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, $meth);
108 'integer' => 0x1000, # HINT_NEW_INTEGER
109 'float' => 0x2000, # HINT_NEW_FLOAT
110 'binary' => 0x4000, # HINT_NEW_BINARY
111 'q' => 0x8000, # HINT_NEW_STRING
112 'qr' => 0x10000, # HINT_NEW_RE
115 %ops = ( with_assign => "+ - * / % ** << >> x .",
116 assign => "+= -= *= /= %= **= <<= >>= x= .=",
117 num_comparison => "< <= > >= == !=",
118 '3way_comparison'=> "<=> cmp",
119 str_comparison => "lt le gt ge eq ne",
123 func => "atan2 cos sin exp abs log sqrt int",
124 conversion => 'bool "" 0+',
126 dereferencing => '${} @{} %{} &{} *{}',
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 =item * I<Increment and decrement>
364 If undefined, addition and subtraction methods can be
365 used instead. These operations are called both in prefix and
368 =item * I<Transcendental functions>
370 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
372 If C<abs> is unavailable, it can be autogenerated using methods
373 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
375 Note that traditionally the Perl function L<int> rounds to 0, thus for
376 floating-point-like types one should follow the same semantic. If
377 C<int> is unavailable, it can be autogenerated using the overloading of
380 =item * I<Boolean, string and numeric conversion>
384 If one or two of these operations are not overloaded, the remaining ones can
385 be used instead. C<bool> is used in the flow control operators
386 (like C<while>) and for the ternary C<?:> operation. These functions can
387 return any arbitrary Perl value. If the corresponding operation for this value
388 is overloaded too, that operation will be called again with this value.
390 As a special case if the overload returns the object itself then it will
391 be used directly. An overloaded conversion returning the object is
392 probably a bug, because you're likely to get something that looks like
393 C<YourPackage=HASH(0x8172b34)>.
399 If not overloaded, the argument will be converted to a filehandle or
400 glob (which may require a stringification). The same overloading
401 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
402 I<globbing> syntax C<E<lt>${var}E<gt>>.
404 B<BUGS> Even in list context, the iterator is currently called only
405 once and with scalar context.
407 =item * I<Dereferencing>
409 '${}', '@{}', '%{}', '&{}', '*{}'.
411 If not overloaded, the argument will be dereferenced I<as is>, thus
412 should be of correct type. These functions should return a reference
413 of correct type, or another object with overloaded dereferencing.
415 As a special case if the overload returns the object itself then it
416 will be used directly (provided it is the correct type).
418 The dereference operators must be specified explicitly they will not be passed to
423 "nomethod", "fallback", "=", "~~",
425 see L<SPECIAL SYMBOLS FOR C<use overload>>.
429 See L<"Fallback"> for an explanation of when a missing method can be
432 A computer-readable form of the above table is available in the hash
433 %overload::ops, with values being space-separated lists of names:
435 with_assign => '+ - * / % ** << >> x .',
436 assign => '+= -= *= /= %= **= <<= >>= x= .=',
437 num_comparison => '< <= > >= == !=',
438 '3way_comparison'=> '<=> cmp',
439 str_comparison => 'lt le gt ge eq ne',
443 func => 'atan2 cos sin exp abs log sqrt',
444 conversion => 'bool "" 0+',
446 dereferencing => '${} @{} %{} &{} *{}',
447 special => 'nomethod fallback ='
449 =head2 Inheritance and overloading
451 Inheritance interacts with overloading in two ways.
455 =item Strings as values of C<use overload> directive
459 use overload key => value;
461 is a string, it is interpreted as a method name.
463 =item Overloading of an operation is inherited by derived classes
465 Any class derived from an overloaded class is also overloaded. The
466 set of overloaded methods is the union of overloaded methods of all
467 the ancestors. If some method is overloaded in several ancestor, then
468 which description will be used is decided by the usual inheritance
471 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
472 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
473 then the subroutine C<D::plus_sub> will be called to implement
474 operation C<+> for an object in package C<A>.
478 Note that since the value of the C<fallback> key is not a subroutine,
479 its inheritance is not governed by the above rules. In the current
480 implementation, the value of C<fallback> in the first overloaded
481 ancestor is used, but this is accidental and subject to change.
