5 $overload::hint_bits = 0x20000; # HINT_LOCALIZE_HH
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;
77 $package = ref $package if ref $package;
78 #my $meth = $package->can('(' . shift);
79 ov_method mycan($package, '(' . shift), $package;
80 #return $meth if $meth ne \&nil;
85 my $package = ref $_[0];
86 return "$_[0]" unless $package;
89 my $class = Scalar::Util::blessed($_[0]);
90 my $class_prefix = defined($class) ? "$class=" : "";
91 my $type = Scalar::Util::reftype($_[0]);
92 my $addr = Scalar::Util::refaddr($_[0]);
93 return sprintf("$class_prefix$type(0x%x)", $addr);
98 sub mycan { # Real can would leave stubs.
99 my ($package, $meth) = @_;
100 return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
102 foreach $p (@{$package . "::ISA"}) {
103 my $out = mycan($p, $meth);
110 'integer' => 0x1000, # HINT_NEW_INTEGER
111 'float' => 0x2000, # HINT_NEW_FLOAT
112 'binary' => 0x4000, # HINT_NEW_BINARY
113 'q' => 0x8000, # HINT_NEW_STRING
114 'qr' => 0x10000, # HINT_NEW_RE
117 %ops = ( with_assign => "+ - * / % ** << >> x .",
118 assign => "+= -= *= /= %= **= <<= >>= x= .=",
119 num_comparison => "< <= > >= == !=",
120 '3way_comparison'=> "<=> cmp",
121 str_comparison => "lt le gt ge eq ne",
125 func => "atan2 cos sin exp abs log sqrt int",
126 conversion => 'bool "" 0+',
128 dereferencing => '${} @{} %{} &{} *{}',
129 special => 'nomethod fallback =');
131 use warnings::register;
133 # Arguments: what, sub
136 warnings::warnif ("Odd number of arguments for overload::constant");
139 elsif (!exists $constants {$_ [0]}) {
140 warnings::warnif ("`$_[0]' is not an overloadable type");
142 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
143 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
144 # blessed, and C<ref> would return the package the ref is blessed into.
145 if (warnings::enabled) {
146 $_ [1] = "undef" unless defined $_ [1];
147 warnings::warn ("`$_[1]' is not a code reference");
152 $^H |= $constants{$_[0]} | $overload::hint_bits;
158 sub remove_constant {
159 # Arguments: what, sub
162 $^H &= ~ $constants{$_[0]};
173 overload - Package for overloading Perl operations
186 $a = new SomeThing 57;
189 if (overload::Overloaded $b) {...}
191 $strval = overload::StrVal $b;
195 =head2 Declaration of overloaded functions
197 The compilation directive
204 declares function Number::add() for addition, and method muas() in
205 the "class" C<Number> (or one of its base classes)
206 for the assignment form C<*=> of multiplication.
208 Arguments of this directive come in (key, value) pairs. Legal values
209 are values legal inside a C<&{ ... }> call, so the name of a
210 subroutine, a reference to a subroutine, or an anonymous subroutine
211 will all work. Note that values specified as strings are
212 interpreted as methods, not subroutines. Legal keys are listed below.
214 The subroutine C<add> will be called to execute C<$a+$b> if $a
215 is a reference to an object blessed into the package C<Number>, or if $a is
216 not an object from a package with defined mathemagic addition, but $b is a
217 reference to a C<Number>. It can also be called in other situations, like
218 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
219 methods refer to methods triggered by an overloaded mathematical
222 Since overloading respects inheritance via the @ISA hierarchy, the
223 above declaration would also trigger overloading of C<+> and C<*=> in
224 all the packages which inherit from C<Number>.
226 =head2 Calling Conventions for Binary Operations
228 The functions specified in the C<use overload ...> directive are called
229 with three (in one particular case with four, see L<Last Resort>)
230 arguments. If the corresponding operation is binary, then the first
231 two arguments are the two arguments of the operation. However, due to
232 general object calling conventions, the first argument should always be
233 an object in the package, so in the situation of C<7+$a>, the
234 order of the arguments is interchanged. It probably does not matter
235 when implementing the addition method, but whether the arguments
236 are reversed is vital to the subtraction method. The method can
237 query this information by examining the third argument, which can take
238 three different values:
244 the order of arguments is as in the current operation.
248 the arguments are reversed.
252 the current operation is an assignment variant (as in
253 C<$a+=7>), but the usual function is called instead. This additional
254 information can be used to generate some optimizations. Compare
255 L<Calling Conventions for Mutators>.
259 =head2 Calling Conventions for Unary Operations
261 Unary operation are considered binary operations with the second
262 argument being C<undef>. Thus the functions that overloads C<{"++"}>
263 is called with arguments C<($a,undef,'')> when $a++ is executed.
