11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
14 if ($_ eq 'fallback') {
18 if (not ref $sub and $sub !~ /::/) {
19 $ {$package . "::(" . $_} = $sub;
22 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
23 *{$package . "::(" . $_} = \&{ $sub };
26 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
30 $package = (caller())[0];
31 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
33 $package->overload::OVERLOAD(@_);
37 $package = (caller())[0];
38 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
41 if ($_ eq 'fallback') {
42 undef $ {$package . "::()"};
44 delete $ {$package . "::"}{"(" . $_};
51 $package = ref $package if ref $package;
57 return undef unless $globref;
58 my $sub = \&{*$globref};
59 return $sub if $sub ne \&nil;
60 return shift->can($ {*$globref});
63 sub OverloadedStringify {
65 $package = ref $package if ref $package;
67 ov_method mycan($package, '(""'), $package
68 or ov_method mycan($package, '(0+'), $package
69 or ov_method mycan($package, '(bool'), $package
70 or ov_method mycan($package, '(nomethod'), $package;
79 $package = Scalar::Util::blessed($package);
80 return undef if !defined $package;
82 #my $meth = $package->can('(' . shift);
83 ov_method mycan($package, '(' . shift), $package;
84 #return $meth if $meth ne \&nil;
89 my $package = ref $_[0];
90 return "$_[0]" unless $package;
95 my $class = Scalar::Util::blessed($_[0]);
96 my $class_prefix = defined($class) ? "$class=" : "";
97 my $type = Scalar::Util::reftype($_[0]);
98 my $addr = Scalar::Util::refaddr($_[0]);
99 return sprintf("$class_prefix$type(0x%x)", $addr);
104 sub mycan { # Real can would leave stubs.
105 my ($package, $meth) = @_;
107 my $mro = mro::get_linear_isa($package);
108 foreach my $p (@$mro) {
109 my $fqmeth = $p . q{::} . $meth;
110 return \*{$fqmeth} if defined &{$fqmeth};
117 'integer' => 0x1000, # HINT_NEW_INTEGER
118 'float' => 0x2000, # HINT_NEW_FLOAT
119 'binary' => 0x4000, # HINT_NEW_BINARY
120 'q' => 0x8000, # HINT_NEW_STRING
121 'qr' => 0x10000, # HINT_NEW_RE
124 %ops = ( with_assign => "+ - * / % ** << >> x .",
125 assign => "+= -= *= /= %= **= <<= >>= x= .=",
126 num_comparison => "< <= > >= == !=",
127 '3way_comparison'=> "<=> cmp",
128 str_comparison => "lt le gt ge eq ne",
129 binary => '& &= | |= ^ ^=',
132 func => "atan2 cos sin exp abs log sqrt int",
133 conversion => 'bool "" 0+',
135 dereferencing => '${} @{} %{} &{} *{}',
136 special => 'nomethod fallback =');
138 use warnings::register;
140 # Arguments: what, sub
143 warnings::warnif ("Odd number of arguments for overload::constant");
146 elsif (!exists $constants {$_ [0]}) {
147 warnings::warnif ("`$_[0]' is not an overloadable type");
149 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
150 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
151 # blessed, and C<ref> would return the package the ref is blessed into.
152 if (warnings::enabled) {
153 $_ [1] = "undef" unless defined $_ [1];
154 warnings::warn ("`$_[1]' is not a code reference");
159 $^H |= $constants{$_[0]};
165 sub remove_constant {
166 # Arguments: what, sub
169 $^H &= ~ $constants{$_[0]};
180 overload - Package for overloading Perl operations
193 $a = new SomeThing 57;
196 if (overload::Overloaded $b) {...}
198 $strval = overload::StrVal $b;
202 =head2 Declaration of overloaded functions
204 The compilation directive
211 declares function Number::add() for addition, and method muas() in
212 the "class" C<Number> (or one of its base classes)
213 for the assignment form C<*=> of multiplication.
215 Arguments of this directive come in (key, value) pairs. Legal values
216 are values legal inside a C<&{ ... }> call, so the name of a
217 subroutine, a reference to a subroutine, or an anonymous subroutine
218 will all work. Note that values specified as strings are
219 interpreted as methods, not subroutines. Legal keys are listed below.
221 The subroutine C<add> will be called to execute C<$a+$b> if $a
222 is a reference to an object blessed into the package C<Number>, or if $a is
223 not an object from a package with defined mathemagic addition, but $b is a
224 reference to a C<Number>. It can also be called in other situations, like
225 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
226 methods refer to methods triggered by an overloaded mathematical
229 Since overloading respects inheritance via the @ISA hierarchy, the
230 above declaration would also trigger overloading of C<+> and C<*=> in
231 all the packages which inherit from C<Number>.
