11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
14 if ($_ eq 'fallback') {
18 if (not ref $sub and $sub !~ /::/) {
19 $ {$package . "::(" . $_} = $sub;
22 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
23 *{$package . "::(" . $_} = \&{ $sub };
26 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
30 $package = (caller())[0];
31 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
33 $package->overload::OVERLOAD(@_);
37 $package = (caller())[0];
38 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
41 if ($_ eq 'fallback') {
42 undef $ {$package . "::()"};
44 delete $ {$package . "::"}{"(" . $_};
51 $package = ref $package if ref $package;
57 return undef unless $globref;
58 my $sub = \&{*$globref};
59 return $sub if $sub ne \&nil;
60 return shift->can($ {*$globref});
63 sub OverloadedStringify {
65 $package = ref $package if ref $package;
67 ov_method mycan($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;
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;
83 my $package = ref $_[0];
84 return "$_[0]" unless $package;
87 my $class = Scalar::Util::blessed($_[0]);
88 my $class_prefix = defined($class) ? "$class=" : "";
89 my $type = Scalar::Util::reftype($_[0]);
90 my $addr = Scalar::Util::refaddr($_[0]);
91 return sprintf("$class_prefix$type(0x%x)", $addr);
96 sub mycan { # Real can would leave stubs.
97 my ($package, $meth) = @_;
99 my $mro = mro::get_linear_isa($package);
100 foreach my $p (@$mro) {
101 my $fqmeth = $p . q{::} . $meth;
102 return \*{$fqmeth} if defined &{$fqmeth};
109 'integer' => 0x1000, # HINT_NEW_INTEGER
110 'float' => 0x2000, # HINT_NEW_FLOAT
111 'binary' => 0x4000, # HINT_NEW_BINARY
112 'q' => 0x8000, # HINT_NEW_STRING
113 'qr' => 0x10000, # HINT_NEW_RE
116 %ops = ( with_assign => "+ - * / % ** << >> x .",
117 assign => "+= -= *= /= %= **= <<= >>= x= .=",
118 num_comparison => "< <= > >= == !=",
119 '3way_comparison'=> "<=> cmp",
120 str_comparison => "lt le gt ge eq ne",
121 binary => '& &= | |= ^ ^=',
124 func => "atan2 cos sin exp abs log sqrt int",
125 conversion => 'bool "" 0+',
127 dereferencing => '${} @{} %{} &{} *{}',
128 special => 'nomethod fallback =');
130 use warnings::register;
132 # Arguments: what, sub
135 warnings::warnif ("Odd number of arguments for overload::constant");
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");
151 $^H |= $constants{$_[0]};
157 sub remove_constant {
158 # Arguments: what, sub
161 $^H &= ~ $constants{$_[0]};
172 overload - Package for overloading Perl operations
185 $a = new SomeThing 57;
188 if (overload::Overloaded $b) {...}
190 $strval = overload::StrVal $b;
194 =head2 Declaration of overloaded functions
196 The compilation directive
203 declares function Number::add() for addition, and method muas() in
204 the "class" C<Number> (or one of its base classes)
205 for the assignment form C<*=> of multiplication.
207 Arguments of this directive come in (key, value) pairs. Legal values
208 are values legal inside a C<&{ ... }> call, so the name of a
209 subroutine, a reference to a subroutine, or an anonymous subroutine
210 will all work. Note that values specified as strings are
211 interpreted as methods, not subroutines. Legal keys are listed below.
213 The subroutine C<add> will be called to execute C<$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<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
218 methods refer to methods triggered by an overloaded mathematical
221 Since overloading respects inheritance via the @ISA hierarchy, the
222 above declaration would also trigger overloading of C<+> and C<*=> in
223 all the packages which inherit from C<Number>.
225 =head2 Calling Conventions for Binary Operations
227 The functions specified in the C<use overload ...> directive are called
228 with three (in one particular case with four, see L<Last Resort>)
229 arguments. If the corresponding operation is binary, then the first
230 two arguments are the two arguments of the operation. However, due to
231 general object calling conventions, the first argument should always be
232 an object in the package, so in the situation of C<7+$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:
243 the order of arguments is as in the current operation.
247 the arguments are reversed.
251 the current operation is an assignment variant (as in
252 C<$a+=7>), but the usual function is called instead. This additional
253 information can be used to generate some optimizations. Compare
254 L<Calling Conventions for Mutators>.
258 =head2 Calling Conventions for Unary Operations
260 Unary operation are considered binary operations with the second
261 argument being C<undef>. Thus the functions that overloads C<{"++"}>
262 is called with arguments C<($a,undef,'')> when $a++ is executed.
