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};
60 return $sub if $sub ne \&nil;
61 return shift->can($ {*$globref});
64 sub OverloadedStringify {
66 $package = ref $package if ref $package;
68 ov_method mycan($package, '(""'), $package
69 or ov_method mycan($package, '(0+'), $package
70 or ov_method mycan($package, '(bool'), $package
71 or ov_method mycan($package, '(nomethod'), $package;
80 $package = Scalar::Util::blessed($package);
81 return undef if !defined $package;
83 #my $meth = $package->can('(' . shift);
84 ov_method mycan($package, '(' . shift), $package;
85 #return $meth if $meth ne \&nil;
90 my $package = ref $_[0];
91 return "$_[0]" unless $package;
96 my $class = Scalar::Util::blessed($_[0]);
97 my $class_prefix = defined($class) ? "$class=" : "";
98 my $type = Scalar::Util::reftype($_[0]);
99 my $addr = Scalar::Util::refaddr($_[0]);
100 return sprintf("$class_prefix$type(0x%x)", $addr);
105 sub mycan { # Real can would leave stubs.
106 my ($package, $meth) = @_;
112 my $mro = mro::get_linear_isa($package);
113 foreach my $p (@$mro) {
114 my $fqmeth = $p . q{::} . $meth;
115 return \*{$fqmeth} if defined &{$fqmeth};
122 'integer' => 0x1000, # HINT_NEW_INTEGER
123 'float' => 0x2000, # HINT_NEW_FLOAT
124 'binary' => 0x4000, # HINT_NEW_BINARY
125 'q' => 0x8000, # HINT_NEW_STRING
126 'qr' => 0x10000, # HINT_NEW_RE
129 %ops = ( with_assign => "+ - * / % ** << >> x .",
130 assign => "+= -= *= /= %= **= <<= >>= x= .=",
131 num_comparison => "< <= > >= == !=",
132 '3way_comparison'=> "<=> cmp",
133 str_comparison => "lt le gt ge eq ne",
134 binary => '& &= | |= ^ ^=',
137 func => "atan2 cos sin exp abs log sqrt int",
138 conversion => 'bool "" 0+',
141 dereferencing => '${} @{} %{} &{} *{}',
143 special => 'nomethod fallback =');
145 use warnings::register;
147 # Arguments: what, sub
150 warnings::warnif ("Odd number of arguments for overload::constant");
153 elsif (!exists $constants {$_ [0]}) {
154 warnings::warnif ("`$_[0]' is not an overloadable type");
156 elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
157 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
158 # blessed, and C<ref> would return the package the ref is blessed into.
159 if (warnings::enabled) {
160 $_ [1] = "undef" unless defined $_ [1];
161 warnings::warn ("`$_[1]' is not a code reference");
166 $^H |= $constants{$_[0]};
172 sub remove_constant {
173 # Arguments: what, sub
176 $^H &= ~ $constants{$_[0]};
187 overload - Package for overloading Perl operations
200 $a = SomeThing->new( 57 );
203 if (overload::Overloaded $b) {...}
205 $strval = overload::StrVal $b;
209 This pragma allows overloading of Perl's operators for a class.
210 To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
212 =head2 Declaration of overloaded functions
214 The compilation directive
221 declares function Number::add() for addition, and method muas() in
222 the "class" C<Number> (or one of its base classes)
223 for the assignment form C<*=> of multiplication.
225 Arguments of this directive come in (key, value) pairs. Legal values
226 are values legal inside a C<&{ ... }> call, so the name of a
227 subroutine, a reference to a subroutine, or an anonymous subroutine
228 will all work. Note that values specified as strings are
229 interpreted as methods, not subroutines. Legal keys are listed below.
231 The subroutine C<add> will be called to execute C<$a+$b> if $a
232 is a reference to an object blessed into the package C<Number>, or if $a is
233 not an object from a package with defined mathemagic addition, but $b is a
234 reference to a C<Number>. It can also be called in other situations, like
235 C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
236 methods refer to methods triggered by an overloaded mathematical
239 Since overloading respects inheritance via the @ISA hierarchy, the
240 above declaration would also trigger overloading of C<+> and C<*=> in
241 all the packages which inherit from C<Number>.
243 =head2 Calling Conventions for Binary Operations
245 The functions specified in the C<use overload ...> directive are called
246 with three (in one particular case with four, see L<Last Resort>)
247 arguments. If the corresponding operation is binary, then the first
248 two arguments are the two arguments of the operation. However, due to
249 general object calling conventions, the first argument should always be
250 an object in the package, so in the situation of C<7+$a>, the
251 order of the arguments is interchanged. It probably does not matter
252 when implementing the addition method, but whether the arguments
253 are reversed is vital to the subtraction method. The method can
254 query this information by examining the third argument, which can take
255 three different values:
261 the order of arguments is as in the current operation.
