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;
75 $package = ref $package if ref $package;
76 #my $meth = $package->can('(' . shift);
77 ov_method mycan($package, '(' . shift), $package;
78 #return $meth if $meth ne \&nil;
83 my $package = ref $_[0];
84 return "$_[0]" unless $package;
87 my $class = Scalar::Util::blessed($_[0]);
88 my $class_prefix = defined($class) ? "$class=" : "";
89 my $type = Scalar::Util::reftype($_[0]);
90 my $addr = Scalar::Util::refaddr($_[0]);
91 return sprintf("$class_prefix$type(0x%x)", $addr);
96 sub mycan { # Real can would leave stubs.
97 my ($package, $meth) = @_;
98 return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
100 foreach $p (@{$package . "::ISA"}) {
101 my $out = mycan($p, $meth);
108 'integer' => 0x1000, # HINT_NEW_INTEGER
109 'float' => 0x2000, # HINT_NEW_FLOAT
110 'binary' => 0x4000, # HINT_NEW_BINARY
111 'q' => 0x8000, # HINT_NEW_STRING
112 'qr' => 0x10000, # HINT_NEW_RE
115 %ops = ( with_assign => "+ - * / % ** << >> x .",
116 assign => "+= -= *= /= %= **= <<= >>= x= .=",
117 num_comparison => "< <= > >= == !=",
118 '3way_comparison'=> "<=> cmp",
119 str_comparison => "lt le gt ge eq ne",
120 binary => '& &= | |= ^ ^=',
123 func => "atan2 cos sin exp abs log sqrt int",
124 conversion => 'bool "" 0+',
126 dereferencing => '${} @{} %{} &{} *{}',
127 special => 'nomethod fallback =');
129 use warnings::register;
131 # Arguments: what, sub
134 warnings::warnif ("Odd number of arguments for overload::constant");
137 elsif (!exists $constants {$_ [0]}) {
138 warnings::warnif ("`$_[0]' is not an overloadable type");
140 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
141 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
142 # blessed, and C<ref> would return the package the ref is blessed into.
143 if (warnings::enabled) {
144 $_ [1] = "undef" unless defined $_ [1];
145 warnings::warn ("`$_[1]' is not a code reference");
150 $^H |= $constants{$_[0]};
156 sub remove_constant {
157 # Arguments: what, sub
160 $^H &= ~ $constants{$_[0]};
171 overload - Package for overloading Perl operations
184 $a = new SomeThing 57;
187 if (overload::Overloaded $b) {...}
189 $strval = overload::StrVal $b;
193 =head2 Declaration of overloaded functions
195 The compilation directive
202 declares function Number::add() for addition, and method muas() in
203 the "class" C<Number> (or one of its base classes)
204 for the assignment form C<*=> of multiplication.
206 Arguments of this directive come in (key, value) pairs. Legal values
207 are values legal inside a C<&{ ... }> call, so the name of a
208 subroutine, a reference to a subroutine, or an anonymous subroutine
209 will all work. Note that values specified as strings are
210 interpreted as methods, not subroutines. Legal keys are listed below.
212 The subroutine C<add> will be called to execute C<$a+$b> if $a
213 is a reference to an object blessed into the package C<Number>, or if $a is
214 not an object from a package with defined mathemagic addition, but $b is a
215 reference to a C<Number>. It can also be called in other situations, like
216 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
217 methods refer to methods triggered by an overloaded mathematical
220 Since overloading respects inheritance via the @ISA hierarchy, the
221 above declaration would also trigger overloading of C<+> and C<*=> in
222 all the packages which inherit from C<Number>.
224 =head2 Calling Conventions for Binary Operations
226 The functions specified in the C<use overload ...> directive are called
227 with three (in one particular case with four, see L<Last Resort>)
228 arguments. If the corresponding operation is binary, then the first
229 two arguments are the two arguments of the operation. However, due to
230 general object calling conventions, the first argument should always be
231 an object in the package, so in the situation of C<7+$a>, the
232 order of the arguments is interchanged. It probably does not matter
233 when implementing the addition method, but whether the arguments
234 are reversed is vital to the subtraction method. The method can
235 query this information by examining the third argument, which can take
236 three different values:
242 the order of arguments is as in the current operation.
246 the arguments are reversed.
250 the current operation is an assignment variant (as in
251 C<$a+=7>), but the usual function is called instead. This additional
252 information can be used to generate some optimizations. Compare
253 L<Calling Conventions for Mutators>.
257 =head2 Calling Conventions for Unary Operations
259 Unary operation are considered binary operations with the second
260 argument being C<undef>. Thus the functions that overloads C<{"++"}>
261 is called with arguments C<($a,undef,'')> when $a++ is executed.