264 =head2 Calling Conventions for Mutators
266 Two types of mutators have different calling conventions:
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
276 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
278 is an appropriate implementation of overloaded C<++>. Note that
280 sub incr { ++$ {$_[0]} ; shift }
282 is OK if used with preincrement and with postincrement. (In the case
283 of postincrement a copying will be performed, see L<Copy Constructor>.)
285 =item C<x=> and other assignment versions
287 There is nothing special about these methods. They may change the
288 value of their arguments, and may leave it as is. The result is going
289 to be assigned to the value in the left-hand-side if different from
292 This allows for the same method to be used as overloaded C<+=> and
293 C<+>. Note that this is I<allowed>, but not recommended, since by the
294 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
295 if C<+=> is not overloaded.
299 B<Warning.> Due to the presence of assignment versions of operations,
300 routines which may be called in assignment context may create
301 self-referential structures. Currently Perl will not free self-referential
302 structures until cycles are C<explicitly> broken. You may get problems
303 when traversing your structures too.
307 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
309 is asking for trouble, since for code C<$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:
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
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 The same remarks in L<"Arithmetic operations"> about
362 assignment-variants and autogeneration apply for
363 bit operations C<"&">, C<"^">, and C<"|"> as well.
365 =item * I<Increment and decrement>
369 If undefined, addition and subtraction methods can be
370 used instead. These operations are called both in prefix and
373 =item * I<Transcendental functions>
375 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
377 If C<abs> is unavailable, it can be autogenerated using methods
378 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
380 Note that traditionally the Perl function L<int> rounds to 0, thus for
381 floating-point-like types one should follow the same semantic. If
382 C<int> is unavailable, it can be autogenerated using the overloading of
385 =item * I<Boolean, string and numeric conversion>
389 If one or two of these operations are not overloaded, the remaining ones can
390 be used instead. C<bool> is used in the flow control operators
391 (like C<while>) and for the ternary C<?:> operation. These functions can
392 return any arbitrary Perl value. If the corresponding operation for this value
393 is overloaded too, that operation will be called again with this value.
395 As a special case if the overload returns the object itself then it will
396 be used directly. An overloaded conversion returning the object is
397 probably a bug, because you're likely to get something that looks like
398 C<YourPackage=HASH(0x8172b34)>.
404 If not overloaded, the argument will be converted to a filehandle or
405 glob (which may require a stringification). The same overloading
406 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
407 I<globbing> syntax C<E<lt>${var}E<gt>>.
409 B<BUGS> Even in list context, the iterator is currently called only
410 once and with scalar context.
412 =item * I<Dereferencing>
414 '${}', '@{}', '%{}', '&{}', '*{}'.
416 If not overloaded, the argument will be dereferenced I<as is>, thus
417 should be of correct type. These functions should return a reference
418 of correct type, or another object with overloaded dereferencing.
420 As a special case if the overload returns the object itself then it
421 will be used directly (provided it is the correct type).
423 The dereference operators must be specified explicitly they will not be passed to
428 "nomethod", "fallback", "=", "~~",
430 see L<SPECIAL SYMBOLS FOR C<use overload>>.
434 See L<"Fallback"> for an explanation of when a missing method can be
437 A computer-readable form of the above table is available in the hash
438 %overload::ops, with values being space-separated lists of names:
440 with_assign => '+ - * / % ** << >> x .',
441 assign => '+= -= *= /= %= **= <<= >>= x= .=',
442 num_comparison => '< <= > >= == !=',
443 '3way_comparison'=> '<=> cmp',
444 str_comparison => 'lt le gt ge eq ne',
445 binary => '& &= | |= ^ ^=',
448 func => 'atan2 cos sin exp abs log sqrt',
449 conversion => 'bool "" 0+',
451 dereferencing => '${} @{} %{} &{} *{}',
452 special => 'nomethod fallback ='
454 =head2 Inheritance and overloading
456 Inheritance interacts with overloading in two ways.
460 =item Strings as values of C<use overload> directive
464 use overload key => value;
466 is a string, it is interpreted as a method name.
468 =item Overloading of an operation is inherited by derived classes
470 Any class derived from an overloaded class is also overloaded. The
471 set of overloaded methods is the union of overloaded methods of all
472 the ancestors. If some method is overloaded in several ancestor, then
473 which description will be used is decided by the usual inheritance
476 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
477 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
478 then the subroutine C<D::plus_sub> will be called to implement
479 operation C<+> for an object in package C<A>.
