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 &{$fqmeth};
121 'integer' => 0x1000, # HINT_NEW_INTEGER
122 'float' => 0x2000, # HINT_NEW_FLOAT
123 'binary' => 0x4000, # HINT_NEW_BINARY
124 'q' => 0x8000, # HINT_NEW_STRING
125 'qr' => 0x10000, # HINT_NEW_RE
128 %ops = ( with_assign => "+ - * / % ** << >> x .",
129 assign => "+= -= *= /= %= **= <<= >>= x= .=",
130 num_comparison => "< <= > >= == !=",
131 '3way_comparison'=> "<=> cmp",
132 str_comparison => "lt le gt ge eq ne",
133 binary => '& &= | |= ^ ^=',
136 func => "atan2 cos sin exp abs log sqrt int",
137 conversion => 'bool "" 0+',
140 dereferencing => '${} @{} %{} &{} *{}',
142 special => 'nomethod fallback =');
144 use warnings::register;
146 # Arguments: what, sub
149 warnings::warnif ("Odd number of arguments for overload::constant");
152 elsif (!exists $constants {$_ [0]}) {
153 warnings::warnif ("`$_[0]' is not an overloadable type");
155 elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
156 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
157 # blessed, and C<ref> would return the package the ref is blessed into.
158 if (warnings::enabled) {
159 $_ [1] = "undef" unless defined $_ [1];
160 warnings::warn ("`$_[1]' is not a code reference");
165 $^H |= $constants{$_[0]};
171 sub remove_constant {
172 # Arguments: what, sub
175 $^H &= ~ $constants{$_[0]};
186 overload - Package for overloading Perl operations
199 $a = SomeThing->new( 57 );
202 if (overload::Overloaded $b) {...}
204 $strval = overload::StrVal $b;
208 This pragma allows overloading of Perl's operators for a class.
209 To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
211 =head2 Declaration of overloaded functions
213 The compilation directive
220 declares function Number::add() for addition, and method muas() in
221 the "class" C<Number> (or one of its base classes)
222 for the assignment form C<*=> of multiplication.
224 Arguments of this directive come in (key, value) pairs. Legal values
225 are values legal inside a C<&{ ... }> call, so the name of a
226 subroutine, a reference to a subroutine, or an anonymous subroutine
227 will all work. Note that values specified as strings are
228 interpreted as methods, not subroutines. Legal keys are listed below.
230 The subroutine C<add> will be called to execute C<$a+$b> if $a
231 is a reference to an object blessed into the package C<Number>, or if $a is
232 not an object from a package with defined mathemagic addition, but $b is a
233 reference to a C<Number>. It can also be called in other situations, like
234 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
235 methods refer to methods triggered by an overloaded mathematical
238 Since overloading respects inheritance via the @ISA hierarchy, the
239 above declaration would also trigger overloading of C<+> and C<*=> in
240 all the packages which inherit from C<Number>.
242 =head2 Calling Conventions for Binary Operations
244 The functions specified in the C<use overload ...> directive are called
245 with three (in one particular case with four, see L<Last Resort>)
246 arguments. If the corresponding operation is binary, then the first
247 two arguments are the two arguments of the operation. However, due to
248 general object calling conventions, the first argument should always be
249 an object in the package, so in the situation of C<7+$a>, the
250 order of the arguments is interchanged. It probably does not matter
251 when implementing the addition method, but whether the arguments
252 are reversed is vital to the subtraction method. The method can
253 query this information by examining the third argument, which can take
254 three different values:
260 the order of arguments is as in the current operation.
264 the arguments are reversed.
268 the current operation is an assignment variant (as in
269 C<$a+=7>), but the usual function is called instead. This additional
270 information can be used to generate some optimizations. Compare
271 L<Calling Conventions for Mutators>.
275 =head2 Calling Conventions for Unary Operations
277 Unary operation are considered binary operations with the second
278 argument being C<undef>. Thus the functions that overloads C<{"++"}>
279 is called with arguments C<($a,undef,'')> when $a++ is executed.
281 =head2 Calling Conventions for Mutators
283 Two types of mutators have different calling conventions:
287 =item C<++> and C<-->
289 The routines which implement these operators are expected to actually
290 I<mutate> their arguments. So, assuming that $obj is a reference to a
293 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
295 is an appropriate implementation of overloaded C<++>. Note that
297 sub incr { ++$ {$_[0]} ; shift }
299 is OK if used with preincrement and with postincrement. (In the case
300 of postincrement a copying will be performed, see L<Copy Constructor>.)
302 =item C<x=> and other assignment versions
304 There is nothing special about these methods. They may change the
305 value of their arguments, and may leave it as is. The result is going
306 to be assigned to the value in the left-hand-side if different from
309 This allows for the same method to be used as overloaded C<+=> and
310 C<+>. Note that this is I<allowed>, but not recommended, since by the
311 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
312 if C<+=> is not overloaded.
