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) = @_;
111 my $mro = mro::get_linear_isa($package);
112 foreach my $p (@$mro) {
113 my $fqmeth = $p . q{::} . $meth;
114 return \*{$fqmeth} if defined &{$fqmeth};
121 'integer' => 0x1000, # HINT_NEW_INTEGER
122 'float' => 0x2000, # HINT_NEW_FLOAT
123 'binary' => 0x4000, # HINT_NEW_BINARY
124 'q' => 0x8000, # HINT_NEW_STRING
125 'qr' => 0x10000, # HINT_NEW_RE
128 %ops = ( with_assign => "+ - * / % ** << >> x .",
129 assign => "+= -= *= /= %= **= <<= >>= x= .=",
130 num_comparison => "< <= > >= == !=",
131 '3way_comparison'=> "<=> cmp",
132 str_comparison => "lt le gt ge eq ne",
133 binary => '& &= | |= ^ ^=',
136 func => "atan2 cos sin exp abs log sqrt int",
137 conversion => 'bool "" 0+',
140 dereferencing => '${} @{} %{} &{} *{}',
141 special => 'nomethod fallback =');
143 use warnings::register;
145 # Arguments: what, sub
148 warnings::warnif ("Odd number of arguments for overload::constant");
151 elsif (!exists $constants {$_ [0]}) {
152 warnings::warnif ("`$_[0]' is not an overloadable type");
154 elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
155 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
156 # blessed, and C<ref> would return the package the ref is blessed into.
157 if (warnings::enabled) {
158 $_ [1] = "undef" unless defined $_ [1];
159 warnings::warn ("`$_[1]' is not a code reference");
164 $^H |= $constants{$_[0]};
170 sub remove_constant {
171 # Arguments: what, sub
174 $^H &= ~ $constants{$_[0]};
185 overload - Package for overloading Perl operations
198 $a = SomeThing->new( 57 );
201 if (overload::Overloaded $b) {...}
203 $strval = overload::StrVal $b;
207 This pragma allows overloading of Perl's operators for a class.
208 To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
210 =head2 Declaration of overloaded functions
212 The compilation directive
219 declares function Number::add() for addition, and method muas() in
220 the "class" C<Number> (or one of its base classes)
221 for the assignment form C<*=> of multiplication.
223 Arguments of this directive come in (key, value) pairs. Legal values
224 are values legal inside a C<&{ ... }> call, so the name of a
225 subroutine, a reference to a subroutine, or an anonymous subroutine
226 will all work. Note that values specified as strings are
227 interpreted as methods, not subroutines. Legal keys are listed below.
229 The subroutine C<add> will be called to execute C<$a+$b> if $a
230 is a reference to an object blessed into the package C<Number>, or if $a is
231 not an object from a package with defined mathemagic addition, but $b is a
232 reference to a C<Number>. It can also be called in other situations, like
233 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
234 methods refer to methods triggered by an overloaded mathematical
237 Since overloading respects inheritance via the @ISA hierarchy, the
238 above declaration would also trigger overloading of C<+> and C<*=> in
239 all the packages which inherit from C<Number>.
241 =head2 Calling Conventions for Binary Operations
243 The functions specified in the C<use overload ...> directive are called
244 with three (in one particular case with four, see L<Last Resort>)
245 arguments. If the corresponding operation is binary, then the first
246 two arguments are the two arguments of the operation. However, due to
247 general object calling conventions, the first argument should always be
248 an object in the package, so in the situation of C<7+$a>, the
249 order of the arguments is interchanged. It probably does not matter
250 when implementing the addition method, but whether the arguments
251 are reversed is vital to the subtraction method. The method can
252 query this information by examining the third argument, which can take
253 three different values:
259 the order of arguments is as in the current operation.
263 the arguments are reversed.
267 the current operation is an assignment variant (as in
268 C<$a+=7>), but the usual function is called instead. This additional
269 information can be used to generate some optimizations. Compare
270 L<Calling Conventions for Mutators>.
274 =head2 Calling Conventions for Unary Operations
276 Unary operation are considered binary operations with the second
277 argument being C<undef>. Thus the functions that overloads C<{"++"}>
278 is called with arguments C<($a,undef,'')> when $a++ is executed.
280 =head2 Calling Conventions for Mutators
282 Two types of mutators have different calling conventions:
286 =item C<++> and C<-->
288 The routines which implement these operators are expected to actually
289 I<mutate> their arguments. So, assuming that $obj is a reference to a
292 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
294 is an appropriate implementation of overloaded C<++>. Note that
296 sub incr { ++$ {$_[0]} ; shift }
298 is OK if used with preincrement and with postincrement. (In the case
299 of postincrement a copying will be performed, see L<Copy Constructor>.)