483 =head1 SPECIAL SYMBOLS FOR C<use overload>
485 Three keys are recognized by Perl that are not covered by the above
490 C<"nomethod"> should be followed by a reference to a function of four
491 parameters. If defined, it is called when the overloading mechanism
492 cannot find a method for some operation. The first three arguments of
493 this function coincide with the arguments for the corresponding method if
494 it were found, the fourth argument is the symbol
495 corresponding to the missing method. If several methods are tried,
496 the last one is used. Say, C<1-$a> can be equivalent to
498 &nomethodMethod($a,1,1,"-")
500 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
501 C<use overload> directive.
503 The C<"nomethod"> mechanism is I<not> used for the dereference operators
504 ( ${} @{} %{} &{} *{} ).
507 If some operation cannot be resolved, and there is no function
508 assigned to C<"nomethod">, then an exception will be raised via die()--
509 unless C<"fallback"> was specified as a key in C<use overload> directive.
514 The key C<"fallback"> governs what to do if a method for a particular
515 operation is not found. Three different cases are possible depending on
516 the value of C<"fallback">:
523 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
524 then tries to calls C<"nomethod"> value; if missing, an exception
529 The same as for the C<undef> value, but no exception is raised. Instead,
530 it silently reverts to what it would have done were there no C<use overload>
533 =item * defined, but FALSE
535 No autogeneration is tried. Perl tries to call
536 C<"nomethod"> value, and if this is missing, raises an exception.
540 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
541 yet, see L<"Inheritance and overloading">.
545 The key C<"~~"> allows you to override the smart matching used by
546 the switch construct. See L<feature>.
548 =head2 Copy Constructor
550 The value for C<"="> is a reference to a function with three
551 arguments, i.e., it looks like the other values in C<use
552 overload>. However, it does not overload the Perl assignment
553 operator. This would go against Camel hair.
555 This operation is called in the situations when a mutator is applied
556 to a reference that shares its object with some other reference, such
562 To make this change $a and not change $b, a copy of C<$$a> is made,
563 and $a is assigned a reference to this new object. This operation is
564 done during execution of the C<++$a>, and not during the assignment,
565 (so before the increment C<$$a> coincides with C<$$b>). This is only
566 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
567 C<nomethod>). Note that if this operation is expressed via C<'+'>
568 a nonmutator, i.e., as in
573 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
574 appear as lvalue when the above code is executed.
576 If the copy constructor is required during the execution of some mutator,
577 but a method for C<'='> was not specified, it can be autogenerated as a
578 string copy if the object is a plain scalar.
584 The actually executed code for
587 Something else which does not modify $a or $b....
593 Something else which does not modify $a or $b....
594 $a = $a->clone(undef,"");
597 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
598 C<'='> was overloaded with C<\&clone>.
602 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
605 =head1 MAGIC AUTOGENERATION
607 If a method for an operation is not found, and the value for C<"fallback"> is
608 TRUE or undefined, Perl tries to autogenerate a substitute method for
609 the missing operation based on the defined operations. Autogenerated method
610 substitutions are possible for the following operations:
614 =item I<Assignment forms of arithmetic operations>
616 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
619 =item I<Conversion operations>
621 String, numeric, and boolean conversion are calculated in terms of one
622 another if not all of them are defined.
624 =item I<Increment and decrement>
626 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
627 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
631 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
635 can be expressed in terms of subtraction.
639 C<!> and C<not> can be expressed in terms of boolean conversion, or
640 string or numerical conversion.
642 =item I<Concatenation>
644 can be expressed in terms of string conversion.
646 =item I<Comparison operations>
648 can be expressed in terms of its "spaceship" counterpart: either
649 C<E<lt>=E<gt>> or C<cmp>:
651 <, >, <=, >=, ==, != in terms of <=>
652 lt, gt, le, ge, eq, ne in terms of cmp
656 <> in terms of builtin operations
658 =item I<Dereferencing>
660 ${} @{} %{} &{} *{} in terms of builtin operations
662 =item I<Copy operator>
664 can be expressed in terms of an assignment to the dereferenced value, if this
665 value is a scalar and not a reference.
669 =head1 Minimal set of overloaded operations
671 Since some operations can be automatically generated from others, there is
672 a minimal set of operations that need to be overloaded in order to have
673 the complete set of overloaded operations at one's disposal.
674 Of course, the autogenerated operations may not do exactly what the user
675 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
680 atan2 cos sin exp log sqrt int
682 Additionally, you need to define at least one of string, boolean or
683 numeric conversions because any one can be used to emulate the others.