265 the arguments are reversed.
269 the current operation is an assignment variant (as in
270 C<$a+=7>), but the usual function is called instead. This additional
271 information can be used to generate some optimizations. Compare
272 L<Calling Conventions for Mutators>.
276 =head2 Calling Conventions for Unary Operations
278 Unary operation are considered binary operations with the second
279 argument being C<undef>. Thus the functions that overloads C<{"++"}>
280 is called with arguments C<($a,undef,'')> when $a++ is executed.
282 =head2 Calling Conventions for Mutators
284 Two types of mutators have different calling conventions:
288 =item C<++> and C<-->
290 The routines which implement these operators are expected to actually
291 I<mutate> their arguments. So, assuming that $obj is a reference to a
294 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
296 is an appropriate implementation of overloaded C<++>. Note that
298 sub incr { ++$ {$_[0]} ; shift }
300 is OK if used with preincrement and with postincrement. (In the case
301 of postincrement a copying will be performed, see L<Copy Constructor>.)
303 =item C<x=> and other assignment versions
305 There is nothing special about these methods. They may change the
306 value of their arguments, and may leave it as is. The result is going
307 to be assigned to the value in the left-hand-side if different from
310 This allows for the same method to be used as overloaded C<+=> and
311 C<+>. Note that this is I<allowed>, but not recommended, since by the
312 semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
313 if C<+=> is not overloaded.
317 B<Warning.> Due to the presence of assignment versions of operations,
318 routines which may be called in assignment context may create
319 self-referential structures. Currently Perl will not free self-referential
320 structures until cycles are C<explicitly> broken. You may get problems
321 when traversing your structures too.
325 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
327 is asking for trouble, since for code C<$obj += $foo> the subroutine
328 is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
329 \$foo]>. If using such a subroutine is an important optimization, one
330 can overload C<+=> explicitly by a non-"optimized" version, or switch
331 to non-optimized version if C<not defined $_[2]> (see
332 L<Calling Conventions for Binary Operations>).
334 Even if no I<explicit> assignment-variants of operators are present in
335 the script, they may be generated by the optimizer. Say, C<",$obj,"> or
336 C<',' . $obj . ','> may be both optimized to
338 my $tmp = ',' . $obj; $tmp .= ',';
340 =head2 Overloadable Operations
342 The following symbols can be specified in C<use overload> directive:
346 =item * I<Arithmetic operations>
348 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
349 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
351 For these operations a substituted non-assignment variant can be called if
352 the assignment variant is not available. Methods for operations C<+>,
353 C<->, C<+=>, and C<-=> can be called to automatically generate
354 increment and decrement methods. The operation C<-> can be used to
355 autogenerate missing methods for unary minus or C<abs>.
357 See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
358 L<"Calling Conventions for Binary Operations">) for details of these
361 =item * I<Comparison operations>
363 "<", "<=", ">", ">=", "==", "!=", "<=>",
364 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
366 If the corresponding "spaceship" variant is available, it can be
367 used to substitute for the missing operation. During C<sort>ing
368 arrays, C<cmp> is used to compare values subject to C<use overload>.
370 =item * I<Bit operations>
372 "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
374 C<neg> stands for unary minus. If the method for C<neg> is not
375 specified, it can be autogenerated using the method for
376 subtraction. If the method for C<!> is not specified, it can be
377 autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
379 The same remarks in L<"Arithmetic operations"> about
380 assignment-variants and autogeneration apply for
381 bit operations C<"&">, C<"^">, and C<"|"> as well.
383 =item * I<Increment and decrement>
387 If undefined, addition and subtraction methods can be
388 used instead. These operations are called both in prefix and
391 =item * I<Transcendental functions>
393 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
395 If C<abs> is unavailable, it can be autogenerated using methods
396 for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
398 Note that traditionally the Perl function L<int> rounds to 0, thus for
399 floating-point-like types one should follow the same semantic. If
400 C<int> is unavailable, it can be autogenerated using the overloading of
403 =item * I<Boolean, string and numeric conversion>
407 If one or two of these operations are not overloaded, the remaining ones can
408 be used instead. C<bool> is used in the flow control operators
409 (like C<while>) and for the ternary C<?:> operation. These functions can
410 return any arbitrary Perl value. If the corresponding operation for this value
411 is overloaded too, that operation will be called again with this value.
413 As a special case if the overload returns the object itself then it will
414 be used directly. An overloaded conversion returning the object is
415 probably a bug, because you're likely to get something that looks like
416 C<YourPackage=HASH(0x8172b34)>.
422 If not overloaded, the argument will be converted to a filehandle or
423 glob (which may require a stringification). The same overloading
424 happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
425 I<globbing> syntax C<E<lt>${var}E<gt>>.
427 B<BUGS> Even in list context, the iterator is currently called only
428 once and with scalar context.
