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. This minimal
681 atan2 cos sin exp log sqrt int
683 Additionally, you need to define at least one of string, boolean or
684 numeric conversions. The string conversion can also be used to
685 emulate concatenation, so we'll assume string conversion is defined.
686 It usually makes sense to explicitly overload all conversion
689 =head1 Losing overloading
691 The restriction for the comparison operation is that even if, for example,
692 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
693 function will produce only a standard logical value based on the
694 numerical value of the result of `C<cmp>'. In particular, a working
695 numeric conversion is needed in this case (possibly expressed in terms of
698 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
699 if the string conversion substitution is applied.
701 When you chop() a mathemagical object it is promoted to a string and its
702 mathemagical properties are lost. The same can happen with other
705 =head1 Run-time Overloading
707 Since all C<use> directives are executed at compile-time, the only way to
708 change overloading during run-time is to
710 eval 'use overload "+" => \&addmethod';
714 eval 'no overload "+", "--", "<="';
716 though the use of these constructs during run-time is questionable.
718 =head1 Public functions
720 Package C<overload.pm> provides the following public functions:
724 =item overload::StrVal(arg)
726 Gives string value of C<arg> as in absence of stringify overloading. If you
727 are using this to get the address of a reference (useful for checking if two
728 references point to the same thing) then you may be better off using
729 C<Scalar::Util::refaddr()>, which is faster.
731 =item overload::Overloaded(arg)
733 Returns true if C<arg> is subject to overloading of some operations.
735 =item overload::Method(obj,op)
737 Returns C<undef> or a reference to the method that implements C<op>.
741 =head1 Overloading constants
743 For some applications, the Perl parser mangles constants too much.
744 It is possible to hook into this process via C<overload::constant()>
745 and C<overload::remove_constant()> functions.
747 These functions take a hash as an argument. The recognized keys of this hash
754 to overload integer constants,
758 to overload floating point constants,
762 to overload octal and hexadecimal constants,
766 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
767 strings and here-documents,
771 to overload constant pieces of regular expressions.
775 The corresponding values are references to functions which take three arguments:
776 the first one is the I<initial> string form of the constant, the second one
777 is how Perl interprets this constant, the third one is how the constant is used.
778 Note that the initial string form does not
779 contain string delimiters, and has backslashes in backslash-delimiter
780 combinations stripped (thus the value of delimiter is not relevant for
781 processing of this string). The return value of this function is how this
782 constant is going to be interpreted by Perl. The third argument is undefined
783 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
784 context (comes from strings, regular expressions, and single-quote HERE
785 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
786 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
788 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
789 it is expected that overloaded constant strings are equipped with reasonable
790 overloaded catenation operator, otherwise absurd results will result.
791 Similarly, negative numbers are considered as negations of positive constants.
793 Note that it is probably meaningless to call the functions overload::constant()
794 and overload::remove_constant() from anywhere but import() and unimport() methods.
795 From these methods they may be called as
800 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
801 overload::constant integer => sub {Math::BigInt->new(shift)};
804 =head1 IMPLEMENTATION
806 What follows is subject to change RSN.
808 The table of methods for all operations is cached in magic for the
809 symbol table hash for the package. The cache is invalidated during
810 processing of C<use overload>, C<no overload>, new function
811 definitions, and changes in @ISA. However, this invalidation remains
812 unprocessed until the next C<bless>ing into the package. Hence if you
813 want to change overloading structure dynamically, you'll need an
814 additional (fake) C<bless>ing to update the table.
816 (Every SVish thing has a magic queue, and magic is an entry in that
817 queue. This is how a single variable may participate in multiple
818 forms of magic simultaneously. For instance, environment variables
819 regularly have two forms at once: their %ENV magic and their taint
820 magic. However, the magic which implements overloading is applied to
821 the stashes, which are rarely used directly, thus should not slow down
824 If an object belongs to a package using overload, it carries a special
825 flag. Thus the only speed penalty during arithmetic operations without
826 overloading is the checking of this flag.
828 In fact, if C<use overload> is not present, there is almost no overhead
829 for overloadable operations, so most programs should not suffer
830 measurable performance penalties. A considerable effort was made to
831 minimize the overhead when overload is used in some package, but the
832 arguments in question do not belong to packages using overload. When
833 in doubt, test your speed with C<use overload> and without it. So far
834 there have been no reports of substantial speed degradation if Perl is
835 compiled with optimization turned on.