301 =item C<x=> and other assignment versions
303 There is nothing special about these methods. They may change the
304 value of their arguments, and may leave it as is. The result is going
305 to be assigned to the value in the left-hand-side if different from
308 This allows for the same method to be used as overloaded C<+=> and
309 C<+>. Note that this is I<allowed>, but not recommended, since by the
310 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
311 if C<+=> is not overloaded.
315 B<Warning.> Due to the presence of assignment versions of operations,
316 routines which may be called in assignment context may create
317 self-referential structures. Currently Perl will not free self-referential
318 structures until cycles are C<explicitly> broken. You may get problems
319 when traversing your structures too.
323 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
325 is asking for trouble, since for code C<$obj += $foo> the subroutine
326 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
327 \$foo]>. If using such a subroutine is an important optimization, one
328 can overload C<+=> explicitly by a non-"optimized" version, or switch
329 to non-optimized version if C<not defined $_[2]> (see
330 L<Calling Conventions for Binary Operations>).
332 Even if no I<explicit> assignment-variants of operators are present in
333 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
334 C<',' . $obj . ','> may be both optimized to
336 my $tmp = ',' . $obj; $tmp .= ',';
338 =head2 Overloadable Operations
340 The following symbols can be specified in C<use overload> directive:
344 =item * I<Arithmetic operations>
346 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
347 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
349 For these operations a substituted non-assignment variant can be called if
350 the assignment variant is not available. Methods for operations C<+>,
351 C<->, C<+=>, and C<-=> can be called to automatically generate
352 increment and decrement methods. The operation C<-> can be used to
353 autogenerate missing methods for unary minus or C<abs>.
355 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
356 L<"Calling Conventions for Binary Operations">) for details of these
359 =item * I<Comparison operations>
361 "<", "<=", ">", ">=", "==", "!=", "<=>",
362 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
364 If the corresponding "spaceship" variant is available, it can be
365 used to substitute for the missing operation. During C<sort>ing
366 arrays, C<cmp> is used to compare values subject to C<use overload>.
368 =item * I<Bit operations>
370 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
372 C<neg> stands for unary minus. If the method for C<neg> is not
373 specified, it can be autogenerated using the method for
374 subtraction. If the method for C<!> is not specified, it can be
375 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
377 The same remarks in L<"Arithmetic operations"> about
378 assignment-variants and autogeneration apply for
379 bit operations C<"&">, C<"^">, and C<"|"> as well.
381 =item * I<Increment and decrement>
385 If undefined, addition and subtraction methods can be
386 used instead. These operations are called both in prefix and
389 =item * I<Transcendental functions>
391 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
393 If C<abs> is unavailable, it can be autogenerated using methods
394 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
396 Note that traditionally the Perl function L<int> rounds to 0, thus for
397 floating-point-like types one should follow the same semantic. If
398 C<int> is unavailable, it can be autogenerated using the overloading of
401 =item * I<Boolean, string and numeric conversion>
405 If one or two of these operations are not overloaded, the remaining ones can
406 be used instead. C<bool> is used in the flow control operators
407 (like C<while>) and for the ternary C<?:> operation. These functions can
408 return any arbitrary Perl value. If the corresponding operation for this value
409 is overloaded too, that operation will be called again with this value.
411 As a special case if the overload returns the object itself then it will
412 be used directly. An overloaded conversion returning the object is
413 probably a bug, because you're likely to get something that looks like
414 C<YourPackage=HASH(0x8172b34)>.
420 If not overloaded, the argument will be converted to a filehandle or
421 glob (which may require a stringification). The same overloading
422 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
423 I<globbing> syntax C<E<lt>${var}E<gt>>.
425 B<BUGS> Even in list context, the iterator is currently called only
426 once and with scalar context.
428 =item * I<File tests>
432 This overload is used for all the filetest operators (C<-f>, C<-x> and
433 so on: see L<perlfunc/-X> for the full list). Even though these are
434 unary operators, the method will be called with a second argument which
435 is a single letter indicating which test was performed. Note that the
436 overload key is the literal string C<"-X">: you can't provide separate
437 overloads for the different tests.
439 Calling an overloaded filetest operator does not affect the stat value
440 associated with the special filehandle C<_>. It still refers to the
441 result of the last C<stat>, C<lstat> or unoverloaded filetest.
443 If not overloaded, these operators will fall back to the default
444 behaviour even without C<< fallback => 1 >>. This means that if the
445 object is a blessed glob or blessed IO ref it will be treated as a
446 filehandle, otherwise string overloading will be invoked and the result
447 treated as a filename.
449 This overload was introduced in perl 5.12.
451 =item * I<Dereferencing>
453 '${}', '@{}', '%{}', '&{}', '*{}'.