430 =item * I<File tests>
434 This overload is used for all the filetest operators (C<-f>, C<-x> and
435 so on: see L<perlfunc/-X> for the full list). Even though these are
436 unary operators, the method will be called with a second argument which
437 is a single letter indicating which test was performed. Note that the
438 overload key is the literal string C<"-X">: you can't provide separate
439 overloads for the different tests.
441 Calling an overloaded filetest operator does not affect the stat value
442 associated with the special filehandle C<_>. It still refers to the
443 result of the last C<stat>, C<lstat> or unoverloaded filetest.
445 If not overloaded, these operators will fall back to the default
446 behaviour even without C<< fallback => 1 >>. This means that if the
447 object is a blessed glob or blessed IO ref it will be treated as a
448 filehandle, otherwise string overloading will be invoked and the result
449 treated as a filename.
451 This overload was introduced in perl 5.12.
455 The key C<"~~"> allows you to override the smart matching logic used by
456 the C<~~> operator and the switch construct (C<given>/C<when>). See
457 L<perlsyn/switch> and L<feature>.
459 Unusually, overloading of the smart match operator does not automatically
460 take precedence over normal smart match behaviour. In particular, in the
464 use overload '~~' => 'match';
466 my $obj = Foo->new();
469 the smart match does I<not> invoke the method call like this:
471 $obj->match([1,2,3],0);
473 rather, the smart match distributive rule takes precedence, so $obj is
474 smart matched against each array element in turn until a match is found,
475 so you may see between one and three of these calls instead:
481 Consult the match table in L<perlsyn/"Smart matching in detail"> for
482 details of when overloading is invoked.
484 =item * I<Dereferencing>
486 '${}', '@{}', '%{}', '&{}', '*{}'.
488 If not overloaded, the argument will be dereferenced I<as is>, thus
489 should be of correct type. These functions should return a reference
490 of correct type, or another object with overloaded dereferencing.
492 As a special case if the overload returns the object itself then it
493 will be used directly (provided it is the correct type).
495 The dereference operators must be specified explicitly they will not be passed to
500 "nomethod", "fallback", "=".
502 see L<SPECIAL SYMBOLS FOR C<use overload>>.
506 See L<"Fallback"> for an explanation of when a missing method can be
509 A computer-readable form of the above table is available in the hash
510 %overload::ops, with values being space-separated lists of names:
512 with_assign => '+ - * / % ** << >> x .',
513 assign => '+= -= *= /= %= **= <<= >>= x= .=',
514 num_comparison => '< <= > >= == !=',
515 '3way_comparison'=> '<=> cmp',
516 str_comparison => 'lt le gt ge eq ne',
517 binary => '& &= | |= ^ ^=',
520 func => 'atan2 cos sin exp abs log sqrt',
521 conversion => 'bool "" 0+',
524 dereferencing => '${} @{} %{} &{} *{}',
526 special => 'nomethod fallback ='
528 =head2 Inheritance and overloading
530 Inheritance interacts with overloading in two ways.
534 =item Strings as values of C<use overload> directive
538 use overload key => value;
540 is a string, it is interpreted as a method name.
542 =item Overloading of an operation is inherited by derived classes
544 Any class derived from an overloaded class is also overloaded. The
545 set of overloaded methods is the union of overloaded methods of all
546 the ancestors. If some method is overloaded in several ancestor, then
547 which description will be used is decided by the usual inheritance
550 If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
551 C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
552 then the subroutine C<D::plus_sub> will be called to implement
553 operation C<+> for an object in package C<A>.
557 Note that since the value of the C<fallback> key is not a subroutine,
558 its inheritance is not governed by the above rules. In the current
559 implementation, the value of C<fallback> in the first overloaded
560 ancestor is used, but this is accidental and subject to change.
562 =head1 SPECIAL SYMBOLS FOR C<use overload>
564 Three keys are recognized by Perl that are not covered by the above
569 C<"nomethod"> should be followed by a reference to a function of four
570 parameters. If defined, it is called when the overloading mechanism
571 cannot find a method for some operation. The first three arguments of
572 this function coincide with the arguments for the corresponding method if
573 it were found, the fourth argument is the symbol
574 corresponding to the missing method. If several methods are tried,
575 the last one is used. Say, C<1-$a> can be equivalent to
577 &nomethodMethod($a,1,1,"-")
579 if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
580 C<use overload> directive.
582 The C<"nomethod"> mechanism is I<not> used for the dereference operators
583 ( ${} @{} %{} &{} *{} ).
586 If some operation cannot be resolved, and there is no function
587 assigned to C<"nomethod">, then an exception will be raised via die()--
588 unless C<"fallback"> was specified as a key in C<use overload> directive.