316 B<Warning.> Due to the presence of assignment versions of operations,
317 routines which may be called in assignment context may create
318 self-referential structures. Currently Perl will not free self-referential
319 structures until cycles are C<explicitly> broken. You may get problems
320 when traversing your structures too.
324 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
326 is asking for trouble, since for code C<$obj += $foo> the subroutine
327 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
328 \$foo]>. If using such a subroutine is an important optimization, one
329 can overload C<+=> explicitly by a non-"optimized" version, or switch
330 to non-optimized version if C<not defined $_[2]> (see
331 L<Calling Conventions for Binary Operations>).
333 Even if no I<explicit> assignment-variants of operators are present in
334 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
335 C<',' . $obj . ','> may be both optimized to
337 my $tmp = ',' . $obj; $tmp .= ',';
339 =head2 Overloadable Operations
341 The following symbols can be specified in C<use overload> directive:
345 =item * I<Arithmetic operations>
347 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
348 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
350 For these operations a substituted non-assignment variant can be called if
351 the assignment variant is not available. Methods for operations C<+>,
352 C<->, C<+=>, and C<-=> can be called to automatically generate
353 increment and decrement methods. The operation C<-> can be used to
354 autogenerate missing methods for unary minus or C<abs>.
356 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
357 L<"Calling Conventions for Binary Operations">) for details of these
360 =item * I<Comparison operations>
362 "<", "<=", ">", ">=", "==", "!=", "<=>",
363 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
365 If the corresponding "spaceship" variant is available, it can be
366 used to substitute for the missing operation. During C<sort>ing
367 arrays, C<cmp> is used to compare values subject to C<use overload>.
369 =item * I<Bit operations>
371 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
373 C<neg> stands for unary minus. If the method for C<neg> is not
374 specified, it can be autogenerated using the method for
375 subtraction. If the method for C<!> is not specified, it can be
376 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
378 The same remarks in L<"Arithmetic operations"> about
379 assignment-variants and autogeneration apply for
380 bit operations C<"&">, C<"^">, and C<"|"> as well.
382 =item * I<Increment and decrement>
386 If undefined, addition and subtraction methods can be
387 used instead. These operations are called both in prefix and
390 =item * I<Transcendental functions>
392 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
394 If C<abs> is unavailable, it can be autogenerated using methods
395 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
397 Note that traditionally the Perl function L<int> rounds to 0, thus for
398 floating-point-like types one should follow the same semantic. If
399 C<int> is unavailable, it can be autogenerated using the overloading of
402 =item * I<Boolean, string and numeric conversion>
406 If one or two of these operations are not overloaded, the remaining ones can
407 be used instead. C<bool> is used in the flow control operators
408 (like C<while>) and for the ternary C<?:> operation. These functions can
409 return any arbitrary Perl value. If the corresponding operation for this value
410 is overloaded too, that operation will be called again with this value.
412 As a special case if the overload returns the object itself then it will
413 be used directly. An overloaded conversion returning the object is
414 probably a bug, because you're likely to get something that looks like
415 C<YourPackage=HASH(0x8172b34)>.
421 If not overloaded, the argument will be converted to a filehandle or
422 glob (which may require a stringification). The same overloading
423 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
424 I<globbing> syntax C<E<lt>${var}E<gt>>.
426 B<BUGS> Even in list context, the iterator is currently called only
427 once and with scalar context.
429 =item * I<File tests>
433 This overload is used for all the filetest operators (C<-f>, C<-x> and
434 so on: see L<perlfunc/-X> for the full list). Even though these are
435 unary operators, the method will be called with a second argument which
436 is a single letter indicating which test was performed. Note that the
437 overload key is the literal string C<"-X">: you can't provide separate
438 overloads for the different tests.
440 Calling an overloaded filetest operator does not affect the stat value
441 associated with the special filehandle C<_>. It still refers to the
442 result of the last C<stat>, C<lstat> or unoverloaded filetest.
444 If not overloaded, these operators will fall back to the default
445 behaviour even without C<< fallback => 1 >>. This means that if the
446 object is a blessed glob or blessed IO ref it will be treated as a
447 filehandle, otherwise string overloading will be invoked and the result
448 treated as a filename.
450 This overload was introduced in perl 5.12.
454 The key C<"~~"> allows you to override the smart matching logic used by
455 the C<~~> operator and the switch construct (C<given>/C<when>). See
456 L<perlsyn/switch> and L<feature>.
458 =item * I<Dereferencing>
460 '${}', '@{}', '%{}', '&{}', '*{}'.