483 Note that since the value of the C<fallback> key is not a subroutine,
484 its inheritance is not governed by the above rules. In the current
485 implementation, the value of C<fallback> in the first overloaded
486 ancestor is used, but this is accidental and subject to change.
488 =head1 SPECIAL SYMBOLS FOR C<use overload>
490 Three keys are recognized by Perl that are not covered by the above
495 C<"nomethod"> should be followed by a reference to a function of four
496 parameters. If defined, it is called when the overloading mechanism
497 cannot find a method for some operation. The first three arguments of
498 this function coincide with the arguments for the corresponding method if
499 it were found, the fourth argument is the symbol
500 corresponding to the missing method. If several methods are tried,
501 the last one is used. Say, C<1-$a> can be equivalent to
503 &nomethodMethod($a,1,1,"-")
505 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
506 C<use overload> directive.
508 The C<"nomethod"> mechanism is I<not> used for the dereference operators
509 ( ${} @{} %{} &{} *{} ).
512 If some operation cannot be resolved, and there is no function
513 assigned to C<"nomethod">, then an exception will be raised via die()--
514 unless C<"fallback"> was specified as a key in C<use overload> directive.
519 The key C<"fallback"> governs what to do if a method for a particular
520 operation is not found. Three different cases are possible depending on
521 the value of C<"fallback">:
528 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
529 then tries to calls C<"nomethod"> value; if missing, an exception
534 The same as for the C<undef> value, but no exception is raised. Instead,
535 it silently reverts to what it would have done were there no C<use overload>
538 =item * defined, but FALSE
540 No autogeneration is tried. Perl tries to call
541 C<"nomethod"> value, and if this is missing, raises an exception.
545 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
546 yet, see L<"Inheritance and overloading">.
550 The key C<"~~"> allows you to override the smart matching used by
551 the switch construct. See L<feature>.
553 =head2 Copy Constructor
555 The value for C<"="> is a reference to a function with three
556 arguments, i.e., it looks like the other values in C<use
557 overload>. However, it does not overload the Perl assignment
558 operator. This would go against Camel hair.
560 This operation is called in the situations when a mutator is applied
561 to a reference that shares its object with some other reference, such
567 To make this change $a and not change $b, a copy of C<$$a> is made,
568 and $a is assigned a reference to this new object. This operation is
569 done during execution of the C<++$a>, and not during the assignment,
570 (so before the increment C<$$a> coincides with C<$$b>). This is only
571 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
572 C<nomethod>). Note that if this operation is expressed via C<'+'>
573 a nonmutator, i.e., as in
578 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
579 appear as lvalue when the above code is executed.
581 If the copy constructor is required during the execution of some mutator,
582 but a method for C<'='> was not specified, it can be autogenerated as a
583 string copy if the object is a plain scalar.
589 The actually executed code for
592 Something else which does not modify $a or $b....
598 Something else which does not modify $a or $b....
599 $a = $a->clone(undef,"");
602 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
603 C<'='> was overloaded with C<\&clone>.
607 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
610 =head1 MAGIC AUTOGENERATION
612 If a method for an operation is not found, and the value for C<"fallback"> is
613 TRUE or undefined, Perl tries to autogenerate a substitute method for
614 the missing operation based on the defined operations. Autogenerated method
615 substitutions are possible for the following operations:
619 =item I<Assignment forms of arithmetic operations>
621 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
624 =item I<Conversion operations>
626 String, numeric, and boolean conversion are calculated in terms of one
627 another if not all of them are defined.
629 =item I<Increment and decrement>
631 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
632 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
636 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
640 can be expressed in terms of subtraction.
644 C<!> and C<not> can be expressed in terms of boolean conversion, or
645 string or numerical conversion.
647 =item I<Concatenation>
649 can be expressed in terms of string conversion.
651 =item I<Comparison operations>
653 can be expressed in terms of its "spaceship" counterpart: either
654 C<E<lt>=E<gt>> or C<cmp>:
656 <, >, <=, >=, ==, != in terms of <=>
657 lt, gt, le, ge, eq, ne in terms of cmp
661 <> in terms of builtin operations
663 =item I<Dereferencing>
665 ${} @{} %{} &{} *{} in terms of builtin operations
667 =item I<Copy operator>
669 can be expressed in terms of an assignment to the dereferenced value, if this
670 value is a scalar and not a reference.