233 =head2 Calling Conventions for Binary Operations
235 The functions specified in the C<use overload ...> directive are called
236 with three (in one particular case with four, see L<Last Resort>)
237 arguments. If the corresponding operation is binary, then the first
238 two arguments are the two arguments of the operation. However, due to
239 general object calling conventions, the first argument should always be
240 an object in the package, so in the situation of C<7+$a>, the
241 order of the arguments is interchanged. It probably does not matter
242 when implementing the addition method, but whether the arguments
243 are reversed is vital to the subtraction method. The method can
244 query this information by examining the third argument, which can take
245 three different values:
251 the order of arguments is as in the current operation.
255 the arguments are reversed.
259 the current operation is an assignment variant (as in
260 C<$a+=7>), but the usual function is called instead. This additional
261 information can be used to generate some optimizations. Compare
262 L<Calling Conventions for Mutators>.
266 =head2 Calling Conventions for Unary Operations
268 Unary operation are considered binary operations with the second
269 argument being C<undef>. Thus the functions that overloads C<{"++"}>
270 is called with arguments C<($a,undef,'')> when $a++ is executed.
272 =head2 Calling Conventions for Mutators
274 Two types of mutators have different calling conventions:
278 =item C<++> and C<-->
280 The routines which implement these operators are expected to actually
281 I<mutate> their arguments. So, assuming that $obj is a reference to a
284 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
286 is an appropriate implementation of overloaded C<++>. Note that
288 sub incr { ++$ {$_[0]} ; shift }
290 is OK if used with preincrement and with postincrement. (In the case
291 of postincrement a copying will be performed, see L<Copy Constructor>.)
293 =item C<x=> and other assignment versions
295 There is nothing special about these methods. They may change the
296 value of their arguments, and may leave it as is. The result is going
297 to be assigned to the value in the left-hand-side if different from
300 This allows for the same method to be used as overloaded C<+=> and
301 C<+>. Note that this is I<allowed>, but not recommended, since by the
302 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
303 if C<+=> is not overloaded.
307 B<Warning.> Due to the presence of assignment versions of operations,
308 routines which may be called in assignment context may create
309 self-referential structures. Currently Perl will not free self-referential
310 structures until cycles are C<explicitly> broken. You may get problems
311 when traversing your structures too.
315 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
317 is asking for trouble, since for code C<$obj += $foo> the subroutine
318 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
319 \$foo]>. If using such a subroutine is an important optimization, one
320 can overload C<+=> explicitly by a non-"optimized" version, or switch
321 to non-optimized version if C<not defined $_[2]> (see
322 L<Calling Conventions for Binary Operations>).
324 Even if no I<explicit> assignment-variants of operators are present in
325 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
326 C<',' . $obj . ','> may be both optimized to
328 my $tmp = ',' . $obj; $tmp .= ',';
330 =head2 Overloadable Operations
332 The following symbols can be specified in C<use overload> directive:
336 =item * I<Arithmetic operations>
338 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
339 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
341 For these operations a substituted non-assignment variant can be called if
342 the assignment variant is not available. Methods for operations C<+>,
343 C<->, C<+=>, and C<-=> can be called to automatically generate
344 increment and decrement methods. The operation C<-> can be used to
345 autogenerate missing methods for unary minus or C<abs>.
347 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
348 L<"Calling Conventions for Binary Operations">) for details of these
351 =item * I<Comparison operations>
353 "<", "<=", ">", ">=", "==", "!=", "<=>",
354 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
356 If the corresponding "spaceship" variant is available, it can be
357 used to substitute for the missing operation. During C<sort>ing
358 arrays, C<cmp> is used to compare values subject to C<use overload>.
360 =item * I<Bit operations>
362 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
364 C<neg> stands for unary minus. If the method for C<neg> is not
365 specified, it can be autogenerated using the method for
366 subtraction. If the method for C<!> is not specified, it can be
367 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
369 The same remarks in L<"Arithmetic operations"> about
370 assignment-variants and autogeneration apply for
371 bit operations C<"&">, C<"^">, and C<"|"> as well.
373 =item * I<Increment and decrement>
377 If undefined, addition and subtraction methods can be
378 used instead. These operations are called both in prefix and
381 =item * I<Transcendental functions>
383 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
385 If C<abs> is unavailable, it can be autogenerated using methods
386 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
388 Note that traditionally the Perl function L<int> rounds to 0, thus for
389 floating-point-like types one should follow the same semantic. If
390 C<int> is unavailable, it can be autogenerated using the overloading of
393 =item * I<Boolean, string and numeric conversion>
397 If one or two of these operations are not overloaded, the remaining ones can
398 be used instead. C<bool> is used in the flow control operators
399 (like C<while>) and for the ternary C<?:> operation. These functions can
400 return any arbitrary Perl value. If the corresponding operation for this value
401 is overloaded too, that operation will be called again with this value.
