses an Operator Implementation>.
711 The value assigned to the key C<'fallback'> tells Perl how hard
712 it should try to find an alternative way to implement a missing
717 =item * defined, but FALSE
719 use overload "fallback" => 0, # ... ;
721 This disables L<Magic Autogeneration>.
725 In the default case where no value is explicitly assigned to
726 C<fallback>, magic autogeneration is enabled.
730 The same as for C<undef>, but if a missing operator cannot be
731 autogenerated then, instead of issuing an error message, Perl
732 is allowed to revert to what it would have done for that
733 operator if there had been no C<use overload> directive.
735 Note: in most cases, particularly the L<Copy Constructor>,
736 this is unlikely to be appropriate behaviour.
740 See L<How Perl Chooses an Operator Implementation>.
742 =head3 Copy Constructor
744 As mentioned L<above|"Mathemagic, Mutators, and Copy Constructors">,
745 this operation is called when a mutator is applied to a reference
746 that shares its object with some other reference.
747 For example, if C<$b> is mathemagical, and C<'++'> is overloaded
748 with C<'incr'>, and C<'='> is overloaded with C<'clone'>, then the
752 # ... (other code which does not modify $a or $b) ...
755 would be executed in a manner equivalent to
759 $b = $b->clone(undef, "");
768 The subroutine for C<'='> does not overload the Perl assignment
769 operator: it is used only to allow mutators to work as described
770 here. (See L</Assignments> above.)
774 As for other operations, the subroutine implementing '=' is passed
775 three arguments, though the last two are always C<undef> and C<''>.
779 The copy constructor is called only before a call to a function
780 declared to implement a mutator, for example, if C<++$b;> in the
781 code above is effected via a method declared for key C<'++'>
782 (or 'nomethod', passed C<'++'> as the fourth argument) or, by
783 autogeneration, C<'+='>.
784 It is not called if the increment operation is effected by a call
785 to the method for C<'+'> since, in the equivalent code,
790 the data referred to by C<$a> is unchanged by the assignment to
791 C<$b> of a reference to new object data.
795 The copy constructor is not called if Perl determines that it is
796 unnecessary because there is no other reference to the data being
801 If C<'fallback'> is undefined or TRUE then a copy constructor
802 can be autogenerated, but only for objects based on scalars.
803 In other cases it needs to be defined explicitly.
804 Where an object's data is stored as, for example, an array of
805 scalars, the following might be appropriate:
807 use overload '=' => sub { bless [ @{$_[0]} ] }, # ...
811 If C<'fallback'> is TRUE and no copy constructor is defined then,
812 for objects not based on scalars, Perl may silently fall back on
813 simple assignment - that is, assignment of the object reference.
814 In effect, this disables the copy constructor mechanism since
815 no new copy of the object data is created.
816 This is almost certainly not what you want.
817 (It is, however, consistent: for example, Perl's fallback for the
818 C<++> operator is to increment the reference itself.)
822 =head2 How Perl Chooses an Operator Implementation
824 Which is checked first, C<nomethod> or C<fallback>?
825 If the two operands of an operator are of different types and
826 both overload the operator, which implementation is used?
827 The following are the precedence rules:
833 If the first operand has declared a subroutine to overload the
834 operator then use that implementation.
838 Otherwise, if fallback is TRUE or undefined for the
839 first operand then see if the
840 L<rules for autogeneration|"Magic Autogeneration">
841 allows another of its operators to be used instead.
845 Unless the operator is an assignment (C<+=>, C<-=>, etc.),
846 repeat step (1) in respect of the second operand.
850 Repeat Step (2) in respect of the second operand.
854 If the first operand has a "nomethod" method then use that.
858 If the second operand has a "nomethod" method then use that.
862 If C<fallback> is TRUE for both operands
863 then perform the usual operation for the operator,
864 treating the operands as numbers, strings, or booleans
865 as appropriate for the operator (see note).
