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;
79 $package = Scalar::Util::blessed($package);
80 return undef if !defined $package;
82 #my $meth = $package->can('(' . shift);
83 ov_method mycan($package, '(' . shift), $package;
84 #return $meth if $meth ne \&nil;
89 my $package = ref $_[0];
90 return "$_[0]" unless $package;
95 my $class = Scalar::Util::blessed($_[0]);
96 my $class_prefix = defined($class) ? "$class=" : "";
97 my $type = Scalar::Util::reftype($_[0]);
98 my $addr = Scalar::Util::refaddr($_[0]);
99 return sprintf("$class_prefix$type(0x%x)", $addr);
104 sub mycan { # Real can would leave stubs.
105 my ($package, $meth) = @_;
111 my $mro = mro::get_linear_isa($package);
112 foreach my $p (@$mro) {
113 my $fqmeth = $p . q{::} . $meth;
114 return \*{$fqmeth} if defined &{$fqmeth};
121 'integer' => 0x1000, # HINT_NEW_INTEGER
122 'float' => 0x2000, # HINT_NEW_FLOAT
123 'binary' => 0x4000, # HINT_NEW_BINARY
124 'q' => 0x8000, # HINT_NEW_STRING
125 'qr' => 0x10000, # HINT_NEW_RE
128 %ops = ( with_assign => "+ - * / % ** << >> x .",
129 assign => "+= -= *= /= %= **= <<= >>= x= .=",
130 num_comparison => "< <= > >= == !=",
131 '3way_comparison'=> "<=> cmp",
132 str_comparison => "lt le gt ge eq ne",
133 binary => '& &= | |= ^ ^=',
136 func => "atan2 cos sin exp abs log sqrt int",
137 conversion => 'bool "" 0+',
139 dereferencing => '${} @{} %{} &{} *{}',
140 special => 'nomethod fallback =');
142 use warnings::register;
144 # Arguments: what, sub
147 warnings::warnif ("Odd number of arguments for overload::constant");
150 elsif (!exists $constants {$_ [0]}) {
151 warnings::warnif ("`$_[0]' is not an overloadable type");
153 elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
154 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
155 # blessed, and C<ref> would return the package the ref is blessed into.
156 if (warnings::enabled) {
157 $_ [1] = "undef" unless defined $_ [1];
158 warnings::warn ("`$_[1]' is not a code reference");
163 $^H |= $constants{$_[0]};
169 sub remove_constant {
170 # Arguments: what, sub
173 $^H &= ~ $constants{$_[0]};
184 overload - Package for overloading Perl operations
197 $a = SomeThing->new( 57 );
200 if (overload::Overloaded $b) {...}
202 $strval = overload::StrVal $b;
206 This pragma allows overloading of Perl's operators for a class.
207 To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
209 =head2 Declaration of overloaded functions
211 The compilation directive
218 declares function Number::add() for addition, and method muas() in
219 the "class" C<Number> (or one of its base classes)
220 for the assignment form C<*=> of multiplication.
222 Arguments of this directive come in (key, value) pairs. Legal values
223 are values legal inside a C<&{ ... }> call, so the name of a
224 subroutine, a reference to a subroutine, or an anonymous subroutine
225 will all work. Note that values specified as strings are
226 interpreted as methods, not subroutines. Legal keys are listed below.
228 The subroutine C<add> will be called to execute C<$a+$b> if $a
229 is a reference to an object blessed into the package C<Number>, or if $a is
230 not an object from a package with defined mathemagic addition, but $b is a
231 reference to a C<Number>. It can also be called in other situations, like
232 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
233 methods refer to methods triggered by an overloaded mathematical
236 Since overloading respects inheritance via the @ISA hierarchy, the
237 above declaration would also trigger overloading of C<+> and C<*=> in
238 all the packages which inherit from C<Number>.
240 =head2 Calling Conventions for Binary Operations
242 The functions specified in the C<use overload ...> directive are called
243 with three (in one particular case with four, see L<Last Resort>)
244 arguments. If the corresponding operation is binary, then the first
245 two arguments are the two arguments of the operation. However, due to
246 general object calling conventions, the first argument should always be
247 an object in the package, so in the situation of C<7+$a>, the
248 order of the arguments is interchanged. It probably does not matter
249 when implementing the addition method, but whether the arguments
250 are reversed is vital to the subtraction method. The method can
251 query this information by examining the third argument, which can take
252 three different values:
258 the order of arguments is as in the current operation.
262 the arguments are reversed.
266 the current operation is an assignment variant (as in
267 C<$a+=7>), but the usual function is called instead. This additional
268 information can be used to generate some optimizations. Compare
269 L<Calling Conventions for Mutators>.
273 =head2 Calling Conventions for Unary Operations
275 Unary operation are considered binary operations with the second
276 argument being C<undef>. Thus the functions that overloads C<{"++"}>
277 is called with arguments C<($a,undef,'')> when $a++ is executed.
