13 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
14 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
16 if ($_ eq 'fallback') {
20 if (not ref $sub and $sub !~ /::/) {
21 $ {$package . "::(" . $_} = $sub;
24 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
25 *{$package . "::(" . $_} = \&{ $sub };
28 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
32 $package = (caller())[0];
33 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
35 $package->overload::OVERLOAD(@_);
39 $package = (caller())[0];
40 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
43 if ($_ eq 'fallback') {
44 undef $ {$package . "::()"};
46 delete $ {$package . "::"}{"(" . $_};
53 $package = ref $package if ref $package;
59 return undef unless $globref;
60 my $sub = \&{*$globref};
61 return $sub if $sub ne \&nil;
62 return shift->can($ {*$globref});
65 sub OverloadedStringify {
67 $package = ref $package if ref $package;
69 ov_method mycan($package, '(""'), $package
70 or ov_method mycan($package, '(0+'), $package
71 or ov_method mycan($package, '(bool'), $package
72 or ov_method mycan($package, '(nomethod'), $package;
78 $package = Scalar::Util::blessed($package);
79 return undef if !defined $package;
81 #my $meth = $package->can('(' . shift);
82 ov_method mycan($package, '(' . shift), $package;
83 #return $meth if $meth ne \&nil;
88 my $package = ref $_[0];
89 return "$_[0]" unless $package;
92 my $class = Scalar::Util::blessed($_[0]);
93 my $class_prefix = defined($class) ? "$class=" : "";
94 my $type = Scalar::Util::reftype($_[0]);
95 my $addr = Scalar::Util::refaddr($_[0]);
96 return sprintf("$class_prefix$type(0x%x)", $addr);
101 sub mycan { # Real can would leave stubs.
102 my ($package, $meth) = @_;
104 my $mro = mro::get_linear_isa($package);
105 foreach my $p (@$mro) {
106 my $fqmeth = $p . q{::} . $meth;
107 return \*{$fqmeth} if defined &{$fqmeth};
114 'integer' => 0x1000, # HINT_NEW_INTEGER
115 'float' => 0x2000, # HINT_NEW_FLOAT
116 'binary' => 0x4000, # HINT_NEW_BINARY
117 'q' => 0x8000, # HINT_NEW_STRING
118 'qr' => 0x10000, # HINT_NEW_RE
121 %ops = ( with_assign => "+ - * / % ** << >> x .",
122 assign => "+= -= *= /= %= **= <<= >>= x= .=",
123 num_comparison => "< <= > >= == !=",
124 '3way_comparison'=> "<=> cmp",
125 str_comparison => "lt le gt ge eq ne",
126 binary => '& &= | |= ^ ^=',
129 func => "atan2 cos sin exp abs log sqrt int",
130 conversion => 'bool "" 0+',
132 dereferencing => '${} @{} %{} &{} *{}',
133 special => 'nomethod fallback =');
135 use warnings::register;
137 # Arguments: what, sub
140 warnings::warnif ("Odd number of arguments for overload::constant");
143 elsif (!exists $constants {$_ [0]}) {
144 warnings::warnif ("`$_[0]' is not an overloadable type");
146 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
147 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
148 # blessed, and C<ref> would return the package the ref is blessed into.
149 if (warnings::enabled) {
150 $_ [1] = "undef" unless defined $_ [1];
151 warnings::warn ("`$_[1]' is not a code reference");
156 $^H |= $constants{$_[0]};
162 sub remove_constant {
163 # Arguments: what, sub
166 $^H &= ~ $constants{$_[0]};
177 overload - Package for overloading Perl operations
190 $a = new SomeThing 57;
193 if (overload::Overloaded $b) {...}
195 $strval = overload::StrVal $b;
199 =head2 Declaration of overloaded functions
201 The compilation directive
208 declares function Number::add() for addition, and method muas() in
209 the "class" C<Number> (or one of its base classes)
210 for the assignment form C<*=> of multiplication.
212 Arguments of this directive come in (key, value) pairs. Legal values
213 are values legal inside a C<&{ ... }> call, so the name of a
214 subroutine, a reference to a subroutine, or an anonymous subroutine
215 will all work. Note that values specified as strings are
216 interpreted as methods, not subroutines. Legal keys are listed below.
218 The subroutine C<add> will be called to execute C<$a+$b> if $a
219 is a reference to an object blessed into the package C<Number>, or if $a is
220 not an object from a package with defined mathemagic addition, but $b is a
221 reference to a C<Number>. It can also be called in other situations, like
222 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
223 methods refer to methods triggered by an overloaded mathematical
226 Since overloading respects inheritance via the @ISA hierarchy, the
227 above declaration would also trigger overloading of C<+> and C<*=> in
228 all the packages which inherit from C<Number>.