674 =head1 Minimal set of overloaded operations
676 Since some operations can be automatically generated from others, there is
677 a minimal set of operations that need to be overloaded in order to have
678 the complete set of overloaded operations at one's disposal.
679 Of course, the autogenerated operations may not do exactly what the user
680 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
685 atan2 cos sin exp log sqrt int
687 Additionally, you need to define at least one of string, boolean or
688 numeric conversions because any one can be used to emulate the others.
689 The string conversion can also be used to emulate concatenation.
691 =head1 Losing overloading
693 The restriction for the comparison operation is that even if, for example,
694 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
695 function will produce only a standard logical value based on the
696 numerical value of the result of `C<cmp>'. In particular, a working
697 numeric conversion is needed in this case (possibly expressed in terms of
700 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
701 if the string conversion substitution is applied.
703 When you chop() a mathemagical object it is promoted to a string and its
704 mathemagical properties are lost. The same can happen with other
707 =head1 Run-time Overloading
709 Since all C<use> directives are executed at compile-time, the only way to
710 change overloading during run-time is to
712 eval 'use overload "+" => \&addmethod';
716 eval 'no overload "+", "--", "<="';
718 though the use of these constructs during run-time is questionable.
720 =head1 Public functions
722 Package C<overload.pm> provides the following public functions:
726 =item overload::StrVal(arg)
728 Gives string value of C<arg> as in absence of stringify overloading. If you
729 are using this to get the address of a reference (useful for checking if two
730 references point to the same thing) then you may be better off using
731 C<Scalar::Util::refaddr()>, which is faster.
733 =item overload::Overloaded(arg)
735 Returns true if C<arg> is subject to overloading of some operations.
737 =item overload::Method(obj,op)
739 Returns C<undef> or a reference to the method that implements C<op>.
743 =head1 Overloading constants
745 For some applications, the Perl parser mangles constants too much.
746 It is possible to hook into this process via C<overload::constant()>
747 and C<overload::remove_constant()> functions.
749 These functions take a hash as an argument. The recognized keys of this hash
756 to overload integer constants,
760 to overload floating point constants,
764 to overload octal and hexadecimal constants,
768 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
769 strings and here-documents,
773 to overload constant pieces of regular expressions.
777 The corresponding values are references to functions which take three arguments:
778 the first one is the I<initial> string form of the constant, the second one
779 is how Perl interprets this constant, the third one is how the constant is used.
780 Note that the initial string form does not
781 contain string delimiters, and has backslashes in backslash-delimiter
782 combinations stripped (thus the value of delimiter is not relevant for
783 processing of this string). The return value of this function is how this
784 constant is going to be interpreted by Perl. The third argument is undefined
785 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
786 context (comes from strings, regular expressions, and single-quote HERE
787 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
788 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
790 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
791 it is expected that overloaded constant strings are equipped with reasonable
792 overloaded catenation operator, otherwise absurd results will result.
793 Similarly, negative numbers are considered as negations of positive constants.
795 Note that it is probably meaningless to call the functions overload::constant()
796 and overload::remove_constant() from anywhere but import() and unimport() methods.
797 From these methods they may be called as
802 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
803 overload::constant integer => sub {Math::BigInt->new(shift)};
806 =head1 IMPLEMENTATION
808 What follows is subject to change RSN.
810 The table of methods for all operations is cached in magic for the
811 symbol table hash for the package. The cache is invalidated during
812 processing of C<use overload>, C<no overload>, new function
813 definitions, and changes in @ISA. However, this invalidation remains
814 unprocessed until the next C<bless>ing into the package. Hence if you
815 want to change overloading structure dynamically, you'll need an
816 additional (fake) C<bless>ing to update the table.
818 (Every SVish thing has a magic queue, and magic is an entry in that
819 queue. This is how a single variable may participate in multiple
820 forms of magic simultaneously. For instance, environment variables
821 regularly have two forms at once: their %ENV magic and their taint
822 magic. However, the magic which implements overloading is applied to
823 the stashes, which are rarely used directly, thus should not slow down
826 If an object belongs to a package using overload, it carries a special
827 flag. Thus the only speed penalty during arithmetic operations without
828 overloading is the checking of this flag.
