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;
77 $package = Scalar::Util::blessed($package);
78 return undef if !defined $package;
80 #my $meth = $package->can('(' . shift);
81 ov_method mycan($package, '(' . shift), $package;
82 #return $meth if $meth ne \&nil;
87 my $package = ref $_[0];
88 return "$_[0]" unless $package;
91 my $class = Scalar::Util::blessed($_[0]);
92 my $class_prefix = defined($class) ? "$class=" : "";
93 my $type = Scalar::Util::reftype($_[0]);
94 my $addr = Scalar::Util::refaddr($_[0]);
95 return sprintf("$class_prefix$type(0x%x)", $addr);
100 sub mycan { # Real can would leave stubs.
101 my ($package, $meth) = @_;
103 my $mro = mro::get_linear_isa($package);
104 foreach my $p (@$mro) {
105 my $fqmeth = $p . q{::} . $meth;
106 return \*{$fqmeth} if defined &{$fqmeth};
113 'integer' => 0x1000, # HINT_NEW_INTEGER
114 'float' => 0x2000, # HINT_NEW_FLOAT
115 'binary' => 0x4000, # HINT_NEW_BINARY
116 'q' => 0x8000, # HINT_NEW_STRING
117 'qr' => 0x10000, # HINT_NEW_RE
120 %ops = ( with_assign => "+ - * / % ** << >> x .",
121 assign => "+= -= *= /= %= **= <<= >>= x= .=",
122 num_comparison => "< <= > >= == !=",
123 '3way_comparison'=> "<=> cmp",
124 str_comparison => "lt le gt ge eq ne",
125 binary => '& &= | |= ^ ^=',
128 func => "atan2 cos sin exp abs log sqrt int",
129 conversion => 'bool "" 0+',
131 dereferencing => '${} @{} %{} &{} *{}',
132 special => 'nomethod fallback =');
134 use warnings::register;
136 # Arguments: what, sub
139 warnings::warnif ("Odd number of arguments for overload::constant");
142 elsif (!exists $constants {$_ [0]}) {
143 warnings::warnif ("`$_[0]' is not an overloadable type");
145 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
146 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
147 # blessed, and C<ref> would return the package the ref is blessed into.
148 if (warnings::enabled) {
149 $_ [1] = "undef" unless defined $_ [1];
150 warnings::warn ("`$_[1]' is not a code reference");
155 $^H |= $constants{$_[0]};
161 sub remove_constant {
162 # Arguments: what, sub
165 $^H &= ~ $constants{$_[0]};
176 overload - Package for overloading Perl operations
189 $a = new SomeThing 57;
192 if (overload::Overloaded $b) {...}
194 $strval = overload::StrVal $b;
198 =head2 Declaration of overloaded functions
200 The compilation directive
207 declares function Number::add() for addition, and method muas() in
208 the "class" C<Number> (or one of its base classes)
209 for the assignment form C<*=> of multiplication.
211 Arguments of this directive come in (key, value) pairs. Legal values
212 are values legal inside a C<&{ ... }> call, so the name of a
213 subroutine, a reference to a subroutine, or an anonymous subroutine
214 will all work. Note that values specified as strings are
215 interpreted as methods, not subroutines. Legal keys are listed below.
217 The subroutine C<add> will be called to execute C<$a+$b> if $a
218 is a reference to an object blessed into the package C<Number>, or if $a is
219 not an object from a package with defined mathemagic addition, but $b is a
220 reference to a C<Number>. It can also be called in other situations, like
221 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
222 methods refer to methods triggered by an overloaded mathematical
225 Since overloading respects inheritance via the @ISA hierarchy, the
226 above declaration would also trigger overloading of C<+> and C<*=> in
227 all the packages which inherit from C<Number>.
229 =head2 Calling Conventions for Binary Operations
231 The functions specified in the C<use overload ...> directive are called
232 with three (in one particular case with four, see L<Last Resort>)
233 arguments. If the corresponding operation is binary, then the first
234 two arguments are the two arguments of the operation. However, due to
235 general object calling conventions, the first argument should always be
236 an object in the package, so in the situation of C<7+$a>, the
237 order of the arguments is interchanged. It probably does not matter
238 when implementing the addition method, but whether the arguments
239 are reversed is vital to the subtraction method. The method can
240 query this information by examining the third argument, which can take
241 three different values:
247 the order of arguments is as in the current operation.
251 the arguments are reversed.
255 the current operation is an assignment variant (as in
256 C<$a+=7>), but the usual function is called instead. This additional
257 information can be used to generate some optimizations. Compare
258 L<Calling Conventions for Mutators>.
262 =head2 Calling Conventions for Unary Operations
264 Unary operation are considered binary operations with the second
265 argument being C<undef>. Thus the functions that overloads C<{"++"}>
266 is called with arguments C<($a,undef,'')> when $a++ is executed.