265 =head2 Calling Conventions for Mutators
267 Two types of mutators have different calling conventions:
271 =item C<++> and C<-->
273 The routines which implement these operators are expected to actually
274 I<mutate> their arguments. So, assuming that $obj is a reference to a
277 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
279 is an appropriate implementation of overloaded C<++>. Note that
281 sub incr { ++$ {$_[0]} ; shift }
283 is OK if used with preincrement and with postincrement. (In the case
284 of postincrement a copying will be performed, see L<Copy Constructor>.)
286 =item C<x=> and other assignment versions
288 There is nothing special about these methods. They may change the
289 value of their arguments, and may leave it as is. The result is going
290 to be assigned to the value in the left-hand-side if different from
293 This allows for the same method to be used as overloaded C<+=> and
294 C<+>. Note that this is I<allowed>, but not recommended, since by the
295 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
296 if C<+=> is not overloaded.
300 B<Warning.> Due to the presence of assignment versions of operations,
301 routines which may be called in assignment context may create
302 self-referential structures. Currently Perl will not free self-referential
303 structures until cycles are C<explicitly> broken. You may get problems
304 when traversing your structures too.
308 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
310 is asking for trouble, since for code C<$obj += $foo> the subroutine
311 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
312 \$foo]>. If using such a subroutine is an important optimization, one
313 can overload C<+=> explicitly by a non-"optimized" version, or switch
314 to non-optimized version if C<not defined $_[2]> (see
315 L<Calling Conventions for Binary Operations>).
317 Even if no I<explicit> assignment-variants of operators are present in
318 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
319 C<',' . $obj . ','> may be both optimized to
321 my $tmp = ',' . $obj; $tmp .= ',';
323 =head2 Overloadable Operations
325 The following symbols can be specified in C<use overload> directive:
329 =item * I<Arithmetic operations>
331 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
332 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
334 For these operations a substituted non-assignment variant can be called if
335 the assignment variant is not available. Methods for operations C<+>,
336 C<->, C<+=>, and C<-=> can be called to automatically generate
337 increment and decrement methods. The operation C<-> can be used to
338 autogenerate missing methods for unary minus or C<abs>.
340 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
341 L<"Calling Conventions for Binary Operations">) for details of these
344 =item * I<Comparison operations>
346 "<", "<=", ">", ">=", "==", "!=", "<=>",
347 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
349 If the corresponding "spaceship" variant is available, it can be
350 used to substitute for the missing operation. During C<sort>ing
351 arrays, C<cmp> is used to compare values subject to C<use overload>.
353 =item * I<Bit operations>
355 "&", "^", "|", "neg", "!", "~",
357 C<neg> stands for unary minus. If the method for C<neg> is not
358 specified, it can be autogenerated using the method for
359 subtraction. If the method for C<!> is not specified, it can be
360 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
362 =item * I<Increment and decrement>
366 If undefined, addition and subtraction methods can be
367 used instead. These operations are called both in prefix and
370 =item * I<Transcendental functions>
372 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
374 If C<abs> is unavailable, it can be autogenerated using methods
375 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
377 Note that traditionally the Perl function L<int> rounds to 0, thus for
378 floating-point-like types one should follow the same semantic. If
379 C<int> is unavailable, it can be autogenerated using the overloading of
382 =item * I<Boolean, string and numeric conversion>
386 If one or two of these operations are not overloaded, the remaining ones can
387 be used instead. C<bool> is used in the flow control operators
388 (like C<while>) and for the ternary C<?:> operation. These functions can
389 return any arbitrary Perl value. If the corresponding operation for this value
390 is overloaded too, that operation will be called again with this value.
392 As a special case if the overload returns the object itself then it will
393 be used directly. An overloaded conversion returning the object is
394 probably a bug, because you're likely to get something that looks like
395 C<YourPackage=HASH(0x8172b34)>.
401 If not overloaded, the argument will be converted to a filehandle or
402 glob (which may require a stringification). The same overloading
403 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
404 I<globbing> syntax C<E<lt>${var}E<gt>>.
406 B<BUGS> Even in list context, the iterator is currently called only
407 once and with scalar context.
409 =item * I<Dereferencing>
411 '${}', '@{}', '%{}', '&{}', '*{}'.
413 If not overloaded, the argument will be dereferenced I<as is>, thus
414 should be of correct type. These functions should return a reference
415 of correct type, or another object with overloaded dereferencing.
417 As a special case if the overload returns the object itself then it
418 will be used directly (provided it is the correct type).