279 =head2 Calling Conventions for Mutators
281 Two types of mutators have different calling conventions:
285 =item C<++> and C<-->
287 The routines which implement these operators are expected to actually
288 I<mutate> their arguments. So, assuming that $obj is a reference to a
291 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
293 is an appropriate implementation of overloaded C<++>. Note that
295 sub incr { ++$ {$_[0]} ; shift }
297 is OK if used with preincrement and with postincrement. (In the case
298 of postincrement a copying will be performed, see L<Copy Constructor>.)
300 =item C<x=> and other assignment versions
302 There is nothing special about these methods. They may change the
303 value of their arguments, and may leave it as is. The result is going
304 to be assigned to the value in the left-hand-side if different from
307 This allows for the same method to be used as overloaded C<+=> and
308 C<+>. Note that this is I<allowed>, but not recommended, since by the
309 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
310 if C<+=> is not overloaded.
314 B<Warning.> Due to the presence of assignment versions of operations,
315 routines which may be called in assignment context may create
316 self-referential structures. Currently Perl will not free self-referential
317 structures until cycles are C<explicitly> broken. You may get problems
318 when traversing your structures too.
322 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
324 is asking for trouble, since for code C<$obj += $foo> the subroutine
325 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
326 \$foo]>. If using such a subroutine is an important optimization, one
327 can overload C<+=> explicitly by a non-"optimized" version, or switch
328 to non-optimized version if C<not defined $_[2]> (see
329 L<Calling Conventions for Binary Operations>).
331 Even if no I<explicit> assignment-variants of operators are present in
332 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
333 C<',' . $obj . ','> may be both optimized to
335 my $tmp = ',' . $obj; $tmp .= ',';
337 =head2 Overloadable Operations
339 The following symbols can be specified in C<use overload> directive:
343 =item * I<Arithmetic operations>
345 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
346 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
348 For these operations a substituted non-assignment variant can be called if
349 the assignment variant is not available. Methods for operations C<+>,
350 C<->, C<+=>, and C<-=> can be called to automatically generate
351 increment and decrement methods. The operation C<-> can be used to
352 autogenerate missing methods for unary minus or C<abs>.
354 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
355 L<"Calling Conventions for Binary Operations">) for details of these
358 =item * I<Comparison operations>
360 "<", "<=", ">", ">=", "==", "!=", "<=>",
361 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
363 If the corresponding "spaceship" variant is available, it can be
364 used to substitute for the missing operation. During C<sort>ing
365 arrays, C<cmp> is used to compare values subject to C<use overload>.
367 =item * I<Bit operations>
369 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
371 C<neg> stands for unary minus. If the method for C<neg> is not
372 specified, it can be autogenerated using the method for
373 subtraction. If the method for C<!> is not specified, it can be
374 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
376 The same remarks in L<"Arithmetic operations"> about
377 assignment-variants and autogeneration apply for
378 bit operations C<"&">, C<"^">, and C<"|"> as well.
380 =item * I<Increment and decrement>
384 If undefined, addition and subtraction methods can be
385 used instead. These operations are called both in prefix and
388 =item * I<Transcendental functions>
390 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
392 If C<abs> is unavailable, it can be autogenerated using methods
393 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
395 Note that traditionally the Perl function L<int> rounds to 0, thus for
396 floating-point-like types one should follow the same semantic. If
397 C<int> is unavailable, it can be autogenerated using the overloading of
400 =item * I<Boolean, string and numeric conversion>
404 If one or two of these operations are not overloaded, the remaining ones can
405 be used instead. C<bool> is used in the flow control operators
406 (like C<while>) and for the ternary C<?:> operation. These functions can
407 return any arbitrary Perl value. If the corresponding operation for this value
408 is overloaded too, that operation will be called again with this value.
410 As a special case if the overload returns the object itself then it will
411 be used directly. An overloaded conversion returning the object is
412 probably a bug, because you're likely to get something that looks like
413 C<YourPackage=HASH(0x8172b34)>.
419 If not overloaded, the argument will be converted to a filehandle or
420 glob (which may require a stringification). The same overloading
421 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
422 I<globbing> syntax C<E<lt>${var}E<gt>>.
424 B<BUGS> Even in list context, the iterator is currently called only
425 once and with scalar context.
427 =item * I<Dereferencing>
429 '${}', '@{}', '%{}', '&{}', '*{}'.
431 If not overloaded, the argument will be dereferenced I<as is>, thus
432 should be of correct type. These functions should return a reference
433 of correct type, or another object with overloaded dereferencing.