403 As a special case if the overload returns the object itself then it will
404 be used directly. An overloaded conversion returning the object is
405 probably a bug, because you're likely to get something that looks like
406 C<YourPackage=HASH(0x8172b34)>.
412 If not overloaded, the argument will be converted to a filehandle or
413 glob (which may require a stringification). The same overloading
414 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
415 I<globbing> syntax C<E<lt>${var}E<gt>>.
417 B<BUGS> Even in list context, the iterator is currently called only
418 once and with scalar context.
420 =item * I<Dereferencing>
422 '${}', '@{}', '%{}', '&{}', '*{}'.
424 If not overloaded, the argument will be dereferenced I<as is>, thus
425 should be of correct type. These functions should return a reference
426 of correct type, or another object with overloaded dereferencing.
428 As a special case if the overload returns the object itself then it
429 will be used directly (provided it is the correct type).
431 The dereference operators must be specified explicitly they will not be passed to
436 "nomethod", "fallback", "=", "~~",
438 see L<SPECIAL SYMBOLS FOR C<use overload>>.
442 See L<"Fallback"> for an explanation of when a missing method can be
445 A computer-readable form of the above table is available in the hash
446 %overload::ops, with values being space-separated lists of names:
448 with_assign => '+ - * / % ** << >> x .',
449 assign => '+= -= *= /= %= **= <<= >>= x= .=',
450 num_comparison => '< <= > >= == !=',
451 '3way_comparison'=> '<=> cmp',
452 str_comparison => 'lt le gt ge eq ne',
453 binary => '& &= | |= ^ ^=',
456 func => 'atan2 cos sin exp abs log sqrt',
457 conversion => 'bool "" 0+',
459 dereferencing => '${} @{} %{} &{} *{}',
460 special => 'nomethod fallback ='
462 =head2 Inheritance and overloading
464 Inheritance interacts with overloading in two ways.
468 =item Strings as values of C<use overload> directive
472 use overload key => value;
474 is a string, it is interpreted as a method name.
476 =item Overloading of an operation is inherited by derived classes
478 Any class derived from an overloaded class is also overloaded. The
479 set of overloaded methods is the union of overloaded methods of all
480 the ancestors. If some method is overloaded in several ancestor, then
481 which description will be used is decided by the usual inheritance
484 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
485 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
486 then the subroutine C<D::plus_sub> will be called to implement
487 operation C<+> for an object in package C<A>.
491 Note that since the value of the C<fallback> key is not a subroutine,
492 its inheritance is not governed by the above rules. In the current
493 implementation, the value of C<fallback> in the first overloaded
494 ancestor is used, but this is accidental and subject to change.
496 =head1 SPECIAL SYMBOLS FOR C<use overload>
498 Three keys are recognized by Perl that are not covered by the above
503 C<"nomethod"> should be followed by a reference to a function of four
504 parameters. If defined, it is called when the overloading mechanism
505 cannot find a method for some operation. The first three arguments of
506 this function coincide with the arguments for the corresponding method if
507 it were found, the fourth argument is the symbol
508 corresponding to the missing method. If several methods are tried,
509 the last one is used. Say, C<1-$a> can be equivalent to
511 &nomethodMethod($a,1,1,"-")
513 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
514 C<use overload> directive.
516 The C<"nomethod"> mechanism is I<not> used for the dereference operators
517 ( ${} @{} %{} &{} *{} ).
520 If some operation cannot be resolved, and there is no function
521 assigned to C<"nomethod">, then an exception will be raised via die()--
522 unless C<"fallback"> was specified as a key in C<use overload> directive.
527 The key C<"fallback"> governs what to do if a method for a particular
528 operation is not found. Three different cases are possible depending on
529 the value of C<"fallback">:
536 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
537 then tries to calls C<"nomethod"> value; if missing, an exception
542 The same as for the C<undef> value, but no exception is raised. Instead,
543 it silently reverts to what it would have done were there no C<use overload>
546 =item * defined, but FALSE
548 No autogeneration is tried. Perl tries to call
549 C<"nomethod"> value, and if this is missing, raises an exception.
553 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
554 yet, see L<"Inheritance and overloading">.
558 The key C<"~~"> allows you to override the smart matching used by
559 the switch construct. See L<feature>.
561 =head2 Copy Constructor
563 The value for C<"="> is a reference to a function with three
564 arguments, i.e., it looks like the other values in C<use
565 overload>. However, it does not overload the Perl assignment
566 operator. This would go against Camel hair.
568 This operation is called in the situations when a mutator is applied
569 to a reference that shares its object with some other reference, such
575 To make this change $a and not change $b, a copy of C<$$a> is made,
576 and $a is assigned a reference to this new object. This operation is
577 done during execution of the C<++$a>, and not during the assignment,
578 (so before the increment C<$$a> coincides with C<$$b>). This is only
579 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
580 C<nomethod>). Note that if this operation is expressed via C<'+'>
581 a nonmutator, i.e., as in
586 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
587 appear as lvalue when the above code is executed.