837 There is no size penalty for data if overload is not used. The only
838 size penalty if overload is used in some package is that I<all> the
839 packages acquire a magic during the next C<bless>ing into the
840 package. This magic is three-words-long for packages without
841 overloading, and carries the cache table if the package is overloaded.
843 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
844 carried out before any operation that can imply an assignment to the
845 object $a (or $b) refers to, like C<$a++>. You can override this
846 behavior by defining your own copy constructor (see L<"Copy Constructor">).
848 It is expected that arguments to methods that are not explicitly supposed
849 to be changed are constant (but this is not enforced).
851 =head1 Metaphor clash
853 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
854 If it I<looks> counter intuitive to you, you are subject to a metaphor
857 Here is a Perl object metaphor:
859 I< object is a reference to blessed data>
861 and an arithmetic metaphor:
863 I< object is a thing by itself>.
865 The I<main> problem of overloading C<=> is the fact that these metaphors
866 imply different actions on the assignment C<$a = $b> if $a and $b are
867 objects. Perl-think implies that $a becomes a reference to whatever
868 $b was referencing. Arithmetic-think implies that the value of "object"
869 $a is changed to become the value of the object $b, preserving the fact
870 that $a and $b are separate entities.
872 The difference is not relevant in the absence of mutators. After
873 a Perl-way assignment an operation which mutates the data referenced by $a
874 would change the data referenced by $b too. Effectively, after
875 C<$a = $b> values of $a and $b become I<indistinguishable>.
877 On the other hand, anyone who has used algebraic notation knows the
878 expressive power of the arithmetic metaphor. Overloading works hard
879 to enable this metaphor while preserving the Perlian way as far as
880 possible. Since it is not possible to freely mix two contradicting
881 metaphors, overloading allows the arithmetic way to write things I<as
882 far as all the mutators are called via overloaded access only>. The
883 way it is done is described in L<Copy Constructor>.
885 If some mutator methods are directly applied to the overloaded values,
886 one may need to I<explicitly unlink> other values which references the
891 $b = $a; # $b is "linked" to $a
893 $a = $a->clone; # Unlink $b from $a
896 Note that overloaded access makes this transparent:
899 $b = $a; # $b is "linked" to $a
900 $a += 4; # would unlink $b automagically
902 However, it would not make
905 $a = 4; # Now $a is a plain 4, not 'Data'
907 preserve "objectness" of $a. But Perl I<has> a way to make assignments
908 to an object do whatever you want. It is just not the overload, but
909 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
910 which returns the object itself, and STORE() method which changes the
911 value of the object, one can reproduce the arithmetic metaphor in its
912 completeness, at least for variables which were tie()d from the start.
914 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
918 Please add examples to what follows!
920 =head2 Two-face scalars
922 Put this in F<two_face.pm> in your Perl library directory:
924 package two_face; # Scalars with separate string and
926 sub new { my $p = shift; bless [@_], $p }
927 use overload '""' => \&str, '0+' => \&num, fallback => 1;
934 my $seven = new two_face ("vii", 7);
935 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
936 print "seven contains `i'\n" if $seven =~ /i/;
938 (The second line creates a scalar which has both a string value, and a
939 numeric value.) This prints:
941 seven=vii, seven=7, eight=8
944 =head2 Two-face references
946 Suppose you want to create an object which is accessible as both an
947 array reference and a hash reference.
950 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
958 tie %h, ref $self, $self;
962 sub TIEHASH { my $p = shift; bless \ shift, $p }
965 $fields{$_} = $i++ foreach qw{zero one two three};
967 my $self = ${shift()};
968 my $key = $fields{shift()};
969 defined $key or die "Out of band access";
970 $$self->[$key] = shift;
973 my $self = ${shift()};
974 my $key = $fields{shift()};
975 defined $key or die "Out of band access";
979 Now one can access an object using both the array and hash syntax:
981 my $bar = new two_refs 3,4,5,6;
983 $bar->{two} == 11 or die 'bad hash fetch';
985 Note several important features of this example. First of all, the
986 I<actual> type of $bar is a scalar reference, and we do not overload
987 the scalar dereference. Thus we can get the I<actual> non-overloaded
988 contents of $bar by just using C<$$bar> (what we do in functions which
989 overload dereference). Similarly, the object returned by the
990 TIEHASH() method is a scalar reference.
992 Second, we create a new tied hash each time the hash syntax is used.
993 This allows us not to worry about a possibility of a reference loop,
994 which would lead to a memory leak.