830 In fact, if C<use overload> is not present, there is almost no overhead
831 for overloadable operations, so most programs should not suffer
832 measurable performance penalties. A considerable effort was made to
833 minimize the overhead when overload is used in some package, but the
834 arguments in question do not belong to packages using overload. When
835 in doubt, test your speed with C<use overload> and without it. So far
836 there have been no reports of substantial speed degradation if Perl is
837 compiled with optimization turned on.
839 There is no size penalty for data if overload is not used. The only
840 size penalty if overload is used in some package is that I<all> the
841 packages acquire a magic during the next C<bless>ing into the
842 package. This magic is three-words-long for packages without
843 overloading, and carries the cache table if the package is overloaded.
845 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
846 carried out before any operation that can imply an assignment to the
847 object $a (or $b) refers to, like C<$a++>. You can override this
848 behavior by defining your own copy constructor (see L<"Copy Constructor">).
850 It is expected that arguments to methods that are not explicitly supposed
851 to be changed are constant (but this is not enforced).
853 =head1 Metaphor clash
855 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
856 If it I<looks> counter intuitive to you, you are subject to a metaphor
859 Here is a Perl object metaphor:
861 I< object is a reference to blessed data>
863 and an arithmetic metaphor:
865 I< object is a thing by itself>.
867 The I<main> problem of overloading C<=> is the fact that these metaphors
868 imply different actions on the assignment C<$a = $b> if $a and $b are
869 objects. Perl-think implies that $a becomes a reference to whatever
870 $b was referencing. Arithmetic-think implies that the value of "object"
871 $a is changed to become the value of the object $b, preserving the fact
872 that $a and $b are separate entities.
874 The difference is not relevant in the absence of mutators. After
875 a Perl-way assignment an operation which mutates the data referenced by $a
876 would change the data referenced by $b too. Effectively, after
877 C<$a = $b> values of $a and $b become I<indistinguishable>.
879 On the other hand, anyone who has used algebraic notation knows the
880 expressive power of the arithmetic metaphor. Overloading works hard
881 to enable this metaphor while preserving the Perlian way as far as
882 possible. Since it is not possible to freely mix two contradicting
883 metaphors, overloading allows the arithmetic way to write things I<as
884 far as all the mutators are called via overloaded access only>. The
885 way it is done is described in L<Copy Constructor>.
887 If some mutator methods are directly applied to the overloaded values,
888 one may need to I<explicitly unlink> other values which references the
893 $b = $a; # $b is "linked" to $a
895 $a = $a->clone; # Unlink $b from $a
898 Note that overloaded access makes this transparent:
901 $b = $a; # $b is "linked" to $a
902 $a += 4; # would unlink $b automagically
904 However, it would not make
907 $a = 4; # Now $a is a plain 4, not 'Data'
909 preserve "objectness" of $a. But Perl I<has> a way to make assignments
910 to an object do whatever you want. It is just not the overload, but
911 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
912 which returns the object itself, and STORE() method which changes the
913 value of the object, one can reproduce the arithmetic metaphor in its
914 completeness, at least for variables which were tie()d from the start.
916 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
920 Please add examples to what follows!
922 =head2 Two-face scalars
924 Put this in F<two_face.pm> in your Perl library directory:
926 package two_face; # Scalars with separate string and
928 sub new { my $p = shift; bless [@_], $p }
929 use overload '""' => \&str, '0+' => \&num, fallback => 1;
936 my $seven = new two_face ("vii", 7);
937 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
938 print "seven contains `i'\n" if $seven =~ /i/;
940 (The second line creates a scalar which has both a string value, and a
941 numeric value.) This prints:
943 seven=vii, seven=7, eight=8
946 =head2 Two-face references
948 Suppose you want to create an object which is accessible as both an
949 array reference and a hash reference.
952 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
960 tie %h, ref $self, $self;
964 sub TIEHASH { my $p = shift; bless \ shift, $p }
967 $fields{$_} = $i++ foreach qw{zero one two three};
969 my $self = ${shift()};
970 my $key = $fields{shift()};
971 defined $key or die "Out of band access";
972 $$self->[$key] = shift;
975 my $self = ${shift()};
976 my $key = $fields{shift()};
977 defined $key or die "Out of band access";
981 Now one can access an object using both the array and hash syntax:
983 my $bar = new two_refs 3,4,5,6;
985 $bar->{two} == 11 or die 'bad hash fetch';
987 Note several important features of this example. First of all, the
988 I<actual> type of $bar is a scalar reference, and we do not overload
989 the scalar dereference. Thus we can get the I<actual> non-overloaded
990 contents of $bar by just using C<$$bar> (what we do in functions which
991 overload dereference). Similarly, the object returned by the
992 TIEHASH() method is a scalar reference.
994 Second, we create a new tied hash each time the hash syntax is used.
995 This allows us not to worry about a possibility of a reference loop,
996 which would lead to a memory leak.