420 The dereference operators must be specified explicitly they will not be passed to
425 "nomethod", "fallback", "=", "~~",
427 see L<SPECIAL SYMBOLS FOR C<use overload>>.
431 See L<"Fallback"> for an explanation of when a missing method can be
434 A computer-readable form of the above table is available in the hash
435 %overload::ops, with values being space-separated lists of names:
437 with_assign => '+ - * / % ** << >> x .',
438 assign => '+= -= *= /= %= **= <<= >>= x= .=',
439 num_comparison => '< <= > >= == !=',
440 '3way_comparison'=> '<=> cmp',
441 str_comparison => 'lt le gt ge eq ne',
445 func => 'atan2 cos sin exp abs log sqrt',
446 conversion => 'bool "" 0+',
448 dereferencing => '${} @{} %{} &{} *{}',
449 special => 'nomethod fallback ='
451 =head2 Inheritance and overloading
453 Inheritance interacts with overloading in two ways.
457 =item Strings as values of C<use overload> directive
461 use overload key => value;
463 is a string, it is interpreted as a method name.
465 =item Overloading of an operation is inherited by derived classes
467 Any class derived from an overloaded class is also overloaded. The
468 set of overloaded methods is the union of overloaded methods of all
469 the ancestors. If some method is overloaded in several ancestor, then
470 which description will be used is decided by the usual inheritance
473 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
474 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
475 then the subroutine C<D::plus_sub> will be called to implement
476 operation C<+> for an object in package C<A>.
480 Note that since the value of the C<fallback> key is not a subroutine,
481 its inheritance is not governed by the above rules. In the current
482 implementation, the value of C<fallback> in the first overloaded
483 ancestor is used, but this is accidental and subject to change.
485 =head1 SPECIAL SYMBOLS FOR C<use overload>
487 Three keys are recognized by Perl that are not covered by the above
492 C<"nomethod"> should be followed by a reference to a function of four
493 parameters. If defined, it is called when the overloading mechanism
494 cannot find a method for some operation. The first three arguments of
495 this function coincide with the arguments for the corresponding method if
496 it were found, the fourth argument is the symbol
497 corresponding to the missing method. If several methods are tried,
498 the last one is used. Say, C<1-$a> can be equivalent to
500 &nomethodMethod($a,1,1,"-")
502 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
503 C<use overload> directive.
505 The C<"nomethod"> mechanism is I<not> used for the dereference operators
506 ( ${} @{} %{} &{} *{} ).
509 If some operation cannot be resolved, and there is no function
510 assigned to C<"nomethod">, then an exception will be raised via die()--
511 unless C<"fallback"> was specified as a key in C<use overload> directive.
516 The key C<"fallback"> governs what to do if a method for a particular
517 operation is not found. Three different cases are possible depending on
518 the value of C<"fallback">:
522 The key C<"~~"> allows you to override the smart matching used by
523 the switch construct. See L<feature>.
530 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
531 then tries to calls C<"nomethod"> value; if missing, an exception
536 The same as for the C<undef> value, but no exception is raised. Instead,
537 it silently reverts to what it would have done were there no C<use overload>
540 =item * defined, but FALSE
542 No autogeneration is tried. Perl tries to call
543 C<"nomethod"> value, and if this is missing, raises an exception.
547 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
548 yet, see L<"Inheritance and overloading">.
550 =head2 Copy Constructor
552 The value for C<"="> is a reference to a function with three
553 arguments, i.e., it looks like the other values in C<use
554 overload>. However, it does not overload the Perl assignment
555 operator. This would go against Camel hair.
557 This operation is called in the situations when a mutator is applied
558 to a reference that shares its object with some other reference, such
564 To make this change $a and not change $b, a copy of C<$$a> is made,
565 and $a is assigned a reference to this new object. This operation is
566 done during execution of the C<++$a>, and not during the assignment,
567 (so before the increment C<$$a> coincides with C<$$b>). This is only
568 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
569 C<nomethod>). Note that if this operation is expressed via C<'+'>
570 a nonmutator, i.e., as in
575 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
576 appear as lvalue when the above code is executed.
578 If the copy constructor is required during the execution of some mutator,
579 but a method for C<'='> was not specified, it can be autogenerated as a
580 string copy if the object is a plain scalar.
586 The actually executed code for
589 Something else which does not modify $a or $b....
595 Something else which does not modify $a or $b....
596 $a = $a->clone(undef,"");
599 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
600 C<'='> was overloaded with C<\&clone>.
604 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
607 =head1 MAGIC AUTOGENERATION
609 If a method for an operation is not found, and the value for C<"fallback"> is
610 TRUE or undefined, Perl tries to autogenerate a substitute method for
611 the missing operation based on the defined operations. Autogenerated method
612 substitutions are possible for the following operations:
616 =item I<Assignment forms of arithmetic operations>
618 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
621 =item I<Conversion operations>
623 String, numeric, and boolean conversion are calculated in terms of one
624 another if not all of them are defined.