435 As a special case if the overload returns the object itself then it
436 will be used directly (provided it is the correct type).
438 The dereference operators must be specified explicitly they will not be passed to
443 "nomethod", "fallback", "=", "~~",
445 see L<SPECIAL SYMBOLS FOR C<use overload>>.
449 See L<"Fallback"> for an explanation of when a missing method can be
452 A computer-readable form of the above table is available in the hash
453 %overload::ops, with values being space-separated lists of names:
455 with_assign => '+ - * / % ** << >> x .',
456 assign => '+= -= *= /= %= **= <<= >>= x= .=',
457 num_comparison => '< <= > >= == !=',
458 '3way_comparison'=> '<=> cmp',
459 str_comparison => 'lt le gt ge eq ne',
460 binary => '& &= | |= ^ ^=',
463 func => 'atan2 cos sin exp abs log sqrt',
464 conversion => 'bool "" 0+',
466 dereferencing => '${} @{} %{} &{} *{}',
467 special => 'nomethod fallback ='
469 =head2 Inheritance and overloading
471 Inheritance interacts with overloading in two ways.
475 =item Strings as values of C<use overload> directive
479 use overload key => value;
481 is a string, it is interpreted as a method name.
483 =item Overloading of an operation is inherited by derived classes
485 Any class derived from an overloaded class is also overloaded. The
486 set of overloaded methods is the union of overloaded methods of all
487 the ancestors. If some method is overloaded in several ancestor, then
488 which description will be used is decided by the usual inheritance
491 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
492 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
493 then the subroutine C<D::plus_sub> will be called to implement
494 operation C<+> for an object in package C<A>.
498 Note that since the value of the C<fallback> key is not a subroutine,
499 its inheritance is not governed by the above rules. In the current
500 implementation, the value of C<fallback> in the first overloaded
501 ancestor is used, but this is accidental and subject to change.
503 =head1 SPECIAL SYMBOLS FOR C<use overload>
505 Three keys are recognized by Perl that are not covered by the above
510 C<"nomethod"> should be followed by a reference to a function of four
511 parameters. If defined, it is called when the overloading mechanism
512 cannot find a method for some operation. The first three arguments of
513 this function coincide with the arguments for the corresponding method if
514 it were found, the fourth argument is the symbol
515 corresponding to the missing method. If several methods are tried,
516 the last one is used. Say, C<1-$a> can be equivalent to
518 &nomethodMethod($a,1,1,"-")
520 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
521 C<use overload> directive.
523 The C<"nomethod"> mechanism is I<not> used for the dereference operators
524 ( ${} @{} %{} &{} *{} ).
527 If some operation cannot be resolved, and there is no function
528 assigned to C<"nomethod">, then an exception will be raised via die()--
529 unless C<"fallback"> was specified as a key in C<use overload> directive.
534 The key C<"fallback"> governs what to do if a method for a particular
535 operation is not found. Three different cases are possible depending on
536 the value of C<"fallback">:
543 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
544 then tries to calls C<"nomethod"> value; if missing, an exception
549 The same as for the C<undef> value, but no exception is raised. Instead,
550 it silently reverts to what it would have done were there no C<use overload>
553 =item * defined, but FALSE
555 No autogeneration is tried. Perl tries to call
556 C<"nomethod"> value, and if this is missing, raises an exception.
560 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
561 yet, see L<"Inheritance and overloading">.
565 The key C<"~~"> allows you to override the smart matching used by
566 the switch construct. See L<feature>.
568 =head2 Copy Constructor
570 The value for C<"="> is a reference to a function with three
571 arguments, i.e., it looks like the other values in C<use
572 overload>. However, it does not overload the Perl assignment
573 operator. This would go against Camel hair.
575 This operation is called in the situations when a mutator is applied
576 to a reference that shares its object with some other reference, such
582 To make this change $a and not change $b, a copy of C<$$a> is made,
583 and $a is assigned a reference to this new object. This operation is
584 done during execution of the C<++$a>, and not during the assignment,
585 (so before the increment C<$$a> coincides with C<$$b>). This is only
586 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
587 C<nomethod>). Note that if this operation is expressed via C<'+'>
588 a nonmutator, i.e., as in
593 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
594 appear as lvalue when the above code is executed.
596 If the copy constructor is required during the execution of some mutator,
597 but a method for C<'='> was not specified, it can be autogenerated as a
598 string copy if the object is a plain scalar or a simple assignment if it
605 The actually executed code for
608 Something else which does not modify $a or $b....
614 Something else which does not modify $a or $b....
615 $a = $a->clone(undef,"");
618 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
619 C<'='> was overloaded with C<\&clone>.
623 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
626 =head1 MAGIC AUTOGENERATION
628 If a method for an operation is not found, and the value for C<"fallback"> is
629 TRUE or undefined, Perl tries to autogenerate a substitute method for
630 the missing operation based on the defined operations. Autogenerated method
631 substitutions are possible for the following operations:
635 =item I<Assignment forms of arithmetic operations>
637 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
640 =item I<Conversion operations>
642 String, numeric, and boolean conversion are calculated in terms of one
643 another if not all of them are defined.