455 If not overloaded, the argument will be dereferenced I<as is>, thus
456 should be of correct type. These functions should return a reference
457 of correct type, or another object with overloaded dereferencing.
459 As a special case if the overload returns the object itself then it
460 will be used directly (provided it is the correct type).
462 The dereference operators must be specified explicitly they will not be passed to
467 "nomethod", "fallback", "=", "~~",
469 see L<SPECIAL SYMBOLS FOR C<use overload>>.
473 See L<"Fallback"> for an explanation of when a missing method can be
476 A computer-readable form of the above table is available in the hash
477 %overload::ops, with values being space-separated lists of names:
479 with_assign => '+ - * / % ** << >> x .',
480 assign => '+= -= *= /= %= **= <<= >>= x= .=',
481 num_comparison => '< <= > >= == !=',
482 '3way_comparison'=> '<=> cmp',
483 str_comparison => 'lt le gt ge eq ne',
484 binary => '& &= | |= ^ ^=',
487 func => 'atan2 cos sin exp abs log sqrt',
488 conversion => 'bool "" 0+',
491 dereferencing => '${} @{} %{} &{} *{}',
492 special => 'nomethod fallback ='
494 =head2 Inheritance and overloading
496 Inheritance interacts with overloading in two ways.
500 =item Strings as values of C<use overload> directive
504 use overload key => value;
506 is a string, it is interpreted as a method name.
508 =item Overloading of an operation is inherited by derived classes
510 Any class derived from an overloaded class is also overloaded. The
511 set of overloaded methods is the union of overloaded methods of all
512 the ancestors. If some method is overloaded in several ancestor, then
513 which description will be used is decided by the usual inheritance
516 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
517 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
518 then the subroutine C<D::plus_sub> will be called to implement
519 operation C<+> for an object in package C<A>.
523 Note that since the value of the C<fallback> key is not a subroutine,
524 its inheritance is not governed by the above rules. In the current
525 implementation, the value of C<fallback> in the first overloaded
526 ancestor is used, but this is accidental and subject to change.
528 =head1 SPECIAL SYMBOLS FOR C<use overload>
530 Three keys are recognized by Perl that are not covered by the above
535 C<"nomethod"> should be followed by a reference to a function of four
536 parameters. If defined, it is called when the overloading mechanism
537 cannot find a method for some operation. The first three arguments of
538 this function coincide with the arguments for the corresponding method if
539 it were found, the fourth argument is the symbol
540 corresponding to the missing method. If several methods are tried,
541 the last one is used. Say, C<1-$a> can be equivalent to
543 &nomethodMethod($a,1,1,"-")
545 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
546 C<use overload> directive.
548 The C<"nomethod"> mechanism is I<not> used for the dereference operators
549 ( ${} @{} %{} &{} *{} ).
552 If some operation cannot be resolved, and there is no function
553 assigned to C<"nomethod">, then an exception will be raised via die()--
554 unless C<"fallback"> was specified as a key in C<use overload> directive.
559 The key C<"fallback"> governs what to do if a method for a particular
560 operation is not found. Three different cases are possible depending on
561 the value of C<"fallback">:
568 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
569 then tries to calls C<"nomethod"> value; if missing, an exception
574 The same as for the C<undef> value, but no exception is raised. Instead,
575 it silently reverts to what it would have done were there no C<use overload>
578 =item * defined, but FALSE
580 No autogeneration is tried. Perl tries to call
581 C<"nomethod"> value, and if this is missing, raises an exception.
585 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
586 yet, see L<"Inheritance and overloading">.
590 The key C<"~~"> allows you to override the smart matching used by
591 the switch construct. See L<feature>.
593 =head2 Copy Constructor
595 The value for C<"="> is a reference to a function with three
596 arguments, i.e., it looks like the other values in C<use
597 overload>. However, it does not overload the Perl assignment
598 operator. This would go against Camel hair.
600 This operation is called in the situations when a mutator is applied
601 to a reference that shares its object with some other reference, such
607 To make this change $a and not change $b, a copy of C<$$a> is made,
608 and $a is assigned a reference to this new object. This operation is
609 done during execution of the C<++$a>, and not during the assignment,
610 (so before the increment C<$$a> coincides with C<$$b>). This is only
611 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
612 C<nomethod>). Note that if this operation is expressed via C<'+'>
613 a nonmutator, i.e., as in
618 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
619 appear as lvalue when the above code is executed.