593 The key C<"fallback"> governs what to do if a method for a particular
594 operation is not found. Three different cases are possible depending on
595 the value of C<"fallback">:
602 substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
603 then tries to calls C<"nomethod"> value; if missing, an exception
608 The same as for the C<undef> value, but no exception is raised. Instead,
609 it silently reverts to what it would have done were there no C<use overload>
612 =item * defined, but FALSE
614 No autogeneration is tried. Perl tries to call
615 C<"nomethod"> value, and if this is missing, raises an exception.
619 B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
620 yet, see L<"Inheritance and overloading">.
622 =head2 Copy Constructor
624 The value for C<"="> is a reference to a function with three
625 arguments, i.e., it looks like the other values in C<use
626 overload>. However, it does not overload the Perl assignment
627 operator. This would go against Camel hair.
629 This operation is called in the situations when a mutator is applied
630 to a reference that shares its object with some other reference, such
636 To make this change $a and not change $b, a copy of C<$$a> is made,
637 and $a is assigned a reference to this new object. This operation is
638 done during execution of the C<++$a>, and not during the assignment,
639 (so before the increment C<$$a> coincides with C<$$b>). This is only
640 done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
641 C<nomethod>). Note that if this operation is expressed via C<'+'>
642 a nonmutator, i.e., as in
647 then C<$a> does not reference a new copy of C<$$a>, since $$a does not
648 appear as lvalue when the above code is executed.
650 If the copy constructor is required during the execution of some mutator,
651 but a method for C<'='> was not specified, it can be autogenerated as a
652 string copy if the object is a plain scalar or a simple assignment if it
659 The actually executed code for
662 Something else which does not modify $a or $b....
668 Something else which does not modify $a or $b....
669 $a = $a->clone(undef,"");
672 if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
673 C<'='> was overloaded with C<\&clone>.
677 Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
680 =head1 MAGIC AUTOGENERATION
682 If a method for an operation is not found, and the value for C<"fallback"> is
683 TRUE or undefined, Perl tries to autogenerate a substitute method for
684 the missing operation based on the defined operations. Autogenerated method
685 substitutions are possible for the following operations:
689 =item I<Assignment forms of arithmetic operations>
691 C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
694 =item I<Conversion operations>
696 String, numeric, and boolean conversion are calculated in terms of one
697 another if not all of them are defined.
699 =item I<Increment and decrement>
701 The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
702 and C<$a--> in terms of C<$a-=1> and C<$a-1>.
706 can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
710 can be expressed in terms of subtraction.
714 C<!> and C<not> can be expressed in terms of boolean conversion, or
715 string or numerical conversion.
717 =item I<Concatenation>
719 can be expressed in terms of string conversion.
721 =item I<Comparison operations>
723 can be expressed in terms of its "spaceship" counterpart: either
724 C<E<lt>=E<gt>> or C<cmp>:
726 <, >, <=, >=, ==, != in terms of <=>
727 lt, gt, le, ge, eq, ne in terms of cmp
731 <> in terms of builtin operations
733 =item I<Dereferencing>
735 ${} @{} %{} &{} *{} in terms of builtin operations
737 =item I<Copy operator>
739 can be expressed in terms of an assignment to the dereferenced value, if this
740 value is a scalar and not a reference, or simply a reference assignment
745 =head1 Minimal set of overloaded operations
747 Since some operations can be automatically generated from others, there is
748 a minimal set of operations that need to be overloaded in order to have
749 the complete set of overloaded operations at one's disposal.
750 Of course, the autogenerated operations may not do exactly what the user
751 expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
756 atan2 cos sin exp log sqrt int
758 Additionally, you need to define at least one of string, boolean or
759 numeric conversions because any one can be used to emulate the others.
760 The string conversion can also be used to emulate concatenation.
762 =head1 Losing overloading
764 The restriction for the comparison operation is that even if, for example,
765 `C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
766 function will produce only a standard logical value based on the
767 numerical value of the result of `C<cmp>'. In particular, a working
768 numeric conversion is needed in this case (possibly expressed in terms of
771 Similarly, C<.=> and C<x=> operators lose their mathemagical properties
772 if the string conversion substitution is applied.
774 When you chop() a mathemagical object it is promoted to a string and its
775 mathemagical properties are lost. The same can happen with other
778 =head1 Run-time Overloading
780 Since all C<use> directives are executed at compile-time, the only way to
781 change overloading during run-time is to
783 eval 'use overload "+" => \&addmethod';
787 eval 'no overload "+", "--", "<="';
789 though the use of these constructs during run-time is questionable.
791 =head1 Public functions
793 Package C<overload.pm> provides the following public functions:
797 =item overload::StrVal(arg)
799 Gives string value of C<arg> as in absence of stringify overloading. If you
800 are using this to get the address of a reference (useful for checking if two
801 references point to the same thing) then you may be better off using
802 C<Scalar::Util::refaddr()>, which is faster.
804 =item overload::Overloaded(arg)
806 Returns true if C<arg> is subject to overloading of some operations.
808 =item overload::Method(obj,op)
810 Returns C<undef> or a reference to the method that implements C<op>.
814 =head1 Overloading constants
816 For some applications, the Perl parser mangles constants too much.
817 It is possible to hook into this process via C<overload::constant()>
818 and C<overload::remove_constant()> functions.