996 Both these problems can be cured. Say, if we want to overload hash
997 dereference on a reference to an object which is I<implemented> as a
998 hash itself, the only problem one has to circumvent is how to access
999 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1000 overloaded dereference operator). Here is one possible fetching routine:
1003 my ($self, $key) = (shift, shift);
1004 my $class = ref $self;
1005 bless $self, 'overload::dummy'; # Disable overloading of %{}
1006 my $out = $self->{$key};
1007 bless $self, $class; # Restore overloading
1011 To remove creation of the tied hash on each access, one may an extra
1012 level of indirection which allows a non-circular structure of references:
1015 use overload '%{}' => sub { ${shift()}->[1] },
1016 '@{}' => sub { ${shift()}->[0] };
1022 bless \ [$a, \%h], $p;
1027 tie %h, ref $self, $self;
1031 sub TIEHASH { my $p = shift; bless \ shift, $p }
1034 $fields{$_} = $i++ foreach qw{zero one two three};
1037 my $key = $fields{shift()};
1038 defined $key or die "Out of band access";
1043 my $key = $fields{shift()};
1044 defined $key or die "Out of band access";
1048 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1049 reference to the intermediate array, which keeps a reference to an
1050 actual array, and the access hash. The tie()ing object for the access
1051 hash is a reference to a reference to the actual array, so
1057 There are no loops of references.
1061 Both "objects" which are blessed into the class C<two_refs1> are
1062 references to a reference to an array, thus references to a I<scalar>.
1063 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1064 overloaded operations.
1068 =head2 Symbolic calculator
1070 Put this in F<symbolic.pm> in your Perl library directory:
1072 package symbolic; # Primitive symbolic calculator
1073 use overload nomethod => \&wrap;
1075 sub new { shift; bless ['n', @_] }
1077 my ($obj, $other, $inv, $meth) = @_;
1078 ($obj, $other) = ($other, $obj) if $inv;
1079 bless [$meth, $obj, $other];
1082 This module is very unusual as overloaded modules go: it does not
1083 provide any usual overloaded operators, instead it provides the L<Last
1084 Resort> operator C<nomethod>. In this example the corresponding
1085 subroutine returns an object which encapsulates operations done over
1086 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1087 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1089 Here is an example of the script which "calculates" the side of
1090 circumscribed octagon using the above package:
1093 my $iter = 1; # 2**($iter+2) = 8
1094 my $side = new symbolic 1;
1098 $side = (sqrt(1 + $side**2) - 1)/$side;
1102 The value of $side is
1104 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1105 undef], 1], ['n', 1]]
1107 Note that while we obtained this value using a nice little script,
1108 there is no simple way to I<use> this value. In fact this value may
1109 be inspected in debugger (see L<perldebug>), but ony if
1110 C<bareStringify> B<O>ption is set, and not via C<p> command.
1112 If one attempts to print this value, then the overloaded operator
1113 C<""> will be called, which will call C<nomethod> operator. The
1114 result of this operator will be stringified again, but this result is
1115 again of type C<symbolic>, which will lead to an infinite loop.
1117 Add a pretty-printer method to the module F<symbolic.pm>:
1120 my ($meth, $a, $b) = @{+shift};
1121 $a = 'u' unless defined $a;
1122 $b = 'u' unless defined $b;
1123 $a = $a->pretty if ref $a;
1124 $b = $b->pretty if ref $b;
1128 Now one can finish the script by
1130 print "side = ", $side->pretty, "\n";
1132 The method C<pretty> is doing object-to-string conversion, so it
1133 is natural to overload the operator C<""> using this method. However,
1134 inside such a method it is not necessary to pretty-print the
1135 I<components> $a and $b of an object. In the above subroutine
1136 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1137 and $b. If these components use overloading, the catenation operator
1138 will look for an overloaded operator C<.>; if not present, it will
1139 look for an overloaded operator C<"">. Thus it is enough to use
1141 use overload nomethod => \&wrap, '""' => \&str;
1143 my ($meth, $a, $b) = @{+shift};
1144 $a = 'u' unless defined $a;
1145 $b = 'u' unless defined $b;
1149 Now one can change the last line of the script to
1151 print "side = $side\n";
1155 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1157 and one can inspect the value in debugger using all the possible
1160 Something is still amiss: consider the loop variable $cnt of the
1161 script. It was a number, not an object. We cannot make this value of
1162 type C<symbolic>, since then the loop will not terminate.