621 If the copy constructor is required during the execution of some mutator,
622 but a method for C<'='> was not specified, it can be autogenerated as a
623 string copy if the object is a plain scalar or a simple assignment if it
630 The actually executed code for
633 Something else which does not modify $a or $b....
639 Something else which does not modify $a or $b....
640 $a = $a->clone(undef,"");
643 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
644 C<'='> was overloaded with C<\&clone>.
648 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
651 =head1 MAGIC AUTOGENERATION
653 If a method for an operation is not found, and the value for C<"fallback"> is
654 TRUE or undefined, Perl tries to autogenerate a substitute method for
655 the missing operation based on the defined operations. Autogenerated method
656 substitutions are possible for the following operations:
660 =item I<Assignment forms of arithmetic operations>
662 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
665 =item I<Conversion operations>
667 String, numeric, and boolean conversion are calculated in terms of one
668 another if not all of them are defined.
670 =item I<Increment and decrement>
672 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
673 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
677 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
681 can be expressed in terms of subtraction.
685 C<!> and C<not> can be expressed in terms of boolean conversion, or
686 string or numerical conversion.
688 =item I<Concatenation>
690 can be expressed in terms of string conversion.
692 =item I<Comparison operations>
694 can be expressed in terms of its "spaceship" counterpart: either
695 C<E<lt>=E<gt>> or C<cmp>:
697 <, >, <=, >=, ==, != in terms of <=>
698 lt, gt, le, ge, eq, ne in terms of cmp
702 <> in terms of builtin operations
704 =item I<Dereferencing>
706 ${} @{} %{} &{} *{} in terms of builtin operations
708 =item I<Copy operator>
710 can be expressed in terms of an assignment to the dereferenced value, if this
711 value is a scalar and not a reference, or simply a reference assignment
716 =head1 Minimal set of overloaded operations
718 Since some operations can be automatically generated from others, there is
719 a minimal set of operations that need to be overloaded in order to have
720 the complete set of overloaded operations at one's disposal.
721 Of course, the autogenerated operations may not do exactly what the user
722 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
727 atan2 cos sin exp log sqrt int
729 Additionally, you need to define at least one of string, boolean or
730 numeric conversions because any one can be used to emulate the others.
731 The string conversion can also be used to emulate concatenation.
733 =head1 Losing overloading
735 The restriction for the comparison operation is that even if, for example,
736 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
737 function will produce only a standard logical value based on the
738 numerical value of the result of `C<cmp>'. In particular, a working
739 numeric conversion is needed in this case (possibly expressed in terms of
742 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
743 if the string conversion substitution is applied.
745 When you chop() a mathemagical object it is promoted to a string and its
746 mathemagical properties are lost. The same can happen with other
749 =head1 Run-time Overloading
751 Since all C<use> directives are executed at compile-time, the only way to
752 change overloading during run-time is to
754 eval 'use overload "+" => \&addmethod';
758 eval 'no overload "+", "--", "<="';
760 though the use of these constructs during run-time is questionable.
762 =head1 Public functions
764 Package C<overload.pm> provides the following public functions:
768 =item overload::StrVal(arg)
770 Gives string value of C<arg> as in absence of stringify overloading. If you
771 are using this to get the address of a reference (useful for checking if two
772 references point to the same thing) then you may be better off using
773 C<Scalar::Util::refaddr()>, which is faster.
775 =item overload::Overloaded(arg)
777 Returns true if C<arg> is subject to overloading of some operations.
779 =item overload::Method(obj,op)
781 Returns C<undef> or a reference to the method that implements C<op>.
785 =head1 Overloading constants
787 For some applications, the Perl parser mangles constants too much.
788 It is possible to hook into this process via C<overload::constant()>
789 and C<overload::remove_constant()> functions.
791 These functions take a hash as an argument. The recognized keys of this hash
798 to overload integer constants,
802 to overload floating point constants,
806 to overload octal and hexadecimal constants,
810 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
811 strings and here-documents,
815 to overload constant pieces of regular expressions.
819 The corresponding values are references to functions which take three arguments:
820 the first one is the I<initial> string form of the constant, the second one
821 is how Perl interprets this constant, the third one is how the constant is used.
822 Note that the initial string form does not
823 contain string delimiters, and has backslashes in backslash-delimiter
824 combinations stripped (thus the value of delimiter is not relevant for
825 processing of this string). The return value of this function is how this
826 constant is going to be interpreted by Perl. The third argument is undefined
827 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
828 context (comes from strings, regular expressions, and single-quote HERE
829 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
830 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
832 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
833 it is expected that overloaded constant strings are equipped with reasonable
834 overloaded catenation operator, otherwise absurd results will result.
835 Similarly, negative numbers are considered as negations of positive constants.
837 Note that it is probably meaningless to call the functions overload::constant()
838 and overload::remove_constant() from anywhere but import() and unimport() methods.