462 If not overloaded, the argument will be dereferenced I<as is>, thus
463 should be of correct type. These functions should return a reference
464 of correct type, or another object with overloaded dereferencing.
466 As a special case if the overload returns the object itself then it
467 will be used directly (provided it is the correct type).
469 The dereference operators must be specified explicitly they will not be passed to
474 "nomethod", "fallback", "=".
476 see L<SPECIAL SYMBOLS FOR C<use overload>>.
480 See L<"Fallback"> for an explanation of when a missing method can be
483 A computer-readable form of the above table is available in the hash
484 %overload::ops, with values being space-separated lists of names:
486 with_assign => '+ - * / % ** << >> x .',
487 assign => '+= -= *= /= %= **= <<= >>= x= .=',
488 num_comparison => '< <= > >= == !=',
489 '3way_comparison'=> '<=> cmp',
490 str_comparison => 'lt le gt ge eq ne',
491 binary => '& &= | |= ^ ^=',
494 func => 'atan2 cos sin exp abs log sqrt',
495 conversion => 'bool "" 0+',
498 dereferencing => '${} @{} %{} &{} *{}',
500 special => 'nomethod fallback ='
502 =head2 Inheritance and overloading
504 Inheritance interacts with overloading in two ways.
508 =item Strings as values of C<use overload> directive
512 use overload key => value;
514 is a string, it is interpreted as a method name.
516 =item Overloading of an operation is inherited by derived classes
518 Any class derived from an overloaded class is also overloaded. The
519 set of overloaded methods is the union of overloaded methods of all
520 the ancestors. If some method is overloaded in several ancestor, then
521 which description will be used is decided by the usual inheritance
524 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
525 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
526 then the subroutine C<D::plus_sub> will be called to implement
527 operation C<+> for an object in package C<A>.
531 Note that since the value of the C<fallback> key is not a subroutine,
532 its inheritance is not governed by the above rules. In the current
533 implementation, the value of C<fallback> in the first overloaded
534 ancestor is used, but this is accidental and subject to change.
536 =head1 SPECIAL SYMBOLS FOR C<use overload>
538 Three keys are recognized by Perl that are not covered by the above
543 C<"nomethod"> should be followed by a reference to a function of four
544 parameters. If defined, it is called when the overloading mechanism
545 cannot find a method for some operation. The first three arguments of
546 this function coincide with the arguments for the corresponding method if
547 it were found, the fourth argument is the symbol
548 corresponding to the missing method. If several methods are tried,
549 the last one is used. Say, C<1-$a> can be equivalent to
551 &nomethodMethod($a,1,1,"-")
553 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
554 C<use overload> directive.
556 The C<"nomethod"> mechanism is I<not> used for the dereference operators
557 ( ${} @{} %{} &{} *{} ).
560 If some operation cannot be resolved, and there is no function
561 assigned to C<"nomethod">, then an exception will be raised via die()--
562 unless C<"fallback"> was specified as a key in C<use overload> directive.
567 The key C<"fallback"> governs what to do if a method for a particular
568 operation is not found. Three different cases are possible depending on
569 the value of C<"fallback">:
576 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
577 then tries to calls C<"nomethod"> value; if missing, an exception
582 The same as for the C<undef> value, but no exception is raised. Instead,
583 it silently reverts to what it would have done were there no C<use overload>
586 =item * defined, but FALSE
588 No autogeneration is tried. Perl tries to call
589 C<"nomethod"> value, and if this is missing, raises an exception.
593 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
594 yet, see L<"Inheritance and overloading">.
596 =head2 Copy Constructor
598 The value for C<"="> is a reference to a function with three
599 arguments, i.e., it looks like the other values in C<use
600 overload>. However, it does not overload the Perl assignment
601 operator. This would go against Camel hair.
603 This operation is called in the situations when a mutator is applied
604 to a reference that shares its object with some other reference, such
610 To make this change $a and not change $b, a copy of C<$$a> is made,
611 and $a is assigned a reference to this new object. This operation is
612 done during execution of the C<++$a>, and not during the assignment,
613 (so before the increment C<$$a> coincides with C<$$b>). This is only
614 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
615 C<nomethod>). Note that if this operation is expressed via C<'+'>
616 a nonmutator, i.e., as in
621 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
622 appear as lvalue when the above code is executed.
624 If the copy constructor is required during the execution of some mutator,
625 but a method for C<'='> was not specified, it can be autogenerated as a
626 string copy if the object is a plain scalar or a simple assignment if it
633 The actually executed code for
636 Something else which does not modify $a or $b....
642 Something else which does not modify $a or $b....
643 $a = $a->clone(undef,"");
646 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
647 C<'='> was overloaded with C<\&clone>.
651 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
654 =head1 MAGIC AUTOGENERATION
656 If a method for an operation is not found, and the value for C<"fallback"> is
657 TRUE or undefined, Perl tries to autogenerate a substitute method for
658 the missing operation based on the defined operations. Autogenerated method
659 substitutions are possible for the following operations:
663 =item I<Assignment forms of arithmetic operations>
665 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
668 =item I<Conversion operations>
670 String, numeric, and boolean conversion are calculated in terms of one
671 another if not all of them are defined.