645 =item I<Increment and decrement>
647 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
648 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
652 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
656 can be expressed in terms of subtraction.
660 C<!> and C<not> can be expressed in terms of boolean conversion, or
661 string or numerical conversion.
663 =item I<Concatenation>
665 can be expressed in terms of string conversion.
667 =item I<Comparison operations>
669 can be expressed in terms of its "spaceship" counterpart: either
670 C<E<lt>=E<gt>> or C<cmp>:
672 <, >, <=, >=, ==, != in terms of <=>
673 lt, gt, le, ge, eq, ne in terms of cmp
677 <> in terms of builtin operations
679 =item I<Dereferencing>
681 ${} @{} %{} &{} *{} in terms of builtin operations
683 =item I<Copy operator>
685 can be expressed in terms of an assignment to the dereferenced value, if this
686 value is a scalar and not a reference, or simply a reference assignment
691 =head1 Minimal set of overloaded operations
693 Since some operations can be automatically generated from others, there is
694 a minimal set of operations that need to be overloaded in order to have
695 the complete set of overloaded operations at one's disposal.
696 Of course, the autogenerated operations may not do exactly what the user
697 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
702 atan2 cos sin exp log sqrt int
704 Additionally, you need to define at least one of string, boolean or
705 numeric conversions because any one can be used to emulate the others.
706 The string conversion can also be used to emulate concatenation.
708 =head1 Losing overloading
710 The restriction for the comparison operation is that even if, for example,
711 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
712 function will produce only a standard logical value based on the
713 numerical value of the result of `C<cmp>'. In particular, a working
714 numeric conversion is needed in this case (possibly expressed in terms of
717 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
718 if the string conversion substitution is applied.
720 When you chop() a mathemagical object it is promoted to a string and its
721 mathemagical properties are lost. The same can happen with other
724 =head1 Run-time Overloading
726 Since all C<use> directives are executed at compile-time, the only way to
727 change overloading during run-time is to
729 eval 'use overload "+" => \&addmethod';
733 eval 'no overload "+", "--", "<="';
735 though the use of these constructs during run-time is questionable.
737 =head1 Public functions
739 Package C<overload.pm> provides the following public functions:
743 =item overload::StrVal(arg)
745 Gives string value of C<arg> as in absence of stringify overloading. If you
746 are using this to get the address of a reference (useful for checking if two
747 references point to the same thing) then you may be better off using
748 C<Scalar::Util::refaddr()>, which is faster.
750 =item overload::Overloaded(arg)
752 Returns true if C<arg> is subject to overloading of some operations.
754 =item overload::Method(obj,op)
756 Returns C<undef> or a reference to the method that implements C<op>.
760 =head1 Overloading constants
762 For some applications, the Perl parser mangles constants too much.
763 It is possible to hook into this process via C<overload::constant()>
764 and C<overload::remove_constant()> functions.
766 These functions take a hash as an argument. The recognized keys of this hash
773 to overload integer constants,
777 to overload floating point constants,
781 to overload octal and hexadecimal constants,
785 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
786 strings and here-documents,
790 to overload constant pieces of regular expressions.
794 The corresponding values are references to functions which take three arguments:
795 the first one is the I<initial> string form of the constant, the second one
796 is how Perl interprets this constant, the third one is how the constant is used.
797 Note that the initial string form does not
798 contain string delimiters, and has backslashes in backslash-delimiter
799 combinations stripped (thus the value of delimiter is not relevant for
800 processing of this string). The return value of this function is how this
801 constant is going to be interpreted by Perl. The third argument is undefined
802 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
803 context (comes from strings, regular expressions, and single-quote HERE
804 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
805 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
807 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
808 it is expected that overloaded constant strings are equipped with reasonable
809 overloaded catenation operator, otherwise absurd results will result.
810 Similarly, negative numbers are considered as negations of positive constants.
812 Note that it is probably meaningless to call the functions overload::constant()
813 and overload::remove_constant() from anywhere but import() and unimport() methods.
814 From these methods they may be called as
819 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
820 overload::constant integer => sub {Math::BigInt->new(shift)};
823 =head1 IMPLEMENTATION
825 What follows is subject to change RSN.
827 The table of methods for all operations is cached in magic for the
828 symbol table hash for the package. The cache is invalidated during
829 processing of C<use overload>, C<no overload>, new function
830 definitions, and changes in @ISA. However, this invalidation remains
831 unprocessed until the next C<bless>ing into the package. Hence if you
832 want to change overloading structure dynamically, you'll need an
833 additional (fake) C<bless>ing to update the table.