820 These functions take a hash as an argument. The recognized keys of this hash
827 to overload integer constants,
831 to overload floating point constants,
835 to overload octal and hexadecimal constants,
839 to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
840 strings and here-documents,
844 to overload constant pieces of regular expressions.
848 The corresponding values are references to functions which take three arguments:
849 the first one is the I<initial> string form of the constant, the second one
850 is how Perl interprets this constant, the third one is how the constant is used.
851 Note that the initial string form does not
852 contain string delimiters, and has backslashes in backslash-delimiter
853 combinations stripped (thus the value of delimiter is not relevant for
854 processing of this string). The return value of this function is how this
855 constant is going to be interpreted by Perl. The third argument is undefined
856 unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
857 context (comes from strings, regular expressions, and single-quote HERE
858 documents), it is C<tr> for arguments of C<tr>/C<y> operators,
859 it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
861 Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
862 it is expected that overloaded constant strings are equipped with reasonable
863 overloaded catenation operator, otherwise absurd results will result.
864 Similarly, negative numbers are considered as negations of positive constants.
866 Note that it is probably meaningless to call the functions overload::constant()
867 and overload::remove_constant() from anywhere but import() and unimport() methods.
868 From these methods they may be called as
873 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
874 overload::constant integer => sub {Math::BigInt->new(shift)};
877 =head1 IMPLEMENTATION
879 What follows is subject to change RSN.
881 The table of methods for all operations is cached in magic for the
882 symbol table hash for the package. The cache is invalidated during
883 processing of C<use overload>, C<no overload>, new function
884 definitions, and changes in @ISA. However, this invalidation remains
885 unprocessed until the next C<bless>ing into the package. Hence if you
886 want to change overloading structure dynamically, you'll need an
887 additional (fake) C<bless>ing to update the table.
889 (Every SVish thing has a magic queue, and magic is an entry in that
890 queue. This is how a single variable may participate in multiple
891 forms of magic simultaneously. For instance, environment variables
892 regularly have two forms at once: their %ENV magic and their taint
893 magic. However, the magic which implements overloading is applied to
894 the stashes, which are rarely used directly, thus should not slow down
897 If an object belongs to a package using overload, it carries a special
898 flag. Thus the only speed penalty during arithmetic operations without
899 overloading is the checking of this flag.
901 In fact, if C<use overload> is not present, there is almost no overhead
902 for overloadable operations, so most programs should not suffer
903 measurable performance penalties. A considerable effort was made to
904 minimize the overhead when overload is used in some package, but the
905 arguments in question do not belong to packages using overload. When
906 in doubt, test your speed with C<use overload> and without it. So far
907 there have been no reports of substantial speed degradation if Perl is
908 compiled with optimization turned on.
910 There is no size penalty for data if overload is not used. The only
911 size penalty if overload is used in some package is that I<all> the
912 packages acquire a magic during the next C<bless>ing into the
913 package. This magic is three-words-long for packages without
914 overloading, and carries the cache table if the package is overloaded.
916 Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
917 carried out before any operation that can imply an assignment to the
918 object $a (or $b) refers to, like C<$a++>. You can override this
919 behavior by defining your own copy constructor (see L<"Copy Constructor">).
921 It is expected that arguments to methods that are not explicitly supposed
922 to be changed are constant (but this is not enforced).
924 =head1 Metaphor clash
926 One may wonder why the semantic of overloaded C<=> is so counter intuitive.
927 If it I<looks> counter intuitive to you, you are subject to a metaphor
930 Here is a Perl object metaphor:
932 I< object is a reference to blessed data>
934 and an arithmetic metaphor:
936 I< object is a thing by itself>.
938 The I<main> problem of overloading C<=> is the fact that these metaphors
939 imply different actions on the assignment C<$a = $b> if $a and $b are
940 objects. Perl-think implies that $a becomes a reference to whatever
941 $b was referencing. Arithmetic-think implies that the value of "object"
942 $a is changed to become the value of the object $b, preserving the fact
943 that $a and $b are separate entities.
945 The difference is not relevant in the absence of mutators. After
946 a Perl-way assignment an operation which mutates the data referenced by $a
947 would change the data referenced by $b too. Effectively, after
948 C<$a = $b> values of $a and $b become I<indistinguishable>.
950 On the other hand, anyone who has used algebraic notation knows the
951 expressive power of the arithmetic metaphor. Overloading works hard
952 to enable this metaphor while preserving the Perlian way as far as
953 possible. Since it is not possible to freely mix two contradicting
954 metaphors, overloading allows the arithmetic way to write things I<as
955 far as all the mutators are called via overloaded access only>. The
956 way it is done is described in L<Copy Constructor>.