869 Nothing worked - die.
873 Where there is only one operand (or only one operand with
874 overloading) the checks in respect of the other operand above are
877 There are exceptions to the above rules for dereference operations
878 (which, if Step 1 fails, always fall back to the normal, built-in
879 implementations - see Dereferencing), and for C<~~> (which has its
880 own set of rules - see C<Matching> under L</Overloadable Operations>
883 Note on Step 7: some operators have a different semantic depending
884 on the type of their operands.
885 As there is no way to instruct Perl to treat the operands as, e.g.,
886 numbers instead of strings, the result here may not be what you
888 See L<BUGS AND PITFALLS>.
890 =head2 Losing Overloading
892 The restriction for the comparison operation is that even if, for example,
893 C<cmp> should return a blessed reference, the autogenerated C<lt>
894 function will produce only a standard logical value based on the
895 numerical value of the result of C<cmp>. In particular, a working
896 numeric conversion is needed in this case (possibly expressed in terms of
899 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
900 if the string conversion substitution is applied.
902 When you chop() a mathemagical object it is promoted to a string and its
903 mathemagical properties are lost. The same can happen with other
906 =head2 Inheritance and Overloading
908 Overloading respects inheritance via the @ISA hierarchy.
909 Inheritance interacts with overloading in two ways.
913 =item Method names in the C<use overload> directive
917 use overload key => value;
919 is a string, it is interpreted as a method name - which may
920 (in the usual way) be inherited from another class.
922 =item Overloading of an operation is inherited by derived classes
924 Any class derived from an overloaded class is also overloaded
925 and inherits its operator implementations.
926 If the same operator is overloaded in more than one ancestor
927 then the implementation is determined by the usual inheritance
930 For example, if C<A> inherits from C<B> and C<C> (in that order),
931 C<B> overloads C<+> with C<\&D::plus_sub>, and C<C> overloads
932 C<+> by C<"plus_meth">, then the subroutine C<D::plus_sub> will
933 be called to implement operation C<+> for an object in package C<A>.
937 Note that since the value of the C<fallback> key is not a subroutine,
938 its inheritance is not governed by the above rules. In the current
939 implementation, the value of C<fallback> in the first overloaded
940 ancestor is used, but this is accidental and subject to change.
942 =head2 Run-time Overloading
944 Since all C<use> directives are executed at compile-time, the only way to
945 change overloading during run-time is to
947 eval 'use overload "+" => \&addmethod';
951 eval 'no overload "+", "--", "<="';
953 though the use of these constructs during run-time is questionable.
955 =head2 Public Functions
957 Package C<overload.pm> provides the following public functions:
961 =item overload::StrVal(arg)
963 Gives string value of C<arg> as in absence of stringify overloading. If you
964 are using this to get the address of a reference (useful for checking if two
965 references point to the same thing) then you may be better off using
966 C<Scalar::Util::refaddr()>, which is faster.
968 =item overload::Overloaded(arg)
970 Returns true if C<arg> is subject to overloading of some operations.
972 =item overload::Method(obj,op)
974 Returns C<undef> or a reference to the method that implements C<op>.
978 =head2 Overloading Constants
980 For some applications, the Perl parser mangles constants too much.
981 It is possible to hook into this process via C<overload::constant()>
982 and C<overload::remove_constant()> functions.
984 These functions take a hash as an argument. The recognized keys of this hash
991 to overload integer constants,
995 to overload floating point constants,
999 to overload octal and hexadecimal constants,
1003 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
1004 strings and here-documents,
1008 to overload constant pieces of regular expressions.
1012 The corresponding values are references to functions which take three arguments:
1013 the first one is the I<initial> string form of the constant, the second one
1014 is how Perl interprets this constant, the third one is how the constant is used.