673 =item I<Increment and decrement>
675 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
676 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
680 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
684 can be expressed in terms of subtraction.
688 C<!> and C<not> can be expressed in terms of boolean conversion, or
689 string or numerical conversion.
691 =item I<Concatenation>
693 can be expressed in terms of string conversion.
695 =item I<Comparison operations>
697 can be expressed in terms of its "spaceship" counterpart: either
698 C<E<lt>=E<gt>> or C<cmp>:
700 <, >, <=, >=, ==, != in terms of <=>
701 lt, gt, le, ge, eq, ne in terms of cmp
705 <> in terms of builtin operations
707 =item I<Dereferencing>
709 ${} @{} %{} &{} *{} in terms of builtin operations
711 =item I<Copy operator>
713 can be expressed in terms of an assignment to the dereferenced value, if this
714 value is a scalar and not a reference, or simply a reference assignment
719 =head1 Minimal set of overloaded operations
721 Since some operations can be automatically generated from others, there is
722 a minimal set of operations that need to be overloaded in order to have
723 the complete set of overloaded operations at one's disposal.
724 Of course, the autogenerated operations may not do exactly what the user
725 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
730 atan2 cos sin exp log sqrt int
732 Additionally, you need to define at least one of string, boolean or
733 numeric conversions because any one can be used to emulate the others.
734 The string conversion can also be used to emulate concatenation.
736 =head1 Losing overloading
738 The restriction for the comparison operation is that even if, for example,
739 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
740 function will produce only a standard logical value based on the
741 numerical value of the result of `C<cmp>'. In particular, a working
742 numeric conversion is needed in this case (possibly expressed in terms of
745 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
746 if the string conversion substitution is applied.
748 When you chop() a mathemagical object it is promoted to a string and its
749 mathemagical properties are lost. The same can happen with other
752 =head1 Run-time Overloading
754 Since all C<use> directives are executed at compile-time, the only way to
755 change overloading during run-time is to
757 eval 'use overload "+" => \&addmethod';
761 eval 'no overload "+", "--", "<="';
763 though the use of these constructs during run-time is questionable.
765 =head1 Public functions
767 Package C<overload.pm> provides the following public functions:
771 =item overload::StrVal(arg)
773 Gives string value of C<arg> as in absence of stringify overloading. If you
774 are using this to get the address of a reference (useful for checking if two
775 references point to the same thing) then you may be better off using
776 C<Scalar::Util::refaddr()>, which is faster.
778 =item overload::Overloaded(arg)
780 Returns true if C<arg> is subject to overloading of some operations.
782 =item overload::Method(obj,op)
784 Returns C<undef> or a reference to the method that implements C<op>.
788 =head1 Overloading constants
790 For some applications, the Perl parser mangles constants too much.
791 It is possible to hook into this process via C<overload::constant()>
792 and C<overload::remove_constant()> functions.
794 These functions take a hash as an argument. The recognized keys of this hash
801 to overload integer constants,
805 to overload floating point constants,
809 to overload octal and hexadecimal constants,
813 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
814 strings and here-documents,
818 to overload constant pieces of regular expressions.
822 The corresponding values are references to functions which take three arguments:
823 the first one is the I<initial> string form of the constant, the second one
824 is how Perl interprets this constant, the third one is how the constant is used.
825 Note that the initial string form does not
826 contain string delimiters, and has backslashes in backslash-delimiter
827 combinations stripped (thus the value of delimiter is not relevant for
828 processing of this string). The return value of this function is how this
829 constant is going to be interpreted by Perl. The third argument is undefined
830 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
831 context (comes from strings, regular expressions, and single-quote HERE
832 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
833 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
835 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
836 it is expected that overloaded constant strings are equipped with reasonable
837 overloaded catenation operator, otherwise absurd results will result.
838 Similarly, negative numbers are considered as negations of positive constants.
840 Note that it is probably meaningless to call the functions overload::constant()
841 and overload::remove_constant() from anywhere but import() and unimport() methods.
842 From these methods they may be called as
847 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
848 overload::constant integer => sub {Math::BigInt->new(shift)};
851 =head1 IMPLEMENTATION
853 What follows is subject to change RSN.
855 The table of methods for all operations is cached in magic for the
856 symbol table hash for the package. The cache is invalidated during
857 processing of C<use overload>, C<no overload>, new function
858 definitions, and changes in @ISA. However, this invalidation remains
859 unprocessed until the next C<bless>ing into the package. Hence if you
860 want to change overloading structure dynamically, you'll need an
861 additional (fake) C<bless>ing to update the table.