230 =head2 Calling Conventions for Binary Operations
232 The functions specified in the C<use overload ...> directive are called
233 with three (in one particular case with four, see L<Last Resort>)
234 arguments. If the corresponding operation is binary, then the first
235 two arguments are the two arguments of the operation. However, due to
236 general object calling conventions, the first argument should always be
237 an object in the package, so in the situation of C<7+$a>, the
238 order of the arguments is interchanged. It probably does not matter
239 when implementing the addition method, but whether the arguments
240 are reversed is vital to the subtraction method. The method can
241 query this information by examining the third argument, which can take
242 three different values:
248 the order of arguments is as in the current operation.
252 the arguments are reversed.
256 the current operation is an assignment variant (as in
257 C<$a+=7>), but the usual function is called instead. This additional
258 information can be used to generate some optimizations. Compare
259 L<Calling Conventions for Mutators>.
263 =head2 Calling Conventions for Unary Operations
265 Unary operation are considered binary operations with the second
266 argument being C<undef>. Thus the functions that overloads C<{"++"}>
267 is called with arguments C<($a,undef,'')> when $a++ is executed.
269 =head2 Calling Conventions for Mutators
271 Two types of mutators have different calling conventions:
275 =item C<++> and C<-->
277 The routines which implement these operators are expected to actually
278 I<mutate> their arguments. So, assuming that $obj is a reference to a
281 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
283 is an appropriate implementation of overloaded C<++>. Note that
285 sub incr { ++$ {$_[0]} ; shift }
287 is OK if used with preincrement and with postincrement. (In the case
288 of postincrement a copying will be performed, see L<Copy Constructor>.)
290 =item C<x=> and other assignment versions
292 There is nothing special about these methods. They may change the
293 value of their arguments, and may leave it as is. The result is going
294 to be assigned to the value in the left-hand-side if different from
297 This allows for the same method to be used as overloaded C<+=> and
298 C<+>. Note that this is I<allowed>, but not recommended, since by the
299 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
300 if C<+=> is not overloaded.
304 B<Warning.> Due to the presence of assignment versions of operations,
305 routines which may be called in assignment context may create
306 self-referential structures. Currently Perl will not free self-referential
307 structures until cycles are C<explicitly> broken. You may get problems
308 when traversing your structures too.
312 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
314 is asking for trouble, since for code C<$obj += $foo> the subroutine
315 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
316 \$foo]>. If using such a subroutine is an important optimization, one
317 can overload C<+=> explicitly by a non-"optimized" version, or switch
318 to non-optimized version if C<not defined $_[2]> (see
319 L<Calling Conventions for Binary Operations>).
321 Even if no I<explicit> assignment-variants of operators are present in
322 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
323 C<',' . $obj . ','> may be both optimized to
325 my $tmp = ',' . $obj; $tmp .= ',';
327 =head2 Overloadable Operations
329 The following symbols can be specified in C<use overload> directive:
333 =item * I<Arithmetic operations>
335 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
336 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
338 For these operations a substituted non-assignment variant can be called if
339 the assignment variant is not available. Methods for operations C<+>,
340 C<->, C<+=>, and C<-=> can be called to automatically generate
341 increment and decrement methods. The operation C<-> can be used to
342 autogenerate missing methods for unary minus or C<abs>.
344 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
345 L<"Calling Conventions for Binary Operations">) for details of these
348 =item * I<Comparison operations>
350 "<", "<=", ">", ">=", "==", "!=", "<=>",
351 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
353 If the corresponding "spaceship" variant is available, it can be
354 used to substitute for the missing operation. During C<sort>ing
355 arrays, C<cmp> is used to compare values subject to C<use overload>.
357 =item * I<Bit operations>
359 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
361 C<neg> stands for unary minus. If the method for C<neg> is not
362 specified, it can be autogenerated using the method for
363 subtraction. If the method for C<!> is not specified, it can be
364 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
366 The same remarks in L<"Arithmetic operations"> about
367 assignment-variants and autogeneration apply for
368 bit operations C<"&">, C<"^">, and C<"|"> as well.
370 =item * I<Increment and decrement>
374 If undefined, addition and subtraction methods can be
375 used instead. These operations are called both in prefix and
378 =item * I<Transcendental functions>
380 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
382 If C<abs> is unavailable, it can be autogenerated using methods
383 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
385 Note that traditionally the Perl function L<int> rounds to 0, thus for
386 floating-point-like types one should follow the same semantic. If
387 C<int> is unavailable, it can be autogenerated using the overloading of
390 =item * I<Boolean, string and numeric conversion>
394 If one or two of these operations are not overloaded, the remaining ones can
395 be used instead. C<bool> is used in the flow control operators
396 (like C<while>) and for the ternary C<?:> operation. These functions can
397 return any arbitrary Perl value. If the corresponding operation for this value
398 is overloaded too, that operation will be called again with this value.