1015 Note that the initial string form does not
1016 contain string delimiters, and has backslashes in backslash-delimiter
1017 combinations stripped (thus the value of delimiter is not relevant for
1018 processing of this string). The return value of this function is how this
1019 constant is going to be interpreted by Perl. The third argument is undefined
1020 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
1021 context (comes from strings, regular expressions, and single-quote HERE
1022 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
1023 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
1025 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
1026 it is expected that overloaded constant strings are equipped with reasonable
1027 overloaded catenation operator, otherwise absurd results will result.
1028 Similarly, negative numbers are considered as negations of positive constants.
1030 Note that it is probably meaningless to call the functions overload::constant()
1031 and overload::remove_constant() from anywhere but import() and unimport() methods.
1032 From these methods they may be called as
1037 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
1038 overload::constant integer => sub {Math::BigInt->new(shift)};
1041 =head1 IMPLEMENTATION
1043 What follows is subject to change RSN.
1045 The table of methods for all operations is cached in magic for the
1046 symbol table hash for the package. The cache is invalidated during
1047 processing of C<use overload>, C<no overload>, new function
1048 definitions, and changes in @ISA. However, this invalidation remains
1049 unprocessed until the next C<bless>ing into the package. Hence if you
1050 want to change overloading structure dynamically, you'll need an
1051 additional (fake) C<bless>ing to update the table.
1053 (Every SVish thing has a magic queue, and magic is an entry in that
1054 queue. This is how a single variable may participate in multiple
1055 forms of magic simultaneously. For instance, environment variables
1056 regularly have two forms at once: their %ENV magic and their taint
1057 magic. However, the magic which implements overloading is applied to
1058 the stashes, which are rarely used directly, thus should not slow down
1061 If an object belongs to a package using overload, it carries a special
1062 flag. Thus the only speed penalty during arithmetic operations without
1063 overloading is the checking of this flag.
1065 In fact, if C<use overload> is not present, there is almost no overhead
1066 for overloadable operations, so most programs should not suffer
1067 measurable performance penalties. A considerable effort was made to
1068 minimize the overhead when overload is used in some package, but the
1069 arguments in question do not belong to packages using overload. When
1070 in doubt, test your speed with C<use overload> and without it. So far
1071 there have been no reports of substantial speed degradation if Perl is
1072 compiled with optimization turned on.
1074 There is no size penalty for data if overload is not used. The only
1075 size penalty if overload is used in some package is that I<all> the
1076 packages acquire a magic during the next C<bless>ing into the
1077 package. This magic is three-words-long for packages without
1078 overloading, and carries the cache table if the package is overloaded.
1080 It is expected that arguments to methods that are not explicitly supposed
1081 to be changed are constant (but this is not enforced).
1085 Please add examples to what follows!
1087 =head2 Two-face Scalars
1089 Put this in F<two_face.pm> in your Perl library directory:
1091 package two_face; # Scalars with separate string and
1093 sub new { my $p = shift; bless [@_], $p }
1094 use overload '""' => \&str, '0+' => \&num, fallback => 1;
1095 sub num {shift->[1]}
1096 sub str {shift->[0]}
1101 my $seven = two_face->new("vii", 7);
1102 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
1103 print "seven contains 'i'\n" if $seven =~ /i/;
1105 (The second line creates a scalar which has both a string value, and a
1106 numeric value.) This prints:
1108 seven=vii, seven=7, eight=8
1111 =head2 Two-face References
1113 Suppose you want to create an object which is accessible as both an
1114 array reference and a hash reference.