589 If the copy constructor is required during the execution of some mutator,
590 but a method for C<'='> was not specified, it can be autogenerated as a
591 string copy if the object is a plain scalar or a simple assignment if it
598 The actually executed code for
601 Something else which does not modify $a or $b....
607 Something else which does not modify $a or $b....
608 $a = $a->clone(undef,"");
611 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
612 C<'='> was overloaded with C<\&clone>.
616 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
619 =head1 MAGIC AUTOGENERATION
621 If a method for an operation is not found, and the value for C<"fallback"> is
622 TRUE or undefined, Perl tries to autogenerate a substitute method for
623 the missing operation based on the defined operations. Autogenerated method
624 substitutions are possible for the following operations:
628 =item I<Assignment forms of arithmetic operations>
630 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
633 =item I<Conversion operations>
635 String, numeric, and boolean conversion are calculated in terms of one
636 another if not all of them are defined.
638 =item I<Increment and decrement>
640 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
641 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
645 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
649 can be expressed in terms of subtraction.
653 C<!> and C<not> can be expressed in terms of boolean conversion, or
654 string or numerical conversion.
656 =item I<Concatenation>
658 can be expressed in terms of string conversion.
660 =item I<Comparison operations>
662 can be expressed in terms of its "spaceship" counterpart: either
663 C<E<lt>=E<gt>> or C<cmp>:
665 <, >, <=, >=, ==, != in terms of <=>
666 lt, gt, le, ge, eq, ne in terms of cmp
670 <> in terms of builtin operations
672 =item I<Dereferencing>
674 ${} @{} %{} &{} *{} in terms of builtin operations
676 =item I<Copy operator>
678 can be expressed in terms of an assignment to the dereferenced value, if this
679 value is a scalar and not a reference, or simply a reference assignment
684 =head1 Minimal set of overloaded operations
686 Since some operations can be automatically generated from others, there is
687 a minimal set of operations that need to be overloaded in order to have
688 the complete set of overloaded operations at one's disposal.
689 Of course, the autogenerated operations may not do exactly what the user
690 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
695 atan2 cos sin exp log sqrt int
697 Additionally, you need to define at least one of string, boolean or
698 numeric conversions because any one can be used to emulate the others.
699 The string conversion can also be used to emulate concatenation.
701 =head1 Losing overloading
703 The restriction for the comparison operation is that even if, for example,
704 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
705 function will produce only a standard logical value based on the
706 numerical value of the result of `C<cmp>'. In particular, a working
707 numeric conversion is needed in this case (possibly expressed in terms of
710 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
711 if the string conversion substitution is applied.
713 When you chop() a mathemagical object it is promoted to a string and its
714 mathemagical properties are lost. The same can happen with other
717 =head1 Run-time Overloading
719 Since all C<use> directives are executed at compile-time, the only way to
720 change overloading during run-time is to
722 eval 'use overload "+" => \&addmethod';
726 eval 'no overload "+", "--", "<="';
728 though the use of these constructs during run-time is questionable.
730 =head1 Public functions
732 Package C<overload.pm> provides the following public functions:
736 =item overload::StrVal(arg)
738 Gives string value of C<arg> as in absence of stringify overloading. If you
739 are using this to get the address of a reference (useful for checking if two
740 references point to the same thing) then you may be better off using
741 C<Scalar::Util::refaddr()>, which is faster.
743 =item overload::Overloaded(arg)
745 Returns true if C<arg> is subject to overloading of some operations.
747 =item overload::Method(obj,op)
749 Returns C<undef> or a reference to the method that implements C<op>.
753 =head1 Overloading constants
755 For some applications, the Perl parser mangles constants too much.
756 It is possible to hook into this process via C<overload::constant()>
757 and C<overload::remove_constant()> functions.
759 These functions take a hash as an argument. The recognized keys of this hash
766 to overload integer constants,
770 to overload floating point constants,
774 to overload octal and hexadecimal constants,
778 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
779 strings and here-documents,
783 to overload constant pieces of regular expressions.
787 The corresponding values are references to functions which take three arguments:
788 the first one is the I<initial> string form of the constant, the second one
789 is how Perl interprets this constant, the third one is how the constant is used.
790 Note that the initial string form does not
791 contain string delimiters, and has backslashes in backslash-delimiter
792 combinations stripped (thus the value of delimiter is not relevant for
793 processing of this string). The return value of this function is how this
794 constant is going to be interpreted by Perl. The third argument is undefined
795 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
796 context (comes from strings, regular expressions, and single-quote HERE
797 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
798 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
800 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
801 it is expected that overloaded constant strings are equipped with reasonable
802 overloaded catenation operator, otherwise absurd results will result.