684 The string conversion can also be used to emulate concatenation.
686 =head1 Losing overloading
688 The restriction for the comparison operation is that even if, for example,
689 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
690 function will produce only a standard logical value based on the
691 numerical value of the result of `C<cmp>'. In particular, a working
692 numeric conversion is needed in this case (possibly expressed in terms of
695 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
696 if the string conversion substitution is applied.
698 When you chop() a mathemagical object it is promoted to a string and its
699 mathemagical properties are lost. The same can happen with other
702 =head1 Run-time Overloading
704 Since all C<use> directives are executed at compile-time, the only way to
705 change overloading during run-time is to
707 eval 'use overload "+" => \&addmethod';
711 eval 'no overload "+", "--", "<="';
713 though the use of these constructs during run-time is questionable.
715 =head1 Public functions
717 Package C<overload.pm> provides the following public functions:
721 =item overload::StrVal(arg)
723 Gives string value of C<arg> as in absence of stringify overloading. If you
724 are using this to get the address of a reference (useful for checking if two
725 references point to the same thing) then you may be better off using
726 C<Scalar::Util::refaddr()>, which is faster.
728 =item overload::Overloaded(arg)
730 Returns true if C<arg> is subject to overloading of some operations.
732 =item overload::Method(obj,op)
734 Returns C<undef> or a reference to the method that implements C<op>.
738 =head1 Overloading constants
740 For some applications, the Perl parser mangles constants too much.
741 It is possible to hook into this process via C<overload::constant()>
742 and C<overload::remove_constant()> functions.
744 These functions take a hash as an argument. The recognized keys of this hash
751 to overload integer constants,
755 to overload floating point constants,
759 to overload octal and hexadecimal constants,
763 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
764 strings and here-documents,
768 to overload constant pieces of regular expressions.
772 The corresponding values are references to functions which take three arguments:
773 the first one is the I<initial> string form of the constant, the second one
774 is how Perl interprets this constant, the third one is how the constant is used.
775 Note that the initial string form does not
776 contain string delimiters, and has backslashes in backslash-delimiter
777 combinations stripped (thus the value of delimiter is not relevant for
778 processing of this string). The return value of this function is how this
779 constant is going to be interpreted by Perl. The third argument is undefined
780 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
781 context (comes from strings, regular expressions, and single-quote HERE
782 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
783 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
785 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
786 it is expected that overloaded constant strings are equipped with reasonable
787 overloaded catenation operator, otherwise absurd results will result.
788 Similarly, negative numbers are considered as negations of positive constants.
790 Note that it is probably meaningless to call the functions overload::constant()
791 and overload::remove_constant() from anywhere but import() and unimport() methods.
792 From these methods they may be called as
797 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
798 overload::constant integer => sub {Math::BigInt->new(shift)};
801 =head1 IMPLEMENTATION
803 What follows is subject to change RSN.
805 The table of methods for all operations is cached in magic for the
806 symbol table hash for the package. The cache is invalidated during
807 processing of C<use overload>, C<no overload>, new function
808 definitions, and changes in @ISA. However, this invalidation remains
809 unprocessed until the next C<bless>ing into the package. Hence if you
810 want to change overloading structure dynamically, you'll need an
811 additional (fake) C<bless>ing to update the table.
813 (Every SVish thing has a magic queue, and magic is an entry in that
814 queue. This is how a single variable may participate in multiple
815 forms of magic simultaneously. For instance, environment variables
816 regularly have two forms at once: their %ENV magic and their taint
817 magic. However, the magic which implements overloading is applied to
818 the stashes, which are rarely used directly, thus should not slow down
821 If an object belongs to a package using overload, it carries a special
822 flag. Thus the only speed penalty during arithmetic operations without
823 overloading is the checking of this flag.
825 In fact, if C<use overload> is not present, there is almost no overhead
826 for overloadable operations, so most programs should not suffer
827 measurable performance penalties. A considerable effort was made to
828 minimize the overhead when overload is used in some package, but the
829 arguments in question do not belong to packages using overload. When
830 in doubt, test your speed with C<use overload> and without it. So far
831 there have been no reports of substantial speed degradation if Perl is
832 compiled with optimization turned on.
834 There is no size penalty for data if overload is not used. The only
835 size penalty if overload is used in some package is that I<all> the
836 packages acquire a magic during the next C<bless>ing into the
837 package. This magic is three-words-long for packages without
838 overloading, and carries the cache table if the package is overloaded.