1117 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
1125 tie %h, ref $self, $self;
1129 sub TIEHASH { my $p = shift; bless \ shift, $p }
1132 $fields{$_} = $i++ foreach qw{zero one two three};
1134 my $self = ${shift()};
1135 my $key = $fields{shift()};
1136 defined $key or die "Out of band access";
1137 $$self->[$key] = shift;
1140 my $self = ${shift()};
1141 my $key = $fields{shift()};
1142 defined $key or die "Out of band access";
1146 Now one can access an object using both the array and hash syntax:
1148 my $bar = two_refs->new(3,4,5,6);
1150 $bar->{two} == 11 or die 'bad hash fetch';
1152 Note several important features of this example. First of all, the
1153 I<actual> type of $bar is a scalar reference, and we do not overload
1154 the scalar dereference. Thus we can get the I<actual> non-overloaded
1155 contents of $bar by just using C<$$bar> (what we do in functions which
1156 overload dereference). Similarly, the object returned by the
1157 TIEHASH() method is a scalar reference.
1159 Second, we create a new tied hash each time the hash syntax is used.
1160 This allows us not to worry about a possibility of a reference loop,
1161 which would lead to a memory leak.
1163 Both these problems can be cured. Say, if we want to overload hash
1164 dereference on a reference to an object which is I<implemented> as a
1165 hash itself, the only problem one has to circumvent is how to access
1166 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1167 overloaded dereference operator). Here is one possible fetching routine:
1170 my ($self, $key) = (shift, shift);
1171 my $class = ref $self;
1172 bless $self, 'overload::dummy'; # Disable overloading of %{}
1173 my $out = $self->{$key};
1174 bless $self, $class; # Restore overloading
1178 To remove creation of the tied hash on each access, one may an extra
1179 level of indirection which allows a non-circular structure of references:
1182 use overload '%{}' => sub { ${shift()}->[1] },
1183 '@{}' => sub { ${shift()}->[0] };
1189 bless \ [$a, \%h], $p;
1194 tie %h, ref $self, $self;
1198 sub TIEHASH { my $p = shift; bless \ shift, $p }
1201 $fields{$_} = $i++ foreach qw{zero one two three};
1204 my $key = $fields{shift()};
1205 defined $key or die "Out of band access";
1210 my $key = $fields{shift()};
1211 defined $key or die "Out of band access";
1215 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1216 reference to the intermediate array, which keeps a reference to an
1217 actual array, and the access hash. The tie()ing object for the access
1218 hash is a reference to a reference to the actual array, so
1224 There are no loops of references.
1228 Both "objects" which are blessed into the class C<two_refs1> are
1229 references to a reference to an array, thus references to a I<scalar>.
1230 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1231 overloaded operations.
1235 =head2 Symbolic Calculator
1237 Put this in F<symbolic.pm> in your Perl library directory:
1239 package symbolic; # Primitive symbolic calculator
1240 use overload nomethod => \&wrap;
1242 sub new { shift; bless ['n', @_] }
1244 my ($obj, $other, $inv, $meth) = @_;
1245 ($obj, $other) = ($other, $obj) if $inv;
1246 bless [$meth, $obj, $other];
1249 This module is very unusual as overloaded modules go: it does not
1250 provide any usual overloaded operators, instead it provides an
1251 implementation for L<C<nomethod>>. In this example the C<nomethod>
1252 subroutine returns an object which encapsulates operations done over
1253 the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
1254 symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
1256 Here is an example of the script which "calculates" the side of
1257 circumscribed octagon using the above package:
1260 my $iter = 1; # 2**($iter+2) = 8
1261 my $side = symbolic->new(1);
1265 $side = (sqrt(1 + $side**2) - 1)/$side;
1269 The value of $side is
1271 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1272 undef], 1], ['n', 1]]
1274 Note that while we obtained this value using a nice little script,
1275 there is no simple way to I<use> this value. In fact this value may
1276 be inspected in debugger (see L<perldebug>), but only if
1277 C<bareStringify> B<O>ption is set, and not via C<p> command.
1279 If one attempts to print this value, then the overloaded operator
1280 C<""> will be called, which will call C<nomethod> operator. The
1281 result of this operator will be stringified again, but this result is
1282 again of type C<symbolic>, which will lead to an infinite loop.