268 =head2 Calling Conventions for Mutators
270 Two types of mutators have different calling conventions:
274 =item C<++> and C<-->
276 The routines which implement these operators are expected to actually
277 I<mutate> their arguments. So, assuming that $obj is a reference to a
280 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
282 is an appropriate implementation of overloaded C<++>. Note that
284 sub incr { ++$ {$_[0]} ; shift }
286 is OK if used with preincrement and with postincrement. (In the case
287 of postincrement a copying will be performed, see L<Copy Constructor>.)
289 =item C<x=> and other assignment versions
291 There is nothing special about these methods. They may change the
292 value of their arguments, and may leave it as is. The result is going
293 to be assigned to the value in the left-hand-side if different from
296 This allows for the same method to be used as overloaded C<+=> and
297 C<+>. Note that this is I<allowed>, but not recommended, since by the
298 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
299 if C<+=> is not overloaded.
303 B<Warning.> Due to the presence of assignment versions of operations,
304 routines which may be called in assignment context may create
305 self-referential structures. Currently Perl will not free self-referential
306 structures until cycles are C<explicitly> broken. You may get problems
307 when traversing your structures too.
311 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
313 is asking for trouble, since for code C<$obj += $foo> the subroutine
314 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
315 \$foo]>. If using such a subroutine is an important optimization, one
316 can overload C<+=> explicitly by a non-"optimized" version, or switch
317 to non-optimized version if C<not defined $_[2]> (see
318 L<Calling Conventions for Binary Operations>).
320 Even if no I<explicit> assignment-variants of operators are present in
321 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
322 C<',' . $obj . ','> may be both optimized to
324 my $tmp = ',' . $obj; $tmp .= ',';
326 =head2 Overloadable Operations
328 The following symbols can be specified in C<use overload> directive:
332 =item * I<Arithmetic operations>
334 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
335 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
337 For these operations a substituted non-assignment variant can be called if
338 the assignment variant is not available. Methods for operations C<+>,
339 C<->, C<+=>, and C<-=> can be called to automatically generate
340 increment and decrement methods. The operation C<-> can be used to
341 autogenerate missing methods for unary minus or C<abs>.
343 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
344 L<"Calling Conventions for Binary Operations">) for details of these
347 =item * I<Comparison operations>
349 "<", "<=", ">", ">=", "==", "!=", "<=>",
350 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
352 If the corresponding "spaceship" variant is available, it can be
353 used to substitute for the missing operation. During C<sort>ing
354 arrays, C<cmp> is used to compare values subject to C<use overload>.
356 =item * I<Bit operations>
358 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
360 C<neg> stands for unary minus. If the method for C<neg> is not
361 specified, it can be autogenerated using the method for
362 subtraction. If the method for C<!> is not specified, it can be
363 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
365 The same remarks in L<"Arithmetic operations"> about
366 assignment-variants and autogeneration apply for
367 bit operations C<"&">, C<"^">, and C<"|"> as well.
369 =item * I<Increment and decrement>
373 If undefined, addition and subtraction methods can be
374 used instead. These operations are called both in prefix and
377 =item * I<Transcendental functions>
379 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
381 If C<abs> is unavailable, it can be autogenerated using methods
382 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
384 Note that traditionally the Perl function L<int> rounds to 0, thus for
385 floating-point-like types one should follow the same semantic. If
386 C<int> is unavailable, it can be autogenerated using the overloading of
389 =item * I<Boolean, string and numeric conversion>
393 If one or two of these operations are not overloaded, the remaining ones can
394 be used instead. C<bool> is used in the flow control operators
395 (like C<while>) and for the ternary C<?:> operation. These functions can
396 return any arbitrary Perl value. If the corresponding operation for this value
397 is overloaded too, that operation will be called again with this value.
399 As a special case if the overload returns the object itself then it will
400 be used directly. An overloaded conversion returning the object is
401 probably a bug, because you're likely to get something that looks like
402 C<YourPackage=HASH(0x8172b34)>.
408 If not overloaded, the argument will be converted to a filehandle or
409 glob (which may require a stringification). The same overloading
410 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
411 I<globbing> syntax C<E<lt>${var}E<gt>>.
413 B<BUGS> Even in list context, the iterator is currently called only
414 once and with scalar context.
416 =item * I<Dereferencing>
418 '${}', '@{}', '%{}', '&{}', '*{}'.
420 If not overloaded, the argument will be dereferenced I<as is>, thus
421 should be of correct type. These functions should return a reference
422 of correct type, or another object with overloaded dereferencing.