998 Both these problems can be cured. Say, if we want to overload hash
999 dereference on a reference to an object which is I<implemented> as a
1000 hash itself, the only problem one has to circumvent is how to access
1001 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1002 overloaded dereference operator). Here is one possible fetching routine:
1005 my ($self, $key) = (shift, shift);
1006 my $class = ref $self;
1007 bless $self, 'overload::dummy'; # Disable overloading of %{}
1008 my $out = $self->{$key};
1009 bless $self, $class; # Restore overloading
1013 To remove creation of the tied hash on each access, one may an extra
1014 level of indirection which allows a non-circular structure of references:
1017 use overload '%{}' => sub { ${shift()}->[1] },
1018 '@{}' => sub { ${shift()}->[0] };
1024 bless \ [$a, \%h], $p;
1029 tie %h, ref $self, $self;
1033 sub TIEHASH { my $p = shift; bless \ shift, $p }
1036 $fields{$_} = $i++ foreach qw{zero one two three};
1039 my $key = $fields{shift()};
1040 defined $key or die "Out of band access";
1045 my $key = $fields{shift()};
1046 defined $key or die "Out of band access";
1050 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1051 reference to the intermediate array, which keeps a reference to an
1052 actual array, and the access hash. The tie()ing object for the access
1053 hash is a reference to a reference to the actual array, so
1059 There are no loops of references.
1063 Both "objects" which are blessed into the class C<two_refs1> are
1064 references to a reference to an array, thus references to a I<scalar>.
1065 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1066 overloaded operations.
1070 =head2 Symbolic calculator
1072 Put this in F<symbolic.pm> in your Perl library directory:
1074 package symbolic; # Primitive symbolic calculator
1075 use overload nomethod => \&wrap;
1077 sub new { shift; bless ['n', @_] }
1079 my ($obj, $other, $inv, $meth) = @_;
1080 ($obj, $other) = ($other, $obj) if $inv;
1081 bless [$meth, $obj, $other];
1084 This module is very unusual as overloaded modules go: it does not
1085 provide any usual overloaded operators, instead it provides the L<Last
1086 Resort> operator C<nomethod>. In this example the corresponding
1087 subroutine returns an object which encapsulates operations done over
1088 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1089 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1091 Here is an example of the script which "calculates" the side of
1092 circumscribed octagon using the above package:
1095 my $iter = 1; # 2**($iter+2) = 8
1096 my $side = new symbolic 1;
1100 $side = (sqrt(1 + $side**2) - 1)/$side;
1104 The value of $side is
1106 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1107 undef], 1], ['n', 1]]
1109 Note that while we obtained this value using a nice little script,
1110 there is no simple way to I<use> this value. In fact this value may
1111 be inspected in debugger (see L<perldebug>), but only if
1112 C<bareStringify> B<O>ption is set, and not via C<p> command.
1114 If one attempts to print this value, then the overloaded operator
1115 C<""> will be called, which will call C<nomethod> operator. The
1116 result of this operator will be stringified again, but this result is
1117 again of type C<symbolic>, which will lead to an infinite loop.
1119 Add a pretty-printer method to the module F<symbolic.pm>:
1122 my ($meth, $a, $b) = @{+shift};
1123 $a = 'u' unless defined $a;
1124 $b = 'u' unless defined $b;
1125 $a = $a->pretty if ref $a;
1126 $b = $b->pretty if ref $b;
1130 Now one can finish the script by
1132 print "side = ", $side->pretty, "\n";
1134 The method C<pretty> is doing object-to-string conversion, so it
1135 is natural to overload the operator C<""> using this method. However,
1136 inside such a method it is not necessary to pretty-print the
1137 I<components> $a and $b of an object. In the above subroutine
1138 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1139 and $b. If these components use overloading, the catenation operator
1140 will look for an overloaded operator C<.>; if not present, it will
1141 look for an overloaded operator C<"">. Thus it is enough to use
1143 use overload nomethod => \&wrap, '""' => \&str;
1145 my ($meth, $a, $b) = @{+shift};
1146 $a = 'u' unless defined $a;
1147 $b = 'u' unless defined $b;
1151 Now one can change the last line of the script to
1153 print "side = $side\n";
1157 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1159 and one can inspect the value in debugger using all the possible
1162 Something is still amiss: consider the loop variable $cnt of the
1163 script. It was a number, not an object. We cannot make this value of
1164 type C<symbolic>, since then the loop will not terminate.