835 (Every SVish thing has a magic queue, and magic is an entry in that
836 queue. This is how a single variable may participate in multiple
837 forms of magic simultaneously. For instance, environment variables
838 regularly have two forms at once: their %ENV magic and their taint
839 magic. However, the magic which implements overloading is applied to
840 the stashes, which are rarely used directly, thus should not slow down
843 If an object belongs to a package using overload, it carries a special
844 flag. Thus the only speed penalty during arithmetic operations without
845 overloading is the checking of this flag.
847 In fact, if C<use overload> is not present, there is almost no overhead
848 for overloadable operations, so most programs should not suffer
849 measurable performance penalties. A considerable effort was made to
850 minimize the overhead when overload is used in some package, but the
851 arguments in question do not belong to packages using overload. When
852 in doubt, test your speed with C<use overload> and without it. So far
853 there have been no reports of substantial speed degradation if Perl is
854 compiled with optimization turned on.
856 There is no size penalty for data if overload is not used. The only
857 size penalty if overload is used in some package is that I<all> the
858 packages acquire a magic during the next C<bless>ing into the
859 package. This magic is three-words-long for packages without
860 overloading, and carries the cache table if the package is overloaded.
862 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
863 carried out before any operation that can imply an assignment to the
864 object $a (or $b) refers to, like C<$a++>. You can override this
865 behavior by defining your own copy constructor (see L<"Copy Constructor">).
867 It is expected that arguments to methods that are not explicitly supposed
868 to be changed are constant (but this is not enforced).
870 =head1 Metaphor clash
872 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
873 If it I<looks> counter intuitive to you, you are subject to a metaphor
876 Here is a Perl object metaphor:
878 I< object is a reference to blessed data>
880 and an arithmetic metaphor:
882 I< object is a thing by itself>.
884 The I<main> problem of overloading C<=> is the fact that these metaphors
885 imply different actions on the assignment C<$a = $b> if $a and $b are
886 objects. Perl-think implies that $a becomes a reference to whatever
887 $b was referencing. Arithmetic-think implies that the value of "object"
888 $a is changed to become the value of the object $b, preserving the fact
889 that $a and $b are separate entities.
891 The difference is not relevant in the absence of mutators. After
892 a Perl-way assignment an operation which mutates the data referenced by $a
893 would change the data referenced by $b too. Effectively, after
894 C<$a = $b> values of $a and $b become I<indistinguishable>.
896 On the other hand, anyone who has used algebraic notation knows the
897 expressive power of the arithmetic metaphor. Overloading works hard
898 to enable this metaphor while preserving the Perlian way as far as
899 possible. Since it is not possible to freely mix two contradicting
900 metaphors, overloading allows the arithmetic way to write things I<as
901 far as all the mutators are called via overloaded access only>. The
902 way it is done is described in L<Copy Constructor>.
904 If some mutator methods are directly applied to the overloaded values,
905 one may need to I<explicitly unlink> other values which references the
910 $b = $a; # $b is "linked" to $a
912 $a = $a->clone; # Unlink $b from $a
915 Note that overloaded access makes this transparent:
918 $b = $a; # $b is "linked" to $a
919 $a += 4; # would unlink $b automagically
921 However, it would not make
924 $a = 4; # Now $a is a plain 4, not 'Data'
926 preserve "objectness" of $a. But Perl I<has> a way to make assignments
927 to an object do whatever you want. It is just not the overload, but
928 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
929 which returns the object itself, and STORE() method which changes the
930 value of the object, one can reproduce the arithmetic metaphor in its
931 completeness, at least for variables which were tie()d from the start.
933 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
937 Please add examples to what follows!
939 =head2 Two-face scalars
941 Put this in F<two_face.pm> in your Perl library directory:
943 package two_face; # Scalars with separate string and
945 sub new { my $p = shift; bless [@_], $p }
946 use overload '""' => \&str, '0+' => \&num, fallback => 1;
953 my $seven = two_face->new("vii", 7);
954 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
955 print "seven contains `i'\n" if $seven =~ /i/;
957 (The second line creates a scalar which has both a string value, and a
958 numeric value.) This prints:
960 seven=vii, seven=7, eight=8
963 =head2 Two-face references
965 Suppose you want to create an object which is accessible as both an
966 array reference and a hash reference.