626 =item I<Increment and decrement>
628 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
629 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
633 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
637 can be expressed in terms of subtraction.
641 C<!> and C<not> can be expressed in terms of boolean conversion, or
642 string or numerical conversion.
644 =item I<Concatenation>
646 can be expressed in terms of string conversion.
648 =item I<Comparison operations>
650 can be expressed in terms of its "spaceship" counterpart: either
651 C<E<lt>=E<gt>> or C<cmp>:
653 <, >, <=, >=, ==, != in terms of <=>
654 lt, gt, le, ge, eq, ne in terms of cmp
658 <> in terms of builtin operations
660 =item I<Dereferencing>
662 ${} @{} %{} &{} *{} in terms of builtin operations
664 =item I<Copy operator>
666 can be expressed in terms of an assignment to the dereferenced value, if this
667 value is a scalar and not a reference.
671 =head1 Minimal set of overloaded operations
673 Since some operations can be automatically generated from others, there is
674 a minimal set of operations that need to be overloaded in order to have
675 the complete set of overloaded operations at one's disposal.
676 Of course, the autogenerated operations may not do exactly what the user
677 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
682 atan2 cos sin exp log sqrt int
684 Additionally, you need to define at least one of string, boolean or
685 numeric conversions because any one can be used to emulate the others.
686 The string conversion can also be used to emulate concatenation.
688 =head1 Losing overloading
690 The restriction for the comparison operation is that even if, for example,
691 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
692 function will produce only a standard logical value based on the
693 numerical value of the result of `C<cmp>'. In particular, a working
694 numeric conversion is needed in this case (possibly expressed in terms of
697 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
698 if the string conversion substitution is applied.
700 When you chop() a mathemagical object it is promoted to a string and its
701 mathemagical properties are lost. The same can happen with other
704 =head1 Run-time Overloading
706 Since all C<use> directives are executed at compile-time, the only way to
707 change overloading during run-time is to
709 eval 'use overload "+" => \&addmethod';
713 eval 'no overload "+", "--", "<="';
715 though the use of these constructs during run-time is questionable.
717 =head1 Public functions
719 Package C<overload.pm> provides the following public functions:
723 =item overload::StrVal(arg)
725 Gives string value of C<arg> as in absence of stringify overloading. If you
726 are using this to get the address of a reference (useful for checking if two
727 references point to the same thing) then you may be better off using
728 C<Scalar::Util::refaddr()>, which is faster.
730 =item overload::Overloaded(arg)
732 Returns true if C<arg> is subject to overloading of some operations.
734 =item overload::Method(obj,op)
736 Returns C<undef> or a reference to the method that implements C<op>.
740 =head1 Overloading constants
742 For some applications, the Perl parser mangles constants too much.
743 It is possible to hook into this process via C<overload::constant()>
744 and C<overload::remove_constant()> functions.
746 These functions take a hash as an argument. The recognized keys of this hash
753 to overload integer constants,
757 to overload floating point constants,
761 to overload octal and hexadecimal constants,
765 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
766 strings and here-documents,
770 to overload constant pieces of regular expressions.
774 The corresponding values are references to functions which take three arguments:
775 the first one is the I<initial> string form of the constant, the second one
776 is how Perl interprets this constant, the third one is how the constant is used.
777 Note that the initial string form does not
778 contain string delimiters, and has backslashes in backslash-delimiter
779 combinations stripped (thus the value of delimiter is not relevant for
780 processing of this string). The return value of this function is how this
781 constant is going to be interpreted by Perl. The third argument is undefined
782 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
783 context (comes from strings, regular expressions, and single-quote HERE
784 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
785 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
787 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
788 it is expected that overloaded constant strings are equipped with reasonable
789 overloaded catenation operator, otherwise absurd results will result.
790 Similarly, negative numbers are considered as negations of positive constants.
792 Note that it is probably meaningless to call the functions overload::constant()
793 and overload::remove_constant() from anywhere but import() and unimport() methods.
794 From these methods they may be called as
799 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
800 overload::constant integer => sub {Math::BigInt->new(shift)};
803 =head1 IMPLEMENTATION
805 What follows is subject to change RSN.
807 The table of methods for all operations is cached in magic for the
808 symbol table hash for the package. The cache is invalidated during
809 processing of C<use overload>, C<no overload>, new function
810 definitions, and changes in @ISA. However, this invalidation remains
811 unprocessed until the next C<bless>ing into the package. Hence if you
812 want to change overloading structure dynamically, you'll need an
813 additional (fake) C<bless>ing to update the table.