400 As a special case if the overload returns the object itself then it will
401 be used directly. An overloaded conversion returning the object is
402 probably a bug, because you're likely to get something that looks like
403 C<YourPackage=HASH(0x8172b34)>.
409 If not overloaded, the argument will be converted to a filehandle or
410 glob (which may require a stringification). The same overloading
411 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
412 I<globbing> syntax C<E<lt>${var}E<gt>>.
414 B<BUGS> Even in list context, the iterator is currently called only
415 once and with scalar context.
417 =item * I<Dereferencing>
419 '${}', '@{}', '%{}', '&{}', '*{}'.
421 If not overloaded, the argument will be dereferenced I<as is>, thus
422 should be of correct type. These functions should return a reference
423 of correct type, or another object with overloaded dereferencing.
425 As a special case if the overload returns the object itself then it
426 will be used directly (provided it is the correct type).
428 The dereference operators must be specified explicitly they will not be passed to
433 "nomethod", "fallback", "=", "~~",
435 see L<SPECIAL SYMBOLS FOR C<use overload>>.
439 See L<"Fallback"> for an explanation of when a missing method can be
442 A computer-readable form of the above table is available in the hash
443 %overload::ops, with values being space-separated lists of names:
445 with_assign => '+ - * / % ** << >> x .',
446 assign => '+= -= *= /= %= **= <<= >>= x= .=',
447 num_comparison => '< <= > >= == !=',
448 '3way_comparison'=> '<=> cmp',
449 str_comparison => 'lt le gt ge eq ne',
450 binary => '& &= | |= ^ ^=',
453 func => 'atan2 cos sin exp abs log sqrt',
454 conversion => 'bool "" 0+',
456 dereferencing => '${} @{} %{} &{} *{}',
457 special => 'nomethod fallback ='
459 =head2 Inheritance and overloading
461 Inheritance interacts with overloading in two ways.
465 =item Strings as values of C<use overload> directive
469 use overload key => value;
471 is a string, it is interpreted as a method name.
473 =item Overloading of an operation is inherited by derived classes
475 Any class derived from an overloaded class is also overloaded. The
476 set of overloaded methods is the union of overloaded methods of all
477 the ancestors. If some method is overloaded in several ancestor, then
478 which description will be used is decided by the usual inheritance
481 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
482 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
483 then the subroutine C<D::plus_sub> will be called to implement
484 operation C<+> for an object in package C<A>.
488 Note that since the value of the C<fallback> key is not a subroutine,
489 its inheritance is not governed by the above rules. In the current
490 implementation, the value of C<fallback> in the first overloaded
491 ancestor is used, but this is accidental and subject to change.
493 =head1 SPECIAL SYMBOLS FOR C<use overload>
495 Three keys are recognized by Perl that are not covered by the above
500 C<"nomethod"> should be followed by a reference to a function of four
501 parameters. If defined, it is called when the overloading mechanism
502 cannot find a method for some operation. The first three arguments of
503 this function coincide with the arguments for the corresponding method if
504 it were found, the fourth argument is the symbol
505 corresponding to the missing method. If several methods are tried,
506 the last one is used. Say, C<1-$a> can be equivalent to
508 &nomethodMethod($a,1,1,"-")
510 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
511 C<use overload> directive.
513 The C<"nomethod"> mechanism is I<not> used for the dereference operators
514 ( ${} @{} %{} &{} *{} ).
517 If some operation cannot be resolved, and there is no function
518 assigned to C<"nomethod">, then an exception will be raised via die()--
519 unless C<"fallback"> was specified as a key in C<use overload> directive.
524 The key C<"fallback"> governs what to do if a method for a particular
525 operation is not found. Three different cases are possible depending on
526 the value of C<"fallback">:
533 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
534 then tries to calls C<"nomethod"> value; if missing, an exception
539 The same as for the C<undef> value, but no exception is raised. Instead,
540 it silently reverts to what it would have done were there no C<use overload>
543 =item * defined, but FALSE
545 No autogeneration is tried. Perl tries to call
546 C<"nomethod"> value, and if this is missing, raises an exception.
550 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
551 yet, see L<"Inheritance and overloading">.
555 The key C<"~~"> allows you to override the smart matching used by
556 the switch construct. See L<feature>.
558 =head2 Copy Constructor
560 The value for C<"="> is a reference to a function with three
561 arguments, i.e., it looks like the other values in C<use
562 overload>. However, it does not overload the Perl assignment
563 operator. This would go against Camel hair.
565 This operation is called in the situations when a mutator is applied
566 to a reference that shares its object with some other reference, such
572 To make this change $a and not change $b, a copy of C<$$a> is made,
573 and $a is assigned a reference to this new object. This operation is
574 done during execution of the C<++$a>, and not during the assignment,
575 (so before the increment C<$$a> coincides with C<$$b>). This is only
576 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
577 C<nomethod>). Note that if this operation is expressed via C<'+'>
578 a nonmutator, i.e., as in
583 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
584 appear as lvalue when the above code is executed.