969 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
977 tie %h, ref $self, $self;
981 sub TIEHASH { my $p = shift; bless \ shift, $p }
984 $fields{$_} = $i++ foreach qw{zero one two three};
986 my $self = ${shift()};
987 my $key = $fields{shift()};
988 defined $key or die "Out of band access";
989 $$self->[$key] = shift;
992 my $self = ${shift()};
993 my $key = $fields{shift()};
994 defined $key or die "Out of band access";
998 Now one can access an object using both the array and hash syntax:
1000 my $bar = two_refs->new(3,4,5,6);
1002 $bar->{two} == 11 or die 'bad hash fetch';
1004 Note several important features of this example. First of all, the
1005 I<actual> type of $bar is a scalar reference, and we do not overload
1006 the scalar dereference. Thus we can get the I<actual> non-overloaded
1007 contents of $bar by just using C<$$bar> (what we do in functions which
1008 overload dereference). Similarly, the object returned by the
1009 TIEHASH() method is a scalar reference.
1011 Second, we create a new tied hash each time the hash syntax is used.
1012 This allows us not to worry about a possibility of a reference loop,
1013 which would lead to a memory leak.
1015 Both these problems can be cured. Say, if we want to overload hash
1016 dereference on a reference to an object which is I<implemented> as a
1017 hash itself, the only problem one has to circumvent is how to access
1018 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1019 overloaded dereference operator). Here is one possible fetching routine:
1022 my ($self, $key) = (shift, shift);
1023 my $class = ref $self;
1024 bless $self, 'overload::dummy'; # Disable overloading of %{}
1025 my $out = $self->{$key};
1026 bless $self, $class; # Restore overloading
1030 To remove creation of the tied hash on each access, one may an extra
1031 level of indirection which allows a non-circular structure of references:
1034 use overload '%{}' => sub { ${shift()}->[1] },
1035 '@{}' => sub { ${shift()}->[0] };
1041 bless \ [$a, \%h], $p;
1046 tie %h, ref $self, $self;
1050 sub TIEHASH { my $p = shift; bless \ shift, $p }
1053 $fields{$_} = $i++ foreach qw{zero one two three};
1056 my $key = $fields{shift()};
1057 defined $key or die "Out of band access";
1062 my $key = $fields{shift()};
1063 defined $key or die "Out of band access";
1067 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1068 reference to the intermediate array, which keeps a reference to an
1069 actual array, and the access hash. The tie()ing object for the access
1070 hash is a reference to a reference to the actual array, so
1076 There are no loops of references.
1080 Both "objects" which are blessed into the class C<two_refs1> are
1081 references to a reference to an array, thus references to a I<scalar>.
1082 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1083 overloaded operations.
1087 =head2 Symbolic calculator
1089 Put this in F<symbolic.pm> in your Perl library directory:
1091 package symbolic; # Primitive symbolic calculator
1092 use overload nomethod => \&wrap;
1094 sub new { shift; bless ['n', @_] }
1096 my ($obj, $other, $inv, $meth) = @_;
1097 ($obj, $other) = ($other, $obj) if $inv;
1098 bless [$meth, $obj, $other];
1101 This module is very unusual as overloaded modules go: it does not
1102 provide any usual overloaded operators, instead it provides the L<Last
1103 Resort> operator C<nomethod>. In this example the corresponding
1104 subroutine returns an object which encapsulates operations done over
1105 the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
1106 symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
1108 Here is an example of the script which "calculates" the side of
1109 circumscribed octagon using the above package:
1112 my $iter = 1; # 2**($iter+2) = 8
1113 my $side = symbolic->new(1);
1117 $side = (sqrt(1 + $side**2) - 1)/$side;
1121 The value of $side is
1123 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1124 undef], 1], ['n', 1]]
1126 Note that while we obtained this value using a nice little script,
1127 there is no simple way to I<use> this value. In fact this value may
1128 be inspected in debugger (see L<perldebug>), but only if
1129 C<bareStringify> B<O>ption is set, and not via C<p> command.
1131 If one attempts to print this value, then the overloaded operator
1132 C<""> will be called, which will call C<nomethod> operator. The
1133 result of this operator will be stringified again, but this result is
1134 again of type C<symbolic>, which will lead to an infinite loop.