815 (Every SVish thing has a magic queue, and magic is an entry in that
816 queue. This is how a single variable may participate in multiple
817 forms of magic simultaneously. For instance, environment variables
818 regularly have two forms at once: their %ENV magic and their taint
819 magic. However, the magic which implements overloading is applied to
820 the stashes, which are rarely used directly, thus should not slow down
823 If an object belongs to a package using overload, it carries a special
824 flag. Thus the only speed penalty during arithmetic operations without
825 overloading is the checking of this flag.
827 In fact, if C<use overload> is not present, there is almost no overhead
828 for overloadable operations, so most programs should not suffer
829 measurable performance penalties. A considerable effort was made to
830 minimize the overhead when overload is used in some package, but the
831 arguments in question do not belong to packages using overload. When
832 in doubt, test your speed with C<use overload> and without it. So far
833 there have been no reports of substantial speed degradation if Perl is
834 compiled with optimization turned on.
836 There is no size penalty for data if overload is not used. The only
837 size penalty if overload is used in some package is that I<all> the
838 packages acquire a magic during the next C<bless>ing into the
839 package. This magic is three-words-long for packages without
840 overloading, and carries the cache table if the package is overloaded.
842 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
843 carried out before any operation that can imply an assignment to the
844 object $a (or $b) refers to, like C<$a++>. You can override this
845 behavior by defining your own copy constructor (see L<"Copy Constructor">).
847 It is expected that arguments to methods that are not explicitly supposed
848 to be changed are constant (but this is not enforced).
850 =head1 Metaphor clash
852 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
853 If it I<looks> counter intuitive to you, you are subject to a metaphor
856 Here is a Perl object metaphor:
858 I< object is a reference to blessed data>
860 and an arithmetic metaphor:
862 I< object is a thing by itself>.
864 The I<main> problem of overloading C<=> is the fact that these metaphors
865 imply different actions on the assignment C<$a = $b> if $a and $b are
866 objects. Perl-think implies that $a becomes a reference to whatever
867 $b was referencing. Arithmetic-think implies that the value of "object"
868 $a is changed to become the value of the object $b, preserving the fact
869 that $a and $b are separate entities.
871 The difference is not relevant in the absence of mutators. After
872 a Perl-way assignment an operation which mutates the data referenced by $a
873 would change the data referenced by $b too. Effectively, after
874 C<$a = $b> values of $a and $b become I<indistinguishable>.
876 On the other hand, anyone who has used algebraic notation knows the
877 expressive power of the arithmetic metaphor. Overloading works hard
878 to enable this metaphor while preserving the Perlian way as far as
879 possible. Since it is not possible to freely mix two contradicting
880 metaphors, overloading allows the arithmetic way to write things I<as
881 far as all the mutators are called via overloaded access only>. The
882 way it is done is described in L<Copy Constructor>.
884 If some mutator methods are directly applied to the overloaded values,
885 one may need to I<explicitly unlink> other values which references the
890 $b = $a; # $b is "linked" to $a
892 $a = $a->clone; # Unlink $b from $a
895 Note that overloaded access makes this transparent:
898 $b = $a; # $b is "linked" to $a
899 $a += 4; # would unlink $b automagically
901 However, it would not make
904 $a = 4; # Now $a is a plain 4, not 'Data'
906 preserve "objectness" of $a. But Perl I<has> a way to make assignments
907 to an object do whatever you want. It is just not the overload, but
908 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
909 which returns the object itself, and STORE() method which changes the
910 value of the object, one can reproduce the arithmetic metaphor in its
911 completeness, at least for variables which were tie()d from the start.
913 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
917 Please add examples to what follows!
919 =head2 Two-face scalars
921 Put this in F<two_face.pm> in your Perl library directory:
923 package two_face; # Scalars with separate string and
925 sub new { my $p = shift; bless [@_], $p }
926 use overload '""' => \&str, '0+' => \&num, fallback => 1;
933 my $seven = new two_face ("vii", 7);
934 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
935 print "seven contains `i'\n" if $seven =~ /i/;
937 (The second line creates a scalar which has both a string value, and a
938 numeric value.) This prints:
940 seven=vii, seven=7, eight=8
943 =head2 Two-face references
945 Suppose you want to create an object which is accessible as both an
946 array reference and a hash reference.