803 Similarly, negative numbers are considered as negations of positive constants.
805 Note that it is probably meaningless to call the functions overload::constant()
806 and overload::remove_constant() from anywhere but import() and unimport() methods.
807 From these methods they may be called as
812 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
813 overload::constant integer => sub {Math::BigInt->new(shift)};
816 =head1 IMPLEMENTATION
818 What follows is subject to change RSN.
820 The table of methods for all operations is cached in magic for the
821 symbol table hash for the package. The cache is invalidated during
822 processing of C<use overload>, C<no overload>, new function
823 definitions, and changes in @ISA. However, this invalidation remains
824 unprocessed until the next C<bless>ing into the package. Hence if you
825 want to change overloading structure dynamically, you'll need an
826 additional (fake) C<bless>ing to update the table.
828 (Every SVish thing has a magic queue, and magic is an entry in that
829 queue. This is how a single variable may participate in multiple
830 forms of magic simultaneously. For instance, environment variables
831 regularly have two forms at once: their %ENV magic and their taint
832 magic. However, the magic which implements overloading is applied to
833 the stashes, which are rarely used directly, thus should not slow down
836 If an object belongs to a package using overload, it carries a special
837 flag. Thus the only speed penalty during arithmetic operations without
838 overloading is the checking of this flag.
840 In fact, if C<use overload> is not present, there is almost no overhead
841 for overloadable operations, so most programs should not suffer
842 measurable performance penalties. A considerable effort was made to
843 minimize the overhead when overload is used in some package, but the
844 arguments in question do not belong to packages using overload. When
845 in doubt, test your speed with C<use overload> and without it. So far
846 there have been no reports of substantial speed degradation if Perl is
847 compiled with optimization turned on.
849 There is no size penalty for data if overload is not used. The only
850 size penalty if overload is used in some package is that I<all> the
851 packages acquire a magic during the next C<bless>ing into the
852 package. This magic is three-words-long for packages without
853 overloading, and carries the cache table if the package is overloaded.
855 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
856 carried out before any operation that can imply an assignment to the
857 object $a (or $b) refers to, like C<$a++>. You can override this
858 behavior by defining your own copy constructor (see L<"Copy Constructor">).
860 It is expected that arguments to methods that are not explicitly supposed
861 to be changed are constant (but this is not enforced).
863 =head1 Metaphor clash
865 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
866 If it I<looks> counter intuitive to you, you are subject to a metaphor
869 Here is a Perl object metaphor:
871 I< object is a reference to blessed data>
873 and an arithmetic metaphor:
875 I< object is a thing by itself>.
877 The I<main> problem of overloading C<=> is the fact that these metaphors
878 imply different actions on the assignment C<$a = $b> if $a and $b are
879 objects. Perl-think implies that $a becomes a reference to whatever
880 $b was referencing. Arithmetic-think implies that the value of "object"
881 $a is changed to become the value of the object $b, preserving the fact
882 that $a and $b are separate entities.
884 The difference is not relevant in the absence of mutators. After
885 a Perl-way assignment an operation which mutates the data referenced by $a
886 would change the data referenced by $b too. Effectively, after
887 C<$a = $b> values of $a and $b become I<indistinguishable>.
889 On the other hand, anyone who has used algebraic notation knows the
890 expressive power of the arithmetic metaphor. Overloading works hard
891 to enable this metaphor while preserving the Perlian way as far as
892 possible. Since it is not possible to freely mix two contradicting
893 metaphors, overloading allows the arithmetic way to write things I<as
894 far as all the mutators are called via overloaded access only>. The
895 way it is done is described in L<Copy Constructor>.
897 If some mutator methods are directly applied to the overloaded values,
898 one may need to I<explicitly unlink> other values which references the
903 $b = $a; # $b is "linked" to $a
905 $a = $a->clone; # Unlink $b from $a
908 Note that overloaded access makes this transparent:
911 $b = $a; # $b is "linked" to $a
912 $a += 4; # would unlink $b automagically
914 However, it would not make
917 $a = 4; # Now $a is a plain 4, not 'Data'
919 preserve "objectness" of $a. But Perl I<has> a way to make assignments
920 to an object do whatever you want. It is just not the overload, but
921 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
922 which returns the object itself, and STORE() method which changes the
923 value of the object, one can reproduce the arithmetic metaphor in its
924 completeness, at least for variables which were tie()d from the start.
926 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
930 Please add examples to what follows!
932 =head2 Two-face scalars
934 Put this in F<two_face.pm> in your Perl library directory:
936 package two_face; # Scalars with separate string and
938 sub new { my $p = shift; bless [@_], $p }
939 use overload '""' => \&str, '0+' => \&num, fallback => 1;
946 my $seven = new two_face ("vii", 7);
947 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
948 print "seven contains `i'\n" if $seven =~ /i/;
950 (The second line creates a scalar which has both a string value, and a
951 numeric value.) This prints:
953 seven=vii, seven=7, eight=8
956 =head2 Two-face references
958 Suppose you want to create an object which is accessible as both an
959 array reference and a hash reference.
