11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 $fb = ${$package . "::()"}; # preserve old fallback value RT#68196
13 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
15 if ($_ eq 'fallback') {
19 if (not ref $sub and $sub !~ /::/) {
20 $ {$package . "::(" . $_} = $sub;
23 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
24 *{$package . "::(" . $_} = \&{ $sub };
27 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
31 $package = (caller())[0];
32 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
34 $package->overload::OVERLOAD(@_);
38 $package = (caller())[0];
39 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
42 if ($_ eq 'fallback') {
43 undef $ {$package . "::()"};
45 delete $ {$package . "::"}{"(" . $_};
52 $package = ref $package if ref $package;
58 return undef unless $globref;
59 my $sub = \&{*$globref};
62 if Scalar::Util::refaddr($sub) != Scalar::Util::refaddr(\&nil);
63 return shift->can($ {*$globref});
66 sub OverloadedStringify {
68 $package = ref $package if ref $package;
70 ov_method mycan($package, '(""'), $package
71 or ov_method mycan($package, '(0+'), $package
72 or ov_method mycan($package, '(bool'), $package
73 or ov_method mycan($package, '(nomethod'), $package;
82 $package = Scalar::Util::blessed($package);
83 return undef if !defined $package;
85 #my $meth = $package->can('(' . shift);
86 ov_method mycan($package, '(' . shift), $package;
87 #return $meth if $meth ne \&nil;
92 my $package = ref $_[0];
93 return "$_[0]" unless $package;
98 my $class = Scalar::Util::blessed($_[0]);
99 my $class_prefix = defined($class) ? "$class=" : "";
100 my $type = Scalar::Util::reftype($_[0]);
101 my $addr = Scalar::Util::refaddr($_[0]);
102 return sprintf("%s%s(0x%x)", $class_prefix, $type, $addr);
107 sub mycan { # Real can would leave stubs.
108 my ($package, $meth) = @_;
114 my $mro = mro::get_linear_isa($package);
115 foreach my $p (@$mro) {
116 my $fqmeth = $p . q{::} . $meth;
117 return \*{$fqmeth} if defined &{$fqmeth};
124 'integer' => 0x1000, # HINT_NEW_INTEGER
125 'float' => 0x2000, # HINT_NEW_FLOAT
126 'binary' => 0x4000, # HINT_NEW_BINARY
127 'q' => 0x8000, # HINT_NEW_STRING
128 'qr' => 0x10000, # HINT_NEW_RE
131 %ops = ( with_assign => "+ - * / % ** << >> x .",
132 assign => "+= -= *= /= %= **= <<= >>= x= .=",
133 num_comparison => "< <= > >= == !=",
134 '3way_comparison'=> "<=> cmp",
135 str_comparison => "lt le gt ge eq ne",
136 binary => '& &= | |= ^ ^=',
139 func => "atan2 cos sin exp abs log sqrt int",
140 conversion => 'bool "" 0+ qr',
143 dereferencing => '${} @{} %{} &{} *{}',
145 special => 'nomethod fallback =');
147 use warnings::register;
149 # Arguments: what, sub
152 warnings::warnif ("Odd number of arguments for overload::constant");
155 elsif (!exists $constants {$_ [0]}) {
156 warnings::warnif ("`$_[0]' is not an overloadable type");
158 elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
159 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
160 # blessed, and C<ref> would return the package the ref is blessed into.
161 if (warnings::enabled) {
162 $_ [1] = "undef" unless defined $_ [1];
163 warnings::warn ("`$_[1]' is not a code reference");
168 $^H |= $constants{$_[0]};
174 sub remove_constant {
175 # Arguments: what, sub
178 $^H &= ~ $constants{$_[0]};
189 overload - Package for overloading Perl operations
202 $a = SomeThing->new( 57 );
205 if (overload::Overloaded $b) {...}
207 $strval = overload::StrVal $b;
211 This pragma allows overloading of Perl's operators for a class.
212 To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
214 =head2 Declaration of overloaded functions
216 The compilation directive
223 declares function Number::add() for addition, and method muas() in
224 the "class" C<Number> (or one of its base classes)
225 for the assignment form C<*=> of multiplication.
227 Arguments of this directive come in (key, value) pairs. Legal values
228 are values legal inside a C<&{ ... }> call, so the name of a
229 subroutine, a reference to a subroutine, or an anonymous subroutine
230 will all work. Note that values specified as strings are
231 interpreted as methods, not subroutines. Legal keys are listed below.
233 The subroutine C<add> will be called to execute C<$a+$b> if $a
234 is a reference to an object blessed into the package C<Number>, or if $a is
235 not an object from a package with defined mathemagic addition, but $b is a
236 reference to a C<Number>. It can also be called in other situations, like
237 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
238 methods refer to methods triggered by an overloaded mathematical
241 Since overloading respects inheritance via the @ISA hierarchy, the
242 above declaration would also trigger overloading of C<+> and C<*=> in
243 all the packages which inherit from C<Number>.