839 From these methods they may be called as
844 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
845 overload::constant integer => sub {Math::BigInt->new(shift)};
848 =head1 IMPLEMENTATION
850 What follows is subject to change RSN.
852 The table of methods for all operations is cached in magic for the
853 symbol table hash for the package. The cache is invalidated during
854 processing of C<use overload>, C<no overload>, new function
855 definitions, and changes in @ISA. However, this invalidation remains
856 unprocessed until the next C<bless>ing into the package. Hence if you
857 want to change overloading structure dynamically, you'll need an
858 additional (fake) C<bless>ing to update the table.
860 (Every SVish thing has a magic queue, and magic is an entry in that
861 queue. This is how a single variable may participate in multiple
862 forms of magic simultaneously. For instance, environment variables
863 regularly have two forms at once: their %ENV magic and their taint
864 magic. However, the magic which implements overloading is applied to
865 the stashes, which are rarely used directly, thus should not slow down
868 If an object belongs to a package using overload, it carries a special
869 flag. Thus the only speed penalty during arithmetic operations without
870 overloading is the checking of this flag.
872 In fact, if C<use overload> is not present, there is almost no overhead
873 for overloadable operations, so most programs should not suffer
874 measurable performance penalties. A considerable effort was made to
875 minimize the overhead when overload is used in some package, but the
876 arguments in question do not belong to packages using overload. When
877 in doubt, test your speed with C<use overload> and without it. So far
878 there have been no reports of substantial speed degradation if Perl is
879 compiled with optimization turned on.
881 There is no size penalty for data if overload is not used. The only
882 size penalty if overload is used in some package is that I<all> the
883 packages acquire a magic during the next C<bless>ing into the
884 package. This magic is three-words-long for packages without
885 overloading, and carries the cache table if the package is overloaded.
887 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
888 carried out before any operation that can imply an assignment to the
889 object $a (or $b) refers to, like C<$a++>. You can override this
890 behavior by defining your own copy constructor (see L<"Copy Constructor">).
892 It is expected that arguments to methods that are not explicitly supposed
893 to be changed are constant (but this is not enforced).
895 =head1 Metaphor clash
897 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
898 If it I<looks> counter intuitive to you, you are subject to a metaphor
901 Here is a Perl object metaphor:
903 I< object is a reference to blessed data>
905 and an arithmetic metaphor:
907 I< object is a thing by itself>.
909 The I<main> problem of overloading C<=> is the fact that these metaphors
910 imply different actions on the assignment C<$a = $b> if $a and $b are
911 objects. Perl-think implies that $a becomes a reference to whatever
912 $b was referencing. Arithmetic-think implies that the value of "object"
913 $a is changed to become the value of the object $b, preserving the fact
914 that $a and $b are separate entities.
916 The difference is not relevant in the absence of mutators. After
917 a Perl-way assignment an operation which mutates the data referenced by $a
918 would change the data referenced by $b too. Effectively, after
919 C<$a = $b> values of $a and $b become I<indistinguishable>.
921 On the other hand, anyone who has used algebraic notation knows the
922 expressive power of the arithmetic metaphor. Overloading works hard
923 to enable this metaphor while preserving the Perlian way as far as
924 possible. Since it is not possible to freely mix two contradicting
925 metaphors, overloading allows the arithmetic way to write things I<as
926 far as all the mutators are called via overloaded access only>. The
927 way it is done is described in L<Copy Constructor>.
929 If some mutator methods are directly applied to the overloaded values,
930 one may need to I<explicitly unlink> other values which references the
935 $b = $a; # $b is "linked" to $a
937 $a = $a->clone; # Unlink $b from $a
940 Note that overloaded access makes this transparent:
943 $b = $a; # $b is "linked" to $a
944 $a += 4; # would unlink $b automagically
946 However, it would not make
949 $a = 4; # Now $a is a plain 4, not 'Data'
951 preserve "objectness" of $a. But Perl I<has> a way to make assignments
952 to an object do whatever you want. It is just not the overload, but
953 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
954 which returns the object itself, and STORE() method which changes the
955 value of the object, one can reproduce the arithmetic metaphor in its
956 completeness, at least for variables which were tie()d from the start.
958 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
962 Please add examples to what follows!
964 =head2 Two-face scalars
966 Put this in F<two_face.pm> in your Perl library directory:
968 package two_face; # Scalars with separate string and
970 sub new { my $p = shift; bless [@_], $p }
971 use overload '""' => \&str, '0+' => \&num, fallback => 1;
978 my $seven = two_face->new("vii", 7);
979 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
980 print "seven contains `i'\n" if $seven =~ /i/;
982 (The second line creates a scalar which has both a string value, and a
983 numeric value.) This prints:
985 seven=vii, seven=7, eight=8
988 =head2 Two-face references
990 Suppose you want to create an object which is accessible as both an
991 array reference and a hash reference.