263 =head2 Calling Conventions for Mutators
265 Two types of mutators have different calling conventions:
269 =item C<++> and C<-->
271 The routines which implement these operators are expected to actually
272 I<mutate> their arguments. So, assuming that $obj is a reference to a
275 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
277 is an appropriate implementation of overloaded C<++>. Note that
279 sub incr { ++$ {$_[0]} ; shift }
281 is OK if used with preincrement and with postincrement. (In the case
282 of postincrement a copying will be performed, see L<Copy Constructor>.)
284 =item C<x=> and other assignment versions
286 There is nothing special about these methods. They may change the
287 value of their arguments, and may leave it as is. The result is going
288 to be assigned to the value in the left-hand-side if different from
291 This allows for the same method to be used as overloaded C<+=> and
292 C<+>. Note that this is I<allowed>, but not recommended, since by the
293 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
294 if C<+=> is not overloaded.
298 B<Warning.> Due to the presence of assignment versions of operations,
299 routines which may be called in assignment context may create
300 self-referential structures. Currently Perl will not free self-referential
301 structures until cycles are C<explicitly> broken. You may get problems
302 when traversing your structures too.
306 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
308 is asking for trouble, since for code C<$obj += $foo> the subroutine
309 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
310 \$foo]>. If using such a subroutine is an important optimization, one
311 can overload C<+=> explicitly by a non-"optimized" version, or switch
312 to non-optimized version if C<not defined $_[2]> (see
313 L<Calling Conventions for Binary Operations>).
315 Even if no I<explicit> assignment-variants of operators are present in
316 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
317 C<',' . $obj . ','> may be both optimized to
319 my $tmp = ',' . $obj; $tmp .= ',';
321 =head2 Overloadable Operations
323 The following symbols can be specified in C<use overload> directive:
327 =item * I<Arithmetic operations>
329 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
330 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
332 For these operations a substituted non-assignment variant can be called if
333 the assignment variant is not available. Methods for operations C<+>,
334 C<->, C<+=>, and C<-=> can be called to automatically generate
335 increment and decrement methods. The operation C<-> can be used to
336 autogenerate missing methods for unary minus or C<abs>.
338 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
339 L<"Calling Conventions for Binary Operations">) for details of these
342 =item * I<Comparison operations>
344 "<", "<=", ">", ">=", "==", "!=", "<=>",
345 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
347 If the corresponding "spaceship" variant is available, it can be
348 used to substitute for the missing operation. During C<sort>ing
349 arrays, C<cmp> is used to compare values subject to C<use overload>.
351 =item * I<Bit operations>
353 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
355 C<neg> stands for unary minus. If the method for C<neg> is not
356 specified, it can be autogenerated using the method for
357 subtraction. If the method for C<!> is not specified, it can be
358 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
360 The same remarks in L<"Arithmetic operations"> about
361 assignment-variants and autogeneration apply for
362 bit operations C<"&">, C<"^">, and C<"|"> as well.
364 =item * I<Increment and decrement>
368 If undefined, addition and subtraction methods can be
369 used instead. These operations are called both in prefix and
372 =item * I<Transcendental functions>
374 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
376 If C<abs> is unavailable, it can be autogenerated using methods
377 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
379 Note that traditionally the Perl function L<int> rounds to 0, thus for
380 floating-point-like types one should follow the same semantic. If
381 C<int> is unavailable, it can be autogenerated using the overloading of
384 =item * I<Boolean, string and numeric conversion>
388 If one or two of these operations are not overloaded, the remaining ones can
389 be used instead. C<bool> is used in the flow control operators
390 (like C<while>) and for the ternary C<?:> operation. These functions can
391 return any arbitrary Perl value. If the corresponding operation for this value
392 is overloaded too, that operation will be called again with this value.
394 As a special case if the overload returns the object itself then it will
395 be used directly. An overloaded conversion returning the object is
396 probably a bug, because you're likely to get something that looks like
397 C<YourPackage=HASH(0x8172b34)>.
403 If not overloaded, the argument will be converted to a filehandle or
404 glob (which may require a stringification). The same overloading
405 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
406 I<globbing> syntax C<E<lt>${var}E<gt>>.
408 B<BUGS> Even in list context, the iterator is currently called only
409 once and with scalar context.
411 =item * I<Dereferencing>
413 '${}', '@{}', '%{}', '&{}', '*{}'.
415 If not overloaded, the argument will be dereferenced I<as is>, thus
416 should be of correct type. These functions should return a reference
417 of correct type, or another object with overloaded dereferencing.
419 As a special case if the overload returns the object itself then it
420 will be used directly (provided it is the correct type).