863 (Every SVish thing has a magic queue, and magic is an entry in that
864 queue. This is how a single variable may participate in multiple
865 forms of magic simultaneously. For instance, environment variables
866 regularly have two forms at once: their %ENV magic and their taint
867 magic. However, the magic which implements overloading is applied to
868 the stashes, which are rarely used directly, thus should not slow down
871 If an object belongs to a package using overload, it carries a special
872 flag. Thus the only speed penalty during arithmetic operations without
873 overloading is the checking of this flag.
875 In fact, if C<use overload> is not present, there is almost no overhead
876 for overloadable operations, so most programs should not suffer
877 measurable performance penalties. A considerable effort was made to
878 minimize the overhead when overload is used in some package, but the
879 arguments in question do not belong to packages using overload. When
880 in doubt, test your speed with C<use overload> and without it. So far
881 there have been no reports of substantial speed degradation if Perl is
882 compiled with optimization turned on.
884 There is no size penalty for data if overload is not used. The only
885 size penalty if overload is used in some package is that I<all> the
886 packages acquire a magic during the next C<bless>ing into the
887 package. This magic is three-words-long for packages without
888 overloading, and carries the cache table if the package is overloaded.
890 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
891 carried out before any operation that can imply an assignment to the
892 object $a (or $b) refers to, like C<$a++>. You can override this
893 behavior by defining your own copy constructor (see L<"Copy Constructor">).
895 It is expected that arguments to methods that are not explicitly supposed
896 to be changed are constant (but this is not enforced).
898 =head1 Metaphor clash
900 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
901 If it I<looks> counter intuitive to you, you are subject to a metaphor
904 Here is a Perl object metaphor:
906 I< object is a reference to blessed data>
908 and an arithmetic metaphor:
910 I< object is a thing by itself>.
912 The I<main> problem of overloading C<=> is the fact that these metaphors
913 imply different actions on the assignment C<$a = $b> if $a and $b are
914 objects. Perl-think implies that $a becomes a reference to whatever
915 $b was referencing. Arithmetic-think implies that the value of "object"
916 $a is changed to become the value of the object $b, preserving the fact
917 that $a and $b are separate entities.
919 The difference is not relevant in the absence of mutators. After
920 a Perl-way assignment an operation which mutates the data referenced by $a
921 would change the data referenced by $b too. Effectively, after
922 C<$a = $b> values of $a and $b become I<indistinguishable>.
924 On the other hand, anyone who has used algebraic notation knows the
925 expressive power of the arithmetic metaphor. Overloading works hard
926 to enable this metaphor while preserving the Perlian way as far as
927 possible. Since it is not possible to freely mix two contradicting
928 metaphors, overloading allows the arithmetic way to write things I<as
929 far as all the mutators are called via overloaded access only>. The
930 way it is done is described in L<Copy Constructor>.
932 If some mutator methods are directly applied to the overloaded values,
933 one may need to I<explicitly unlink> other values which references the
938 $b = $a; # $b is "linked" to $a
940 $a = $a->clone; # Unlink $b from $a
943 Note that overloaded access makes this transparent:
946 $b = $a; # $b is "linked" to $a
947 $a += 4; # would unlink $b automagically
949 However, it would not make
952 $a = 4; # Now $a is a plain 4, not 'Data'
954 preserve "objectness" of $a. But Perl I<has> a way to make assignments
955 to an object do whatever you want. It is just not the overload, but
956 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
957 which returns the object itself, and STORE() method which changes the
958 value of the object, one can reproduce the arithmetic metaphor in its
959 completeness, at least for variables which were tie()d from the start.
961 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
965 Please add examples to what follows!
967 =head2 Two-face scalars
969 Put this in F<two_face.pm> in your Perl library directory:
971 package two_face; # Scalars with separate string and
973 sub new { my $p = shift; bless [@_], $p }
974 use overload '""' => \&str, '0+' => \&num, fallback => 1;
981 my $seven = two_face->new("vii", 7);
982 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
983 print "seven contains `i'\n" if $seven =~ /i/;
985 (The second line creates a scalar which has both a string value, and a
986 numeric value.) This prints:
988 seven=vii, seven=7, eight=8
991 =head2 Two-face references
993 Suppose you want to create an object which is accessible as both an
994 array reference and a hash reference.