424 As a special case if the overload returns the object itself then it
425 will be used directly (provided it is the correct type).
427 The dereference operators must be specified explicitly they will not be passed to
432 "nomethod", "fallback", "=", "~~",
434 see L<SPECIAL SYMBOLS FOR C<use overload>>.
438 See L<"Fallback"> for an explanation of when a missing method can be
441 A computer-readable form of the above table is available in the hash
442 %overload::ops, with values being space-separated lists of names:
444 with_assign => '+ - * / % ** << >> x .',
445 assign => '+= -= *= /= %= **= <<= >>= x= .=',
446 num_comparison => '< <= > >= == !=',
447 '3way_comparison'=> '<=> cmp',
448 str_comparison => 'lt le gt ge eq ne',
449 binary => '& &= | |= ^ ^=',
452 func => 'atan2 cos sin exp abs log sqrt',
453 conversion => 'bool "" 0+',
455 dereferencing => '${} @{} %{} &{} *{}',
456 special => 'nomethod fallback ='
458 =head2 Inheritance and overloading
460 Inheritance interacts with overloading in two ways.
464 =item Strings as values of C<use overload> directive
468 use overload key => value;
470 is a string, it is interpreted as a method name.
472 =item Overloading of an operation is inherited by derived classes
474 Any class derived from an overloaded class is also overloaded. The
475 set of overloaded methods is the union of overloaded methods of all
476 the ancestors. If some method is overloaded in several ancestor, then
477 which description will be used is decided by the usual inheritance
480 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
481 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
482 then the subroutine C<D::plus_sub> will be called to implement
483 operation C<+> for an object in package C<A>.
487 Note that since the value of the C<fallback> key is not a subroutine,
488 its inheritance is not governed by the above rules. In the current
489 implementation, the value of C<fallback> in the first overloaded
490 ancestor is used, but this is accidental and subject to change.
492 =head1 SPECIAL SYMBOLS FOR C<use overload>
494 Three keys are recognized by Perl that are not covered by the above
499 C<"nomethod"> should be followed by a reference to a function of four
500 parameters. If defined, it is called when the overloading mechanism
501 cannot find a method for some operation. The first three arguments of
502 this function coincide with the arguments for the corresponding method if
503 it were found, the fourth argument is the symbol
504 corresponding to the missing method. If several methods are tried,
505 the last one is used. Say, C<1-$a> can be equivalent to
507 &nomethodMethod($a,1,1,"-")
509 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
510 C<use overload> directive.
512 The C<"nomethod"> mechanism is I<not> used for the dereference operators
513 ( ${} @{} %{} &{} *{} ).
516 If some operation cannot be resolved, and there is no function
517 assigned to C<"nomethod">, then an exception will be raised via die()--
518 unless C<"fallback"> was specified as a key in C<use overload> directive.
523 The key C<"fallback"> governs what to do if a method for a particular
524 operation is not found. Three different cases are possible depending on
525 the value of C<"fallback">:
532 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
533 then tries to calls C<"nomethod"> value; if missing, an exception
538 The same as for the C<undef> value, but no exception is raised. Instead,
539 it silently reverts to what it would have done were there no C<use overload>
542 =item * defined, but FALSE
544 No autogeneration is tried. Perl tries to call
545 C<"nomethod"> value, and if this is missing, raises an exception.
549 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
550 yet, see L<"Inheritance and overloading">.
554 The key C<"~~"> allows you to override the smart matching used by
555 the switch construct. See L<feature>.
557 =head2 Copy Constructor
559 The value for C<"="> is a reference to a function with three
560 arguments, i.e., it looks like the other values in C<use
561 overload>. However, it does not overload the Perl assignment
562 operator. This would go against Camel hair.
564 This operation is called in the situations when a mutator is applied
565 to a reference that shares its object with some other reference, such
571 To make this change $a and not change $b, a copy of C<$$a> is made,
572 and $a is assigned a reference to this new object. This operation is
573 done during execution of the C<++$a>, and not during the assignment,
574 (so before the increment C<$$a> coincides with C<$$b>). This is only
575 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
576 C<nomethod>). Note that if this operation is expressed via C<'+'>
577 a nonmutator, i.e., as in
582 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
583 appear as lvalue when the above code is executed.
585 If the copy constructor is required during the execution of some mutator,
586 but a method for C<'='> was not specified, it can be autogenerated as a
587 string copy if the object is a plain scalar.