840 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
841 carried out before any operation that can imply an assignment to the
842 object $a (or $b) refers to, like C<$a++>. You can override this
843 behavior by defining your own copy constructor (see L<"Copy Constructor">).
845 It is expected that arguments to methods that are not explicitly supposed
846 to be changed are constant (but this is not enforced).
848 =head1 Metaphor clash
850 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
851 If it I<looks> counter intuitive to you, you are subject to a metaphor
854 Here is a Perl object metaphor:
856 I< object is a reference to blessed data>
858 and an arithmetic metaphor:
860 I< object is a thing by itself>.
862 The I<main> problem of overloading C<=> is the fact that these metaphors
863 imply different actions on the assignment C<$a = $b> if $a and $b are
864 objects. Perl-think implies that $a becomes a reference to whatever
865 $b was referencing. Arithmetic-think implies that the value of "object"
866 $a is changed to become the value of the object $b, preserving the fact
867 that $a and $b are separate entities.
869 The difference is not relevant in the absence of mutators. After
870 a Perl-way assignment an operation which mutates the data referenced by $a
871 would change the data referenced by $b too. Effectively, after
872 C<$a = $b> values of $a and $b become I<indistinguishable>.
874 On the other hand, anyone who has used algebraic notation knows the
875 expressive power of the arithmetic metaphor. Overloading works hard
876 to enable this metaphor while preserving the Perlian way as far as
877 possible. Since it is not possible to freely mix two contradicting
878 metaphors, overloading allows the arithmetic way to write things I<as
879 far as all the mutators are called via overloaded access only>. The
880 way it is done is described in L<Copy Constructor>.
882 If some mutator methods are directly applied to the overloaded values,
883 one may need to I<explicitly unlink> other values which references the
888 $b = $a; # $b is "linked" to $a
890 $a = $a->clone; # Unlink $b from $a
893 Note that overloaded access makes this transparent:
896 $b = $a; # $b is "linked" to $a
897 $a += 4; # would unlink $b automagically
899 However, it would not make
902 $a = 4; # Now $a is a plain 4, not 'Data'
904 preserve "objectness" of $a. But Perl I<has> a way to make assignments
905 to an object do whatever you want. It is just not the overload, but
906 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
907 which returns the object itself, and STORE() method which changes the
908 value of the object, one can reproduce the arithmetic metaphor in its
909 completeness, at least for variables which were tie()d from the start.
911 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
915 Please add examples to what follows!
917 =head2 Two-face scalars
919 Put this in F<two_face.pm> in your Perl library directory:
921 package two_face; # Scalars with separate string and
923 sub new { my $p = shift; bless [@_], $p }
924 use overload '""' => \&str, '0+' => \&num, fallback => 1;
931 my $seven = new two_face ("vii", 7);
932 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
933 print "seven contains `i'\n" if $seven =~ /i/;
935 (The second line creates a scalar which has both a string value, and a
936 numeric value.) This prints:
938 seven=vii, seven=7, eight=8
941 =head2 Two-face references
943 Suppose you want to create an object which is accessible as both an
944 array reference and a hash reference.
947 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
955 tie %h, ref $self, $self;
959 sub TIEHASH { my $p = shift; bless \ shift, $p }
962 $fields{$_} = $i++ foreach qw{zero one two three};
964 my $self = ${shift()};
965 my $key = $fields{shift()};
966 defined $key or die "Out of band access";
967 $$self->[$key] = shift;
970 my $self = ${shift()};
971 my $key = $fields{shift()};
972 defined $key or die "Out of band access";
976 Now one can access an object using both the array and hash syntax:
978 my $bar = new two_refs 3,4,5,6;
980 $bar->{two} == 11 or die 'bad hash fetch';
982 Note several important features of this example. First of all, the
983 I<actual> type of $bar is a scalar reference, and we do not overload
984 the scalar dereference. Thus we can get the I<actual> non-overloaded
985 contents of $bar by just using C<$$bar> (what we do in functions which
986 overload dereference). Similarly, the object returned by the
987 TIEHASH() method is a scalar reference.
989 Second, we create a new tied hash each time the hash syntax is used.
990 This allows us not to worry about a possibility of a reference loop,
991 which would lead to a memory leak.