962 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
970 tie %h, ref $self, $self;
974 sub TIEHASH { my $p = shift; bless \ shift, $p }
977 $fields{$_} = $i++ foreach qw{zero one two three};
979 my $self = ${shift()};
980 my $key = $fields{shift()};
981 defined $key or die "Out of band access";
982 $$self->[$key] = shift;
985 my $self = ${shift()};
986 my $key = $fields{shift()};
987 defined $key or die "Out of band access";
991 Now one can access an object using both the array and hash syntax:
993 my $bar = new two_refs 3,4,5,6;
995 $bar->{two} == 11 or die 'bad hash fetch';
997 Note several important features of this example. First of all, the
998 I<actual> type of $bar is a scalar reference, and we do not overload
999 the scalar dereference. Thus we can get the I<actual> non-overloaded
1000 contents of $bar by just using C<$$bar> (what we do in functions which
1001 overload dereference). Similarly, the object returned by the
1002 TIEHASH() method is a scalar reference.
1004 Second, we create a new tied hash each time the hash syntax is used.
1005 This allows us not to worry about a possibility of a reference loop,
1006 which would lead to a memory leak.
1008 Both these problems can be cured. Say, if we want to overload hash
1009 dereference on a reference to an object which is I<implemented> as a
1010 hash itself, the only problem one has to circumvent is how to access
1011 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1012 overloaded dereference operator). Here is one possible fetching routine:
1015 my ($self, $key) = (shift, shift);
1016 my $class = ref $self;
1017 bless $self, 'overload::dummy'; # Disable overloading of %{}
1018 my $out = $self->{$key};
1019 bless $self, $class; # Restore overloading
1023 To remove creation of the tied hash on each access, one may an extra
1024 level of indirection which allows a non-circular structure of references:
1027 use overload '%{}' => sub { ${shift()}->[1] },
1028 '@{}' => sub { ${shift()}->[0] };
1034 bless \ [$a, \%h], $p;
1039 tie %h, ref $self, $self;
1043 sub TIEHASH { my $p = shift; bless \ shift, $p }
1046 $fields{$_} = $i++ foreach qw{zero one two three};
1049 my $key = $fields{shift()};
1050 defined $key or die "Out of band access";
1055 my $key = $fields{shift()};
1056 defined $key or die "Out of band access";
1060 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1061 reference to the intermediate array, which keeps a reference to an
1062 actual array, and the access hash. The tie()ing object for the access
1063 hash is a reference to a reference to the actual array, so
1069 There are no loops of references.
1073 Both "objects" which are blessed into the class C<two_refs1> are
1074 references to a reference to an array, thus references to a I<scalar>.
1075 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1076 overloaded operations.
1080 =head2 Symbolic calculator
1082 Put this in F<symbolic.pm> in your Perl library directory:
1084 package symbolic; # Primitive symbolic calculator
1085 use overload nomethod => \&wrap;
1087 sub new { shift; bless ['n', @_] }
1089 my ($obj, $other, $inv, $meth) = @_;
1090 ($obj, $other) = ($other, $obj) if $inv;
1091 bless [$meth, $obj, $other];
1094 This module is very unusual as overloaded modules go: it does not
1095 provide any usual overloaded operators, instead it provides the L<Last
1096 Resort> operator C<nomethod>. In this example the corresponding
1097 subroutine returns an object which encapsulates operations done over
1098 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1099 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1101 Here is an example of the script which "calculates" the side of
1102 circumscribed octagon using the above package:
1105 my $iter = 1; # 2**($iter+2) = 8
1106 my $side = new symbolic 1;
1110 $side = (sqrt(1 + $side**2) - 1)/$side;
1114 The value of $side is
1116 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1117 undef], 1], ['n', 1]]
1119 Note that while we obtained this value using a nice little script,
1120 there is no simple way to I<use> this value. In fact this value may
1121 be inspected in debugger (see L<perldebug>), but only if
1122 C<bareStringify> B<O>ption is set, and not via C<p> command.
1124 If one attempts to print this value, then the overloaded operator
1125 C<""> will be called, which will call C<nomethod> operator. The
1126 result of this operator will be stringified again, but this result is
1127 again of type C<symbolic>, which will lead to an infinite loop.