593 The actually executed code for
596 Something else which does not modify $a or $b....
602 Something else which does not modify $a or $b....
603 $a = $a->clone(undef,"");
606 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
607 C<'='> was overloaded with C<\&clone>.
611 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
614 =head1 MAGIC AUTOGENERATION
616 If a method for an operation is not found, and the value for C<"fallback"> is
617 TRUE or undefined, Perl tries to autogenerate a substitute method for
618 the missing operation based on the defined operations. Autogenerated method
619 substitutions are possible for the following operations:
623 =item I<Assignment forms of arithmetic operations>
625 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
628 =item I<Conversion operations>
630 String, numeric, and boolean conversion are calculated in terms of one
631 another if not all of them are defined.
633 =item I<Increment and decrement>
635 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
636 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
640 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
644 can be expressed in terms of subtraction.
648 C<!> and C<not> can be expressed in terms of boolean conversion, or
649 string or numerical conversion.
651 =item I<Concatenation>
653 can be expressed in terms of string conversion.
655 =item I<Comparison operations>
657 can be expressed in terms of its "spaceship" counterpart: either
658 C<E<lt>=E<gt>> or C<cmp>:
660 <, >, <=, >=, ==, != in terms of <=>
661 lt, gt, le, ge, eq, ne in terms of cmp
665 <> in terms of builtin operations
667 =item I<Dereferencing>
669 ${} @{} %{} &{} *{} in terms of builtin operations
671 =item I<Copy operator>
673 can be expressed in terms of an assignment to the dereferenced value, if this
674 value is a scalar and not a reference.
678 =head1 Minimal set of overloaded operations
680 Since some operations can be automatically generated from others, there is
681 a minimal set of operations that need to be overloaded in order to have
682 the complete set of overloaded operations at one's disposal.
683 Of course, the autogenerated operations may not do exactly what the user
684 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
689 atan2 cos sin exp log sqrt int
691 Additionally, you need to define at least one of string, boolean or
692 numeric conversions because any one can be used to emulate the others.
693 The string conversion can also be used to emulate concatenation.
695 =head1 Losing overloading
697 The restriction for the comparison operation is that even if, for example,
698 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
699 function will produce only a standard logical value based on the
700 numerical value of the result of `C<cmp>'. In particular, a working
701 numeric conversion is needed in this case (possibly expressed in terms of
704 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
705 if the string conversion substitution is applied.
707 When you chop() a mathemagical object it is promoted to a string and its
708 mathemagical properties are lost. The same can happen with other
711 =head1 Run-time Overloading
713 Since all C<use> directives are executed at compile-time, the only way to
714 change overloading during run-time is to
716 eval 'use overload "+" => \&addmethod';
720 eval 'no overload "+", "--", "<="';
722 though the use of these constructs during run-time is questionable.
724 =head1 Public functions
726 Package C<overload.pm> provides the following public functions:
730 =item overload::StrVal(arg)
732 Gives string value of C<arg> as in absence of stringify overloading. If you
733 are using this to get the address of a reference (useful for checking if two
734 references point to the same thing) then you may be better off using
735 C<Scalar::Util::refaddr()>, which is faster.
737 =item overload::Overloaded(arg)
739 Returns true if C<arg> is subject to overloading of some operations.
741 =item overload::Method(obj,op)
743 Returns C<undef> or a reference to the method that implements C<op>.
747 =head1 Overloading constants
749 For some applications, the Perl parser mangles constants too much.
750 It is possible to hook into this process via C<overload::constant()>
751 and C<overload::remove_constant()> functions.
753 These functions take a hash as an argument. The recognized keys of this hash
760 to overload integer constants,
764 to overload floating point constants,
768 to overload octal and hexadecimal constants,
772 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
773 strings and here-documents,
777 to overload constant pieces of regular expressions.
781 The corresponding values are references to functions which take three arguments:
782 the first one is the I<initial> string form of the constant, the second one
783 is how Perl interprets this constant, the third one is how the constant is used.
784 Note that the initial string form does not
785 contain string delimiters, and has backslashes in backslash-delimiter
786 combinations stripped (thus the value of delimiter is not relevant for
787 processing of this string). The return value of this function is how this
788 constant is going to be interpreted by Perl. The third argument is undefined
789 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
790 context (comes from strings, regular expressions, and single-quote HERE
791 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
792 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
794 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
795 it is expected that overloaded constant strings are equipped with reasonable
796 overloaded catenation operator, otherwise absurd results will result.
797 Similarly, negative numbers are considered as negations of positive constants.
799 Note that it is probably meaningless to call the functions overload::constant()
800 and overload::remove_constant() from anywhere but import() and unimport() methods.
801 From these methods they may be called as
806 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
807 overload::constant integer => sub {Math::BigInt->new(shift)};
810 =head1 IMPLEMENTATION
812 What follows is subject to change RSN.
814 The table of methods for all operations is cached in magic for the
815 symbol table hash for the package. The cache is invalidated during
816 processing of C<use overload>, C<no overload>, new function
817 definitions, and changes in @ISA. However, this invalidation remains
818 unprocessed until the next C<bless>ing into the package. Hence if you
819 want to change overloading structure dynamically, you'll need an
820 additional (fake) C<bless>ing to update the table.