993 Both these problems can be cured. Say, if we want to overload hash
994 dereference on a reference to an object which is I<implemented> as a
995 hash itself, the only problem one has to circumvent is how to access
996 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
997 overloaded dereference operator). Here is one possible fetching routine:
1000 my ($self, $key) = (shift, shift);
1001 my $class = ref $self;
1002 bless $self, 'overload::dummy'; # Disable overloading of %{}
1003 my $out = $self->{$key};
1004 bless $self, $class; # Restore overloading
1008 To remove creation of the tied hash on each access, one may an extra
1009 level of indirection which allows a non-circular structure of references:
1012 use overload '%{}' => sub { ${shift()}->[1] },
1013 '@{}' => sub { ${shift()}->[0] };
1019 bless \ [$a, \%h], $p;
1024 tie %h, ref $self, $self;
1028 sub TIEHASH { my $p = shift; bless \ shift, $p }
1031 $fields{$_} = $i++ foreach qw{zero one two three};
1034 my $key = $fields{shift()};
1035 defined $key or die "Out of band access";
1040 my $key = $fields{shift()};
1041 defined $key or die "Out of band access";
1045 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1046 reference to the intermediate array, which keeps a reference to an
1047 actual array, and the access hash. The tie()ing object for the access
1048 hash is a reference to a reference to the actual array, so
1054 There are no loops of references.
1058 Both "objects" which are blessed into the class C<two_refs1> are
1059 references to a reference to an array, thus references to a I<scalar>.
1060 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1061 overloaded operations.
1065 =head2 Symbolic calculator
1067 Put this in F<symbolic.pm> in your Perl library directory:
1069 package symbolic; # Primitive symbolic calculator
1070 use overload nomethod => \&wrap;
1072 sub new { shift; bless ['n', @_] }
1074 my ($obj, $other, $inv, $meth) = @_;
1075 ($obj, $other) = ($other, $obj) if $inv;
1076 bless [$meth, $obj, $other];
1079 This module is very unusual as overloaded modules go: it does not
1080 provide any usual overloaded operators, instead it provides the L<Last
1081 Resort> operator C<nomethod>. In this example the corresponding
1082 subroutine returns an object which encapsulates operations done over
1083 the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1084 symbolic 3> contains C<['+', 2, ['n', 3]]>.
1086 Here is an example of the script which "calculates" the side of
1087 circumscribed octagon using the above package:
1090 my $iter = 1; # 2**($iter+2) = 8
1091 my $side = new symbolic 1;
1095 $side = (sqrt(1 + $side**2) - 1)/$side;
1099 The value of $side is
1101 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1102 undef], 1], ['n', 1]]
1104 Note that while we obtained this value using a nice little script,
1105 there is no simple way to I<use> this value. In fact this value may
1106 be inspected in debugger (see L<perldebug>), but ony if
1107 C<bareStringify> B<O>ption is set, and not via C<p> command.
1109 If one attempts to print this value, then the overloaded operator
1110 C<""> will be called, which will call C<nomethod> operator. The
1111 result of this operator will be stringified again, but this result is
1112 again of type C<symbolic>, which will lead to an infinite loop.
1114 Add a pretty-printer method to the module F<symbolic.pm>:
1117 my ($meth, $a, $b) = @{+shift};
1118 $a = 'u' unless defined $a;
1119 $b = 'u' unless defined $b;
1120 $a = $a->pretty if ref $a;
1121 $b = $b->pretty if ref $b;
1125 Now one can finish the script by
1127 print "side = ", $side->pretty, "\n";
1129 The method C<pretty> is doing object-to-string conversion, so it
1130 is natural to overload the operator C<""> using this method. However,
1131 inside such a method it is not necessary to pretty-print the
1132 I<components> $a and $b of an object. In the above subroutine
1133 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1134 and $b. If these components use overloading, the catenation operator
1135 will look for an overloaded operator C<.>; if not present, it will
1136 look for an overloaded operator C<"">. Thus it is enough to use
1138 use overload nomethod => \&wrap, '""' => \&str;
1140 my ($meth, $a, $b) = @{+shift};
1141 $a = 'u' unless defined $a;
1142 $b = 'u' unless defined $b;
1146 Now one can change the last line of the script to
1148 print "side = $side\n";
1152 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1154 and one can inspect the value in debugger using all the possible
1157 Something is still amiss: consider the loop variable $cnt of the
1158 script. It was a number, not an object. We cannot make this value of
1159 type C<symbolic>, since then the loop will not terminate.