949 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
957 tie %h, ref $self, $self;
961 sub TIEHASH { my $p = shift; bless \ shift, $p }
964 $fields{$_} = $i++ foreach qw{zero one two three};
966 my $self = ${shift()};
967 my $key = $fields{shift()};
968 defined $key or die "Out of band access";
969 $$self->[$key] = shift;
972 my $self = ${shift()};
973 my $key = $fields{shift()};
974 defined $key or die "Out of band access";
978 Now one can access an object using both the array and hash syntax:
980 my $bar = new two_refs 3,4,5,6;
982 $bar->{two} == 11 or die 'bad hash fetch';
984 Note several important features of this example. First of all, the
985 I<actual> type of $bar is a scalar reference, and we do not overload
986 the scalar dereference. Thus we can get the I<actual> non-overloaded
987 contents of $bar by just using C<$$bar> (what we do in functions which
988 overload dereference). Similarly, the object returned by the
989 TIEHASH() method is a scalar reference.
991 Second, we create a new tied hash each time the hash syntax is used.
992 This allows us not to worry about a possibility of a reference loop,
993 which would lead to a memory leak.
995 Both these problems can be cured. Say, if we want to overload hash
996 dereference on a reference to an object which is I<implemented> as a
997 hash itself, the only problem one has to circumvent is how to access
998 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
999 overloaded dereference operator). Here is one possible fetching routine:
1002 my ($self, $key) = (shift, shift);
1003 my $class = ref $self;
1004 bless $self, 'overload::dummy'; # Disable overloading of %{}
1005 my $out = $self->{$key};
1006 bless $self, $class; # Restore overloading
1010 To remove creation of the tied hash on each access, one may an extra
1011 level of indirection which allows a non-circular structure of references:
1014 use overload '%{}' => sub { ${shift()}->[1] },
1015 '@{}' => sub { ${shift()}->[0] };
1021 bless \ [$a, \%h], $p;
1026 tie %h, ref $self, $self;
1030 sub TIEHASH { my $p = shift; bless \ shift, $p }
1033 $fields{$_} = $i++ foreach qw{zero one two three};
1036 my $key = $fields{shift()};
1037 defined $key or die "Out of band access";
1042 my $key = $fields{shift()};
1043 defined $key or die "Out of band access";
1047 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1048 reference to the intermediate array, which keeps a reference to an
1049 actual array, and the access hash. The tie()ing object for the access
1050 hash is a reference to a reference to the actual array, so
1056 There are no loops of references.
1060 Both "objects" which are blessed into the class C<two_refs1> are
1061 references to a reference to an array, thus references to a I<scalar>.
1062 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1063 overloaded operations.
1067 =head2 Symbolic calculator
1069 Put this in F<symbolic.pm> in your Perl library directory:
1071 package symbolic; # Primitive symbolic calculator
1072 use overload nomethod => \&wrap;
1074 sub new { shift; bless ['n', @_] }
1076 my ($obj, $other, $inv, $meth) = @_;
1077 ($obj, $other) = ($other, $obj) if $inv;
1078 bless [$meth, $obj, $other];
1081 This module is very unusual as overloaded modules go: it does not
1082 provide any usual overloaded operators, instead it provides the L<Last
1083 Resort> operator C<nomethod>. In this example the corresponding
1084 subroutine returns an object which encapsulates operations done over
1085 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1086 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1088 Here is an example of the script which "calculates" the side of
1089 circumscribed octagon using the above package:
1092 my $iter = 1; # 2**($iter+2) = 8
1093 my $side = new symbolic 1;
1097 $side = (sqrt(1 + $side**2) - 1)/$side;
1101 The value of $side is
1103 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1104 undef], 1], ['n', 1]]
1106 Note that while we obtained this value using a nice little script,
1107 there is no simple way to I<use> this value. In fact this value may
1108 be inspected in debugger (see L<perldebug>), but ony if
1109 C<bareStringify> B<O>ption is set, and not via C<p> command.
1111 If one attempts to print this value, then the overloaded operator
1112 C<""> will be called, which will call C<nomethod> operator. The
1113 result of this operator will be stringified again, but this result is
1114 again of type C<symbolic>, which will lead to an infinite loop.