997 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
1005 tie %h, ref $self, $self;
1009 sub TIEHASH { my $p = shift; bless \ shift, $p }
1012 $fields{$_} = $i++ foreach qw{zero one two three};
1014 my $self = ${shift()};
1015 my $key = $fields{shift()};
1016 defined $key or die "Out of band access";
1017 $$self->[$key] = shift;
1020 my $self = ${shift()};
1021 my $key = $fields{shift()};
1022 defined $key or die "Out of band access";
1026 Now one can access an object using both the array and hash syntax:
1028 my $bar = two_refs->new(3,4,5,6);
1030 $bar->{two} == 11 or die 'bad hash fetch';
1032 Note several important features of this example. First of all, the
1033 I<actual> type of $bar is a scalar reference, and we do not overload
1034 the scalar dereference. Thus we can get the I<actual> non-overloaded
1035 contents of $bar by just using C<$$bar> (what we do in functions which
1036 overload dereference). Similarly, the object returned by the
1037 TIEHASH() method is a scalar reference.
1039 Second, we create a new tied hash each time the hash syntax is used.
1040 This allows us not to worry about a possibility of a reference loop,
1041 which would lead to a memory leak.
1043 Both these problems can be cured. Say, if we want to overload hash
1044 dereference on a reference to an object which is I<implemented> as a
1045 hash itself, the only problem one has to circumvent is how to access
1046 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1047 overloaded dereference operator). Here is one possible fetching routine:
1050 my ($self, $key) = (shift, shift);
1051 my $class = ref $self;
1052 bless $self, 'overload::dummy'; # Disable overloading of %{}
1053 my $out = $self->{$key};
1054 bless $self, $class; # Restore overloading
1058 To remove creation of the tied hash on each access, one may an extra
1059 level of indirection which allows a non-circular structure of references:
1062 use overload '%{}' => sub { ${shift()}->[1] },
1063 '@{}' => sub { ${shift()}->[0] };
1069 bless \ [$a, \%h], $p;
1074 tie %h, ref $self, $self;
1078 sub TIEHASH { my $p = shift; bless \ shift, $p }
1081 $fields{$_} = $i++ foreach qw{zero one two three};
1084 my $key = $fields{shift()};
1085 defined $key or die "Out of band access";
1090 my $key = $fields{shift()};
1091 defined $key or die "Out of band access";
1095 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1096 reference to the intermediate array, which keeps a reference to an
1097 actual array, and the access hash. The tie()ing object for the access
1098 hash is a reference to a reference to the actual array, so
1104 There are no loops of references.
1108 Both "objects" which are blessed into the class C<two_refs1> are
1109 references to a reference to an array, thus references to a I<scalar>.
1110 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1111 overloaded operations.
1115 =head2 Symbolic calculator
1117 Put this in F<symbolic.pm> in your Perl library directory:
1119 package symbolic; # Primitive symbolic calculator
1120 use overload nomethod => \&wrap;
1122 sub new { shift; bless ['n', @_] }
1124 my ($obj, $other, $inv, $meth) = @_;
1125 ($obj, $other) = ($other, $obj) if $inv;
1126 bless [$meth, $obj, $other];
1129 This module is very unusual as overloaded modules go: it does not
1130 provide any usual overloaded operators, instead it provides the L<Last
1131 Resort> operator C<nomethod>. In this example the corresponding
1132 subroutine returns an object which encapsulates operations done over
1133 the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
1134 symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
1136 Here is an example of the script which "calculates" the side of
1137 circumscribed octagon using the above package:
1140 my $iter = 1; # 2**($iter+2) = 8
1141 my $side = symbolic->new(1);
1145 $side = (sqrt(1 + $side**2) - 1)/$side;
1149 The value of $side is
1151 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1152 undef], 1], ['n', 1]]
1154 Note that while we obtained this value using a nice little script,
1155 there is no simple way to I<use> this value. In fact this value may
1156 be inspected in debugger (see L<perldebug>), but only if
1157 C<bareStringify> B<O>ption is set, and not via C<p> command.
1159 If one attempts to print this value, then the overloaded operator
1160 C<""> will be called, which will call C<nomethod> operator. The
1161 result of this operator will be stringified again, but this result is
1162 again of type C<symbolic>, which will lead to an infinite loop.