1129 Add a pretty-printer method to the module F<symbolic.pm>:
1132 my ($meth, $a, $b) = @{+shift};
1133 $a = 'u' unless defined $a;
1134 $b = 'u' unless defined $b;
1135 $a = $a->pretty if ref $a;
1136 $b = $b->pretty if ref $b;
1140 Now one can finish the script by
1142 print "side = ", $side->pretty, "\n";
1144 The method C<pretty> is doing object-to-string conversion, so it
1145 is natural to overload the operator C<""> using this method. However,
1146 inside such a method it is not necessary to pretty-print the
1147 I<components> $a and $b of an object. In the above subroutine
1148 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1149 and $b. If these components use overloading, the catenation operator
1150 will look for an overloaded operator C<.>; if not present, it will
1151 look for an overloaded operator C<"">. Thus it is enough to use
1153 use overload nomethod => \&wrap, '""' => \&str;
1155 my ($meth, $a, $b) = @{+shift};
1156 $a = 'u' unless defined $a;
1157 $b = 'u' unless defined $b;
1161 Now one can change the last line of the script to
1163 print "side = $side\n";
1167 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1169 and one can inspect the value in debugger using all the possible
1172 Something is still amiss: consider the loop variable $cnt of the
1173 script. It was a number, not an object. We cannot make this value of
1174 type C<symbolic>, since then the loop will not terminate.
1176 Indeed, to terminate the cycle, the $cnt should become false.
1177 However, the operator C<bool> for checking falsity is overloaded (this
1178 time via overloaded C<"">), and returns a long string, thus any object
1179 of type C<symbolic> is true. To overcome this, we need a way to
1180 compare an object to 0. In fact, it is easier to write a numeric
1183 Here is the text of F<symbolic.pm> with such a routine added (and
1184 slightly modified str()):
1186 package symbolic; # Primitive symbolic calculator
1188 nomethod => \&wrap, '""' => \&str, '0+' => \#
1190 sub new { shift; bless ['n', @_] }
1192 my ($obj, $other, $inv, $meth) = @_;
1193 ($obj, $other) = ($other, $obj) if $inv;
1194 bless [$meth, $obj, $other];
1197 my ($meth, $a, $b) = @{+shift};
1198 $a = 'u' unless defined $a;
1205 my %subr = ( n => sub {$_[0]},
1206 sqrt => sub {sqrt $_[0]},
1207 '-' => sub {shift() - shift()},
1208 '+' => sub {shift() + shift()},
1209 '/' => sub {shift() / shift()},
1210 '*' => sub {shift() * shift()},
1211 '**' => sub {shift() ** shift()},
1214 my ($meth, $a, $b) = @{+shift};
1215 my $subr = $subr{$meth}
1216 or die "Do not know how to ($meth) in symbolic";
1217 $a = $a->num if ref $a eq __PACKAGE__;
1218 $b = $b->num if ref $b eq __PACKAGE__;
1222 All the work of numeric conversion is done in %subr and num(). Of
1223 course, %subr is not complete, it contains only operators used in the
1224 example below. Here is the extra-credit question: why do we need an
1225 explicit recursion in num()? (Answer is at the end of this section.)
1227 Use this module like this:
1230 my $iter = new symbolic 2; # 16-gon
1231 my $side = new symbolic 1;
1235 $cnt = $cnt - 1; # Mutator `--' not implemented
1236 $side = (sqrt(1 + $side**2) - 1)/$side;
1238 printf "%s=%f\n", $side, $side;
1239 printf "pi=%f\n", $side*(2**($iter+2));
1241 It prints (without so many line breaks)
1243 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1245 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1248 The above module is very primitive. It does not implement
1249 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1250 (not required without mutators!), and implements only those arithmetic
1251 operations which are used in the example.
1253 To implement most arithmetic operations is easy; one should just use
1254 the tables of operations, and change the code which fills %subr to
1256 my %subr = ( 'n' => sub {$_[0]} );
1257 foreach my $op (split " ", $overload::ops{with_assign}) {
1258 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1260 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1261 foreach my $op (split " ", "@overload::ops{ @bins }") {
1262 $subr{$op} = eval "sub {shift() $op shift()}";
1264 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1265 print "defining `$op'\n";
1266 $subr{$op} = eval "sub {$op shift()}";
1269 Due to L<Calling Conventions for Mutators>, we do not need anything
1270 special to make C<+=> and friends work, except filling C<+=> entry of
1271 %subr, and defining a copy constructor (needed since Perl has no
1272 way to know that the implementation of C<'+='> does not mutate
1273 the argument, compare L<Copy Constructor>).