1116 Add a pretty-printer method to the module F<symbolic.pm>:
1119 my ($meth, $a, $b) = @{+shift};
1120 $a = 'u' unless defined $a;
1121 $b = 'u' unless defined $b;
1122 $a = $a->pretty if ref $a;
1123 $b = $b->pretty if ref $b;
1127 Now one can finish the script by
1129 print "side = ", $side->pretty, "\n";
1131 The method C<pretty> is doing object-to-string conversion, so it
1132 is natural to overload the operator C<""> using this method. However,
1133 inside such a method it is not necessary to pretty-print the
1134 I<components> $a and $b of an object. In the above subroutine
1135 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1136 and $b. If these components use overloading, the catenation operator
1137 will look for an overloaded operator C<.>; if not present, it will
1138 look for an overloaded operator C<"">. Thus it is enough to use
1140 use overload nomethod => \&wrap, '""' => \&str;
1142 my ($meth, $a, $b) = @{+shift};
1143 $a = 'u' unless defined $a;
1144 $b = 'u' unless defined $b;
1148 Now one can change the last line of the script to
1150 print "side = $side\n";
1154 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1156 and one can inspect the value in debugger using all the possible
1159 Something is still amiss: consider the loop variable $cnt of the
1160 script. It was a number, not an object. We cannot make this value of
1161 type C<symbolic>, since then the loop will not terminate.
1163 Indeed, to terminate the cycle, the $cnt should become false.
1164 However, the operator C<bool> for checking falsity is overloaded (this
1165 time via overloaded C<"">), and returns a long string, thus any object
1166 of type C<symbolic> is true. To overcome this, we need a way to
1167 compare an object to 0. In fact, it is easier to write a numeric
1170 Here is the text of F<symbolic.pm> with such a routine added (and
1171 slightly modified str()):
1173 package symbolic; # Primitive symbolic calculator
1175 nomethod => \&wrap, '""' => \&str, '0+' => \#
1177 sub new { shift; bless ['n', @_] }
1179 my ($obj, $other, $inv, $meth) = @_;
1180 ($obj, $other) = ($other, $obj) if $inv;
1181 bless [$meth, $obj, $other];
1184 my ($meth, $a, $b) = @{+shift};
1185 $a = 'u' unless defined $a;
1192 my %subr = ( n => sub {$_[0]},
1193 sqrt => sub {sqrt $_[0]},
1194 '-' => sub {shift() - shift()},
1195 '+' => sub {shift() + shift()},
1196 '/' => sub {shift() / shift()},
1197 '*' => sub {shift() * shift()},
1198 '**' => sub {shift() ** shift()},
1201 my ($meth, $a, $b) = @{+shift};
1202 my $subr = $subr{$meth}
1203 or die "Do not know how to ($meth) in symbolic";
1204 $a = $a->num if ref $a eq __PACKAGE__;
1205 $b = $b->num if ref $b eq __PACKAGE__;
1209 All the work of numeric conversion is done in %subr and num(). Of
1210 course, %subr is not complete, it contains only operators used in the
1211 example below. Here is the extra-credit question: why do we need an
1212 explicit recursion in num()? (Answer is at the end of this section.)
1214 Use this module like this:
1217 my $iter = new symbolic 2; # 16-gon
1218 my $side = new symbolic 1;
1222 $cnt = $cnt - 1; # Mutator `--' not implemented
1223 $side = (sqrt(1 + $side**2) - 1)/$side;
1225 printf "%s=%f\n", $side, $side;
1226 printf "pi=%f\n", $side*(2**($iter+2));
1228 It prints (without so many line breaks)
1230 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1232 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1235 The above module is very primitive. It does not implement
1236 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1237 (not required without mutators!), and implements only those arithmetic
1238 operations which are used in the example.
1240 To implement most arithmetic operations is easy; one should just use
1241 the tables of operations, and change the code which fills %subr to
1243 my %subr = ( 'n' => sub {$_[0]} );
1244 foreach my $op (split " ", $overload::ops{with_assign}) {
1245 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1247 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1248 foreach my $op (split " ", "@overload::ops{ @bins }") {
1249 $subr{$op} = eval "sub {shift() $op shift()}";
1251 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1252 print "defining `$op'\n";
1253 $subr{$op} = eval "sub {$op shift()}";
1256 Due to L<Calling Conventions for Mutators>, we do not need anything
1257 special to make C<+=> and friends work, except filling C<+=> entry of
1258 %subr, and defining a copy constructor (needed since Perl has no
1259 way to know that the implementation of C<'+='> does not mutate
1260 the argument, compare L<Copy Constructor>).
1262 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1263 line, and code (this code assumes that mutators change things one level
1264 deep only, so recursive copying is not needed):
1268 bless [@$self], ref $self;
1271 To make C<++> and C<--> work, we need to implement actual mutators,
1272 either directly, or in C<nomethod>. We continue to do things inside
1273 C<nomethod>, thus add
1275 if ($meth eq '++' or $meth eq '--') {
1276 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1280 after the first line of wrap(). This is not a most effective
1281 implementation, one may consider
1283 sub inc { $_[0] = bless ['++', shift, 1]; }
1287 As a final remark, note that one can fill %subr by
1289 my %subr = ( 'n' => sub {$_[0]} );
1290 foreach my $op (split " ", $overload::ops{with_assign}) {
1291 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1293 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1294 foreach my $op (split " ", "@overload::ops{ @bins }") {
1295 $subr{$op} = eval "sub {shift() $op shift()}";
1297 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1298 $subr{$op} = eval "sub {$op shift()}";
1300 $subr{'++'} = $subr{'+'};
1301 $subr{'--'} = $subr{'-'};
1303 This finishes implementation of a primitive symbolic calculator in
1304 50 lines of Perl code. Since the numeric values of subexpressions
1305 are not cached, the calculator is very slow.