422 The dereference operators must be specified explicitly they will not be passed to
427 "nomethod", "fallback", "=", "~~",
429 see L<SPECIAL SYMBOLS FOR C<use overload>>.
433 See L<"Fallback"> for an explanation of when a missing method can be
436 A computer-readable form of the above table is available in the hash
437 %overload::ops, with values being space-separated lists of names:
439 with_assign => '+ - * / % ** << >> x .',
440 assign => '+= -= *= /= %= **= <<= >>= x= .=',
441 num_comparison => '< <= > >= == !=',
442 '3way_comparison'=> '<=> cmp',
443 str_comparison => 'lt le gt ge eq ne',
444 binary => '& &= | |= ^ ^=',
447 func => 'atan2 cos sin exp abs log sqrt',
448 conversion => 'bool "" 0+',
450 dereferencing => '${} @{} %{} &{} *{}',
451 special => 'nomethod fallback ='
453 =head2 Inheritance and overloading
455 Inheritance interacts with overloading in two ways.
459 =item Strings as values of C<use overload> directive
463 use overload key => value;
465 is a string, it is interpreted as a method name.
467 =item Overloading of an operation is inherited by derived classes
469 Any class derived from an overloaded class is also overloaded. The
470 set of overloaded methods is the union of overloaded methods of all
471 the ancestors. If some method is overloaded in several ancestor, then
472 which description will be used is decided by the usual inheritance
475 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
476 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
477 then the subroutine C<D::plus_sub> will be called to implement
478 operation C<+> for an object in package C<A>.
482 Note that since the value of the C<fallback> key is not a subroutine,
483 its inheritance is not governed by the above rules. In the current
484 implementation, the value of C<fallback> in the first overloaded
485 ancestor is used, but this is accidental and subject to change.
487 =head1 SPECIAL SYMBOLS FOR C<use overload>
489 Three keys are recognized by Perl that are not covered by the above
494 C<"nomethod"> should be followed by a reference to a function of four
495 parameters. If defined, it is called when the overloading mechanism
496 cannot find a method for some operation. The first three arguments of
497 this function coincide with the arguments for the corresponding method if
498 it were found, the fourth argument is the symbol
499 corresponding to the missing method. If several methods are tried,
500 the last one is used. Say, C<1-$a> can be equivalent to
502 &nomethodMethod($a,1,1,"-")
504 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
505 C<use overload> directive.
507 The C<"nomethod"> mechanism is I<not> used for the dereference operators
508 ( ${} @{} %{} &{} *{} ).
511 If some operation cannot be resolved, and there is no function
512 assigned to C<"nomethod">, then an exception will be raised via die()--
513 unless C<"fallback"> was specified as a key in C<use overload> directive.
518 The key C<"fallback"> governs what to do if a method for a particular
519 operation is not found. Three different cases are possible depending on
520 the value of C<"fallback">:
527 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
528 then tries to calls C<"nomethod"> value; if missing, an exception
533 The same as for the C<undef> value, but no exception is raised. Instead,
534 it silently reverts to what it would have done were there no C<use overload>
537 =item * defined, but FALSE
539 No autogeneration is tried. Perl tries to call
540 C<"nomethod"> value, and if this is missing, raises an exception.
544 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
545 yet, see L<"Inheritance and overloading">.
549 The key C<"~~"> allows you to override the smart matching used by
550 the switch construct. See L<feature>.
552 =head2 Copy Constructor
554 The value for C<"="> is a reference to a function with three
555 arguments, i.e., it looks like the other values in C<use
556 overload>. However, it does not overload the Perl assignment
557 operator. This would go against Camel hair.
559 This operation is called in the situations when a mutator is applied
560 to a reference that shares its object with some other reference, such
566 To make this change $a and not change $b, a copy of C<$$a> is made,
567 and $a is assigned a reference to this new object. This operation is
568 done during execution of the C<++$a>, and not during the assignment,
569 (so before the increment C<$$a> coincides with C<$$b>). This is only
570 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
571 C<nomethod>). Note that if this operation is expressed via C<'+'>
572 a nonmutator, i.e., as in
577 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
578 appear as lvalue when the above code is executed.
580 If the copy constructor is required during the execution of some mutator,
581 but a method for C<'='> was not specified, it can be autogenerated as a
582 string copy if the object is a plain scalar.
588 The actually executed code for
591 Something else which does not modify $a or $b....
597 Something else which does not modify $a or $b....
598 $a = $a->clone(undef,"");
601 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
602 C<'='> was overloaded with C<\&clone>.
606 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
609 =head1 MAGIC AUTOGENERATION
611 If a method for an operation is not found, and the value for C<"fallback"> is
612 TRUE or undefined, Perl tries to autogenerate a substitute method for
613 the missing operation based on the defined operations. Autogenerated method
614 substitutions are possible for the following operations:
618 =item I<Assignment forms of arithmetic operations>
620 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
623 =item I<Conversion operations>
625 String, numeric, and boolean conversion are calculated in terms of one
626 another if not all of them are defined.