822 (Every SVish thing has a magic queue, and magic is an entry in that
823 queue. This is how a single variable may participate in multiple
824 forms of magic simultaneously. For instance, environment variables
825 regularly have two forms at once: their %ENV magic and their taint
826 magic. However, the magic which implements overloading is applied to
827 the stashes, which are rarely used directly, thus should not slow down
830 If an object belongs to a package using overload, it carries a special
831 flag. Thus the only speed penalty during arithmetic operations without
832 overloading is the checking of this flag.
834 In fact, if C<use overload> is not present, there is almost no overhead
835 for overloadable operations, so most programs should not suffer
836 measurable performance penalties. A considerable effort was made to
837 minimize the overhead when overload is used in some package, but the
838 arguments in question do not belong to packages using overload. When
839 in doubt, test your speed with C<use overload> and without it. So far
840 there have been no reports of substantial speed degradation if Perl is
841 compiled with optimization turned on.
843 There is no size penalty for data if overload is not used. The only
844 size penalty if overload is used in some package is that I<all> the
845 packages acquire a magic during the next C<bless>ing into the
846 package. This magic is three-words-long for packages without
847 overloading, and carries the cache table if the package is overloaded.
849 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
850 carried out before any operation that can imply an assignment to the
851 object $a (or $b) refers to, like C<$a++>. You can override this
852 behavior by defining your own copy constructor (see L<"Copy Constructor">).
854 It is expected that arguments to methods that are not explicitly supposed
855 to be changed are constant (but this is not enforced).
857 =head1 Metaphor clash
859 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
860 If it I<looks> counter intuitive to you, you are subject to a metaphor
863 Here is a Perl object metaphor:
865 I< object is a reference to blessed data>
867 and an arithmetic metaphor:
869 I< object is a thing by itself>.
871 The I<main> problem of overloading C<=> is the fact that these metaphors
872 imply different actions on the assignment C<$a = $b> if $a and $b are
873 objects. Perl-think implies that $a becomes a reference to whatever
874 $b was referencing. Arithmetic-think implies that the value of "object"
875 $a is changed to become the value of the object $b, preserving the fact
876 that $a and $b are separate entities.
878 The difference is not relevant in the absence of mutators. After
879 a Perl-way assignment an operation which mutates the data referenced by $a
880 would change the data referenced by $b too. Effectively, after
881 C<$a = $b> values of $a and $b become I<indistinguishable>.
883 On the other hand, anyone who has used algebraic notation knows the
884 expressive power of the arithmetic metaphor. Overloading works hard
885 to enable this metaphor while preserving the Perlian way as far as
886 possible. Since it is not possible to freely mix two contradicting
887 metaphors, overloading allows the arithmetic way to write things I<as
888 far as all the mutators are called via overloaded access only>. The
889 way it is done is described in L<Copy Constructor>.
891 If some mutator methods are directly applied to the overloaded values,
892 one may need to I<explicitly unlink> other values which references the
897 $b = $a; # $b is "linked" to $a
899 $a = $a->clone; # Unlink $b from $a
902 Note that overloaded access makes this transparent:
905 $b = $a; # $b is "linked" to $a
906 $a += 4; # would unlink $b automagically
908 However, it would not make
911 $a = 4; # Now $a is a plain 4, not 'Data'
913 preserve "objectness" of $a. But Perl I<has> a way to make assignments
914 to an object do whatever you want. It is just not the overload, but
915 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
916 which returns the object itself, and STORE() method which changes the
917 value of the object, one can reproduce the arithmetic metaphor in its
918 completeness, at least for variables which were tie()d from the start.
920 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
924 Please add examples to what follows!
926 =head2 Two-face scalars
928 Put this in F<two_face.pm> in your Perl library directory:
930 package two_face; # Scalars with separate string and
932 sub new { my $p = shift; bless [@_], $p }
933 use overload '""' => \&str, '0+' => \&num, fallback => 1;
940 my $seven = new two_face ("vii", 7);
941 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
942 print "seven contains `i'\n" if $seven =~ /i/;
944 (The second line creates a scalar which has both a string value, and a
945 numeric value.) This prints:
947 seven=vii, seven=7, eight=8
950 =head2 Two-face references
952 Suppose you want to create an object which is accessible as both an
953 array reference and a hash reference.