245 =head2 Calling Conventions for Binary Operations
247 The functions specified in the C<use overload ...> directive are called
248 with three (in one particular case with four, see L<Last Resort>)
249 arguments. If the corresponding operation is binary, then the first
250 two arguments are the two arguments of the operation. However, due to
251 general object calling conventions, the first argument should always be
252 an object in the package, so in the situation of C<7+$a>, the
253 order of the arguments is interchanged. It probably does not matter
254 when implementing the addition method, but whether the arguments
255 are reversed is vital to the subtraction method. The method can
256 query this information by examining the third argument, which can take
257 three different values:
263 the order of arguments is as in the current operation.
267 the arguments are reversed.
271 the current operation is an assignment variant (as in
272 C<$a+=7>), but the usual function is called instead. This additional
273 information can be used to generate some optimizations. Compare
274 L<Calling Conventions for Mutators>.
278 =head2 Calling Conventions for Unary Operations
280 Unary operation are considered binary operations with the second
281 argument being C<undef>. Thus the functions that overloads C<{"++"}>
282 is called with arguments C<($a,undef,'')> when $a++ is executed.
284 =head2 Calling Conventions for Mutators
286 Two types of mutators have different calling conventions:
290 =item C<++> and C<-->
292 The routines which implement these operators are expected to actually
293 I<mutate> their arguments. So, assuming that $obj is a reference to a
296 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
298 is an appropriate implementation of overloaded C<++>. Note that
300 sub incr { ++$ {$_[0]} ; shift }
302 is OK if used with preincrement and with postincrement. (In the case
303 of postincrement a copying will be performed, see L<Copy Constructor>.)
305 =item C<x=> and other assignment versions
307 There is nothing special about these methods. They may change the
308 value of their arguments, and may leave it as is. The result is going
309 to be assigned to the value in the left-hand-side if different from
312 This allows for the same method to be used as overloaded C<+=> and
313 C<+>. Note that this is I<allowed>, but not recommended, since by the
314 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
315 if C<+=> is not overloaded.
319 B<Warning.> Due to the presence of assignment versions of operations,
320 routines which may be called in assignment context may create
321 self-referential structures. Currently Perl will not free self-referential
322 structures until cycles are C<explicitly> broken. You may get problems
323 when traversing your structures too.
327 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
329 is asking for trouble, since for code C<$obj += $foo> the subroutine
330 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
331 \$foo]>. If using such a subroutine is an important optimization, one
332 can overload C<+=> explicitly by a non-"optimized" version, or switch
333 to non-optimized version if C<not defined $_[2]> (see
334 L<Calling Conventions for Binary Operations>).
336 Even if no I<explicit> assignment-variants of operators are present in
337 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
338 C<',' . $obj . ','> may be both optimized to
340 my $tmp = ',' . $obj; $tmp .= ',';
342 =head2 Overloadable Operations
344 The following symbols can be specified in C<use overload> directive:
348 =item * I<Arithmetic operations>
350 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
351 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
353 For these operations a substituted non-assignment variant can be called if
354 the assignment variant is not available. Methods for operations C<+>,
355 C<->, C<+=>, and C<-=> can be called to automatically generate
356 increment and decrement methods. The operation C<-> can be used to
357 autogenerate missing methods for unary minus or C<abs>.
359 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
360 L<"Calling Conventions for Binary Operations">) for details of these
363 =item * I<Comparison operations>
365 "<", "<=", ">", ">=", "==", "!=", "<=>",
366 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
368 If the corresponding "spaceship" variant is available, it can be
369 used to substitute for the missing operation. During C<sort>ing
370 arrays, C<cmp> is used to compare values subject to C<use overload>.
372 =item * I<Bit operations>
374 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
376 C<neg> stands for unary minus. If the method for C<neg> is not
377 specified, it can be autogenerated using the method for
378 subtraction. If the method for C<!> is not specified, it can be
379 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
381 The same remarks in L<"Arithmetic operations"> about
382 assignment-variants and autogeneration apply for
383 bit operations C<"&">, C<"^">, and C<"|"> as well.
385 =item * I<Increment and decrement>
389 If undefined, addition and subtraction methods can be
390 used instead. These operations are called both in prefix and
393 =item * I<Transcendental functions>
395 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
397 If C<abs> is unavailable, it can be autogenerated using methods
398 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
400 Note that traditionally the Perl function L<int> rounds to 0, thus for
401 floating-point-like types one should follow the same semantic. If
402 C<int> is unavailable, it can be autogenerated using the overloading of
405 =item * I<Boolean, string, numeric and regexp conversions>
407 'bool', '""', '0+', 'qr'
409 If one or two of these operations are not overloaded, the remaining ones
410 can be used instead. C<bool> is used in the flow control operators
411 (like C<while>) and for the ternary C<?:> operation; C<qr> is used for
412 the RHS of C<=~> and when an object is interpolated into a regexp.
414 C<bool>, C<"">, and C<0+> can return any arbitrary Perl value. If the
415 corresponding operation for this value is overloaded too, that operation
416 will be called again with this value. C<qr> must return a compiled
417 regexp, or a ref to a compiled regexp (such as C<qr//> returns), and any
418 further overloading on the return value will be ignored.