628 =item I<Increment and decrement>
630 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
631 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
635 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
639 can be expressed in terms of subtraction.
643 C<!> and C<not> can be expressed in terms of boolean conversion, or
644 string or numerical conversion.
646 =item I<Concatenation>
648 can be expressed in terms of string conversion.
650 =item I<Comparison operations>
652 can be expressed in terms of its "spaceship" counterpart: either
653 C<E<lt>=E<gt>> or C<cmp>:
655 <, >, <=, >=, ==, != in terms of <=>
656 lt, gt, le, ge, eq, ne in terms of cmp
660 <> in terms of builtin operations
662 =item I<Dereferencing>
664 ${} @{} %{} &{} *{} in terms of builtin operations
666 =item I<Copy operator>
668 can be expressed in terms of an assignment to the dereferenced value, if this
669 value is a scalar and not a reference.
673 =head1 Minimal set of overloaded operations
675 Since some operations can be automatically generated from others, there is
676 a minimal set of operations that need to be overloaded in order to have
677 the complete set of overloaded operations at one's disposal.
678 Of course, the autogenerated operations may not do exactly what the user
679 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
684 atan2 cos sin exp log sqrt int
686 Additionally, you need to define at least one of string, boolean or
687 numeric conversions because any one can be used to emulate the others.
688 The string conversion can also be used to emulate concatenation.
690 =head1 Losing overloading
692 The restriction for the comparison operation is that even if, for example,
693 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
694 function will produce only a standard logical value based on the
695 numerical value of the result of `C<cmp>'. In particular, a working
696 numeric conversion is needed in this case (possibly expressed in terms of
699 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
700 if the string conversion substitution is applied.
702 When you chop() a mathemagical object it is promoted to a string and its
703 mathemagical properties are lost. The same can happen with other
706 =head1 Run-time Overloading
708 Since all C<use> directives are executed at compile-time, the only way to
709 change overloading during run-time is to
711 eval 'use overload "+" => \&addmethod';
715 eval 'no overload "+", "--", "<="';
717 though the use of these constructs during run-time is questionable.
719 =head1 Public functions
721 Package C<overload.pm> provides the following public functions:
725 =item overload::StrVal(arg)
727 Gives string value of C<arg> as in absence of stringify overloading. If you
728 are using this to get the address of a reference (useful for checking if two
729 references point to the same thing) then you may be better off using
730 C<Scalar::Util::refaddr()>, which is faster.
732 =item overload::Overloaded(arg)
734 Returns true if C<arg> is subject to overloading of some operations.
736 =item overload::Method(obj,op)
738 Returns C<undef> or a reference to the method that implements C<op>.
742 =head1 Overloading constants
744 For some applications, the Perl parser mangles constants too much.
745 It is possible to hook into this process via C<overload::constant()>
746 and C<overload::remove_constant()> functions.
748 These functions take a hash as an argument. The recognized keys of this hash
755 to overload integer constants,
759 to overload floating point constants,
763 to overload octal and hexadecimal constants,
767 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
768 strings and here-documents,
772 to overload constant pieces of regular expressions.
776 The corresponding values are references to functions which take three arguments:
777 the first one is the I<initial> string form of the constant, the second one
778 is how Perl interprets this constant, the third one is how the constant is used.
779 Note that the initial string form does not
780 contain string delimiters, and has backslashes in backslash-delimiter
781 combinations stripped (thus the value of delimiter is not relevant for
782 processing of this string). The return value of this function is how this
783 constant is going to be interpreted by Perl. The third argument is undefined
784 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
785 context (comes from strings, regular expressions, and single-quote HERE
786 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
787 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
789 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
790 it is expected that overloaded constant strings are equipped with reasonable
791 overloaded catenation operator, otherwise absurd results will result.
792 Similarly, negative numbers are considered as negations of positive constants.
794 Note that it is probably meaningless to call the functions overload::constant()
795 and overload::remove_constant() from anywhere but import() and unimport() methods.
796 From these methods they may be called as
801 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
802 overload::constant integer => sub {Math::BigInt->new(shift)};
805 =head1 IMPLEMENTATION
807 What follows is subject to change RSN.
809 The table of methods for all operations is cached in magic for the
810 symbol table hash for the package. The cache is invalidated during
811 processing of C<use overload>, C<no overload>, new function
812 definitions, and changes in @ISA. However, this invalidation remains
813 unprocessed until the next C<bless>ing into the package. Hence if you
814 want to change overloading structure dynamically, you'll need an
815 additional (fake) C<bless>ing to update the table.