956 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
964 tie %h, ref $self, $self;
968 sub TIEHASH { my $p = shift; bless \ shift, $p }
971 $fields{$_} = $i++ foreach qw{zero one two three};
973 my $self = ${shift()};
974 my $key = $fields{shift()};
975 defined $key or die "Out of band access";
976 $$self->[$key] = shift;
979 my $self = ${shift()};
980 my $key = $fields{shift()};
981 defined $key or die "Out of band access";
985 Now one can access an object using both the array and hash syntax:
987 my $bar = new two_refs 3,4,5,6;
989 $bar->{two} == 11 or die 'bad hash fetch';
991 Note several important features of this example. First of all, the
992 I<actual> type of $bar is a scalar reference, and we do not overload
993 the scalar dereference. Thus we can get the I<actual> non-overloaded
994 contents of $bar by just using C<$$bar> (what we do in functions which
995 overload dereference). Similarly, the object returned by the
996 TIEHASH() method is a scalar reference.
998 Second, we create a new tied hash each time the hash syntax is used.
999 This allows us not to worry about a possibility of a reference loop,
1000 which would lead to a memory leak.
1002 Both these problems can be cured. Say, if we want to overload hash
1003 dereference on a reference to an object which is I<implemented> as a
1004 hash itself, the only problem one has to circumvent is how to access
1005 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1006 overloaded dereference operator). Here is one possible fetching routine:
1009 my ($self, $key) = (shift, shift);
1010 my $class = ref $self;
1011 bless $self, 'overload::dummy'; # Disable overloading of %{}
1012 my $out = $self->{$key};
1013 bless $self, $class; # Restore overloading
1017 To remove creation of the tied hash on each access, one may an extra
1018 level of indirection which allows a non-circular structure of references:
1021 use overload '%{}' => sub { ${shift()}->[1] },
1022 '@{}' => sub { ${shift()}->[0] };
1028 bless \ [$a, \%h], $p;
1033 tie %h, ref $self, $self;
1037 sub TIEHASH { my $p = shift; bless \ shift, $p }
1040 $fields{$_} = $i++ foreach qw{zero one two three};
1043 my $key = $fields{shift()};
1044 defined $key or die "Out of band access";
1049 my $key = $fields{shift()};
1050 defined $key or die "Out of band access";
1054 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1055 reference to the intermediate array, which keeps a reference to an
1056 actual array, and the access hash. The tie()ing object for the access
1057 hash is a reference to a reference to the actual array, so
1063 There are no loops of references.
1067 Both "objects" which are blessed into the class C<two_refs1> are
1068 references to a reference to an array, thus references to a I<scalar>.
1069 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1070 overloaded operations.
1074 =head2 Symbolic calculator
1076 Put this in F<symbolic.pm> in your Perl library directory:
1078 package symbolic; # Primitive symbolic calculator
1079 use overload nomethod => \&wrap;
1081 sub new { shift; bless ['n', @_] }
1083 my ($obj, $other, $inv, $meth) = @_;
1084 ($obj, $other) = ($other, $obj) if $inv;
1085 bless [$meth, $obj, $other];
1088 This module is very unusual as overloaded modules go: it does not
1089 provide any usual overloaded operators, instead it provides the L<Last
1090 Resort> operator C<nomethod>. In this example the corresponding
1091 subroutine returns an object which encapsulates operations done over
1092 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1093 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1095 Here is an example of the script which "calculates" the side of
1096 circumscribed octagon using the above package:
1099 my $iter = 1; # 2**($iter+2) = 8
1100 my $side = new symbolic 1;
1104 $side = (sqrt(1 + $side**2) - 1)/$side;
1108 The value of $side is
1110 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1111 undef], 1], ['n', 1]]
1113 Note that while we obtained this value using a nice little script,
1114 there is no simple way to I<use> this value. In fact this value may
1115 be inspected in debugger (see L<perldebug>), but only if
1116 C<bareStringify> B<O>ption is set, and not via C<p> command.
1118 If one attempts to print this value, then the overloaded operator
1119 C<""> will be called, which will call C<nomethod> operator. The
1120 result of this operator will be stringified again, but this result is
1121 again of type C<symbolic>, which will lead to an infinite loop.