420 As a special case if the overload returns the object itself then it will
421 be used directly. An overloaded conversion returning the object is
422 probably a bug, because you're likely to get something that looks like
423 C<YourPackage=HASH(0x8172b34)>.
429 If not overloaded, the argument will be converted to a filehandle or
430 glob (which may require a stringification). The same overloading
431 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
432 I<globbing> syntax C<E<lt>${var}E<gt>>.
434 B<BUGS> Even in list context, the iterator is currently called only
435 once and with scalar context.
437 =item * I<File tests>
441 This overload is used for all the filetest operators (C<-f>, C<-x> and
442 so on: see L<perlfunc/-X> for the full list). Even though these are
443 unary operators, the method will be called with a second argument which
444 is a single letter indicating which test was performed. Note that the
445 overload key is the literal string C<"-X">: you can't provide separate
446 overloads for the different tests.
448 Calling an overloaded filetest operator does not affect the stat value
449 associated with the special filehandle C<_>. It still refers to the
450 result of the last C<stat>, C<lstat> or unoverloaded filetest.
452 If not overloaded, these operators will fall back to the default
453 behaviour even without C<< fallback => 1 >>. This means that if the
454 object is a blessed glob or blessed IO ref it will be treated as a
455 filehandle, otherwise string overloading will be invoked and the result
456 treated as a filename.
458 This overload was introduced in perl 5.12.
462 The key C<"~~"> allows you to override the smart matching logic used by
463 the C<~~> operator and the switch construct (C<given>/C<when>). See
464 L<perlsyn/switch> and L<feature>.
466 Unusually, overloading of the smart match operator does not automatically
467 take precedence over normal smart match behaviour. In particular, in the
471 use overload '~~' => 'match';
473 my $obj = Foo->new();
476 the smart match does I<not> invoke the method call like this:
478 $obj->match([1,2,3],0);
480 rather, the smart match distributive rule takes precedence, so $obj is
481 smart matched against each array element in turn until a match is found,
482 so you may see between one and three of these calls instead:
488 Consult the match table in L<perlsyn/"Smart matching in detail"> for
489 details of when overloading is invoked.
491 =item * I<Dereferencing>
493 '${}', '@{}', '%{}', '&{}', '*{}'.
495 If not overloaded, the argument will be dereferenced I<as is>, thus
496 should be of correct type. These functions should return a reference
497 of correct type, or another object with overloaded dereferencing.
499 As a special case if the overload returns the object itself then it
500 will be used directly (provided it is the correct type).
502 The dereference operators must be specified explicitly they will not be passed to
507 "nomethod", "fallback", "=".
509 see L<SPECIAL SYMBOLS FOR C<use overload>>.
513 See L<"Fallback"> for an explanation of when a missing method can be
516 A computer-readable form of the above table is available in the hash
517 %overload::ops, with values being space-separated lists of names:
519 with_assign => '+ - * / % ** << >> x .',
520 assign => '+= -= *= /= %= **= <<= >>= x= .=',
521 num_comparison => '< <= > >= == !=',
522 '3way_comparison'=> '<=> cmp',
523 str_comparison => 'lt le gt ge eq ne',
524 binary => '& &= | |= ^ ^=',
527 func => 'atan2 cos sin exp abs log sqrt',
528 conversion => 'bool "" 0+ qr',
531 dereferencing => '${} @{} %{} &{} *{}',
533 special => 'nomethod fallback ='
535 =head2 Inheritance and overloading
537 Inheritance interacts with overloading in two ways.
541 =item Strings as values of C<use overload> directive
545 use overload key => value;
547 is a string, it is interpreted as a method name.
549 =item Overloading of an operation is inherited by derived classes
551 Any class derived from an overloaded class is also overloaded. The
552 set of overloaded methods is the union of overloaded methods of all
553 the ancestors. If some method is overloaded in several ancestor, then
554 which description will be used is decided by the usual inheritance
557 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
558 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
559 then the subroutine C<D::plus_sub> will be called to implement
560 operation C<+> for an object in package C<A>.
564 Note that since the value of the C<fallback> key is not a subroutine,
565 its inheritance is not governed by the above rules. In the current
566 implementation, the value of C<fallback> in the first overloaded
567 ancestor is used, but this is accidental and subject to change.
569 =head1 SPECIAL SYMBOLS FOR C<use overload>
571 Three keys are recognized by Perl that are not covered by the above
576 C<"nomethod"> should be followed by a reference to a function of four
577 parameters. If defined, it is called when the overloading mechanism
578 cannot find a method for some operation. The first three arguments of
579 this function coincide with the arguments for the corresponding method if
580 it were found, the fourth argument is the symbol
581 corresponding to the missing method. If several methods are tried,
582 the last one is used. Say, C<1-$a> can be equivalent to
584 &nomethodMethod($a,1,1,"-")
586 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
587 C<use overload> directive.
589 The C<"nomethod"> mechanism is I<not> used for the dereference operators
590 ( ${} @{} %{} &{} *{} ).
593 If some operation cannot be resolved, and there is no function
594 assigned to C<"nomethod">, then an exception will be raised via die()--
595 unless C<"fallback"> was specified as a key in C<use overload> directive.