1161 Indeed, to terminate the cycle, the $cnt should become false.
1162 However, the operator C<bool> for checking falsity is overloaded (this
1163 time via overloaded C<"">), and returns a long string, thus any object
1164 of type C<symbolic> is true. To overcome this, we need a way to
1165 compare an object to 0. In fact, it is easier to write a numeric
1168 Here is the text of F<symbolic.pm> with such a routine added (and
1169 slightly modified str()):
1171 package symbolic; # Primitive symbolic calculator
1173 nomethod => \&wrap, '""' => \&str, '0+' => \#
1175 sub new { shift; bless ['n', @_] }
1177 my ($obj, $other, $inv, $meth) = @_;
1178 ($obj, $other) = ($other, $obj) if $inv;
1179 bless [$meth, $obj, $other];
1182 my ($meth, $a, $b) = @{+shift};
1183 $a = 'u' unless defined $a;
1190 my %subr = ( n => sub {$_[0]},
1191 sqrt => sub {sqrt $_[0]},
1192 '-' => sub {shift() - shift()},
1193 '+' => sub {shift() + shift()},
1194 '/' => sub {shift() / shift()},
1195 '*' => sub {shift() * shift()},
1196 '**' => sub {shift() ** shift()},
1199 my ($meth, $a, $b) = @{+shift};
1200 my $subr = $subr{$meth}
1201 or die "Do not know how to ($meth) in symbolic";
1202 $a = $a->num if ref $a eq __PACKAGE__;
1203 $b = $b->num if ref $b eq __PACKAGE__;
1207 All the work of numeric conversion is done in %subr and num(). Of
1208 course, %subr is not complete, it contains only operators used in the
1209 example below. Here is the extra-credit question: why do we need an
1210 explicit recursion in num()? (Answer is at the end of this section.)
1212 Use this module like this:
1215 my $iter = new symbolic 2; # 16-gon
1216 my $side = new symbolic 1;
1220 $cnt = $cnt - 1; # Mutator `--' not implemented
1221 $side = (sqrt(1 + $side**2) - 1)/$side;
1223 printf "%s=%f\n", $side, $side;
1224 printf "pi=%f\n", $side*(2**($iter+2));
1226 It prints (without so many line breaks)
1228 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1230 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1233 The above module is very primitive. It does not implement
1234 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1235 (not required without mutators!), and implements only those arithmetic
1236 operations which are used in the example.
1238 To implement most arithmetic operations is easy; one should just use
1239 the tables of operations, and change the code which fills %subr to
1241 my %subr = ( 'n' => sub {$_[0]} );
1242 foreach my $op (split " ", $overload::ops{with_assign}) {
1243 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1245 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1246 foreach my $op (split " ", "@overload::ops{ @bins }") {
1247 $subr{$op} = eval "sub {shift() $op shift()}";
1249 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1250 print "defining `$op'\n";
1251 $subr{$op} = eval "sub {$op shift()}";
1254 Due to L<Calling Conventions for Mutators>, we do not need anything
1255 special to make C<+=> and friends work, except filling C<+=> entry of
1256 %subr, and defining a copy constructor (needed since Perl has no
1257 way to know that the implementation of C<'+='> does not mutate
1258 the argument, compare L<Copy Constructor>).
1260 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1261 line, and code (this code assumes that mutators change things one level
1262 deep only, so recursive copying is not needed):
1266 bless [@$self], ref $self;
1269 To make C<++> and C<--> work, we need to implement actual mutators,
1270 either directly, or in C<nomethod>. We continue to do things inside
1271 C<nomethod>, thus add
1273 if ($meth eq '++' or $meth eq '--') {
1274 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1278 after the first line of wrap(). This is not a most effective
1279 implementation, one may consider
1281 sub inc { $_[0] = bless ['++', shift, 1]; }
1285 As a final remark, note that one can fill %subr by
1287 my %subr = ( 'n' => sub {$_[0]} );
1288 foreach my $op (split " ", $overload::ops{with_assign}) {
1289 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1291 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1292 foreach my $op (split " ", "@overload::ops{ @bins }") {
1293 $subr{$op} = eval "sub {shift() $op shift()}";
1295 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1296 $subr{$op} = eval "sub {$op shift()}";
1298 $subr{'++'} = $subr{'+'};
1299 $subr{'--'} = $subr{'-'};
1301 This finishes implementation of a primitive symbolic calculator in
1302 50 lines of Perl code. Since the numeric values of subexpressions
1303 are not cached, the calculator is very slow.