1166 Indeed, to terminate the cycle, the $cnt should become false.
1167 However, the operator C<bool> for checking falsity is overloaded (this
1168 time via overloaded C<"">), and returns a long string, thus any object
1169 of type C<symbolic> is true. To overcome this, we need a way to
1170 compare an object to 0. In fact, it is easier to write a numeric
1173 Here is the text of F<symbolic.pm> with such a routine added (and
1174 slightly modified str()):
1176 package symbolic; # Primitive symbolic calculator
1178 nomethod => \&wrap, '""' => \&str, '0+' => \#
1180 sub new { shift; bless ['n', @_] }
1182 my ($obj, $other, $inv, $meth) = @_;
1183 ($obj, $other) = ($other, $obj) if $inv;
1184 bless [$meth, $obj, $other];
1187 my ($meth, $a, $b) = @{+shift};
1188 $a = 'u' unless defined $a;
1195 my %subr = ( n => sub {$_[0]},
1196 sqrt => sub {sqrt $_[0]},
1197 '-' => sub {shift() - shift()},
1198 '+' => sub {shift() + shift()},
1199 '/' => sub {shift() / shift()},
1200 '*' => sub {shift() * shift()},
1201 '**' => sub {shift() ** shift()},
1204 my ($meth, $a, $b) = @{+shift};
1205 my $subr = $subr{$meth}
1206 or die "Do not know how to ($meth) in symbolic";
1207 $a = $a->num if ref $a eq __PACKAGE__;
1208 $b = $b->num if ref $b eq __PACKAGE__;
1212 All the work of numeric conversion is done in %subr and num(). Of
1213 course, %subr is not complete, it contains only operators used in the
1214 example below. Here is the extra-credit question: why do we need an
1215 explicit recursion in num()? (Answer is at the end of this section.)
1217 Use this module like this:
1220 my $iter = new symbolic 2; # 16-gon
1221 my $side = new symbolic 1;
1225 $cnt = $cnt - 1; # Mutator `--' not implemented
1226 $side = (sqrt(1 + $side**2) - 1)/$side;
1228 printf "%s=%f\n", $side, $side;
1229 printf "pi=%f\n", $side*(2**($iter+2));
1231 It prints (without so many line breaks)
1233 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1235 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1238 The above module is very primitive. It does not implement
1239 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1240 (not required without mutators!), and implements only those arithmetic
1241 operations which are used in the example.
1243 To implement most arithmetic operations is easy; one should just use
1244 the tables of operations, and change the code which fills %subr to
1246 my %subr = ( 'n' => sub {$_[0]} );
1247 foreach my $op (split " ", $overload::ops{with_assign}) {
1248 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1250 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1251 foreach my $op (split " ", "@overload::ops{ @bins }") {
1252 $subr{$op} = eval "sub {shift() $op shift()}";
1254 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1255 print "defining `$op'\n";
1256 $subr{$op} = eval "sub {$op shift()}";
1259 Due to L<Calling Conventions for Mutators>, we do not need anything
1260 special to make C<+=> and friends work, except filling C<+=> entry of
1261 %subr, and defining a copy constructor (needed since Perl has no
1262 way to know that the implementation of C<'+='> does not mutate
1263 the argument, compare L<Copy Constructor>).
1265 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1266 line, and code (this code assumes that mutators change things one level
1267 deep only, so recursive copying is not needed):
1271 bless [@$self], ref $self;
1274 To make C<++> and C<--> work, we need to implement actual mutators,
1275 either directly, or in C<nomethod>. We continue to do things inside
1276 C<nomethod>, thus add
1278 if ($meth eq '++' or $meth eq '--') {
1279 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1283 after the first line of wrap(). This is not a most effective
1284 implementation, one may consider
1286 sub inc { $_[0] = bless ['++', shift, 1]; }
1290 As a final remark, note that one can fill %subr by
1292 my %subr = ( 'n' => sub {$_[0]} );
1293 foreach my $op (split " ", $overload::ops{with_assign}) {
1294 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1296 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1297 foreach my $op (split " ", "@overload::ops{ @bins }") {
1298 $subr{$op} = eval "sub {shift() $op shift()}";
1300 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1301 $subr{$op} = eval "sub {$op shift()}";
1303 $subr{'++'} = $subr{'+'};
1304 $subr{'--'} = $subr{'-'};
1306 This finishes implementation of a primitive symbolic calculator in
1307 50 lines of Perl code. Since the numeric values of subexpressions
1308 are not cached, the calculator is very slow.