586 If the copy constructor is required during the execution of some mutator,
587 but a method for C<'='> was not specified, it can be autogenerated as a
588 string copy if the object is a plain scalar.
594 The actually executed code for
597 Something else which does not modify $a or $b....
603 Something else which does not modify $a or $b....
604 $a = $a->clone(undef,"");
607 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
608 C<'='> was overloaded with C<\&clone>.
612 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
615 =head1 MAGIC AUTOGENERATION
617 If a method for an operation is not found, and the value for C<"fallback"> is
618 TRUE or undefined, Perl tries to autogenerate a substitute method for
619 the missing operation based on the defined operations. Autogenerated method
620 substitutions are possible for the following operations:
624 =item I<Assignment forms of arithmetic operations>
626 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
629 =item I<Conversion operations>
631 String, numeric, and boolean conversion are calculated in terms of one
632 another if not all of them are defined.
634 =item I<Increment and decrement>
636 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
637 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
641 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
645 can be expressed in terms of subtraction.
649 C<!> and C<not> can be expressed in terms of boolean conversion, or
650 string or numerical conversion.
652 =item I<Concatenation>
654 can be expressed in terms of string conversion.
656 =item I<Comparison operations>
658 can be expressed in terms of its "spaceship" counterpart: either
659 C<E<lt>=E<gt>> or C<cmp>:
661 <, >, <=, >=, ==, != in terms of <=>
662 lt, gt, le, ge, eq, ne in terms of cmp
666 <> in terms of builtin operations
668 =item I<Dereferencing>
670 ${} @{} %{} &{} *{} in terms of builtin operations
672 =item I<Copy operator>
674 can be expressed in terms of an assignment to the dereferenced value, if this
675 value is a scalar and not a reference.
679 =head1 Minimal set of overloaded operations
681 Since some operations can be automatically generated from others, there is
682 a minimal set of operations that need to be overloaded in order to have
683 the complete set of overloaded operations at one's disposal.
684 Of course, the autogenerated operations may not do exactly what the user
685 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
690 atan2 cos sin exp log sqrt int
692 Additionally, you need to define at least one of string, boolean or
693 numeric conversions because any one can be used to emulate the others.
694 The string conversion can also be used to emulate concatenation.
696 =head1 Losing overloading
698 The restriction for the comparison operation is that even if, for example,
699 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
700 function will produce only a standard logical value based on the
701 numerical value of the result of `C<cmp>'. In particular, a working
702 numeric conversion is needed in this case (possibly expressed in terms of
705 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
706 if the string conversion substitution is applied.
708 When you chop() a mathemagical object it is promoted to a string and its
709 mathemagical properties are lost. The same can happen with other
712 =head1 Run-time Overloading
714 Since all C<use> directives are executed at compile-time, the only way to
715 change overloading during run-time is to
717 eval 'use overload "+" => \&addmethod';
721 eval 'no overload "+", "--", "<="';
723 though the use of these constructs during run-time is questionable.
725 =head1 Public functions
727 Package C<overload.pm> provides the following public functions:
731 =item overload::StrVal(arg)
733 Gives string value of C<arg> as in absence of stringify overloading. If you
734 are using this to get the address of a reference (useful for checking if two
735 references point to the same thing) then you may be better off using
736 C<Scalar::Util::refaddr()>, which is faster.
738 =item overload::Overloaded(arg)
740 Returns true if C<arg> is subject to overloading of some operations.
742 =item overload::Method(obj,op)
744 Returns C<undef> or a reference to the method that implements C<op>.
748 =head1 Overloading constants
750 For some applications, the Perl parser mangles constants too much.
751 It is possible to hook into this process via C<overload::constant()>
752 and C<overload::remove_constant()> functions.
754 These functions take a hash as an argument. The recognized keys of this hash
761 to overload integer constants,
765 to overload floating point constants,
769 to overload octal and hexadecimal constants,
773 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
774 strings and here-documents,
778 to overload constant pieces of regular expressions.
782 The corresponding values are references to functions which take three arguments:
783 the first one is the I<initial> string form of the constant, the second one
784 is how Perl interprets this constant, the third one is how the constant is used.
785 Note that the initial string form does not
786 contain string delimiters, and has backslashes in backslash-delimiter
787 combinations stripped (thus the value of delimiter is not relevant for
788 processing of this string). The return value of this function is how this
789 constant is going to be interpreted by Perl. The third argument is undefined
790 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
791 context (comes from strings, regular expressions, and single-quote HERE
792 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
793 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
795 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
796 it is expected that overloaded constant strings are equipped with reasonable
797 overloaded catenation operator, otherwise absurd results will result.