1284 Add a pretty-printer method to the module F<symbolic.pm>:
1287 my ($meth, $a, $b) = @{+shift};
1288 $a = 'u' unless defined $a;
1289 $b = 'u' unless defined $b;
1290 $a = $a->pretty if ref $a;
1291 $b = $b->pretty if ref $b;
1295 Now one can finish the script by
1297 print "side = ", $side->pretty, "\n";
1299 The method C<pretty> is doing object-to-string conversion, so it
1300 is natural to overload the operator C<""> using this method. However,
1301 inside such a method it is not necessary to pretty-print the
1302 I<components> $a and $b of an object. In the above subroutine
1303 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1304 and $b. If these components use overloading, the catenation operator
1305 will look for an overloaded operator C<.>; if not present, it will
1306 look for an overloaded operator C<"">. Thus it is enough to use
1308 use overload nomethod => \&wrap, '""' => \&str;
1310 my ($meth, $a, $b) = @{+shift};
1311 $a = 'u' unless defined $a;
1312 $b = 'u' unless defined $b;
1316 Now one can change the last line of the script to
1318 print "side = $side\n";
1322 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1324 and one can inspect the value in debugger using all the possible
1327 Something is still amiss: consider the loop variable $cnt of the
1328 script. It was a number, not an object. We cannot make this value of
1329 type C<symbolic>, since then the loop will not terminate.
1331 Indeed, to terminate the cycle, the $cnt should become false.
1332 However, the operator C<bool> for checking falsity is overloaded (this
1333 time via overloaded C<"">), and returns a long string, thus any object
1334 of type C<symbolic> is true. To overcome this, we need a way to
1335 compare an object to 0. In fact, it is easier to write a numeric
1338 Here is the text of F<symbolic.pm> with such a routine added (and
1339 slightly modified str()):
1341 package symbolic; # Primitive symbolic calculator
1343 nomethod => \&wrap, '""' => \&str, '0+' => \#
1345 sub new { shift; bless ['n', @_] }
1347 my ($obj, $other, $inv, $meth) = @_;
1348 ($obj, $other) = ($other, $obj) if $inv;
1349 bless [$meth, $obj, $other];
1352 my ($meth, $a, $b) = @{+shift};
1353 $a = 'u' unless defined $a;
1360 my %subr = ( n => sub {$_[0]},
1361 sqrt => sub {sqrt $_[0]},
1362 '-' => sub {shift() - shift()},
1363 '+' => sub {shift() + shift()},
1364 '/' => sub {shift() / shift()},
1365 '*' => sub {shift() * shift()},
1366 '**' => sub {shift() ** shift()},
1369 my ($meth, $a, $b) = @{+shift};
1370 my $subr = $subr{$meth}
1371 or die "Do not know how to ($meth) in symbolic";
1372 $a = $a->num if ref $a eq __PACKAGE__;
1373 $b = $b->num if ref $b eq __PACKAGE__;
1377 All the work of numeric conversion is done in %subr and num(). Of
1378 course, %subr is not complete, it contains only operators used in the
1379 example below. Here is the extra-credit question: why do we need an
1380 explicit recursion in num()? (Answer is at the end of this section.)
1382 Use this module like this:
1385 my $iter = symbolic->new(2); # 16-gon
1386 my $side = symbolic->new(1);
1390 $cnt = $cnt - 1; # Mutator '--' not implemented
1391 $side = (sqrt(1 + $side**2) - 1)/$side;
1393 printf "%s=%f\n", $side, $side;
1394 printf "pi=%f\n", $side*(2**($iter+2));
1396 It prints (without so many line breaks)
1398 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1400 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1403 The above module is very primitive. It does not implement
1404 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1405 (not required without mutators!), and implements only those arithmetic
1406 operations which are used in the example.