817 (Every SVish thing has a magic queue, and magic is an entry in that
818 queue. This is how a single variable may participate in multiple
819 forms of magic simultaneously. For instance, environment variables
820 regularly have two forms at once: their %ENV magic and their taint
821 magic. However, the magic which implements overloading is applied to
822 the stashes, which are rarely used directly, thus should not slow down
825 If an object belongs to a package using overload, it carries a special
826 flag. Thus the only speed penalty during arithmetic operations without
827 overloading is the checking of this flag.
829 In fact, if C<use overload> is not present, there is almost no overhead
830 for overloadable operations, so most programs should not suffer
831 measurable performance penalties. A considerable effort was made to
832 minimize the overhead when overload is used in some package, but the
833 arguments in question do not belong to packages using overload. When
834 in doubt, test your speed with C<use overload> and without it. So far
835 there have been no reports of substantial speed degradation if Perl is
836 compiled with optimization turned on.
838 There is no size penalty for data if overload is not used. The only
839 size penalty if overload is used in some package is that I<all> the
840 packages acquire a magic during the next C<bless>ing into the
841 package. This magic is three-words-long for packages without
842 overloading, and carries the cache table if the package is overloaded.
844 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
845 carried out before any operation that can imply an assignment to the
846 object $a (or $b) refers to, like C<$a++>. You can override this
847 behavior by defining your own copy constructor (see L<"Copy Constructor">).
849 It is expected that arguments to methods that are not explicitly supposed
850 to be changed are constant (but this is not enforced).
852 =head1 Metaphor clash
854 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
855 If it I<looks> counter intuitive to you, you are subject to a metaphor
858 Here is a Perl object metaphor:
860 I< object is a reference to blessed data>
862 and an arithmetic metaphor:
864 I< object is a thing by itself>.
866 The I<main> problem of overloading C<=> is the fact that these metaphors
867 imply different actions on the assignment C<$a = $b> if $a and $b are
868 objects. Perl-think implies that $a becomes a reference to whatever
869 $b was referencing. Arithmetic-think implies that the value of "object"
870 $a is changed to become the value of the object $b, preserving the fact
871 that $a and $b are separate entities.
873 The difference is not relevant in the absence of mutators. After
874 a Perl-way assignment an operation which mutates the data referenced by $a
875 would change the data referenced by $b too. Effectively, after
876 C<$a = $b> values of $a and $b become I<indistinguishable>.
878 On the other hand, anyone who has used algebraic notation knows the
879 expressive power of the arithmetic metaphor. Overloading works hard
880 to enable this metaphor while preserving the Perlian way as far as
881 possible. Since it is not possible to freely mix two contradicting
882 metaphors, overloading allows the arithmetic way to write things I<as
883 far as all the mutators are called via overloaded access only>. The
884 way it is done is described in L<Copy Constructor>.
886 If some mutator methods are directly applied to the overloaded values,
887 one may need to I<explicitly unlink> other values which references the
892 $b = $a; # $b is "linked" to $a
894 $a = $a->clone; # Unlink $b from $a
897 Note that overloaded access makes this transparent:
900 $b = $a; # $b is "linked" to $a
901 $a += 4; # would unlink $b automagically
903 However, it would not make
906 $a = 4; # Now $a is a plain 4, not 'Data'
908 preserve "objectness" of $a. But Perl I<has> a way to make assignments
909 to an object do whatever you want. It is just not the overload, but
910 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
911 which returns the object itself, and STORE() method which changes the
912 value of the object, one can reproduce the arithmetic metaphor in its
913 completeness, at least for variables which were tie()d from the start.
915 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
919 Please add examples to what follows!
921 =head2 Two-face scalars
923 Put this in F<two_face.pm> in your Perl library directory:
925 package two_face; # Scalars with separate string and
927 sub new { my $p = shift; bless [@_], $p }
928 use overload '""' => \&str, '0+' => \&num, fallback => 1;
935 my $seven = new two_face ("vii", 7);
936 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
937 print "seven contains `i'\n" if $seven =~ /i/;
939 (The second line creates a scalar which has both a string value, and a
940 numeric value.) This prints:
942 seven=vii, seven=7, eight=8
945 =head2 Two-face references
947 Suppose you want to create an object which is accessible as both an
948 array reference and a hash reference.