1275 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1276 line, and code (this code assumes that mutators change things one level
1277 deep only, so recursive copying is not needed):
1281 bless [@$self], ref $self;
1284 To make C<++> and C<--> work, we need to implement actual mutators,
1285 either directly, or in C<nomethod>. We continue to do things inside
1286 C<nomethod>, thus add
1288 if ($meth eq '++' or $meth eq '--') {
1289 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1293 after the first line of wrap(). This is not a most effective
1294 implementation, one may consider
1296 sub inc { $_[0] = bless ['++', shift, 1]; }
1300 As a final remark, note that one can fill %subr by
1302 my %subr = ( 'n' => sub {$_[0]} );
1303 foreach my $op (split " ", $overload::ops{with_assign}) {
1304 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1306 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1307 foreach my $op (split " ", "@overload::ops{ @bins }") {
1308 $subr{$op} = eval "sub {shift() $op shift()}";
1310 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1311 $subr{$op} = eval "sub {$op shift()}";
1313 $subr{'++'} = $subr{'+'};
1314 $subr{'--'} = $subr{'-'};
1316 This finishes implementation of a primitive symbolic calculator in
1317 50 lines of Perl code. Since the numeric values of subexpressions
1318 are not cached, the calculator is very slow.
1320 Here is the answer for the exercise: In the case of str(), we need no
1321 explicit recursion since the overloaded C<.>-operator will fall back
1322 to an existing overloaded operator C<"">. Overloaded arithmetic
1323 operators I<do not> fall back to numeric conversion if C<fallback> is
1324 not explicitly requested. Thus without an explicit recursion num()
1325 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1326 the argument of num().
1328 If you wonder why defaults for conversion are different for str() and
1329 num(), note how easy it was to write the symbolic calculator. This
1330 simplicity is due to an appropriate choice of defaults. One extra
1331 note: due to the explicit recursion num() is more fragile than sym():
1332 we need to explicitly check for the type of $a and $b. If components
1333 $a and $b happen to be of some related type, this may lead to problems.
1335 =head2 I<Really> symbolic calculator
1337 One may wonder why we call the above calculator symbolic. The reason
1338 is that the actual calculation of the value of expression is postponed
1339 until the value is I<used>.
1341 To see it in action, add a method
1346 @$obj->[0,1] = ('=', shift);
1349 to the package C<symbolic>. After this change one can do
1351 my $a = new symbolic 3;
1352 my $b = new symbolic 4;
1353 my $c = sqrt($a**2 + $b**2);
1355 and the numeric value of $c becomes 5. However, after calling
1357 $a->STORE(12); $b->STORE(5);
1359 the numeric value of $c becomes 13. There is no doubt now that the module
1360 symbolic provides a I<symbolic> calculator indeed.
1362 To hide the rough edges under the hood, provide a tie()d interface to the
1363 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1365 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1367 sub nop { } # Around a bug
1369 (the bug is described in L<"BUGS">). One can use this new interface as
1371 tie $a, 'symbolic', 3;
1372 tie $b, 'symbolic', 4;
1373 $a->nop; $b->nop; # Around a bug
1375 my $c = sqrt($a**2 + $b**2);
1377 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1378 of $c becomes 13. To insulate the user of the module add a method
1380 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1385 symbolic->vars($a, $b);
1386 my $c = sqrt($a**2 + $b**2);
1389 printf "c5 %s=%f\n", $c, $c;
1392 printf "c13 %s=%f\n", $c, $c;
1394 shows that the numeric value of $c follows changes to the values of $a
1399 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1403 When Perl is run with the B<-Do> switch or its equivalent, overloading
1404 induces diagnostic messages.
1406 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1407 deduce which operations are overloaded (and which ancestor triggers
1408 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1409 is shown by debugger. The method C<()> corresponds to the C<fallback>
1410 key (in fact a presence of this method shows that this package has
1411 overloading enabled, and it is what is used by the C<Overloaded>
1412 function of module C<overload>).
1414 The module might issue the following warnings:
1418 =item Odd number of arguments for overload::constant
1420 (W) The call to overload::constant contained an odd number of arguments.
1421 The arguments should come in pairs.
1423 =item `%s' is not an overloadable type
1425 (W) You tried to overload a constant type the overload package is unaware of.
1427 =item `%s' is not a code reference
1429 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1430 to be a code reference. Either an anonymous subroutine, or a reference
1437 Because it is used for overloading, the per-package hash %OVERLOAD now
1438 has a special meaning in Perl. The symbol table is filled with names
1439 looking like line-noise.
1441 For the purpose of inheritance every overloaded package behaves as if
1442 C<fallback> is present (possibly undefined). This may create
1443 interesting effects if some package is not overloaded, but inherits
1444 from two overloaded packages.
1446 Relation between overloading and tie()ing is broken. Overloading is
1447 triggered or not basing on the I<previous> class of tie()d value.
1449 This happens because the presence of overloading is checked too early,
1450 before any tie()d access is attempted. If the FETCH()ed class of the
1451 tie()d value does not change, a simple workaround is to access the value
1452 immediately after tie()ing, so that after this call the I<previous> class
1453 coincides with the current one.
1455 B<Needed:> a way to fix this without a speed penalty.
1457 Barewords are not covered by overloaded string constants.
1459 This document is confusing. There are grammos and misleading language
1460 used in places. It would seem a total rewrite is needed.