1136 Add a pretty-printer method to the module F<symbolic.pm>:
1139 my ($meth, $a, $b) = @{+shift};
1140 $a = 'u' unless defined $a;
1141 $b = 'u' unless defined $b;
1142 $a = $a->pretty if ref $a;
1143 $b = $b->pretty if ref $b;
1147 Now one can finish the script by
1149 print "side = ", $side->pretty, "\n";
1151 The method C<pretty> is doing object-to-string conversion, so it
1152 is natural to overload the operator C<""> using this method. However,
1153 inside such a method it is not necessary to pretty-print the
1154 I<components> $a and $b of an object. In the above subroutine
1155 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1156 and $b. If these components use overloading, the catenation operator
1157 will look for an overloaded operator C<.>; if not present, it will
1158 look for an overloaded operator C<"">. Thus it is enough to use
1160 use overload nomethod => \&wrap, '""' => \&str;
1162 my ($meth, $a, $b) = @{+shift};
1163 $a = 'u' unless defined $a;
1164 $b = 'u' unless defined $b;
1168 Now one can change the last line of the script to
1170 print "side = $side\n";
1174 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1176 and one can inspect the value in debugger using all the possible
1179 Something is still amiss: consider the loop variable $cnt of the
1180 script. It was a number, not an object. We cannot make this value of
1181 type C<symbolic>, since then the loop will not terminate.
1183 Indeed, to terminate the cycle, the $cnt should become false.
1184 However, the operator C<bool> for checking falsity is overloaded (this
1185 time via overloaded C<"">), and returns a long string, thus any object
1186 of type C<symbolic> is true. To overcome this, we need a way to
1187 compare an object to 0. In fact, it is easier to write a numeric
1190 Here is the text of F<symbolic.pm> with such a routine added (and
1191 slightly modified str()):
1193 package symbolic; # Primitive symbolic calculator
1195 nomethod => \&wrap, '""' => \&str, '0+' => \#
1197 sub new { shift; bless ['n', @_] }
1199 my ($obj, $other, $inv, $meth) = @_;
1200 ($obj, $other) = ($other, $obj) if $inv;
1201 bless [$meth, $obj, $other];
1204 my ($meth, $a, $b) = @{+shift};
1205 $a = 'u' unless defined $a;
1212 my %subr = ( n => sub {$_[0]},
1213 sqrt => sub {sqrt $_[0]},
1214 '-' => sub {shift() - shift()},
1215 '+' => sub {shift() + shift()},
1216 '/' => sub {shift() / shift()},
1217 '*' => sub {shift() * shift()},
1218 '**' => sub {shift() ** shift()},
1221 my ($meth, $a, $b) = @{+shift};
1222 my $subr = $subr{$meth}
1223 or die "Do not know how to ($meth) in symbolic";
1224 $a = $a->num if ref $a eq __PACKAGE__;
1225 $b = $b->num if ref $b eq __PACKAGE__;
1229 All the work of numeric conversion is done in %subr and num(). Of
1230 course, %subr is not complete, it contains only operators used in the
1231 example below. Here is the extra-credit question: why do we need an
1232 explicit recursion in num()? (Answer is at the end of this section.)
1234 Use this module like this:
1237 my $iter = symbolic->new(2); # 16-gon
1238 my $side = symbolic->new(1);
1242 $cnt = $cnt - 1; # Mutator `--' not implemented
1243 $side = (sqrt(1 + $side**2) - 1)/$side;
1245 printf "%s=%f\n", $side, $side;
1246 printf "pi=%f\n", $side*(2**($iter+2));
1248 It prints (without so many line breaks)
1250 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1252 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1255 The above module is very primitive. It does not implement
1256 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1257 (not required without mutators!), and implements only those arithmetic
1258 operations which are used in the example.
1260 To implement most arithmetic operations is easy; one should just use
1261 the tables of operations, and change the code which fills %subr to
1263 my %subr = ( 'n' => sub {$_[0]} );
1264 foreach my $op (split " ", $overload::ops{with_assign}) {
1265 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1267 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1268 foreach my $op (split " ", "@overload::ops{ @bins }") {
1269 $subr{$op} = eval "sub {shift() $op shift()}";
1271 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1272 print "defining `$op'\n";
1273 $subr{$op} = eval "sub {$op shift()}";
1276 Due to L<Calling Conventions for Mutators>, we do not need anything
1277 special to make C<+=> and friends work, except filling C<+=> entry of
1278 %subr, and defining a copy constructor (needed since Perl has no
1279 way to know that the implementation of C<'+='> does not mutate
1280 the argument, compare L<Copy Constructor>).
1282 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1283 line, and code (this code assumes that mutators change things one level
1284 deep only, so recursive copying is not needed):
1288 bless [@$self], ref $self;
1291 To make C<++> and C<--> work, we need to implement actual mutators,
1292 either directly, or in C<nomethod>. We continue to do things inside
1293 C<nomethod>, thus add
1295 if ($meth eq '++' or $meth eq '--') {
1296 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1300 after the first line of wrap(). This is not a most effective
1301 implementation, one may consider
1303 sub inc { $_[0] = bless ['++', shift, 1]; }
1307 As a final remark, note that one can fill %subr by
1309 my %subr = ( 'n' => sub {$_[0]} );
1310 foreach my $op (split " ", $overload::ops{with_assign}) {
1311 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1313 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1314 foreach my $op (split " ", "@overload::ops{ @bins }") {
1315 $subr{$op} = eval "sub {shift() $op shift()}";
1317 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1318 $subr{$op} = eval "sub {$op shift()}";
1320 $subr{'++'} = $subr{'+'};
1321 $subr{'--'} = $subr{'-'};
1323 This finishes implementation of a primitive symbolic calculator in
1324 50 lines of Perl code. Since the numeric values of subexpressions
1325 are not cached, the calculator is very slow.