600 The key C<"fallback"> governs what to do if a method for a particular
601 operation is not found. Three different cases are possible depending on
602 the value of C<"fallback">:
609 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
610 then tries to calls C<"nomethod"> value; if missing, an exception
615 The same as for the C<undef> value, but no exception is raised. Instead,
616 it silently reverts to what it would have done were there no C<use overload>
619 =item * defined, but FALSE
621 No autogeneration is tried. Perl tries to call
622 C<"nomethod"> value, and if this is missing, raises an exception.
626 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
627 yet, see L<"Inheritance and overloading">.
629 =head2 Copy Constructor
631 The value for C<"="> is a reference to a function with three
632 arguments, i.e., it looks like the other values in C<use
633 overload>. However, it does not overload the Perl assignment
634 operator. This would go against Camel hair.
636 This operation is called in the situations when a mutator is applied
637 to a reference that shares its object with some other reference, such
643 To make this change $a and not change $b, a copy of C<$$a> is made,
644 and $a is assigned a reference to this new object. This operation is
645 done during execution of the C<++$a>, and not during the assignment,
646 (so before the increment C<$$a> coincides with C<$$b>). This is only
647 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
648 C<nomethod>). Note that if this operation is expressed via C<'+'>
649 a nonmutator, i.e., as in
654 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
655 appear as lvalue when the above code is executed.
657 If the copy constructor is required during the execution of some mutator,
658 but a method for C<'='> was not specified, it can be autogenerated as a
659 string copy if the object is a plain scalar or a simple assignment if it
666 The actually executed code for
669 Something else which does not modify $a or $b....
675 Something else which does not modify $a or $b....
676 $a = $a->clone(undef,"");
679 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
680 C<'='> was overloaded with C<\&clone>.
684 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
687 =head1 MAGIC AUTOGENERATION
689 If a method for an operation is not found, and the value for C<"fallback"> is
690 TRUE or undefined, Perl tries to autogenerate a substitute method for
691 the missing operation based on the defined operations. Autogenerated method
692 substitutions are possible for the following operations:
696 =item I<Assignment forms of arithmetic operations>
698 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
701 =item I<Conversion operations>
703 String, numeric, boolean and regexp conversions are calculated in terms
704 of one another if not all of them are defined.
706 =item I<Increment and decrement>
708 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
709 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
713 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
717 can be expressed in terms of subtraction.
721 C<!> and C<not> can be expressed in terms of boolean conversion, or
722 string or numerical conversion.
724 =item I<Concatenation>
726 can be expressed in terms of string conversion.
728 =item I<Comparison operations>
730 can be expressed in terms of its "spaceship" counterpart: either
731 C<E<lt>=E<gt>> or C<cmp>:
733 <, >, <=, >=, ==, != in terms of <=>
734 lt, gt, le, ge, eq, ne in terms of cmp
738 <> in terms of builtin operations
740 =item I<Dereferencing>
742 ${} @{} %{} &{} *{} in terms of builtin operations
744 =item I<Copy operator>
746 can be expressed in terms of an assignment to the dereferenced value, if this
747 value is a scalar and not a reference, or simply a reference assignment
752 =head1 Minimal set of overloaded operations
754 Since some operations can be automatically generated from others, there is
755 a minimal set of operations that need to be overloaded in order to have
756 the complete set of overloaded operations at one's disposal.
757 Of course, the autogenerated operations may not do exactly what the user
758 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
763 atan2 cos sin exp log sqrt int
765 Additionally, you need to define at least one of string, boolean or
766 numeric conversions because any one can be used to emulate the others.
767 The string conversion can also be used to emulate concatenation.
769 =head1 Losing overloading
771 The restriction for the comparison operation is that even if, for example,
772 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
773 function will produce only a standard logical value based on the
774 numerical value of the result of `C<cmp>'. In particular, a working
775 numeric conversion is needed in this case (possibly expressed in terms of
778 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
779 if the string conversion substitution is applied.
781 When you chop() a mathemagical object it is promoted to a string and its
782 mathemagical properties are lost. The same can happen with other
785 =head1 Run-time Overloading
787 Since all C<use> directives are executed at compile-time, the only way to
788 change overloading during run-time is to
790 eval 'use overload "+" => \&addmethod';
794 eval 'no overload "+", "--", "<="';
796 though the use of these constructs during run-time is questionable.
798 =head1 Public functions
800 Package C<overload.pm> provides the following public functions:
804 =item overload::StrVal(arg)
806 Gives string value of C<arg> as in absence of stringify overloading. If you
807 are using this to get the address of a reference (useful for checking if two
808 references point to the same thing) then you may be better off using
809 C<Scalar::Util::refaddr()>, which is faster.
811 =item overload::Overloaded(arg)
813 Returns true if C<arg> is subject to overloading of some operations.
815 =item overload::Method(obj,op)
817 Returns C<undef> or a reference to the method that implements C<op>.
821 =head1 Overloading constants
823 For some applications, the Perl parser mangles constants too much.
824 It is possible to hook into this process via C<overload::constant()>
825 and C<overload::remove_constant()> functions.