958 If some mutator methods are directly applied to the overloaded values,
959 one may need to I<explicitly unlink> other values which references the
964 $b = $a; # $b is "linked" to $a
966 $a = $a->clone; # Unlink $b from $a
969 Note that overloaded access makes this transparent:
972 $b = $a; # $b is "linked" to $a
973 $a += 4; # would unlink $b automagically
975 However, it would not make
978 $a = 4; # Now $a is a plain 4, not 'Data'
980 preserve "objectness" of $a. But Perl I<has> a way to make assignments
981 to an object do whatever you want. It is just not the overload, but
982 tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
983 which returns the object itself, and STORE() method which changes the
984 value of the object, one can reproduce the arithmetic metaphor in its
985 completeness, at least for variables which were tie()d from the start.
987 (Note that a workaround for a bug may be needed, see L<"BUGS">.)
991 Please add examples to what follows!
993 =head2 Two-face scalars
995 Put this in F<two_face.pm> in your Perl library directory:
997 package two_face; # Scalars with separate string and
999 sub new { my $p = shift; bless [@_], $p }
1000 use overload '""' => \&str, '0+' => \&num, fallback => 1;
1001 sub num {shift->[1]}
1002 sub str {shift->[0]}
1007 my $seven = two_face->new("vii", 7);
1008 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
1009 print "seven contains `i'\n" if $seven =~ /i/;
1011 (The second line creates a scalar which has both a string value, and a
1012 numeric value.) This prints:
1014 seven=vii, seven=7, eight=8
1017 =head2 Two-face references
1019 Suppose you want to create an object which is accessible as both an
1020 array reference and a hash reference.
1023 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
1031 tie %h, ref $self, $self;
1035 sub TIEHASH { my $p = shift; bless \ shift, $p }
1038 $fields{$_} = $i++ foreach qw{zero one two three};
1040 my $self = ${shift()};
1041 my $key = $fields{shift()};
1042 defined $key or die "Out of band access";
1043 $$self->[$key] = shift;
1046 my $self = ${shift()};
1047 my $key = $fields{shift()};
1048 defined $key or die "Out of band access";
1052 Now one can access an object using both the array and hash syntax:
1054 my $bar = two_refs->new(3,4,5,6);
1056 $bar->{two} == 11 or die 'bad hash fetch';
1058 Note several important features of this example. First of all, the
1059 I<actual> type of $bar is a scalar reference, and we do not overload
1060 the scalar dereference. Thus we can get the I<actual> non-overloaded
1061 contents of $bar by just using C<$$bar> (what we do in functions which
1062 overload dereference). Similarly, the object returned by the
1063 TIEHASH() method is a scalar reference.
1065 Second, we create a new tied hash each time the hash syntax is used.
1066 This allows us not to worry about a possibility of a reference loop,
1067 which would lead to a memory leak.
1069 Both these problems can be cured. Say, if we want to overload hash
1070 dereference on a reference to an object which is I<implemented> as a
1071 hash itself, the only problem one has to circumvent is how to access
1072 this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
1073 overloaded dereference operator). Here is one possible fetching routine:
1076 my ($self, $key) = (shift, shift);
1077 my $class = ref $self;
1078 bless $self, 'overload::dummy'; # Disable overloading of %{}
1079 my $out = $self->{$key};
1080 bless $self, $class; # Restore overloading
1084 To remove creation of the tied hash on each access, one may an extra
1085 level of indirection which allows a non-circular structure of references:
1088 use overload '%{}' => sub { ${shift()}->[1] },
1089 '@{}' => sub { ${shift()}->[0] };
1095 bless \ [$a, \%h], $p;
1100 tie %h, ref $self, $self;
1104 sub TIEHASH { my $p = shift; bless \ shift, $p }
1107 $fields{$_} = $i++ foreach qw{zero one two three};
1110 my $key = $fields{shift()};
1111 defined $key or die "Out of band access";
1116 my $key = $fields{shift()};
1117 defined $key or die "Out of band access";
1121 Now if $baz is overloaded like this, then C<$baz> is a reference to a
1122 reference to the intermediate array, which keeps a reference to an
1123 actual array, and the access hash. The tie()ing object for the access
1124 hash is a reference to a reference to the actual array, so
1130 There are no loops of references.
1134 Both "objects" which are blessed into the class C<two_refs1> are
1135 references to a reference to an array, thus references to a I<scalar>.
1136 Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1137 overloaded operations.