1327 Here is the answer for the exercise: In the case of str(), we need no
1328 explicit recursion since the overloaded C<.>-operator will fall back
1329 to an existing overloaded operator C<"">. Overloaded arithmetic
1330 operators I<do not> fall back to numeric conversion if C<fallback> is
1331 not explicitly requested. Thus without an explicit recursion num()
1332 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1333 the argument of num().
1335 If you wonder why defaults for conversion are different for str() and
1336 num(), note how easy it was to write the symbolic calculator. This
1337 simplicity is due to an appropriate choice of defaults. One extra
1338 note: due to the explicit recursion num() is more fragile than sym():
1339 we need to explicitly check for the type of $a and $b. If components
1340 $a and $b happen to be of some related type, this may lead to problems.
1342 =head2 I<Really> symbolic calculator
1344 One may wonder why we call the above calculator symbolic. The reason
1345 is that the actual calculation of the value of expression is postponed
1346 until the value is I<used>.
1348 To see it in action, add a method
1353 @$obj->[0,1] = ('=', shift);
1356 to the package C<symbolic>. After this change one can do
1358 my $a = symbolic->new(3);
1359 my $b = symbolic->new(4);
1360 my $c = sqrt($a**2 + $b**2);
1362 and the numeric value of $c becomes 5. However, after calling
1364 $a->STORE(12); $b->STORE(5);
1366 the numeric value of $c becomes 13. There is no doubt now that the module
1367 symbolic provides a I<symbolic> calculator indeed.
1369 To hide the rough edges under the hood, provide a tie()d interface to the
1370 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1372 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1374 sub nop { } # Around a bug
1376 (the bug is described in L<"BUGS">). One can use this new interface as
1378 tie $a, 'symbolic', 3;
1379 tie $b, 'symbolic', 4;
1380 $a->nop; $b->nop; # Around a bug
1382 my $c = sqrt($a**2 + $b**2);
1384 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1385 of $c becomes 13. To insulate the user of the module add a method
1387 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1392 symbolic->vars($a, $b);
1393 my $c = sqrt($a**2 + $b**2);
1396 printf "c5 %s=%f\n", $c, $c;
1399 printf "c13 %s=%f\n", $c, $c;
1401 shows that the numeric value of $c follows changes to the values of $a
1406 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1410 The L<overloading> pragma can be used to enable or disable overloaded
1411 operations within a lexical scope.
1415 When Perl is run with the B<-Do> switch or its equivalent, overloading
1416 induces diagnostic messages.
1418 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1419 deduce which operations are overloaded (and which ancestor triggers
1420 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1421 is shown by debugger. The method C<()> corresponds to the C<fallback>
1422 key (in fact a presence of this method shows that this package has
1423 overloading enabled, and it is what is used by the C<Overloaded>
1424 function of module C<overload>).
1426 The module might issue the following warnings:
1430 =item Odd number of arguments for overload::constant
1432 (W) The call to overload::constant contained an odd number of arguments.
1433 The arguments should come in pairs.
1435 =item `%s' is not an overloadable type
1437 (W) You tried to overload a constant type the overload package is unaware of.
1439 =item `%s' is not a code reference
1441 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1442 to be a code reference. Either an anonymous subroutine, or a reference
1449 Because it is used for overloading, the per-package hash %OVERLOAD now
1450 has a special meaning in Perl. The symbol table is filled with names
1451 looking like line-noise.
1453 For the purpose of inheritance every overloaded package behaves as if
1454 C<fallback> is present (possibly undefined). This may create
1455 interesting effects if some package is not overloaded, but inherits
1456 from two overloaded packages.
1458 Relation between overloading and tie()ing is broken. Overloading is
1459 triggered or not basing on the I<previous> class of tie()d value.
1461 This happens because the presence of overloading is checked too early,
1462 before any tie()d access is attempted. If the FETCH()ed class of the
1463 tie()d value does not change, a simple workaround is to access the value
1464 immediately after tie()ing, so that after this call the I<previous> class
1465 coincides with the current one.
1467 B<Needed:> a way to fix this without a speed penalty.
1469 Barewords are not covered by overloaded string constants.
1471 This document is confusing. There are grammos and misleading language
1472 used in places. It would seem a total rewrite is needed.