1408 To implement most arithmetic operations is easy; one should just use
1409 the tables of operations, and change the code which fills %subr to
1411 my %subr = ( 'n' => sub {$_[0]} );
1412 foreach my $op (split " ", $overload::ops{with_assign}) {
1413 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1415 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1416 foreach my $op (split " ", "@overload::ops{ @bins }") {
1417 $subr{$op} = eval "sub {shift() $op shift()}";
1419 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1420 print "defining '$op'\n";
1421 $subr{$op} = eval "sub {$op shift()}";
1424 Since subroutines implementing assignment operators are not required
1425 to modify their operands (see L<Overloadable Operations> above),
1426 we do not need anything special to make C<+=> and friends work,
1427 besides adding these operators to %subr and defining a copy
1428 constructor (needed since Perl has no way to know that the
1429 implementation of C<'+='> does not mutate the argument -
1430 see L<Copy Constructor>).
1432 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1433 line, and code (this code assumes that mutators change things one level
1434 deep only, so recursive copying is not needed):
1438 bless [@$self], ref $self;
1441 To make C<++> and C<--> work, we need to implement actual mutators,
1442 either directly, or in C<nomethod>. We continue to do things inside
1443 C<nomethod>, thus add
1445 if ($meth eq '++' or $meth eq '--') {
1446 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1450 after the first line of wrap(). This is not a most effective
1451 implementation, one may consider
1453 sub inc { $_[0] = bless ['++', shift, 1]; }
1457 As a final remark, note that one can fill %subr by
1459 my %subr = ( 'n' => sub {$_[0]} );
1460 foreach my $op (split " ", $overload::ops{with_assign}) {
1461 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1463 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1464 foreach my $op (split " ", "@overload::ops{ @bins }") {
1465 $subr{$op} = eval "sub {shift() $op shift()}";
1467 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1468 $subr{$op} = eval "sub {$op shift()}";
1470 $subr{'++'} = $subr{'+'};
1471 $subr{'--'} = $subr{'-'};
1473 This finishes implementation of a primitive symbolic calculator in
1474 50 lines of Perl code. Since the numeric values of subexpressions
1475 are not cached, the calculator is very slow.
1477 Here is the answer for the exercise: In the case of str(), we need no
1478 explicit recursion since the overloaded C<.>-operator will fall back
1479 to an existing overloaded operator C<"">. Overloaded arithmetic
1480 operators I<do not> fall back to numeric conversion if C<fallback> is
1481 not explicitly requested. Thus without an explicit recursion num()
1482 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1483 the argument of num().
1485 If you wonder why defaults for conversion are different for str() and
1486 num(), note how easy it was to write the symbolic calculator. This
1487 simplicity is due to an appropriate choice of defaults. One extra
1488 note: due to the explicit recursion num() is more fragile than sym():
1489 we need to explicitly check for the type of $a and $b. If components
1490 $a and $b happen to be of some related type, this may lead to problems.
1492 =head2 I<Really> Symbolic Calculator
1494 One may wonder why we call the above calculator symbolic. The reason
1495 is that the actual calculation of the value of expression is postponed
1496 until the value is I<used>.
1498 To see it in action, add a method
1503 @$obj->[0,1] = ('=', shift);
1506 to the package C<symbolic>. After this change one can do
1508 my $a = symbolic->new(3);
1509 my $b = symbolic->new(4);
1510 my $c = sqrt($a**2 + $b**2);
1512 and the numeric value of $c becomes 5. However, after calling
1514 $a->STORE(12); $b->STORE(5);
1516 the numeric value of $c becomes 13. There is no doubt now that the module
1517 symbolic provides a I<symbolic> calculator indeed.
1519 To hide the rough edges under the hood, provide a tie()d interface to the
1520 package C<symbolic>. Add methods
1522 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1524 sub nop { } # Around a bug
1526 (the bug, fixed in Perl 5.14, is described in L<"BUGS">). One can use this
1529 tie $a, 'symbolic', 3;
1530 tie $b, 'symbolic', 4;
1531 $a->nop; $b->nop; # Around a bug
1533 my $c = sqrt($a**2 + $b**2);
1535 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1536 of $c becomes 13. To insulate the user of the module add a method
1538 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1543 symbolic->vars($a, $b);
1544 my $c = sqrt($a**2 + $b**2);
1547 printf "c5 %s=%f\n", $c, $c;
1550 printf "c13 %s=%f\n", $c, $c;
1552 shows that the numeric value of $c follows changes to the values of $a
1557 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1561 The C<overloading> pragma can be used to enable or disable overloaded
1562 operations within a lexical scope - see L<overloading>.