1123 Add a pretty-printer method to the module F<symbolic.pm>:
1126 my ($meth, $a, $b) = @{+shift};
1127 $a = 'u' unless defined $a;
1128 $b = 'u' unless defined $b;
1129 $a = $a->pretty if ref $a;
1130 $b = $b->pretty if ref $b;
1134 Now one can finish the script by
1136 print "side = ", $side->pretty, "\n";
1138 The method C<pretty> is doing object-to-string conversion, so it
1139 is natural to overload the operator C<""> using this method. However,
1140 inside such a method it is not necessary to pretty-print the
1141 I<components> $a and $b of an object. In the above subroutine
1142 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1143 and $b. If these components use overloading, the catenation operator
1144 will look for an overloaded operator C<.>; if not present, it will
1145 look for an overloaded operator C<"">. Thus it is enough to use
1147 use overload nomethod => \&wrap, '""' => \&str;
1149 my ($meth, $a, $b) = @{+shift};
1150 $a = 'u' unless defined $a;
1151 $b = 'u' unless defined $b;
1155 Now one can change the last line of the script to
1157 print "side = $side\n";
1161 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1163 and one can inspect the value in debugger using all the possible
1166 Something is still amiss: consider the loop variable $cnt of the
1167 script. It was a number, not an object. We cannot make this value of
1168 type C<symbolic>, since then the loop will not terminate.
1170 Indeed, to terminate the cycle, the $cnt should become false.
1171 However, the operator C<bool> for checking falsity is overloaded (this
1172 time via overloaded C<"">), and returns a long string, thus any object
1173 of type C<symbolic> is true. To overcome this, we need a way to
1174 compare an object to 0. In fact, it is easier to write a numeric
1177 Here is the text of F<symbolic.pm> with such a routine added (and
1178 slightly modified str()):
1180 package symbolic; # Primitive symbolic calculator
1182 nomethod => \&wrap, '""' => \&str, '0+' => \#
1184 sub new { shift; bless ['n', @_] }
1186 my ($obj, $other, $inv, $meth) = @_;
1187 ($obj, $other) = ($other, $obj) if $inv;
1188 bless [$meth, $obj, $other];
1191 my ($meth, $a, $b) = @{+shift};
1192 $a = 'u' unless defined $a;
1199 my %subr = ( n => sub {$_[0]},
1200 sqrt => sub {sqrt $_[0]},
1201 '-' => sub {shift() - shift()},
1202 '+' => sub {shift() + shift()},
1203 '/' => sub {shift() / shift()},
1204 '*' => sub {shift() * shift()},
1205 '**' => sub {shift() ** shift()},
1208 my ($meth, $a, $b) = @{+shift};
1209 my $subr = $subr{$meth}
1210 or die "Do not know how to ($meth) in symbolic";
1211 $a = $a->num if ref $a eq __PACKAGE__;
1212 $b = $b->num if ref $b eq __PACKAGE__;
1216 All the work of numeric conversion is done in %subr and num(). Of
1217 course, %subr is not complete, it contains only operators used in the
1218 example below. Here is the extra-credit question: why do we need an
1219 explicit recursion in num()? (Answer is at the end of this section.)
1221 Use this module like this:
1224 my $iter = new symbolic 2; # 16-gon
1225 my $side = new symbolic 1;
1229 $cnt = $cnt - 1; # Mutator `--' not implemented
1230 $side = (sqrt(1 + $side**2) - 1)/$side;
1232 printf "%s=%f\n", $side, $side;
1233 printf "pi=%f\n", $side*(2**($iter+2));
1235 It prints (without so many line breaks)
1237 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1239 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1242 The above module is very primitive. It does not implement
1243 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1244 (not required without mutators!), and implements only those arithmetic
1245 operations which are used in the example.
1247 To implement most arithmetic operations is easy; one should just use
1248 the tables of operations, and change the code which fills %subr to
1250 my %subr = ( 'n' => sub {$_[0]} );
1251 foreach my $op (split " ", $overload::ops{with_assign}) {
1252 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1254 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1255 foreach my $op (split " ", "@overload::ops{ @bins }") {
1256 $subr{$op} = eval "sub {shift() $op shift()}";
1258 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1259 print "defining `$op'\n";
1260 $subr{$op} = eval "sub {$op shift()}";
1263 Due to L<Calling Conventions for Mutators>, we do not need anything
1264 special to make C<+=> and friends work, except filling C<+=> entry of
1265 %subr, and defining a copy constructor (needed since Perl has no
1266 way to know that the implementation of C<'+='> does not mutate
1267 the argument, compare L<Copy Constructor>).
1269 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1270 line, and code (this code assumes that mutators change things one level
1271 deep only, so recursive copying is not needed):
1275 bless [@$self], ref $self;
1278 To make C<++> and C<--> work, we need to implement actual mutators,
1279 either directly, or in C<nomethod>. We continue to do things inside
1280 C<nomethod>, thus add
1282 if ($meth eq '++' or $meth eq '--') {
1283 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1287 after the first line of wrap(). This is not a most effective
1288 implementation, one may consider
1290 sub inc { $_[0] = bless ['++', shift, 1]; }
1294 As a final remark, note that one can fill %subr by
1296 my %subr = ( 'n' => sub {$_[0]} );
1297 foreach my $op (split " ", $overload::ops{with_assign}) {
1298 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1300 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1301 foreach my $op (split " ", "@overload::ops{ @bins }") {
1302 $subr{$op} = eval "sub {shift() $op shift()}";
1304 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1305 $subr{$op} = eval "sub {$op shift()}";
1307 $subr{'++'} = $subr{'+'};
1308 $subr{'--'} = $subr{'-'};
1310 This finishes implementation of a primitive symbolic calculator in
1311 50 lines of Perl code. Since the numeric values of subexpressions
1312 are not cached, the calculator is very slow.