827 These functions take a hash as an argument. The recognized keys of this hash
834 to overload integer constants,
838 to overload floating point constants,
842 to overload octal and hexadecimal constants,
846 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
847 strings and here-documents,
851 to overload constant pieces of regular expressions.
855 The corresponding values are references to functions which take three arguments:
856 the first one is the I<initial> string form of the constant, the second one
857 is how Perl interprets this constant, the third one is how the constant is used.
858 Note that the initial string form does not
859 contain string delimiters, and has backslashes in backslash-delimiter
860 combinations stripped (thus the value of delimiter is not relevant for
861 processing of this string). The return value of this function is how this
862 constant is going to be interpreted by Perl. The third argument is undefined
863 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
864 context (comes from strings, regular expressions, and single-quote HERE
865 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
866 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
868 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
869 it is expected that overloaded constant strings are equipped with reasonable
870 overloaded catenation operator, otherwise absurd results will result.
871 Similarly, negative numbers are considered as negations of positive constants.
873 Note that it is probably meaningless to call the functions overload::constant()
874 and overload::remove_constant() from anywhere but import() and unimport() methods.
875 From these methods they may be called as
880 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
881 overload::constant integer => sub {Math::BigInt->new(shift)};
884 =head1 IMPLEMENTATION
886 What follows is subject to change RSN.
888 The table of methods for all operations is cached in magic for the
889 symbol table hash for the package. The cache is invalidated during
890 processing of C<use overload>, C<no overload>, new function
891 definitions, and changes in @ISA. However, this invalidation remains
892 unprocessed until the next C<bless>ing into the package. Hence if you
893 want to change overloading structure dynamically, you'll need an
894 additional (fake) C<bless>ing to update the table.
896 (Every SVish thing has a magic queue, and magic is an entry in that
897 queue. This is how a single variable may participate in multiple
898 forms of magic simultaneously. For instance, environment variables
899 regularly have two forms at once: their %ENV magic and their taint
900 magic. However, the magic which implements overloading is applied to
901 the stashes, which are rarely used directly, thus should not slow down
904 If an object belongs to a package using overload, it carries a special
905 flag. Thus the only speed penalty during arithmetic operations without
906 overloading is the checking of this flag.
908 In fact, if C<use overload> is not present, there is almost no overhead
909 for overloadable operations, so most programs should not suffer
910 measurable performance penalties. A considerable effort was made to
911 minimize the overhead when overload is used in some package, but the
912 arguments in question do not belong to packages using overload. When
913 in doubt, test your speed with C<use overload> and without it. So far
914 there have been no reports of substantial speed degradation if Perl is
915 compiled with optimization turned on.
917 There is no size penalty for data if overload is not used. The only
918 size penalty if overload is used in some package is that I<all> the
919 packages acquire a magic during the next C<bless>ing into the
920 package. This magic is three-words-long for packages without
921 overloading, and carries the cache table if the package is overloaded.
923 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
924 carried out before any operation that can imply an assignment to the
925 object $a (or $b) refers to, like C<$a++>. You can override this
926 behavior by defining your own copy constructor (see L<"Copy Constructor">).
928 It is expected that arguments to methods that are not explicitly supposed
929 to be changed are constant (but this is not enforced).
931 =head1 Metaphor clash
933 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
934 If it I<looks> counter intuitive to you, you are subject to a metaphor
937 Here is a Perl object metaphor:
939 I< object is a reference to blessed data>
941 and an arithmetic metaphor:
943 I< object is a thing by itself>.
945 The I<main> problem of overloading C<=> is the fact that these metaphors
946 imply different actions on the assignment C<$a = $b> if $a and $b are
947 objects. Perl-think implies that $a becomes a reference to whatever
948 $b was referencing. Arithmetic-think implies that the value of "object"
949 $a is changed to become the value of the object $b, preserving the fact
950 that $a and $b are separate entities.
952 The difference is not relevant in the absence of mutators. After
953 a Perl-way assignment an operation which mutates the data referenced by $a
954 would change the data referenced by $b too. Effectively, after
955 C<$a = $b> values of $a and $b become I<indistinguishable>.
957 On the other hand, anyone who has used algebraic notation knows the
958 expressive power of the arithmetic metaphor. Overloading works hard
959 to enable this metaphor while preserving the Perlian way as far as
960 possible. Since it is not possible to freely mix two contradicting
961 metaphors, overloading allows the arithmetic way to write things I<as
962 far as all the mutators are called via overloaded access only>. The
963 way it is done is described in L<Copy Constructor>.
965 If some mutator methods are directly applied to the overloaded values,
966 one may need to I<explicitly unlink> other values which references the
971 $b = $a; # $b is "linked" to $a
973 $a = $a->clone; # Unlink $b from $a
976 Note that overloaded access makes this transparent:
979 $b = $a; # $b is "linked" to $a
980 $a += 4; # would unlink $b automagically
982 However, it would not make
985 $a = 4; # Now $a is a plain 4, not 'Data'
987 preserve "objectness" of $a. But Perl I<has> a way to make assignments
988 to an object do whatever you want. It is just not the overload, but
989 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
990 which returns the object itself, and STORE() method which changes the
991 value of the object, one can reproduce the arithmetic metaphor in its
992 completeness, at least for variables which were tie()d from the start.