951 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
959 tie %h, ref $self, $self;
963 sub TIEHASH { my $p = shift; bless \ shift, $p }
966 $fields{$_} = $i++ foreach qw{zero one two three};
968 my $self = ${shift()};
969 my $key = $fields{shift()};
970 defined $key or die "Out of band access";
971 $$self->[$key] = shift;
974 my $self = ${shift()};
975 my $key = $fields{shift()};
976 defined $key or die "Out of band access";
980 Now one can access an object using both the array and hash syntax:
982 my $bar = new two_refs 3,4,5,6;
984 $bar->{two} == 11 or die 'bad hash fetch';
986 Note several important features of this example. First of all, the
987 I<actual> type of $bar is a scalar reference, and we do not overload
988 the scalar dereference. Thus we can get the I<actual> non-overloaded
989 contents of $bar by just using C<$$bar> (what we do in functions which
990 overload dereference). Similarly, the object returned by the
991 TIEHASH() method is a scalar reference.
993 Second, we create a new tied hash each time the hash syntax is used.
994 This allows us not to worry about a possibility of a reference loop,
995 which would lead to a memory leak.
997 Both these problems can be cured. Say, if we want to overload hash
998 dereference on a reference to an object which is I<implemented> as a
999 hash itself, the only problem one has to circumvent is how to access
1000 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1001 overloaded dereference operator). Here is one possible fetching routine:
1004 my ($self, $key) = (shift, shift);
1005 my $class = ref $self;
1006 bless $self, 'overload::dummy'; # Disable overloading of %{}
1007 my $out = $self->{$key};
1008 bless $self, $class; # Restore overloading
1012 To remove creation of the tied hash on each access, one may an extra
1013 level of indirection which allows a non-circular structure of references:
1016 use overload '%{}' => sub { ${shift()}->[1] },
1017 '@{}' => sub { ${shift()}->[0] };
1023 bless \ [$a, \%h], $p;
1028 tie %h, ref $self, $self;
1032 sub TIEHASH { my $p = shift; bless \ shift, $p }
1035 $fields{$_} = $i++ foreach qw{zero one two three};
1038 my $key = $fields{shift()};
1039 defined $key or die "Out of band access";
1044 my $key = $fields{shift()};
1045 defined $key or die "Out of band access";
1049 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1050 reference to the intermediate array, which keeps a reference to an
1051 actual array, and the access hash. The tie()ing object for the access
1052 hash is a reference to a reference to the actual array, so
1058 There are no loops of references.
1062 Both "objects" which are blessed into the class C<two_refs1> are
1063 references to a reference to an array, thus references to a I<scalar>.
1064 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1065 overloaded operations.
1069 =head2 Symbolic calculator
1071 Put this in F<symbolic.pm> in your Perl library directory:
1073 package symbolic; # Primitive symbolic calculator
1074 use overload nomethod => \&wrap;
1076 sub new { shift; bless ['n', @_] }
1078 my ($obj, $other, $inv, $meth) = @_;
1079 ($obj, $other) = ($other, $obj) if $inv;
1080 bless [$meth, $obj, $other];
1083 This module is very unusual as overloaded modules go: it does not
1084 provide any usual overloaded operators, instead it provides the L<Last
1085 Resort> operator C<nomethod>. In this example the corresponding
1086 subroutine returns an object which encapsulates operations done over
1087 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1088 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1090 Here is an example of the script which "calculates" the side of
1091 circumscribed octagon using the above package:
1094 my $iter = 1; # 2**($iter+2) = 8
1095 my $side = new symbolic 1;
1099 $side = (sqrt(1 + $side**2) - 1)/$side;
1103 The value of $side is
1105 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1106 undef], 1], ['n', 1]]
1108 Note that while we obtained this value using a nice little script,
1109 there is no simple way to I<use> this value. In fact this value may
1110 be inspected in debugger (see L<perldebug>), but only if
1111 C<bareStringify> B<O>ption is set, and not via C<p> command.
1113 If one attempts to print this value, then the overloaded operator
1114 C<""> will be called, which will call C<nomethod> operator. The
1115 result of this operator will be stringified again, but this result is
1116 again of type C<symbolic>, which will lead to an infinite loop.