798 Similarly, negative numbers are considered as negations of positive constants.
800 Note that it is probably meaningless to call the functions overload::constant()
801 and overload::remove_constant() from anywhere but import() and unimport() methods.
802 From these methods they may be called as
807 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
808 overload::constant integer => sub {Math::BigInt->new(shift)};
811 =head1 IMPLEMENTATION
813 What follows is subject to change RSN.
815 The table of methods for all operations is cached in magic for the
816 symbol table hash for the package. The cache is invalidated during
817 processing of C<use overload>, C<no overload>, new function
818 definitions, and changes in @ISA. However, this invalidation remains
819 unprocessed until the next C<bless>ing into the package. Hence if you
820 want to change overloading structure dynamically, you'll need an
821 additional (fake) C<bless>ing to update the table.
823 (Every SVish thing has a magic queue, and magic is an entry in that
824 queue. This is how a single variable may participate in multiple
825 forms of magic simultaneously. For instance, environment variables
826 regularly have two forms at once: their %ENV magic and their taint
827 magic. However, the magic which implements overloading is applied to
828 the stashes, which are rarely used directly, thus should not slow down
831 If an object belongs to a package using overload, it carries a special
832 flag. Thus the only speed penalty during arithmetic operations without
833 overloading is the checking of this flag.
835 In fact, if C<use overload> is not present, there is almost no overhead
836 for overloadable operations, so most programs should not suffer
837 measurable performance penalties. A considerable effort was made to
838 minimize the overhead when overload is used in some package, but the
839 arguments in question do not belong to packages using overload. When
840 in doubt, test your speed with C<use overload> and without it. So far
841 there have been no reports of substantial speed degradation if Perl is
842 compiled with optimization turned on.
844 There is no size penalty for data if overload is not used. The only
845 size penalty if overload is used in some package is that I<all> the
846 packages acquire a magic during the next C<bless>ing into the
847 package. This magic is three-words-long for packages without
848 overloading, and carries the cache table if the package is overloaded.
850 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
851 carried out before any operation that can imply an assignment to the
852 object $a (or $b) refers to, like C<$a++>. You can override this
853 behavior by defining your own copy constructor (see L<"Copy Constructor">).
855 It is expected that arguments to methods that are not explicitly supposed
856 to be changed are constant (but this is not enforced).
858 =head1 Metaphor clash
860 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
861 If it I<looks> counter intuitive to you, you are subject to a metaphor
864 Here is a Perl object metaphor:
866 I< object is a reference to blessed data>
868 and an arithmetic metaphor:
870 I< object is a thing by itself>.
872 The I<main> problem of overloading C<=> is the fact that these metaphors
873 imply different actions on the assignment C<$a = $b> if $a and $b are
874 objects. Perl-think implies that $a becomes a reference to whatever
875 $b was referencing. Arithmetic-think implies that the value of "object"
876 $a is changed to become the value of the object $b, preserving the fact
877 that $a and $b are separate entities.
879 The difference is not relevant in the absence of mutators. After
880 a Perl-way assignment an operation which mutates the data referenced by $a
881 would change the data referenced by $b too. Effectively, after
882 C<$a = $b> values of $a and $b become I<indistinguishable>.
884 On the other hand, anyone who has used algebraic notation knows the
885 expressive power of the arithmetic metaphor. Overloading works hard
886 to enable this metaphor while preserving the Perlian way as far as
887 possible. Since it is not possible to freely mix two contradicting
888 metaphors, overloading allows the arithmetic way to write things I<as
889 far as all the mutators are called via overloaded access only>. The
890 way it is done is described in L<Copy Constructor>.
892 If some mutator methods are directly applied to the overloaded values,
893 one may need to I<explicitly unlink> other values which references the
898 $b = $a; # $b is "linked" to $a
900 $a = $a->clone; # Unlink $b from $a
903 Note that overloaded access makes this transparent:
906 $b = $a; # $b is "linked" to $a
907 $a += 4; # would unlink $b automagically
909 However, it would not make
912 $a = 4; # Now $a is a plain 4, not 'Data'
914 preserve "objectness" of $a. But Perl I<has> a way to make assignments
915 to an object do whatever you want. It is just not the overload, but
916 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
917 which returns the object itself, and STORE() method which changes the
918 value of the object, one can reproduce the arithmetic metaphor in its
919 completeness, at least for variables which were tie()d from the start.
921 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
925 Please add examples to what follows!
927 =head2 Two-face scalars
929 Put this in F<two_face.pm> in your Perl library directory:
931 package two_face; # Scalars with separate string and
933 sub new { my $p = shift; bless [@_], $p }
934 use overload '""' => \&str, '0+' => \&num, fallback => 1;
941 my $seven = new two_face ("vii", 7);
942 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
943 print "seven contains `i'\n" if $seven =~ /i/;
945 (The second line creates a scalar which has both a string value, and a
946 numeric value.) This prints:
948 seven=vii, seven=7, eight=8
951 =head2 Two-face references
953 Suppose you want to create an object which is accessible as both an
954 array reference and a hash reference.