1164 Indeed, to terminate the cycle, the $cnt should become false.
1165 However, the operator C<bool> for checking falsity is overloaded (this
1166 time via overloaded C<"">), and returns a long string, thus any object
1167 of type C<symbolic> is true. To overcome this, we need a way to
1168 compare an object to 0. In fact, it is easier to write a numeric
1171 Here is the text of F<symbolic.pm> with such a routine added (and
1172 slightly modified str()):
1174 package symbolic; # Primitive symbolic calculator
1176 nomethod => \&wrap, '""' => \&str, '0+' => \#
1178 sub new { shift; bless ['n', @_] }
1180 my ($obj, $other, $inv, $meth) = @_;
1181 ($obj, $other) = ($other, $obj) if $inv;
1182 bless [$meth, $obj, $other];
1185 my ($meth, $a, $b) = @{+shift};
1186 $a = 'u' unless defined $a;
1193 my %subr = ( n => sub {$_[0]},
1194 sqrt => sub {sqrt $_[0]},
1195 '-' => sub {shift() - shift()},
1196 '+' => sub {shift() + shift()},
1197 '/' => sub {shift() / shift()},
1198 '*' => sub {shift() * shift()},
1199 '**' => sub {shift() ** shift()},
1202 my ($meth, $a, $b) = @{+shift};
1203 my $subr = $subr{$meth}
1204 or die "Do not know how to ($meth) in symbolic";
1205 $a = $a->num if ref $a eq __PACKAGE__;
1206 $b = $b->num if ref $b eq __PACKAGE__;
1210 All the work of numeric conversion is done in %subr and num(). Of
1211 course, %subr is not complete, it contains only operators used in the
1212 example below. Here is the extra-credit question: why do we need an
1213 explicit recursion in num()? (Answer is at the end of this section.)
1215 Use this module like this:
1218 my $iter = new symbolic 2; # 16-gon
1219 my $side = new symbolic 1;
1223 $cnt = $cnt - 1; # Mutator `--' not implemented
1224 $side = (sqrt(1 + $side**2) - 1)/$side;
1226 printf "%s=%f\n", $side, $side;
1227 printf "pi=%f\n", $side*(2**($iter+2));
1229 It prints (without so many line breaks)
1231 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1233 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1236 The above module is very primitive. It does not implement
1237 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1238 (not required without mutators!), and implements only those arithmetic
1239 operations which are used in the example.
1241 To implement most arithmetic operations is easy; one should just use
1242 the tables of operations, and change the code which fills %subr to
1244 my %subr = ( 'n' => sub {$_[0]} );
1245 foreach my $op (split " ", $overload::ops{with_assign}) {
1246 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1248 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1249 foreach my $op (split " ", "@overload::ops{ @bins }") {
1250 $subr{$op} = eval "sub {shift() $op shift()}";
1252 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1253 print "defining `$op'\n";
1254 $subr{$op} = eval "sub {$op shift()}";
1257 Due to L<Calling Conventions for Mutators>, we do not need anything
1258 special to make C<+=> and friends work, except filling C<+=> entry of
1259 %subr, and defining a copy constructor (needed since Perl has no
1260 way to know that the implementation of C<'+='> does not mutate
1261 the argument, compare L<Copy Constructor>).
1263 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1264 line, and code (this code assumes that mutators change things one level
1265 deep only, so recursive copying is not needed):
1269 bless [@$self], ref $self;
1272 To make C<++> and C<--> work, we need to implement actual mutators,
1273 either directly, or in C<nomethod>. We continue to do things inside
1274 C<nomethod>, thus add
1276 if ($meth eq '++' or $meth eq '--') {
1277 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1281 after the first line of wrap(). This is not a most effective
1282 implementation, one may consider
1284 sub inc { $_[0] = bless ['++', shift, 1]; }
1288 As a final remark, note that one can fill %subr by
1290 my %subr = ( 'n' => sub {$_[0]} );
1291 foreach my $op (split " ", $overload::ops{with_assign}) {
1292 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1294 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1295 foreach my $op (split " ", "@overload::ops{ @bins }") {
1296 $subr{$op} = eval "sub {shift() $op shift()}";
1298 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1299 $subr{$op} = eval "sub {$op shift()}";
1301 $subr{'++'} = $subr{'+'};
1302 $subr{'--'} = $subr{'-'};
1304 This finishes implementation of a primitive symbolic calculator in
1305 50 lines of Perl code. Since the numeric values of subexpressions
1306 are not cached, the calculator is very slow.