1118 Add a pretty-printer method to the module F<symbolic.pm>:
1121 my ($meth, $a, $b) = @{+shift};
1122 $a = 'u' unless defined $a;
1123 $b = 'u' unless defined $b;
1124 $a = $a->pretty if ref $a;
1125 $b = $b->pretty if ref $b;
1129 Now one can finish the script by
1131 print "side = ", $side->pretty, "\n";
1133 The method C<pretty> is doing object-to-string conversion, so it
1134 is natural to overload the operator C<""> using this method. However,
1135 inside such a method it is not necessary to pretty-print the
1136 I<components> $a and $b of an object. In the above subroutine
1137 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1138 and $b. If these components use overloading, the catenation operator
1139 will look for an overloaded operator C<.>; if not present, it will
1140 look for an overloaded operator C<"">. Thus it is enough to use
1142 use overload nomethod => \&wrap, '""' => \&str;
1144 my ($meth, $a, $b) = @{+shift};
1145 $a = 'u' unless defined $a;
1146 $b = 'u' unless defined $b;
1150 Now one can change the last line of the script to
1152 print "side = $side\n";
1156 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1158 and one can inspect the value in debugger using all the possible
1161 Something is still amiss: consider the loop variable $cnt of the
1162 script. It was a number, not an object. We cannot make this value of
1163 type C<symbolic>, since then the loop will not terminate.
1165 Indeed, to terminate the cycle, the $cnt should become false.
1166 However, the operator C<bool> for checking falsity is overloaded (this
1167 time via overloaded C<"">), and returns a long string, thus any object
1168 of type C<symbolic> is true. To overcome this, we need a way to
1169 compare an object to 0. In fact, it is easier to write a numeric
1172 Here is the text of F<symbolic.pm> with such a routine added (and
1173 slightly modified str()):
1175 package symbolic; # Primitive symbolic calculator
1177 nomethod => \&wrap, '""' => \&str, '0+' => \#
1179 sub new { shift; bless ['n', @_] }
1181 my ($obj, $other, $inv, $meth) = @_;
1182 ($obj, $other) = ($other, $obj) if $inv;
1183 bless [$meth, $obj, $other];
1186 my ($meth, $a, $b) = @{+shift};
1187 $a = 'u' unless defined $a;
1194 my %subr = ( n => sub {$_[0]},
1195 sqrt => sub {sqrt $_[0]},
1196 '-' => sub {shift() - shift()},
1197 '+' => sub {shift() + shift()},
1198 '/' => sub {shift() / shift()},
1199 '*' => sub {shift() * shift()},
1200 '**' => sub {shift() ** shift()},
1203 my ($meth, $a, $b) = @{+shift};
1204 my $subr = $subr{$meth}
1205 or die "Do not know how to ($meth) in symbolic";
1206 $a = $a->num if ref $a eq __PACKAGE__;
1207 $b = $b->num if ref $b eq __PACKAGE__;
1211 All the work of numeric conversion is done in %subr and num(). Of
1212 course, %subr is not complete, it contains only operators used in the
1213 example below. Here is the extra-credit question: why do we need an
1214 explicit recursion in num()? (Answer is at the end of this section.)
1216 Use this module like this:
1219 my $iter = new symbolic 2; # 16-gon
1220 my $side = new symbolic 1;
1224 $cnt = $cnt - 1; # Mutator `--' not implemented
1225 $side = (sqrt(1 + $side**2) - 1)/$side;
1227 printf "%s=%f\n", $side, $side;
1228 printf "pi=%f\n", $side*(2**($iter+2));
1230 It prints (without so many line breaks)
1232 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1234 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1237 The above module is very primitive. It does not implement
1238 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1239 (not required without mutators!), and implements only those arithmetic
1240 operations which are used in the example.
1242 To implement most arithmetic operations is easy; one should just use
1243 the tables of operations, and change the code which fills %subr to
1245 my %subr = ( 'n' => sub {$_[0]} );
1246 foreach my $op (split " ", $overload::ops{with_assign}) {
1247 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1249 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1250 foreach my $op (split " ", "@overload::ops{ @bins }") {
1251 $subr{$op} = eval "sub {shift() $op shift()}";
1253 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1254 print "defining `$op'\n";
1255 $subr{$op} = eval "sub {$op shift()}";
1258 Due to L<Calling Conventions for Mutators>, we do not need anything
1259 special to make C<+=> and friends work, except filling C<+=> entry of
1260 %subr, and defining a copy constructor (needed since Perl has no
1261 way to know that the implementation of C<'+='> does not mutate
1262 the argument, compare L<Copy Constructor>).
1264 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1265 line, and code (this code assumes that mutators change things one level
1266 deep only, so recursive copying is not needed):
1270 bless [@$self], ref $self;
1273 To make C<++> and C<--> work, we need to implement actual mutators,
1274 either directly, or in C<nomethod>. We continue to do things inside
1275 C<nomethod>, thus add
1277 if ($meth eq '++' or $meth eq '--') {
1278 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1282 after the first line of wrap(). This is not a most effective
1283 implementation, one may consider
1285 sub inc { $_[0] = bless ['++', shift, 1]; }
1289 As a final remark, note that one can fill %subr by
1291 my %subr = ( 'n' => sub {$_[0]} );
1292 foreach my $op (split " ", $overload::ops{with_assign}) {
1293 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1295 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1296 foreach my $op (split " ", "@overload::ops{ @bins }") {
1297 $subr{$op} = eval "sub {shift() $op shift()}";
1299 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1300 $subr{$op} = eval "sub {$op shift()}";
1302 $subr{'++'} = $subr{'+'};
1303 $subr{'--'} = $subr{'-'};
1305 This finishes implementation of a primitive symbolic calculator in
1306 50 lines of Perl code. Since the numeric values of subexpressions
1307 are not cached, the calculator is very slow.