1141 =head2 Symbolic calculator
1143 Put this in F<symbolic.pm> in your Perl library directory:
1145 package symbolic; # Primitive symbolic calculator
1146 use overload nomethod => \&wrap;
1148 sub new { shift; bless ['n', @_] }
1150 my ($obj, $other, $inv, $meth) = @_;
1151 ($obj, $other) = ($other, $obj) if $inv;
1152 bless [$meth, $obj, $other];
1155 This module is very unusual as overloaded modules go: it does not
1156 provide any usual overloaded operators, instead it provides the L<Last
1157 Resort> operator C<nomethod>. In this example the corresponding
1158 subroutine returns an object which encapsulates operations done over
1159 the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
1160 symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
1162 Here is an example of the script which "calculates" the side of
1163 circumscribed octagon using the above package:
1166 my $iter = 1; # 2**($iter+2) = 8
1167 my $side = symbolic->new(1);
1171 $side = (sqrt(1 + $side**2) - 1)/$side;
1175 The value of $side is
1177 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1178 undef], 1], ['n', 1]]
1180 Note that while we obtained this value using a nice little script,
1181 there is no simple way to I<use> this value. In fact this value may
1182 be inspected in debugger (see L<perldebug>), but only if
1183 C<bareStringify> B<O>ption is set, and not via C<p> command.
1185 If one attempts to print this value, then the overloaded operator
1186 C<""> will be called, which will call C<nomethod> operator. The
1187 result of this operator will be stringified again, but this result is
1188 again of type C<symbolic>, which will lead to an infinite loop.
1190 Add a pretty-printer method to the module F<symbolic.pm>:
1193 my ($meth, $a, $b) = @{+shift};
1194 $a = 'u' unless defined $a;
1195 $b = 'u' unless defined $b;
1196 $a = $a->pretty if ref $a;
1197 $b = $b->pretty if ref $b;
1201 Now one can finish the script by
1203 print "side = ", $side->pretty, "\n";
1205 The method C<pretty> is doing object-to-string conversion, so it
1206 is natural to overload the operator C<""> using this method. However,
1207 inside such a method it is not necessary to pretty-print the
1208 I<components> $a and $b of an object. In the above subroutine
1209 C<"[$meth $a $b]"> is a catenation of some strings and components $a
1210 and $b. If these components use overloading, the catenation operator
1211 will look for an overloaded operator C<.>; if not present, it will
1212 look for an overloaded operator C<"">. Thus it is enough to use
1214 use overload nomethod => \&wrap, '""' => \&str;
1216 my ($meth, $a, $b) = @{+shift};
1217 $a = 'u' unless defined $a;
1218 $b = 'u' unless defined $b;
1222 Now one can change the last line of the script to
1224 print "side = $side\n";
1228 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1230 and one can inspect the value in debugger using all the possible
1233 Something is still amiss: consider the loop variable $cnt of the
1234 script. It was a number, not an object. We cannot make this value of
1235 type C<symbolic>, since then the loop will not terminate.
1237 Indeed, to terminate the cycle, the $cnt should become false.
1238 However, the operator C<bool> for checking falsity is overloaded (this
1239 time via overloaded C<"">), and returns a long string, thus any object
1240 of type C<symbolic> is true. To overcome this, we need a way to
1241 compare an object to 0. In fact, it is easier to write a numeric
1244 Here is the text of F<symbolic.pm> with such a routine added (and
1245 slightly modified str()):
1247 package symbolic; # Primitive symbolic calculator
1249 nomethod => \&wrap, '""' => \&str, '0+' => \#
1251 sub new { shift; bless ['n', @_] }
1253 my ($obj, $other, $inv, $meth) = @_;
1254 ($obj, $other) = ($other, $obj) if $inv;
1255 bless [$meth, $obj, $other];
1258 my ($meth, $a, $b) = @{+shift};
1259 $a = 'u' unless defined $a;
1266 my %subr = ( n => sub {$_[0]},
1267 sqrt => sub {sqrt $_[0]},
1268 '-' => sub {shift() - shift()},
1269 '+' => sub {shift() + shift()},
1270 '/' => sub {shift() / shift()},
1271 '*' => sub {shift() * shift()},
1272 '**' => sub {shift() ** shift()},
1275 my ($meth, $a, $b) = @{+shift};
1276 my $subr = $subr{$meth}
1277 or die "Do not know how to ($meth) in symbolic";
1278 $a = $a->num if ref $a eq __PACKAGE__;
1279 $b = $b->num if ref $b eq __PACKAGE__;
1283 All the work of numeric conversion is done in %subr and num(). Of
1284 course, %subr is not complete, it contains only operators used in the
1285 example below. Here is the extra-credit question: why do we need an
1286 explicit recursion in num()? (Answer is at the end of this section.)