994 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
1002 tie %h, ref $self, $self;
1006 sub TIEHASH { my $p = shift; bless \ shift, $p }
1009 $fields{$_} = $i++ foreach qw{zero one two three};
1011 my $self = ${shift()};
1012 my $key = $fields{shift()};
1013 defined $key or die "Out of band access";
1014 $$self->[$key] = shift;
1017 my $self = ${shift()};
1018 my $key = $fields{shift()};
1019 defined $key or die "Out of band access";
1023 Now one can access an object using both the array and hash syntax:
1025 my $bar = two_refs->new(3,4,5,6);
1027 $bar->{two} == 11 or die 'bad hash fetch';
1029 Note several important features of this example. First of all, the
1030 I<actual> type of $bar is a scalar reference, and we do not overload
1031 the scalar dereference. Thus we can get the I<actual> non-overloaded
1032 contents of $bar by just using C<$$bar> (what we do in functions which
1033 overload dereference). Similarly, the object returned by the
1034 TIEHASH() method is a scalar reference.
1036 Second, we create a new tied hash each time the hash syntax is used.
1037 This allows us not to worry about a possibility of a reference loop,
1038 which would lead to a memory leak.
1040 Both these problems can be cured. Say, if we want to overload hash
1041 dereference on a reference to an object which is I<implemented> as a
1042 hash itself, the only problem one has to circumvent is how to access
1043 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1044 overloaded dereference operator). Here is one possible fetching routine:
1047 my ($self, $key) = (shift, shift);
1048 my $class = ref $self;
1049 bless $self, 'overload::dummy'; # Disable overloading of %{}
1050 my $out = $self->{$key};
1051 bless $self, $class; # Restore overloading
1055 To remove creation of the tied hash on each access, one may an extra
1056 level of indirection which allows a non-circular structure of references:
1059 use overload '%{}' => sub { ${shift()}->[1] },
1060 '@{}' => sub { ${shift()}->[0] };
1066 bless \ [$a, \%h], $p;
1071 tie %h, ref $self, $self;
1075 sub TIEHASH { my $p = shift; bless \ shift, $p }
1078 $fields{$_} = $i++ foreach qw{zero one two three};
1081 my $key = $fields{shift()};
1082 defined $key or die "Out of band access";
1087 my $key = $fields{shift()};
1088 defined $key or die "Out of band access";
1092 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1093 reference to the intermediate array, which keeps a reference to an
1094 actual array, and the access hash. The tie()ing object for the access
1095 hash is a reference to a reference to the actual array, so
1101 There are no loops of references.
1105 Both "objects" which are blessed into the class C<two_refs1> are
1106 references to a reference to an array, thus references to a I<scalar>.
1107 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1108 overloaded operations.
1112 =head2 Symbolic calculator
1114 Put this in F<symbolic.pm> in your Perl library directory:
1116 package symbolic; # Primitive symbolic calculator
1117 use overload nomethod => \&wrap;
1119 sub new { shift; bless ['n', @_] }
1121 my ($obj, $other, $inv, $meth) = @_;
1122 ($obj, $other) = ($other, $obj) if $inv;
1123 bless [$meth, $obj, $other];
1126 This module is very unusual as overloaded modules go: it does not
1127 provide any usual overloaded operators, instead it provides the L<Last
1128 Resort> operator C<nomethod>. In this example the corresponding
1129 subroutine returns an object which encapsulates operations done over
1130 the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
1131 symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
1133 Here is an example of the script which "calculates" the side of
1134 circumscribed octagon using the above package:
1137 my $iter = 1; # 2**($iter+2) = 8
1138 my $side = symbolic->new(1);
1142 $side = (sqrt(1 + $side**2) - 1)/$side;
1146 The value of $side is
1148 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1149 undef], 1], ['n', 1]]
1151 Note that while we obtained this value using a nice little script,
1152 there is no simple way to I<use> this value. In fact this value may
1153 be inspected in debugger (see L<perldebug>), but only if
1154 C<bareStringify> B<O>ption is set, and not via C<p> command.
1156 If one attempts to print this value, then the overloaded operator
1157 C<""> will be called, which will call C<nomethod> operator. The
1158 result of this operator will be stringified again, but this result is
1159 again of type C<symbolic>, which will lead to an infinite loop.