1310 Here is the answer for the exercise: In the case of str(), we need no
1311 explicit recursion since the overloaded C<.>-operator will fall back
1312 to an existing overloaded operator C<"">. Overloaded arithmetic
1313 operators I<do not> fall back to numeric conversion if C<fallback> is
1314 not explicitly requested. Thus without an explicit recursion num()
1315 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1316 the argument of num().
1318 If you wonder why defaults for conversion are different for str() and
1319 num(), note how easy it was to write the symbolic calculator. This
1320 simplicity is due to an appropriate choice of defaults. One extra
1321 note: due to the explicit recursion num() is more fragile than sym():
1322 we need to explicitly check for the type of $a and $b. If components
1323 $a and $b happen to be of some related type, this may lead to problems.
1325 =head2 I<Really> symbolic calculator
1327 One may wonder why we call the above calculator symbolic. The reason
1328 is that the actual calculation of the value of expression is postponed
1329 until the value is I<used>.
1331 To see it in action, add a method
1336 @$obj->[0,1] = ('=', shift);
1339 to the package C<symbolic>. After this change one can do
1341 my $a = new symbolic 3;
1342 my $b = new symbolic 4;
1343 my $c = sqrt($a**2 + $b**2);
1345 and the numeric value of $c becomes 5. However, after calling
1347 $a->STORE(12); $b->STORE(5);
1349 the numeric value of $c becomes 13. There is no doubt now that the module
1350 symbolic provides a I<symbolic> calculator indeed.
1352 To hide the rough edges under the hood, provide a tie()d interface to the
1353 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1355 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1357 sub nop { } # Around a bug
1359 (the bug is described in L<"BUGS">). One can use this new interface as
1361 tie $a, 'symbolic', 3;
1362 tie $b, 'symbolic', 4;
1363 $a->nop; $b->nop; # Around a bug
1365 my $c = sqrt($a**2 + $b**2);
1367 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1368 of $c becomes 13. To insulate the user of the module add a method
1370 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1375 symbolic->vars($a, $b);
1376 my $c = sqrt($a**2 + $b**2);
1379 printf "c5 %s=%f\n", $c, $c;
1382 printf "c13 %s=%f\n", $c, $c;
1384 shows that the numeric value of $c follows changes to the values of $a
1389 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1393 When Perl is run with the B<-Do> switch or its equivalent, overloading
1394 induces diagnostic messages.
1396 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1397 deduce which operations are overloaded (and which ancestor triggers
1398 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1399 is shown by debugger. The method C<()> corresponds to the C<fallback>
1400 key (in fact a presence of this method shows that this package has
1401 overloading enabled, and it is what is used by the C<Overloaded>
1402 function of module C<overload>).
1404 The module might issue the following warnings:
1408 =item Odd number of arguments for overload::constant
1410 (W) The call to overload::constant contained an odd number of arguments.
1411 The arguments should come in pairs.
1413 =item `%s' is not an overloadable type
1415 (W) You tried to overload a constant type the overload package is unaware of.
1417 =item `%s' is not a code reference
1419 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1420 to be a code reference. Either an anonymous subroutine, or a reference
1427 Because it is used for overloading, the per-package hash %OVERLOAD now
1428 has a special meaning in Perl. The symbol table is filled with names
1429 looking like line-noise.
1431 For the purpose of inheritance every overloaded package behaves as if
1432 C<fallback> is present (possibly undefined). This may create
1433 interesting effects if some package is not overloaded, but inherits
1434 from two overloaded packages.
1436 Relation between overloading and tie()ing is broken. Overloading is
1437 triggered or not basing on the I<previous> class of tie()d value.
1439 This happens because the presence of overloading is checked too early,
1440 before any tie()d access is attempted. If the FETCH()ed class of the
1441 tie()d value does not change, a simple workaround is to access the value
1442 immediately after tie()ing, so that after this call the I<previous> class
1443 coincides with the current one.
1445 B<Needed:> a way to fix this without a speed penalty.
1447 Barewords are not covered by overloaded string constants.
1449 This document is confusing. There are grammos and misleading language
1450 used in places. It would seem a total rewrite is needed.