994 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
998 Please add examples to what follows!
1000 =head2 Two-face scalars
1002 Put this in F<two_face.pm> in your Perl library directory:
1004 package two_face; # Scalars with separate string and
1006 sub new { my $p = shift; bless [@_], $p }
1007 use overload '""' => \&str, '0+' => \&num, fallback => 1;
1008 sub num {shift->[1]}
1009 sub str {shift->[0]}
1014 my $seven = two_face->new("vii", 7);
1015 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
1016 print "seven contains `i'\n" if $seven =~ /i/;
1018 (The second line creates a scalar which has both a string value, and a
1019 numeric value.) This prints:
1021 seven=vii, seven=7, eight=8
1024 =head2 Two-face references
1026 Suppose you want to create an object which is accessible as both an
1027 array reference and a hash reference.
1030 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
1038 tie %h, ref $self, $self;
1042 sub TIEHASH { my $p = shift; bless \ shift, $p }
1045 $fields{$_} = $i++ foreach qw{zero one two three};
1047 my $self = ${shift()};
1048 my $key = $fields{shift()};
1049 defined $key or die "Out of band access";
1050 $$self->[$key] = shift;
1053 my $self = ${shift()};
1054 my $key = $fields{shift()};
1055 defined $key or die "Out of band access";
1059 Now one can access an object using both the array and hash syntax:
1061 my $bar = two_refs->new(3,4,5,6);
1063 $bar->{two} == 11 or die 'bad hash fetch';
1065 Note several important features of this example. First of all, the
1066 I<actual> type of $bar is a scalar reference, and we do not overload
1067 the scalar dereference. Thus we can get the I<actual> non-overloaded
1068 contents of $bar by just using C<$$bar> (what we do in functions which
1069 overload dereference). Similarly, the object returned by the
1070 TIEHASH() method is a scalar reference.
1072 Second, we create a new tied hash each time the hash syntax is used.
1073 This allows us not to worry about a possibility of a reference loop,
1074 which would lead to a memory leak.
1076 Both these problems can be cured. Say, if we want to overload hash
1077 dereference on a reference to an object which is I<implemented> as a
1078 hash itself, the only problem one has to circumvent is how to access
1079 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1080 overloaded dereference operator). Here is one possible fetching routine:
1083 my ($self, $key) = (shift, shift);
1084 my $class = ref $self;
1085 bless $self, 'overload::dummy'; # Disable overloading of %{}
1086 my $out = $self->{$key};
1087 bless $self, $class; # Restore overloading
1091 To remove creation of the tied hash on each access, one may an extra
1092 level of indirection which allows a non-circular structure of references:
1095 use overload '%{}' => sub { ${shift()}->[1] },
1096 '@{}' => sub { ${shift()}->[0] };
1102 bless \ [$a, \%h], $p;
1107 tie %h, ref $self, $self;
1111 sub TIEHASH { my $p = shift; bless \ shift, $p }
1114 $fields{$_} = $i++ foreach qw{zero one two three};
1117 my $key = $fields{shift()};
1118 defined $key or die "Out of band access";
1123 my $key = $fields{shift()};
1124 defined $key or die "Out of band access";
1128 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1129 reference to the intermediate array, which keeps a reference to an
1130 actual array, and the access hash. The tie()ing object for the access
1131 hash is a reference to a reference to the actual array, so
1137 There are no loops of references.
1141 Both "objects" which are blessed into the class C<two_refs1> are
1142 references to a reference to an array, thus references to a I<scalar>.
1143 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1144 overloaded operations.
1148 =head2 Symbolic calculator
1150 Put this in F<symbolic.pm> in your Perl library directory:
1152 package symbolic; # Primitive symbolic calculator
1153 use overload nomethod => \&wrap;
1155 sub new { shift; bless ['n', @_] }
1157 my ($obj, $other, $inv, $meth) = @_;
1158 ($obj, $other) = ($other, $obj) if $inv;
1159 bless [$meth, $obj, $other];
1162 This module is very unusual as overloaded modules go: it does not
1163 provide any usual overloaded operators, instead it provides the L<Last
1164 Resort> operator C<nomethod>. In this example the corresponding
1165 subroutine returns an object which encapsulates operations done over
1166 the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
1167 symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
1169 Here is an example of the script which "calculates" the side of
1170 circumscribed octagon using the above package:
1173 my $iter = 1; # 2**($iter+2) = 8
1174 my $side = symbolic->new(1);
1178 $side = (sqrt(1 + $side**2) - 1)/$side;
1182 The value of $side is
1184 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1185 undef], 1], ['n', 1]]
1187 Note that while we obtained this value using a nice little script,
1188 there is no simple way to I<use> this value. In fact this value may
1189 be inspected in debugger (see L<perldebug>), but only if
1190 C<bareStringify> B<O>ption is set, and not via C<p> command.
1192 If one attempts to print this value, then the overloaded operator
1193 C<""> will be called, which will call C<nomethod> operator. The
1194 result of this operator will be stringified again, but this result is
1195 again of type C<symbolic>, which will lead to an infinite loop.