1161 Add a pretty-printer method to the module F<symbolic.pm>:
1164 my ($meth, $a, $b) = @{+shift};
1165 $a = 'u' unless defined $a;
1166 $b = 'u' unless defined $b;
1167 $a = $a->pretty if ref $a;
1168 $b = $b->pretty if ref $b;
1172 Now one can finish the script by
1174 print "side = ", $side->pretty, "\n";
1176 The method C<pretty> is doing object-to-string conversion, so it
1177 is natural to overload the operator C<""> using this method. However,
1178 inside such a method it is not necessary to pretty-print the
1179 I<components> $a and $b of an object. In the above subroutine
1180 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1181 and $b. If these components use overloading, the catenation operator
1182 will look for an overloaded operator C<.>; if not present, it will
1183 look for an overloaded operator C<"">. Thus it is enough to use
1185 use overload nomethod => \&wrap, '""' => \&str;
1187 my ($meth, $a, $b) = @{+shift};
1188 $a = 'u' unless defined $a;
1189 $b = 'u' unless defined $b;
1193 Now one can change the last line of the script to
1195 print "side = $side\n";
1199 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1201 and one can inspect the value in debugger using all the possible
1204 Something is still amiss: consider the loop variable $cnt of the
1205 script. It was a number, not an object. We cannot make this value of
1206 type C<symbolic>, since then the loop will not terminate.
1208 Indeed, to terminate the cycle, the $cnt should become false.
1209 However, the operator C<bool> for checking falsity is overloaded (this
1210 time via overloaded C<"">), and returns a long string, thus any object
1211 of type C<symbolic> is true. To overcome this, we need a way to
1212 compare an object to 0. In fact, it is easier to write a numeric
1215 Here is the text of F<symbolic.pm> with such a routine added (and
1216 slightly modified str()):
1218 package symbolic; # Primitive symbolic calculator
1220 nomethod => \&wrap, '""' => \&str, '0+' => \#
1222 sub new { shift; bless ['n', @_] }
1224 my ($obj, $other, $inv, $meth) = @_;
1225 ($obj, $other) = ($other, $obj) if $inv;
1226 bless [$meth, $obj, $other];
1229 my ($meth, $a, $b) = @{+shift};
1230 $a = 'u' unless defined $a;
1237 my %subr = ( n => sub {$_[0]},
1238 sqrt => sub {sqrt $_[0]},
1239 '-' => sub {shift() - shift()},
1240 '+' => sub {shift() + shift()},
1241 '/' => sub {shift() / shift()},
1242 '*' => sub {shift() * shift()},
1243 '**' => sub {shift() ** shift()},
1246 my ($meth, $a, $b) = @{+shift};
1247 my $subr = $subr{$meth}
1248 or die "Do not know how to ($meth) in symbolic";
1249 $a = $a->num if ref $a eq __PACKAGE__;
1250 $b = $b->num if ref $b eq __PACKAGE__;
1254 All the work of numeric conversion is done in %subr and num(). Of
1255 course, %subr is not complete, it contains only operators used in the
1256 example below. Here is the extra-credit question: why do we need an
1257 explicit recursion in num()? (Answer is at the end of this section.)
1259 Use this module like this:
1262 my $iter = symbolic->new(2); # 16-gon
1263 my $side = symbolic->new(1);
1267 $cnt = $cnt - 1; # Mutator `--' not implemented
1268 $side = (sqrt(1 + $side**2) - 1)/$side;
1270 printf "%s=%f\n", $side, $side;
1271 printf "pi=%f\n", $side*(2**($iter+2));
1273 It prints (without so many line breaks)
1275 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1277 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1280 The above module is very primitive. It does not implement
1281 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1282 (not required without mutators!), and implements only those arithmetic
1283 operations which are used in the example.
1285 To implement most arithmetic operations is easy; one should just use
1286 the tables of operations, and change the code which fills %subr to
1288 my %subr = ( 'n' => sub {$_[0]} );
1289 foreach my $op (split " ", $overload::ops{with_assign}) {
1290 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1292 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1293 foreach my $op (split " ", "@overload::ops{ @bins }") {
1294 $subr{$op} = eval "sub {shift() $op shift()}";
1296 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1297 print "defining `$op'\n";
1298 $subr{$op} = eval "sub {$op shift()}";
1301 Due to L<Calling Conventions for Mutators>, we do not need anything
1302 special to make C<+=> and friends work, except filling C<+=> entry of
1303 %subr, and defining a copy constructor (needed since Perl has no
1304 way to know that the implementation of C<'+='> does not mutate
1305 the argument, compare L<Copy Constructor>).
1307 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1308 line, and code (this code assumes that mutators change things one level
1309 deep only, so recursive copying is not needed):
1313 bless [@$self], ref $self;
1316 To make C<++> and C<--> work, we need to implement actual mutators,
1317 either directly, or in C<nomethod>. We continue to do things inside
1318 C<nomethod>, thus add
1320 if ($meth eq '++' or $meth eq '--') {
1321 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1325 after the first line of wrap(). This is not a most effective
1326 implementation, one may consider
1328 sub inc { $_[0] = bless ['++', shift, 1]; }
1332 As a final remark, note that one can fill %subr by
1334 my %subr = ( 'n' => sub {$_[0]} );
1335 foreach my $op (split " ", $overload::ops{with_assign}) {
1336 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1338 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1339 foreach my $op (split " ", "@overload::ops{ @bins }") {
1340 $subr{$op} = eval "sub {shift() $op shift()}";
1342 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1343 $subr{$op} = eval "sub {$op shift()}";
1345 $subr{'++'} = $subr{'+'};
1346 $subr{'--'} = $subr{'-'};
1348 This finishes implementation of a primitive symbolic calculator in
1349 50 lines of Perl code. Since the numeric values of subexpressions
1350 are not cached, the calculator is very slow.