1307 Here is the answer for the exercise: In the case of str(), we need no
1308 explicit recursion since the overloaded C<.>-operator will fall back
1309 to an existing overloaded operator C<"">. Overloaded arithmetic
1310 operators I<do not> fall back to numeric conversion if C<fallback> is
1311 not explicitly requested. Thus without an explicit recursion num()
1312 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1313 the argument of num().
1315 If you wonder why defaults for conversion are different for str() and
1316 num(), note how easy it was to write the symbolic calculator. This
1317 simplicity is due to an appropriate choice of defaults. One extra
1318 note: due to the explicit recursion num() is more fragile than sym():
1319 we need to explicitly check for the type of $a and $b. If components
1320 $a and $b happen to be of some related type, this may lead to problems.
1322 =head2 I<Really> symbolic calculator
1324 One may wonder why we call the above calculator symbolic. The reason
1325 is that the actual calculation of the value of expression is postponed
1326 until the value is I<used>.
1328 To see it in action, add a method
1333 @$obj->[0,1] = ('=', shift);
1336 to the package C<symbolic>. After this change one can do
1338 my $a = new symbolic 3;
1339 my $b = new symbolic 4;
1340 my $c = sqrt($a**2 + $b**2);
1342 and the numeric value of $c becomes 5. However, after calling
1344 $a->STORE(12); $b->STORE(5);
1346 the numeric value of $c becomes 13. There is no doubt now that the module
1347 symbolic provides a I<symbolic> calculator indeed.
1349 To hide the rough edges under the hood, provide a tie()d interface to the
1350 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1352 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1354 sub nop { } # Around a bug
1356 (the bug is described in L<"BUGS">). One can use this new interface as
1358 tie $a, 'symbolic', 3;
1359 tie $b, 'symbolic', 4;
1360 $a->nop; $b->nop; # Around a bug
1362 my $c = sqrt($a**2 + $b**2);
1364 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1365 of $c becomes 13. To insulate the user of the module add a method
1367 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1372 symbolic->vars($a, $b);
1373 my $c = sqrt($a**2 + $b**2);
1376 printf "c5 %s=%f\n", $c, $c;
1379 printf "c13 %s=%f\n", $c, $c;
1381 shows that the numeric value of $c follows changes to the values of $a
1386 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1390 When Perl is run with the B<-Do> switch or its equivalent, overloading
1391 induces diagnostic messages.
1393 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1394 deduce which operations are overloaded (and which ancestor triggers
1395 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1396 is shown by debugger. The method C<()> corresponds to the C<fallback>
1397 key (in fact a presence of this method shows that this package has
1398 overloading enabled, and it is what is used by the C<Overloaded>
1399 function of module C<overload>).
1401 The module might issue the following warnings:
1405 =item Odd number of arguments for overload::constant
1407 (W) The call to overload::constant contained an odd number of arguments.
1408 The arguments should come in pairs.
1410 =item `%s' is not an overloadable type
1412 (W) You tried to overload a constant type the overload package is unaware of.
1414 =item `%s' is not a code reference
1416 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1417 to be a code reference. Either an anonymous subroutine, or a reference
1424 Because it is used for overloading, the per-package hash %OVERLOAD now
1425 has a special meaning in Perl. The symbol table is filled with names
1426 looking like line-noise.
1428 For the purpose of inheritance every overloaded package behaves as if
1429 C<fallback> is present (possibly undefined). This may create
1430 interesting effects if some package is not overloaded, but inherits
1431 from two overloaded packages.
1433 Relation between overloading and tie()ing is broken. Overloading is
1434 triggered or not basing on the I<previous> class of tie()d value.
1436 This happens because the presence of overloading is checked too early,
1437 before any tie()d access is attempted. If the FETCH()ed class of the
1438 tie()d value does not change, a simple workaround is to access the value
1439 immediately after tie()ing, so that after this call the I<previous> class
1440 coincides with the current one.
1442 B<Needed:> a way to fix this without a speed penalty.
1444 Barewords are not covered by overloaded string constants.
1446 This document is confusing. There are grammos and misleading language
1447 used in places. It would seem a total rewrite is needed.