1314 Here is the answer for the exercise: In the case of str(), we need no
1315 explicit recursion since the overloaded C<.>-operator will fall back
1316 to an existing overloaded operator C<"">. Overloaded arithmetic
1317 operators I<do not> fall back to numeric conversion if C<fallback> is
1318 not explicitly requested. Thus without an explicit recursion num()
1319 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1320 the argument of num().
1322 If you wonder why defaults for conversion are different for str() and
1323 num(), note how easy it was to write the symbolic calculator. This
1324 simplicity is due to an appropriate choice of defaults. One extra
1325 note: due to the explicit recursion num() is more fragile than sym():
1326 we need to explicitly check for the type of $a and $b. If components
1327 $a and $b happen to be of some related type, this may lead to problems.
1329 =head2 I<Really> symbolic calculator
1331 One may wonder why we call the above calculator symbolic. The reason
1332 is that the actual calculation of the value of expression is postponed
1333 until the value is I<used>.
1335 To see it in action, add a method
1340 @$obj->[0,1] = ('=', shift);
1343 to the package C<symbolic>. After this change one can do
1345 my $a = new symbolic 3;
1346 my $b = new symbolic 4;
1347 my $c = sqrt($a**2 + $b**2);
1349 and the numeric value of $c becomes 5. However, after calling
1351 $a->STORE(12); $b->STORE(5);
1353 the numeric value of $c becomes 13. There is no doubt now that the module
1354 symbolic provides a I<symbolic> calculator indeed.
1356 To hide the rough edges under the hood, provide a tie()d interface to the
1357 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1359 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1361 sub nop { } # Around a bug
1363 (the bug is described in L<"BUGS">). One can use this new interface as
1365 tie $a, 'symbolic', 3;
1366 tie $b, 'symbolic', 4;
1367 $a->nop; $b->nop; # Around a bug
1369 my $c = sqrt($a**2 + $b**2);
1371 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1372 of $c becomes 13. To insulate the user of the module add a method
1374 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1379 symbolic->vars($a, $b);
1380 my $c = sqrt($a**2 + $b**2);
1383 printf "c5 %s=%f\n", $c, $c;
1386 printf "c13 %s=%f\n", $c, $c;
1388 shows that the numeric value of $c follows changes to the values of $a
1393 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1397 When Perl is run with the B<-Do> switch or its equivalent, overloading
1398 induces diagnostic messages.
1400 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1401 deduce which operations are overloaded (and which ancestor triggers
1402 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1403 is shown by debugger. The method C<()> corresponds to the C<fallback>
1404 key (in fact a presence of this method shows that this package has
1405 overloading enabled, and it is what is used by the C<Overloaded>
1406 function of module C<overload>).
1408 The module might issue the following warnings:
1412 =item Odd number of arguments for overload::constant
1414 (W) The call to overload::constant contained an odd number of arguments.
1415 The arguments should come in pairs.
1417 =item `%s' is not an overloadable type
1419 (W) You tried to overload a constant type the overload package is unaware of.
1421 =item `%s' is not a code reference
1423 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1424 to be a code reference. Either an anonymous subroutine, or a reference
1431 Because it is used for overloading, the per-package hash %OVERLOAD now
1432 has a special meaning in Perl. The symbol table is filled with names
1433 looking like line-noise.
1435 For the purpose of inheritance every overloaded package behaves as if
1436 C<fallback> is present (possibly undefined). This may create
1437 interesting effects if some package is not overloaded, but inherits
1438 from two overloaded packages.
1440 Relation between overloading and tie()ing is broken. Overloading is
1441 triggered or not basing on the I<previous> class of tie()d value.
1443 This happens because the presence of overloading is checked too early,
1444 before any tie()d access is attempted. If the FETCH()ed class of the
1445 tie()d value does not change, a simple workaround is to access the value
1446 immediately after tie()ing, so that after this call the I<previous> class
1447 coincides with the current one.
1449 B<Needed:> a way to fix this without a speed penalty.
1451 Barewords are not covered by overloaded string constants.
1453 This document is confusing. There are grammos and misleading language
1454 used in places. It would seem a total rewrite is needed.