957 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
965 tie %h, ref $self, $self;
969 sub TIEHASH { my $p = shift; bless \ shift, $p }
972 $fields{$_} = $i++ foreach qw{zero one two three};
974 my $self = ${shift()};
975 my $key = $fields{shift()};
976 defined $key or die "Out of band access";
977 $$self->[$key] = shift;
980 my $self = ${shift()};
981 my $key = $fields{shift()};
982 defined $key or die "Out of band access";
986 Now one can access an object using both the array and hash syntax:
988 my $bar = new two_refs 3,4,5,6;
990 $bar->{two} == 11 or die 'bad hash fetch';
992 Note several important features of this example. First of all, the
993 I<actual> type of $bar is a scalar reference, and we do not overload
994 the scalar dereference. Thus we can get the I<actual> non-overloaded
995 contents of $bar by just using C<$$bar> (what we do in functions which
996 overload dereference). Similarly, the object returned by the
997 TIEHASH() method is a scalar reference.
999 Second, we create a new tied hash each time the hash syntax is used.
1000 This allows us not to worry about a possibility of a reference loop,
1001 which would lead to a memory leak.
1003 Both these problems can be cured. Say, if we want to overload hash
1004 dereference on a reference to an object which is I<implemented> as a
1005 hash itself, the only problem one has to circumvent is how to access
1006 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1007 overloaded dereference operator). Here is one possible fetching routine:
1010 my ($self, $key) = (shift, shift);
1011 my $class = ref $self;
1012 bless $self, 'overload::dummy'; # Disable overloading of %{}
1013 my $out = $self->{$key};
1014 bless $self, $class; # Restore overloading
1018 To remove creation of the tied hash on each access, one may an extra
1019 level of indirection which allows a non-circular structure of references:
1022 use overload '%{}' => sub { ${shift()}->[1] },
1023 '@{}' => sub { ${shift()}->[0] };
1029 bless \ [$a, \%h], $p;
1034 tie %h, ref $self, $self;
1038 sub TIEHASH { my $p = shift; bless \ shift, $p }
1041 $fields{$_} = $i++ foreach qw{zero one two three};
1044 my $key = $fields{shift()};
1045 defined $key or die "Out of band access";
1050 my $key = $fields{shift()};
1051 defined $key or die "Out of band access";
1055 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1056 reference to the intermediate array, which keeps a reference to an
1057 actual array, and the access hash. The tie()ing object for the access
1058 hash is a reference to a reference to the actual array, so
1064 There are no loops of references.
1068 Both "objects" which are blessed into the class C<two_refs1> are
1069 references to a reference to an array, thus references to a I<scalar>.
1070 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1071 overloaded operations.
1075 =head2 Symbolic calculator
1077 Put this in F<symbolic.pm> in your Perl library directory:
1079 package symbolic; # Primitive symbolic calculator
1080 use overload nomethod => \&wrap;
1082 sub new { shift; bless ['n', @_] }
1084 my ($obj, $other, $inv, $meth) = @_;
1085 ($obj, $other) = ($other, $obj) if $inv;
1086 bless [$meth, $obj, $other];
1089 This module is very unusual as overloaded modules go: it does not
1090 provide any usual overloaded operators, instead it provides the L<Last
1091 Resort> operator C<nomethod>. In this example the corresponding
1092 subroutine returns an object which encapsulates operations done over
1093 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1094 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1096 Here is an example of the script which "calculates" the side of
1097 circumscribed octagon using the above package:
1100 my $iter = 1; # 2**($iter+2) = 8
1101 my $side = new symbolic 1;
1105 $side = (sqrt(1 + $side**2) - 1)/$side;
1109 The value of $side is
1111 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1112 undef], 1], ['n', 1]]
1114 Note that while we obtained this value using a nice little script,
1115 there is no simple way to I<use> this value. In fact this value may
1116 be inspected in debugger (see L<perldebug>), but only if
1117 C<bareStringify> B<O>ption is set, and not via C<p> command.
1119 If one attempts to print this value, then the overloaded operator
1120 C<""> will be called, which will call C<nomethod> operator. The
1121 result of this operator will be stringified again, but this result is
1122 again of type C<symbolic>, which will lead to an infinite loop.