1305 Here is the answer for the exercise: In the case of str(), we need no
1306 explicit recursion since the overloaded C<.>-operator will fall back
1307 to an existing overloaded operator C<"">. Overloaded arithmetic
1308 operators I<do not> fall back to numeric conversion if C<fallback> is
1309 not explicitly requested. Thus without an explicit recursion num()
1310 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1311 the argument of num().
1313 If you wonder why defaults for conversion are different for str() and
1314 num(), note how easy it was to write the symbolic calculator. This
1315 simplicity is due to an appropriate choice of defaults. One extra
1316 note: due to the explicit recursion num() is more fragile than sym():
1317 we need to explicitly check for the type of $a and $b. If components
1318 $a and $b happen to be of some related type, this may lead to problems.
1320 =head2 I<Really> symbolic calculator
1322 One may wonder why we call the above calculator symbolic. The reason
1323 is that the actual calculation of the value of expression is postponed
1324 until the value is I<used>.
1326 To see it in action, add a method
1331 @$obj->[0,1] = ('=', shift);
1334 to the package C<symbolic>. After this change one can do
1336 my $a = new symbolic 3;
1337 my $b = new symbolic 4;
1338 my $c = sqrt($a**2 + $b**2);
1340 and the numeric value of $c becomes 5. However, after calling
1342 $a->STORE(12); $b->STORE(5);
1344 the numeric value of $c becomes 13. There is no doubt now that the module
1345 symbolic provides a I<symbolic> calculator indeed.
1347 To hide the rough edges under the hood, provide a tie()d interface to the
1348 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1350 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1352 sub nop { } # Around a bug
1354 (the bug is described in L<"BUGS">). One can use this new interface as
1356 tie $a, 'symbolic', 3;
1357 tie $b, 'symbolic', 4;
1358 $a->nop; $b->nop; # Around a bug
1360 my $c = sqrt($a**2 + $b**2);
1362 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1363 of $c becomes 13. To insulate the user of the module add a method
1365 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1370 symbolic->vars($a, $b);
1371 my $c = sqrt($a**2 + $b**2);
1374 printf "c5 %s=%f\n", $c, $c;
1377 printf "c13 %s=%f\n", $c, $c;
1379 shows that the numeric value of $c follows changes to the values of $a
1384 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1388 When Perl is run with the B<-Do> switch or its equivalent, overloading
1389 induces diagnostic messages.
1391 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1392 deduce which operations are overloaded (and which ancestor triggers
1393 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1394 is shown by debugger. The method C<()> corresponds to the C<fallback>
1395 key (in fact a presence of this method shows that this package has
1396 overloading enabled, and it is what is used by the C<Overloaded>
1397 function of module C<overload>).
1399 The module might issue the following warnings:
1403 =item Odd number of arguments for overload::constant
1405 (W) The call to overload::constant contained an odd number of arguments.
1406 The arguments should come in pairs.
1408 =item `%s' is not an overloadable type
1410 (W) You tried to overload a constant type the overload package is unaware of.
1412 =item `%s' is not a code reference
1414 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1415 to be a code reference. Either an anonymous subroutine, or a reference
1422 Because it is used for overloading, the per-package hash %OVERLOAD now
1423 has a special meaning in Perl. The symbol table is filled with names
1424 looking like line-noise.
1426 For the purpose of inheritance every overloaded package behaves as if
1427 C<fallback> is present (possibly undefined). This may create
1428 interesting effects if some package is not overloaded, but inherits
1429 from two overloaded packages.
1431 Relation between overloading and tie()ing is broken. Overloading is
1432 triggered or not basing on the I<previous> class of tie()d value.
1434 This happens because the presence of overloading is checked too early,
1435 before any tie()d access is attempted. If the FETCH()ed class of the
1436 tie()d value does not change, a simple workaround is to access the value
1437 immediately after tie()ing, so that after this call the I<previous> class
1438 coincides with the current one.
1440 B<Needed:> a way to fix this without a speed penalty.
1442 Barewords are not covered by overloaded string constants.
1444 This document is confusing. There are grammos and misleading language
1445 used in places. It would seem a total rewrite is needed.