1164 Add a pretty-printer method to the module F<symbolic.pm>:
1167 my ($meth, $a, $b) = @{+shift};
1168 $a = 'u' unless defined $a;
1169 $b = 'u' unless defined $b;
1170 $a = $a->pretty if ref $a;
1171 $b = $b->pretty if ref $b;
1175 Now one can finish the script by
1177 print "side = ", $side->pretty, "\n";
1179 The method C<pretty> is doing object-to-string conversion, so it
1180 is natural to overload the operator C<""> using this method. However,
1181 inside such a method it is not necessary to pretty-print the
1182 I<components> $a and $b of an object. In the above subroutine
1183 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1184 and $b. If these components use overloading, the catenation operator
1185 will look for an overloaded operator C<.>; if not present, it will
1186 look for an overloaded operator C<"">. Thus it is enough to use
1188 use overload nomethod => \&wrap, '""' => \&str;
1190 my ($meth, $a, $b) = @{+shift};
1191 $a = 'u' unless defined $a;
1192 $b = 'u' unless defined $b;
1196 Now one can change the last line of the script to
1198 print "side = $side\n";
1202 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1204 and one can inspect the value in debugger using all the possible
1207 Something is still amiss: consider the loop variable $cnt of the
1208 script. It was a number, not an object. We cannot make this value of
1209 type C<symbolic>, since then the loop will not terminate.
1211 Indeed, to terminate the cycle, the $cnt should become false.
1212 However, the operator C<bool> for checking falsity is overloaded (this
1213 time via overloaded C<"">), and returns a long string, thus any object
1214 of type C<symbolic> is true. To overcome this, we need a way to
1215 compare an object to 0. In fact, it is easier to write a numeric
1218 Here is the text of F<symbolic.pm> with such a routine added (and
1219 slightly modified str()):
1221 package symbolic; # Primitive symbolic calculator
1223 nomethod => \&wrap, '""' => \&str, '0+' => \#
1225 sub new { shift; bless ['n', @_] }
1227 my ($obj, $other, $inv, $meth) = @_;
1228 ($obj, $other) = ($other, $obj) if $inv;
1229 bless [$meth, $obj, $other];
1232 my ($meth, $a, $b) = @{+shift};
1233 $a = 'u' unless defined $a;
1240 my %subr = ( n => sub {$_[0]},
1241 sqrt => sub {sqrt $_[0]},
1242 '-' => sub {shift() - shift()},
1243 '+' => sub {shift() + shift()},
1244 '/' => sub {shift() / shift()},
1245 '*' => sub {shift() * shift()},
1246 '**' => sub {shift() ** shift()},
1249 my ($meth, $a, $b) = @{+shift};
1250 my $subr = $subr{$meth}
1251 or die "Do not know how to ($meth) in symbolic";
1252 $a = $a->num if ref $a eq __PACKAGE__;
1253 $b = $b->num if ref $b eq __PACKAGE__;
1257 All the work of numeric conversion is done in %subr and num(). Of
1258 course, %subr is not complete, it contains only operators used in the
1259 example below. Here is the extra-credit question: why do we need an
1260 explicit recursion in num()? (Answer is at the end of this section.)
1262 Use this module like this:
1265 my $iter = symbolic->new(2); # 16-gon
1266 my $side = symbolic->new(1);
1270 $cnt = $cnt - 1; # Mutator `--' not implemented
1271 $side = (sqrt(1 + $side**2) - 1)/$side;
1273 printf "%s=%f\n", $side, $side;
1274 printf "pi=%f\n", $side*(2**($iter+2));
1276 It prints (without so many line breaks)
1278 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1280 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1283 The above module is very primitive. It does not implement
1284 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1285 (not required without mutators!), and implements only those arithmetic
1286 operations which are used in the example.
1288 To implement most arithmetic operations is easy; one should just use
1289 the tables of operations, and change the code which fills %subr to
1291 my %subr = ( 'n' => sub {$_[0]} );
1292 foreach my $op (split " ", $overload::ops{with_assign}) {
1293 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1295 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1296 foreach my $op (split " ", "@overload::ops{ @bins }") {
1297 $subr{$op} = eval "sub {shift() $op shift()}";
1299 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1300 print "defining `$op'\n";
1301 $subr{$op} = eval "sub {$op shift()}";
1304 Due to L<Calling Conventions for Mutators>, we do not need anything
1305 special to make C<+=> and friends work, except filling C<+=> entry of
1306 %subr, and defining a copy constructor (needed since Perl has no
1307 way to know that the implementation of C<'+='> does not mutate
1308 the argument, compare L<Copy Constructor>).
1310 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1311 line, and code (this code assumes that mutators change things one level
1312 deep only, so recursive copying is not needed):
1316 bless [@$self], ref $self;
1319 To make C<++> and C<--> work, we need to implement actual mutators,
1320 either directly, or in C<nomethod>. We continue to do things inside
1321 C<nomethod>, thus add
1323 if ($meth eq '++' or $meth eq '--') {
1324 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1328 after the first line of wrap(). This is not a most effective
1329 implementation, one may consider
1331 sub inc { $_[0] = bless ['++', shift, 1]; }
1335 As a final remark, note that one can fill %subr by
1337 my %subr = ( 'n' => sub {$_[0]} );
1338 foreach my $op (split " ", $overload::ops{with_assign}) {
1339 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1341 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1342 foreach my $op (split " ", "@overload::ops{ @bins }") {
1343 $subr{$op} = eval "sub {shift() $op shift()}";
1345 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1346 $subr{$op} = eval "sub {$op shift()}";
1348 $subr{'++'} = $subr{'+'};
1349 $subr{'--'} = $subr{'-'};
1351 This finishes implementation of a primitive symbolic calculator in
1352 50 lines of Perl code. Since the numeric values of subexpressions
1353 are not cached, the calculator is very slow.