1124 Add a pretty-printer method to the module F<symbolic.pm>:
1127 my ($meth, $a, $b) = @{+shift};
1128 $a = 'u' unless defined $a;
1129 $b = 'u' unless defined $b;
1130 $a = $a->pretty if ref $a;
1131 $b = $b->pretty if ref $b;
1135 Now one can finish the script by
1137 print "side = ", $side->pretty, "\n";
1139 The method C<pretty> is doing object-to-string conversion, so it
1140 is natural to overload the operator C<""> using this method. However,
1141 inside such a method it is not necessary to pretty-print the
1142 I<components> $a and $b of an object. In the above subroutine
1143 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1144 and $b. If these components use overloading, the catenation operator
1145 will look for an overloaded operator C<.>; if not present, it will
1146 look for an overloaded operator C<"">. Thus it is enough to use
1148 use overload nomethod => \&wrap, '""' => \&str;
1150 my ($meth, $a, $b) = @{+shift};
1151 $a = 'u' unless defined $a;
1152 $b = 'u' unless defined $b;
1156 Now one can change the last line of the script to
1158 print "side = $side\n";
1162 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1164 and one can inspect the value in debugger using all the possible
1167 Something is still amiss: consider the loop variable $cnt of the
1168 script. It was a number, not an object. We cannot make this value of
1169 type C<symbolic>, since then the loop will not terminate.
1171 Indeed, to terminate the cycle, the $cnt should become false.
1172 However, the operator C<bool> for checking falsity is overloaded (this
1173 time via overloaded C<"">), and returns a long string, thus any object
1174 of type C<symbolic> is true. To overcome this, we need a way to
1175 compare an object to 0. In fact, it is easier to write a numeric
1178 Here is the text of F<symbolic.pm> with such a routine added (and
1179 slightly modified str()):
1181 package symbolic; # Primitive symbolic calculator
1183 nomethod => \&wrap, '""' => \&str, '0+' => \#
1185 sub new { shift; bless ['n', @_] }
1187 my ($obj, $other, $inv, $meth) = @_;
1188 ($obj, $other) = ($other, $obj) if $inv;
1189 bless [$meth, $obj, $other];
1192 my ($meth, $a, $b) = @{+shift};
1193 $a = 'u' unless defined $a;
1200 my %subr = ( n => sub {$_[0]},
1201 sqrt => sub {sqrt $_[0]},
1202 '-' => sub {shift() - shift()},
1203 '+' => sub {shift() + shift()},
1204 '/' => sub {shift() / shift()},
1205 '*' => sub {shift() * shift()},
1206 '**' => sub {shift() ** shift()},
1209 my ($meth, $a, $b) = @{+shift};
1210 my $subr = $subr{$meth}
1211 or die "Do not know how to ($meth) in symbolic";
1212 $a = $a->num if ref $a eq __PACKAGE__;
1213 $b = $b->num if ref $b eq __PACKAGE__;
1217 All the work of numeric conversion is done in %subr and num(). Of
1218 course, %subr is not complete, it contains only operators used in the
1219 example below. Here is the extra-credit question: why do we need an
1220 explicit recursion in num()? (Answer is at the end of this section.)
1222 Use this module like this:
1225 my $iter = new symbolic 2; # 16-gon
1226 my $side = new symbolic 1;
1230 $cnt = $cnt - 1; # Mutator `--' not implemented
1231 $side = (sqrt(1 + $side**2) - 1)/$side;
1233 printf "%s=%f\n", $side, $side;
1234 printf "pi=%f\n", $side*(2**($iter+2));
1236 It prints (without so many line breaks)
1238 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1240 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1243 The above module is very primitive. It does not implement
1244 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1245 (not required without mutators!), and implements only those arithmetic
1246 operations which are used in the example.
1248 To implement most arithmetic operations is easy; one should just use
1249 the tables of operations, and change the code which fills %subr to
1251 my %subr = ( 'n' => sub {$_[0]} );
1252 foreach my $op (split " ", $overload::ops{with_assign}) {
1253 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1255 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1256 foreach my $op (split " ", "@overload::ops{ @bins }") {
1257 $subr{$op} = eval "sub {shift() $op shift()}";
1259 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1260 print "defining `$op'\n";
1261 $subr{$op} = eval "sub {$op shift()}";
1264 Due to L<Calling Conventions for Mutators>, we do not need anything
1265 special to make C<+=> and friends work, except filling C<+=> entry of
1266 %subr, and defining a copy constructor (needed since Perl has no
1267 way to know that the implementation of C<'+='> does not mutate
1268 the argument, compare L<Copy Constructor>).