1309 Here is the answer for the exercise: In the case of str(), we need no
1310 explicit recursion since the overloaded C<.>-operator will fall back
1311 to an existing overloaded operator C<"">. Overloaded arithmetic
1312 operators I<do not> fall back to numeric conversion if C<fallback> is
1313 not explicitly requested. Thus without an explicit recursion num()
1314 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1315 the argument of num().
1317 If you wonder why defaults for conversion are different for str() and
1318 num(), note how easy it was to write the symbolic calculator. This
1319 simplicity is due to an appropriate choice of defaults. One extra
1320 note: due to the explicit recursion num() is more fragile than sym():
1321 we need to explicitly check for the type of $a and $b. If components
1322 $a and $b happen to be of some related type, this may lead to problems.
1324 =head2 I<Really> symbolic calculator
1326 One may wonder why we call the above calculator symbolic. The reason
1327 is that the actual calculation of the value of expression is postponed
1328 until the value is I<used>.
1330 To see it in action, add a method
1335 @$obj->[0,1] = ('=', shift);
1338 to the package C<symbolic>. After this change one can do
1340 my $a = new symbolic 3;
1341 my $b = new symbolic 4;
1342 my $c = sqrt($a**2 + $b**2);
1344 and the numeric value of $c becomes 5. However, after calling
1346 $a->STORE(12); $b->STORE(5);
1348 the numeric value of $c becomes 13. There is no doubt now that the module
1349 symbolic provides a I<symbolic> calculator indeed.
1351 To hide the rough edges under the hood, provide a tie()d interface to the
1352 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1354 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1356 sub nop { } # Around a bug
1358 (the bug is described in L<"BUGS">). One can use this new interface as
1360 tie $a, 'symbolic', 3;
1361 tie $b, 'symbolic', 4;
1362 $a->nop; $b->nop; # Around a bug
1364 my $c = sqrt($a**2 + $b**2);
1366 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1367 of $c becomes 13. To insulate the user of the module add a method
1369 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1374 symbolic->vars($a, $b);
1375 my $c = sqrt($a**2 + $b**2);
1378 printf "c5 %s=%f\n", $c, $c;
1381 printf "c13 %s=%f\n", $c, $c;
1383 shows that the numeric value of $c follows changes to the values of $a
1388 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1392 When Perl is run with the B<-Do> switch or its equivalent, overloading
1393 induces diagnostic messages.
1395 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1396 deduce which operations are overloaded (and which ancestor triggers
1397 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1398 is shown by debugger. The method C<()> corresponds to the C<fallback>
1399 key (in fact a presence of this method shows that this package has
1400 overloading enabled, and it is what is used by the C<Overloaded>
1401 function of module C<overload>).
1403 The module might issue the following warnings:
1407 =item Odd number of arguments for overload::constant
1409 (W) The call to overload::constant contained an odd number of arguments.
1410 The arguments should come in pairs.
1412 =item `%s' is not an overloadable type
1414 (W) You tried to overload a constant type the overload package is unaware of.
1416 =item `%s' is not a code reference
1418 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1419 to be a code reference. Either an anonymous subroutine, or a reference
1426 Because it is used for overloading, the per-package hash %OVERLOAD now
1427 has a special meaning in Perl. The symbol table is filled with names
1428 looking like line-noise.
1430 For the purpose of inheritance every overloaded package behaves as if
1431 C<fallback> is present (possibly undefined). This may create
1432 interesting effects if some package is not overloaded, but inherits
1433 from two overloaded packages.
1435 Relation between overloading and tie()ing is broken. Overloading is
1436 triggered or not basing on the I<previous> class of tie()d value.
1438 This happens because the presence of overloading is checked too early,
1439 before any tie()d access is attempted. If the FETCH()ed class of the
1440 tie()d value does not change, a simple workaround is to access the value
1441 immediately after tie()ing, so that after this call the I<previous> class
1442 coincides with the current one.
1444 B<Needed:> a way to fix this without a speed penalty.
1446 Barewords are not covered by overloaded string constants.
1448 This document is confusing. There are grammos and misleading language
1449 used in places. It would seem a total rewrite is needed.