1197 Add a pretty-printer method to the module F<symbolic.pm>:
1200 my ($meth, $a, $b) = @{+shift};
1201 $a = 'u' unless defined $a;
1202 $b = 'u' unless defined $b;
1203 $a = $a->pretty if ref $a;
1204 $b = $b->pretty if ref $b;
1208 Now one can finish the script by
1210 print "side = ", $side->pretty, "\n";
1212 The method C<pretty> is doing object-to-string conversion, so it
1213 is natural to overload the operator C<""> using this method. However,
1214 inside such a method it is not necessary to pretty-print the
1215 I<components> $a and $b of an object. In the above subroutine
1216 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1217 and $b. If these components use overloading, the catenation operator
1218 will look for an overloaded operator C<.>; if not present, it will
1219 look for an overloaded operator C<"">. Thus it is enough to use
1221 use overload nomethod => \&wrap, '""' => \&str;
1223 my ($meth, $a, $b) = @{+shift};
1224 $a = 'u' unless defined $a;
1225 $b = 'u' unless defined $b;
1229 Now one can change the last line of the script to
1231 print "side = $side\n";
1235 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1237 and one can inspect the value in debugger using all the possible
1240 Something is still amiss: consider the loop variable $cnt of the
1241 script. It was a number, not an object. We cannot make this value of
1242 type C<symbolic>, since then the loop will not terminate.
1244 Indeed, to terminate the cycle, the $cnt should become false.
1245 However, the operator C<bool> for checking falsity is overloaded (this
1246 time via overloaded C<"">), and returns a long string, thus any object
1247 of type C<symbolic> is true. To overcome this, we need a way to
1248 compare an object to 0. In fact, it is easier to write a numeric
1251 Here is the text of F<symbolic.pm> with such a routine added (and
1252 slightly modified str()):
1254 package symbolic; # Primitive symbolic calculator
1256 nomethod => \&wrap, '""' => \&str, '0+' => \#
1258 sub new { shift; bless ['n', @_] }
1260 my ($obj, $other, $inv, $meth) = @_;
1261 ($obj, $other) = ($other, $obj) if $inv;
1262 bless [$meth, $obj, $other];
1265 my ($meth, $a, $b) = @{+shift};
1266 $a = 'u' unless defined $a;
1273 my %subr = ( n => sub {$_[0]},
1274 sqrt => sub {sqrt $_[0]},
1275 '-' => sub {shift() - shift()},
1276 '+' => sub {shift() + shift()},
1277 '/' => sub {shift() / shift()},
1278 '*' => sub {shift() * shift()},
1279 '**' => sub {shift() ** shift()},
1282 my ($meth, $a, $b) = @{+shift};
1283 my $subr = $subr{$meth}
1284 or die "Do not know how to ($meth) in symbolic";
1285 $a = $a->num if ref $a eq __PACKAGE__;
1286 $b = $b->num if ref $b eq __PACKAGE__;
1290 All the work of numeric conversion is done in %subr and num(). Of
1291 course, %subr is not complete, it contains only operators used in the
1292 example below. Here is the extra-credit question: why do we need an
1293 explicit recursion in num()? (Answer is at the end of this section.)
1295 Use this module like this:
1298 my $iter = symbolic->new(2); # 16-gon
1299 my $side = symbolic->new(1);
1303 $cnt = $cnt - 1; # Mutator `--' not implemented
1304 $side = (sqrt(1 + $side**2) - 1)/$side;
1306 printf "%s=%f\n", $side, $side;
1307 printf "pi=%f\n", $side*(2**($iter+2));
1309 It prints (without so many line breaks)
1311 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1313 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1316 The above module is very primitive. It does not implement
1317 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1318 (not required without mutators!), and implements only those arithmetic
1319 operations which are used in the example.
1321 To implement most arithmetic operations is easy; one should just use
1322 the tables of operations, and change the code which fills %subr to
1324 my %subr = ( 'n' => sub {$_[0]} );
1325 foreach my $op (split " ", $overload::ops{with_assign}) {
1326 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1328 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1329 foreach my $op (split " ", "@overload::ops{ @bins }") {
1330 $subr{$op} = eval "sub {shift() $op shift()}";
1332 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1333 print "defining `$op'\n";
1334 $subr{$op} = eval "sub {$op shift()}";
1337 Due to L<Calling Conventions for Mutators>, we do not need anything
1338 special to make C<+=> and friends work, except filling C<+=> entry of
1339 %subr, and defining a copy constructor (needed since Perl has no
1340 way to know that the implementation of C<'+='> does not mutate
1341 the argument, compare L<Copy Constructor>).