1308 Here is the answer for the exercise: In the case of str(), we need no
1309 explicit recursion since the overloaded C<.>-operator will fall back
1310 to an existing overloaded operator C<"">. Overloaded arithmetic
1311 operators I<do not> fall back to numeric conversion if C<fallback> is
1312 not explicitly requested. Thus without an explicit recursion num()
1313 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1314 the argument of num().
1316 If you wonder why defaults for conversion are different for str() and
1317 num(), note how easy it was to write the symbolic calculator. This
1318 simplicity is due to an appropriate choice of defaults. One extra
1319 note: due to the explicit recursion num() is more fragile than sym():
1320 we need to explicitly check for the type of $a and $b. If components
1321 $a and $b happen to be of some related type, this may lead to problems.
1323 =head2 I<Really> symbolic calculator
1325 One may wonder why we call the above calculator symbolic. The reason
1326 is that the actual calculation of the value of expression is postponed
1327 until the value is I<used>.
1329 To see it in action, add a method
1334 @$obj->[0,1] = ('=', shift);
1337 to the package C<symbolic>. After this change one can do
1339 my $a = new symbolic 3;
1340 my $b = new symbolic 4;
1341 my $c = sqrt($a**2 + $b**2);
1343 and the numeric value of $c becomes 5. However, after calling
1345 $a->STORE(12); $b->STORE(5);
1347 the numeric value of $c becomes 13. There is no doubt now that the module
1348 symbolic provides a I<symbolic> calculator indeed.
1350 To hide the rough edges under the hood, provide a tie()d interface to the
1351 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1353 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1355 sub nop { } # Around a bug
1357 (the bug is described in L<"BUGS">). One can use this new interface as
1359 tie $a, 'symbolic', 3;
1360 tie $b, 'symbolic', 4;
1361 $a->nop; $b->nop; # Around a bug
1363 my $c = sqrt($a**2 + $b**2);
1365 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1366 of $c becomes 13. To insulate the user of the module add a method
1368 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1373 symbolic->vars($a, $b);
1374 my $c = sqrt($a**2 + $b**2);
1377 printf "c5 %s=%f\n", $c, $c;
1380 printf "c13 %s=%f\n", $c, $c;
1382 shows that the numeric value of $c follows changes to the values of $a
1387 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1391 When Perl is run with the B<-Do> switch or its equivalent, overloading
1392 induces diagnostic messages.
1394 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1395 deduce which operations are overloaded (and which ancestor triggers
1396 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1397 is shown by debugger. The method C<()> corresponds to the C<fallback>
1398 key (in fact a presence of this method shows that this package has
1399 overloading enabled, and it is what is used by the C<Overloaded>
1400 function of module C<overload>).
1402 The module might issue the following warnings:
1406 =item Odd number of arguments for overload::constant
1408 (W) The call to overload::constant contained an odd number of arguments.
1409 The arguments should come in pairs.
1411 =item `%s' is not an overloadable type
1413 (W) You tried to overload a constant type the overload package is unaware of.
1415 =item `%s' is not a code reference
1417 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1418 to be a code reference. Either an anonymous subroutine, or a reference
1425 Because it is used for overloading, the per-package hash %OVERLOAD now
1426 has a special meaning in Perl. The symbol table is filled with names
1427 looking like line-noise.
1429 For the purpose of inheritance every overloaded package behaves as if
1430 C<fallback> is present (possibly undefined). This may create
1431 interesting effects if some package is not overloaded, but inherits
1432 from two overloaded packages.
1434 Relation between overloading and tie()ing is broken. Overloading is
1435 triggered or not basing on the I<previous> class of tie()d value.
1437 This happens because the presence of overloading is checked too early,
1438 before any tie()d access is attempted. If the FETCH()ed class of the
1439 tie()d value does not change, a simple workaround is to access the value
1440 immediately after tie()ing, so that after this call the I<previous> class
1441 coincides with the current one.
1443 B<Needed:> a way to fix this without a speed penalty.
1445 Barewords are not covered by overloaded string constants.
1447 This document is confusing. There are grammos and misleading language
1448 used in places. It would seem a total rewrite is needed.