1566 When Perl is run with the B<-Do> switch or its equivalent, overloading
1567 induces diagnostic messages.
1569 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1570 deduce which operations are overloaded (and which ancestor triggers
1571 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1572 is shown by debugger. The method C<()> corresponds to the C<fallback>
1573 key (in fact a presence of this method shows that this package has
1574 overloading enabled, and it is what is used by the C<Overloaded>
1575 function of module C<overload>).
1577 The module might issue the following warnings:
1581 =item Odd number of arguments for overload::constant
1583 (W) The call to overload::constant contained an odd number of arguments.
1584 The arguments should come in pairs.
1586 =item '%s' is not an overloadable type
1588 (W) You tried to overload a constant type the overload package is unaware of.
1590 =item '%s' is not a code reference
1592 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1593 to be a code reference. Either an anonymous subroutine, or a reference
1598 =head1 BUGS AND PITFALLS
1604 No warning is issued for invalid C<use overload> keys.
1605 Such errors are not always obvious:
1607 use overload "+0" => sub { ...; }, # should be "0+"
1608 "not" => sub { ...; }; # should be "!"
1614 A pitfall when fallback is TRUE and Perl resorts to a built-in
1615 implementation of an operator is that some operators have more
1616 than one semantic, for example C<|>:
1618 use overload '0+' => sub { $_[0]->{n}; },
1620 my $x = bless { n => 4 }, "main";
1621 my $y = bless { n => 8 }, "main";
1622 print $x | $y, "\n";
1624 You might expect this to output "12".
1625 In fact, it prints "<": the ASCII result of treating "|"
1626 as a bitwise string operator - that is, the result of treating
1627 the operands as the strings "4" and "8" rather than numbers.
1628 The fact that numify (C<0+>) is implemented but stringify
1629 (C<"">) isn't makes no difference since the latter is simply
1630 autogenerated from the former.
1632 The only way to change this is to provide your own subroutine
1637 Magic autogeneration increases the potential for inadvertently
1638 creating self-referential structures.
1639 Currently Perl will not free self-referential
1640 structures until cycles are explicitly broken.
1643 use overload '+' => 'add';
1644 sub add { bless [ \$_[0], \$_[1] ] };
1646 is asking for trouble, since
1650 will effectively become
1652 $obj = add($obj, $y, undef);
1654 with the same result as
1656 $obj = [\$obj, \$foo];
1658 Even if no I<explicit> assignment-variants of operators are present in
1659 the script, they may be generated by the optimizer.
1666 my $tmp = 'obj = ' . $obj; $tmp .= "\n";
1670 Because it is used for overloading, the per-package hash
1671 C<%OVERLOAD> now has a special meaning in Perl.
1672 The symbol table is filled with names looking like line-noise.
1676 For the purpose of inheritance every overloaded package behaves as if
1677 C<fallback> is present (possibly undefined). This may create
1678 interesting effects if some package is not overloaded, but inherits
1679 from two overloaded packages.
1683 Before Perl 5.14, the relation between overloading and tie()ing was broken.
1684 Overloading is triggered or not basing on the I<previous> class of the
1687 This happened because the presence of overloading was checked
1688 too early, before any tie()d access was attempted. If the
1689 class of the value FETCH()ed from the tied variable does not
1690 change, a simple workaround for code that is to run on older Perl
1691 versions is to access the value (via C<() = $foo> or some such)
1692 immediately after tie()ing, so that after this call the I<previous> class
1693 coincides with the current one.
1697 Barewords are not covered by overloaded string constants.