1355 Here is the answer for the exercise: In the case of str(), we need no
1356 explicit recursion since the overloaded C<.>-operator will fall back
1357 to an existing overloaded operator C<"">. Overloaded arithmetic
1358 operators I<do not> fall back to numeric conversion if C<fallback> is
1359 not explicitly requested. Thus without an explicit recursion num()
1360 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1361 the argument of num().
1363 If you wonder why defaults for conversion are different for str() and
1364 num(), note how easy it was to write the symbolic calculator. This
1365 simplicity is due to an appropriate choice of defaults. One extra
1366 note: due to the explicit recursion num() is more fragile than sym():
1367 we need to explicitly check for the type of $a and $b. If components
1368 $a and $b happen to be of some related type, this may lead to problems.
1370 =head2 I<Really> symbolic calculator
1372 One may wonder why we call the above calculator symbolic. The reason
1373 is that the actual calculation of the value of expression is postponed
1374 until the value is I<used>.
1376 To see it in action, add a method
1381 @$obj->[0,1] = ('=', shift);
1384 to the package C<symbolic>. After this change one can do
1386 my $a = symbolic->new(3);
1387 my $b = symbolic->new(4);
1388 my $c = sqrt($a**2 + $b**2);
1390 and the numeric value of $c becomes 5. However, after calling
1392 $a->STORE(12); $b->STORE(5);
1394 the numeric value of $c becomes 13. There is no doubt now that the module
1395 symbolic provides a I<symbolic> calculator indeed.
1397 To hide the rough edges under the hood, provide a tie()d interface to the
1398 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1400 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1402 sub nop { } # Around a bug
1404 (the bug is described in L<"BUGS">). One can use this new interface as
1406 tie $a, 'symbolic', 3;
1407 tie $b, 'symbolic', 4;
1408 $a->nop; $b->nop; # Around a bug
1410 my $c = sqrt($a**2 + $b**2);
1412 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1413 of $c becomes 13. To insulate the user of the module add a method
1415 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1420 symbolic->vars($a, $b);
1421 my $c = sqrt($a**2 + $b**2);
1424 printf "c5 %s=%f\n", $c, $c;
1427 printf "c13 %s=%f\n", $c, $c;
1429 shows that the numeric value of $c follows changes to the values of $a
1434 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1438 The L<overloading> pragma can be used to enable or disable overloaded
1439 operations within a lexical scope.
1443 When Perl is run with the B<-Do> switch or its equivalent, overloading
1444 induces diagnostic messages.
1446 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1447 deduce which operations are overloaded (and which ancestor triggers
1448 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1449 is shown by debugger. The method C<()> corresponds to the C<fallback>
1450 key (in fact a presence of this method shows that this package has
1451 overloading enabled, and it is what is used by the C<Overloaded>
1452 function of module C<overload>).
1454 The module might issue the following warnings:
1458 =item Odd number of arguments for overload::constant
1460 (W) The call to overload::constant contained an odd number of arguments.
1461 The arguments should come in pairs.
1463 =item `%s' is not an overloadable type
1465 (W) You tried to overload a constant type the overload package is unaware of.
1467 =item `%s' is not a code reference
1469 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1470 to be a code reference. Either an anonymous subroutine, or a reference
1477 Because it is used for overloading, the per-package hash %OVERLOAD now
1478 has a special meaning in Perl. The symbol table is filled with names
1479 looking like line-noise.
1481 For the purpose of inheritance every overloaded package behaves as if
1482 C<fallback> is present (possibly undefined). This may create
1483 interesting effects if some package is not overloaded, but inherits
1484 from two overloaded packages.
1486 Relation between overloading and tie()ing is broken. Overloading is
1487 triggered or not basing on the I<previous> class of tie()d value.
1489 This happens because the presence of overloading is checked too early,
1490 before any tie()d access is attempted. If the FETCH()ed class of the
1491 tie()d value does not change, a simple workaround is to access the value
1492 immediately after tie()ing, so that after this call the I<previous> class
1493 coincides with the current one.
1495 B<Needed:> a way to fix this without a speed penalty.
1497 Barewords are not covered by overloaded string constants.
1499 This document is confusing. There are grammos and misleading language
1500 used in places. It would seem a total rewrite is needed.