1343 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1344 line, and code (this code assumes that mutators change things one level
1345 deep only, so recursive copying is not needed):
1349 bless [@$self], ref $self;
1352 To make C<++> and C<--> work, we need to implement actual mutators,
1353 either directly, or in C<nomethod>. We continue to do things inside
1354 C<nomethod>, thus add
1356 if ($meth eq '++' or $meth eq '--') {
1357 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1361 after the first line of wrap(). This is not a most effective
1362 implementation, one may consider
1364 sub inc { $_[0] = bless ['++', shift, 1]; }
1368 As a final remark, note that one can fill %subr by
1370 my %subr = ( 'n' => sub {$_[0]} );
1371 foreach my $op (split " ", $overload::ops{with_assign}) {
1372 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1374 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1375 foreach my $op (split " ", "@overload::ops{ @bins }") {
1376 $subr{$op} = eval "sub {shift() $op shift()}";
1378 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1379 $subr{$op} = eval "sub {$op shift()}";
1381 $subr{'++'} = $subr{'+'};
1382 $subr{'--'} = $subr{'-'};
1384 This finishes implementation of a primitive symbolic calculator in
1385 50 lines of Perl code. Since the numeric values of subexpressions
1386 are not cached, the calculator is very slow.
1388 Here is the answer for the exercise: In the case of str(), we need no
1389 explicit recursion since the overloaded C<.>-operator will fall back
1390 to an existing overloaded operator C<"">. Overloaded arithmetic
1391 operators I<do not> fall back to numeric conversion if C<fallback> is
1392 not explicitly requested. Thus without an explicit recursion num()
1393 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1394 the argument of num().
1396 If you wonder why defaults for conversion are different for str() and
1397 num(), note how easy it was to write the symbolic calculator. This
1398 simplicity is due to an appropriate choice of defaults. One extra
1399 note: due to the explicit recursion num() is more fragile than sym():
1400 we need to explicitly check for the type of $a and $b. If components
1401 $a and $b happen to be of some related type, this may lead to problems.
1403 =head2 I<Really> symbolic calculator
1405 One may wonder why we call the above calculator symbolic. The reason
1406 is that the actual calculation of the value of expression is postponed
1407 until the value is I<used>.
1409 To see it in action, add a method
1414 @$obj->[0,1] = ('=', shift);
1417 to the package C<symbolic>. After this change one can do
1419 my $a = symbolic->new(3);
1420 my $b = symbolic->new(4);
1421 my $c = sqrt($a**2 + $b**2);
1423 and the numeric value of $c becomes 5. However, after calling
1425 $a->STORE(12); $b->STORE(5);
1427 the numeric value of $c becomes 13. There is no doubt now that the module
1428 symbolic provides a I<symbolic> calculator indeed.
1430 To hide the rough edges under the hood, provide a tie()d interface to the
1431 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1433 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1435 sub nop { } # Around a bug
1437 (the bug is described in L<"BUGS">). One can use this new interface as
1439 tie $a, 'symbolic', 3;
1440 tie $b, 'symbolic', 4;
1441 $a->nop; $b->nop; # Around a bug
1443 my $c = sqrt($a**2 + $b**2);
1445 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1446 of $c becomes 13. To insulate the user of the module add a method
1448 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1453 symbolic->vars($a, $b);
1454 my $c = sqrt($a**2 + $b**2);
1457 printf "c5 %s=%f\n", $c, $c;
1460 printf "c13 %s=%f\n", $c, $c;
1462 shows that the numeric value of $c follows changes to the values of $a
1467 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1471 The L<overloading> pragma can be used to enable or disable overloaded
1472 operations within a lexical scope.
1476 When Perl is run with the B<-Do> switch or its equivalent, overloading
1477 induces diagnostic messages.
1479 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1480 deduce which operations are overloaded (and which ancestor triggers
1481 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1482 is shown by debugger. The method C<()> corresponds to the C<fallback>
1483 key (in fact a presence of this method shows that this package has
1484 overloading enabled, and it is what is used by the C<Overloaded>
1485 function of module C<overload>).
1487 The module might issue the following warnings:
1491 =item Odd number of arguments for overload::constant
1493 (W) The call to overload::constant contained an odd number of arguments.
1494 The arguments should come in pairs.
1496 =item `%s' is not an overloadable type
1498 (W) You tried to overload a constant type the overload package is unaware of.
1500 =item `%s' is not a code reference
1502 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1503 to be a code reference. Either an anonymous subroutine, or a reference
1510 Because it is used for overloading, the per-package hash %OVERLOAD now
1511 has a special meaning in Perl. The symbol table is filled with names
1512 looking like line-noise.
1514 For the purpose of inheritance every overloaded package behaves as if
1515 C<fallback> is present (possibly undefined). This may create
1516 interesting effects if some package is not overloaded, but inherits
1517 from two overloaded packages.
1519 Relation between overloading and tie()ing is broken. Overloading is
1520 triggered or not basing on the I<previous> class of tie()d value.
1522 This happens because the presence of overloading is checked too early,
1523 before any tie()d access is attempted. If the FETCH()ed class of the
1524 tie()d value does not change, a simple workaround is to access the value
1525 immediately after tie()ing, so that after this call the I<previous> class
1526 coincides with the current one.
1528 B<Needed:> a way to fix this without a speed penalty.
1530 Barewords are not covered by overloaded string constants.
1532 This document is confusing. There are grammos and misleading language
1533 used in places. It would seem a total rewrite is needed.