1352 Here is the answer for the exercise: In the case of str(), we need no
1353 explicit recursion since the overloaded C<.>-operator will fall back
1354 to an existing overloaded operator C<"">. Overloaded arithmetic
1355 operators I<do not> fall back to numeric conversion if C<fallback> is
1356 not explicitly requested. Thus without an explicit recursion num()
1357 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1358 the argument of num().
1360 If you wonder why defaults for conversion are different for str() and
1361 num(), note how easy it was to write the symbolic calculator. This
1362 simplicity is due to an appropriate choice of defaults. One extra
1363 note: due to the explicit recursion num() is more fragile than sym():
1364 we need to explicitly check for the type of $a and $b. If components
1365 $a and $b happen to be of some related type, this may lead to problems.
1367 =head2 I<Really> symbolic calculator
1369 One may wonder why we call the above calculator symbolic. The reason
1370 is that the actual calculation of the value of expression is postponed
1371 until the value is I<used>.
1373 To see it in action, add a method
1378 @$obj->[0,1] = ('=', shift);
1381 to the package C<symbolic>. After this change one can do
1383 my $a = symbolic->new(3);
1384 my $b = symbolic->new(4);
1385 my $c = sqrt($a**2 + $b**2);
1387 and the numeric value of $c becomes 5. However, after calling
1389 $a->STORE(12); $b->STORE(5);
1391 the numeric value of $c becomes 13. There is no doubt now that the module
1392 symbolic provides a I<symbolic> calculator indeed.
1394 To hide the rough edges under the hood, provide a tie()d interface to the
1395 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1397 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1399 sub nop { } # Around a bug
1401 (the bug is described in L<"BUGS">). One can use this new interface as
1403 tie $a, 'symbolic', 3;
1404 tie $b, 'symbolic', 4;
1405 $a->nop; $b->nop; # Around a bug
1407 my $c = sqrt($a**2 + $b**2);
1409 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1410 of $c becomes 13. To insulate the user of the module add a method
1412 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1417 symbolic->vars($a, $b);
1418 my $c = sqrt($a**2 + $b**2);
1421 printf "c5 %s=%f\n", $c, $c;
1424 printf "c13 %s=%f\n", $c, $c;
1426 shows that the numeric value of $c follows changes to the values of $a
1431 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1435 The L<overloading> pragma can be used to enable or disable overloaded
1436 operations within a lexical scope.
1440 When Perl is run with the B<-Do> switch or its equivalent, overloading
1441 induces diagnostic messages.
1443 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1444 deduce which operations are overloaded (and which ancestor triggers
1445 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1446 is shown by debugger. The method C<()> corresponds to the C<fallback>
1447 key (in fact a presence of this method shows that this package has
1448 overloading enabled, and it is what is used by the C<Overloaded>
1449 function of module C<overload>).
1451 The module might issue the following warnings:
1455 =item Odd number of arguments for overload::constant
1457 (W) The call to overload::constant contained an odd number of arguments.
1458 The arguments should come in pairs.
1460 =item `%s' is not an overloadable type
1462 (W) You tried to overload a constant type the overload package is unaware of.
1464 =item `%s' is not a code reference
1466 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1467 to be a code reference. Either an anonymous subroutine, or a reference
1474 Because it is used for overloading, the per-package hash %OVERLOAD now
1475 has a special meaning in Perl. The symbol table is filled with names
1476 looking like line-noise.
1478 For the purpose of inheritance every overloaded package behaves as if
1479 C<fallback> is present (possibly undefined). This may create
1480 interesting effects if some package is not overloaded, but inherits
1481 from two overloaded packages.
1483 Relation between overloading and tie()ing is broken. Overloading is
1484 triggered or not basing on the I<previous> class of tie()d value.
1486 This happens because the presence of overloading is checked too early,
1487 before any tie()d access is attempted. If the FETCH()ed class of the
1488 tie()d value does not change, a simple workaround is to access the value
1489 immediately after tie()ing, so that after this call the I<previous> class
1490 coincides with the current one.
1492 B<Needed:> a way to fix this without a speed penalty.
1494 Barewords are not covered by overloaded string constants.
1496 This document is confusing. There are grammos and misleading language
1497 used in places. It would seem a total rewrite is needed.