1270 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1271 line, and code (this code assumes that mutators change things one level
1272 deep only, so recursive copying is not needed):
1276 bless [@$self], ref $self;
1279 To make C<++> and C<--> work, we need to implement actual mutators,
1280 either directly, or in C<nomethod>. We continue to do things inside
1281 C<nomethod>, thus add
1283 if ($meth eq '++' or $meth eq '--') {
1284 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1288 after the first line of wrap(). This is not a most effective
1289 implementation, one may consider
1291 sub inc { $_[0] = bless ['++', shift, 1]; }
1295 As a final remark, note that one can fill %subr by
1297 my %subr = ( 'n' => sub {$_[0]} );
1298 foreach my $op (split " ", $overload::ops{with_assign}) {
1299 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1301 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1302 foreach my $op (split " ", "@overload::ops{ @bins }") {
1303 $subr{$op} = eval "sub {shift() $op shift()}";
1305 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1306 $subr{$op} = eval "sub {$op shift()}";
1308 $subr{'++'} = $subr{'+'};
1309 $subr{'--'} = $subr{'-'};
1311 This finishes implementation of a primitive symbolic calculator in
1312 50 lines of Perl code. Since the numeric values of subexpressions
1313 are not cached, the calculator is very slow.
1315 Here is the answer for the exercise: In the case of str(), we need no
1316 explicit recursion since the overloaded C<.>-operator will fall back
1317 to an existing overloaded operator C<"">. Overloaded arithmetic
1318 operators I<do not> fall back to numeric conversion if C<fallback> is
1319 not explicitly requested. Thus without an explicit recursion num()
1320 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1321 the argument of num().
1323 If you wonder why defaults for conversion are different for str() and
1324 num(), note how easy it was to write the symbolic calculator. This
1325 simplicity is due to an appropriate choice of defaults. One extra
1326 note: due to the explicit recursion num() is more fragile than sym():
1327 we need to explicitly check for the type of $a and $b. If components
1328 $a and $b happen to be of some related type, this may lead to problems.
1330 =head2 I<Really> symbolic calculator
1332 One may wonder why we call the above calculator symbolic. The reason
1333 is that the actual calculation of the value of expression is postponed
1334 until the value is I<used>.
1336 To see it in action, add a method
1341 @$obj->[0,1] = ('=', shift);
1344 to the package C<symbolic>. After this change one can do
1346 my $a = new symbolic 3;
1347 my $b = new symbolic 4;
1348 my $c = sqrt($a**2 + $b**2);
1350 and the numeric value of $c becomes 5. However, after calling
1352 $a->STORE(12); $b->STORE(5);
1354 the numeric value of $c becomes 13. There is no doubt now that the module
1355 symbolic provides a I<symbolic> calculator indeed.
1357 To hide the rough edges under the hood, provide a tie()d interface to the
1358 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1360 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1362 sub nop { } # Around a bug
1364 (the bug is described in L<"BUGS">). One can use this new interface as
1366 tie $a, 'symbolic', 3;
1367 tie $b, 'symbolic', 4;
1368 $a->nop; $b->nop; # Around a bug
1370 my $c = sqrt($a**2 + $b**2);
1372 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1373 of $c becomes 13. To insulate the user of the module add a method
1375 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1380 symbolic->vars($a, $b);
1381 my $c = sqrt($a**2 + $b**2);
1384 printf "c5 %s=%f\n", $c, $c;
1387 printf "c13 %s=%f\n", $c, $c;
1389 shows that the numeric value of $c follows changes to the values of $a
1394 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1398 When Perl is run with the B<-Do> switch or its equivalent, overloading
1399 induces diagnostic messages.
1401 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1402 deduce which operations are overloaded (and which ancestor triggers
1403 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1404 is shown by debugger. The method C<()> corresponds to the C<fallback>
1405 key (in fact a presence of this method shows that this package has
1406 overloading enabled, and it is what is used by the C<Overloaded>
1407 function of module C<overload>).
1409 The module might issue the following warnings:
1413 =item Odd number of arguments for overload::constant
1415 (W) The call to overload::constant contained an odd number of arguments.
1416 The arguments should come in pairs.
1418 =item `%s' is not an overloadable type
1420 (W) You tried to overload a constant type the overload package is unaware of.
1422 =item `%s' is not a code reference
1424 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1425 to be a code reference. Either an anonymous subroutine, or a reference
1432 Because it is used for overloading, the per-package hash %OVERLOAD now
1433 has a special meaning in Perl. The symbol table is filled with names
1434 looking like line-noise.
1436 For the purpose of inheritance every overloaded package behaves as if
1437 C<fallback> is present (possibly undefined). This may create
1438 interesting effects if some package is not overloaded, but inherits
1439 from two overloaded packages.
1441 Relation between overloading and tie()ing is broken. Overloading is
1442 triggered or not basing on the I<previous> class of tie()d value.
1444 This happens because the presence of overloading is checked too early,
1445 before any tie()d access is attempted. If the FETCH()ed class of the
1446 tie()d value does not change, a simple workaround is to access the value
1447 immediately after tie()ing, so that after this call the I<previous> class
1448 coincides with the current one.
1450 B<Needed:> a way to fix this without a speed penalty.
1452 Barewords are not covered by overloaded string constants.
1454 This document is confusing. There are grammos and misleading language
1455 used in places. It would seem a total rewrite is needed.