1288 Use this module like this:
1291 my $iter = symbolic->new(2); # 16-gon
1292 my $side = symbolic->new(1);
1296 $cnt = $cnt - 1; # Mutator `--' not implemented
1297 $side = (sqrt(1 + $side**2) - 1)/$side;
1299 printf "%s=%f\n", $side, $side;
1300 printf "pi=%f\n", $side*(2**($iter+2));
1302 It prints (without so many line breaks)
1304 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1306 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1309 The above module is very primitive. It does not implement
1310 mutator methods (C<++>, C<-=> and so on), does not do deep copying
1311 (not required without mutators!), and implements only those arithmetic
1312 operations which are used in the example.
1314 To implement most arithmetic operations is easy; one should just use
1315 the tables of operations, and change the code which fills %subr to
1317 my %subr = ( 'n' => sub {$_[0]} );
1318 foreach my $op (split " ", $overload::ops{with_assign}) {
1319 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1321 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1322 foreach my $op (split " ", "@overload::ops{ @bins }") {
1323 $subr{$op} = eval "sub {shift() $op shift()}";
1325 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1326 print "defining `$op'\n";
1327 $subr{$op} = eval "sub {$op shift()}";
1330 Due to L<Calling Conventions for Mutators>, we do not need anything
1331 special to make C<+=> and friends work, except filling C<+=> entry of
1332 %subr, and defining a copy constructor (needed since Perl has no
1333 way to know that the implementation of C<'+='> does not mutate
1334 the argument, compare L<Copy Constructor>).
1336 To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1337 line, and code (this code assumes that mutators change things one level
1338 deep only, so recursive copying is not needed):
1342 bless [@$self], ref $self;
1345 To make C<++> and C<--> work, we need to implement actual mutators,
1346 either directly, or in C<nomethod>. We continue to do things inside
1347 C<nomethod>, thus add
1349 if ($meth eq '++' or $meth eq '--') {
1350 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1354 after the first line of wrap(). This is not a most effective
1355 implementation, one may consider
1357 sub inc { $_[0] = bless ['++', shift, 1]; }
1361 As a final remark, note that one can fill %subr by
1363 my %subr = ( 'n' => sub {$_[0]} );
1364 foreach my $op (split " ", $overload::ops{with_assign}) {
1365 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1367 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1368 foreach my $op (split " ", "@overload::ops{ @bins }") {
1369 $subr{$op} = eval "sub {shift() $op shift()}";
1371 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1372 $subr{$op} = eval "sub {$op shift()}";
1374 $subr{'++'} = $subr{'+'};
1375 $subr{'--'} = $subr{'-'};
1377 This finishes implementation of a primitive symbolic calculator in
1378 50 lines of Perl code. Since the numeric values of subexpressions
1379 are not cached, the calculator is very slow.
1381 Here is the answer for the exercise: In the case of str(), we need no
1382 explicit recursion since the overloaded C<.>-operator will fall back
1383 to an existing overloaded operator C<"">. Overloaded arithmetic
1384 operators I<do not> fall back to numeric conversion if C<fallback> is
1385 not explicitly requested. Thus without an explicit recursion num()
1386 would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1387 the argument of num().
1389 If you wonder why defaults for conversion are different for str() and
1390 num(), note how easy it was to write the symbolic calculator. This
1391 simplicity is due to an appropriate choice of defaults. One extra
1392 note: due to the explicit recursion num() is more fragile than sym():
1393 we need to explicitly check for the type of $a and $b. If components
1394 $a and $b happen to be of some related type, this may lead to problems.
1396 =head2 I<Really> symbolic calculator
1398 One may wonder why we call the above calculator symbolic. The reason
1399 is that the actual calculation of the value of expression is postponed
1400 until the value is I<used>.
1402 To see it in action, add a method
1407 @$obj->[0,1] = ('=', shift);
1410 to the package C<symbolic>. After this change one can do
1412 my $a = symbolic->new(3);
1413 my $b = symbolic->new(4);
1414 my $c = sqrt($a**2 + $b**2);
1416 and the numeric value of $c becomes 5. However, after calling
1418 $a->STORE(12); $b->STORE(5);
1420 the numeric value of $c becomes 13. There is no doubt now that the module
1421 symbolic provides a I<symbolic> calculator indeed.
1423 To hide the rough edges under the hood, provide a tie()d interface to the
1424 package C<symbolic> (compare with L<Metaphor clash>). Add methods
1426 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1428 sub nop { } # Around a bug
1430 (the bug is described in L<"BUGS">). One can use this new interface as
1432 tie $a, 'symbolic', 3;
1433 tie $b, 'symbolic', 4;
1434 $a->nop; $b->nop; # Around a bug
1436 my $c = sqrt($a**2 + $b**2);
1438 Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1439 of $c becomes 13. To insulate the user of the module add a method
1441 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1446 symbolic->vars($a, $b);
1447 my $c = sqrt($a**2 + $b**2);
1450 printf "c5 %s=%f\n", $c, $c;
1453 printf "c13 %s=%f\n", $c, $c;
1455 shows that the numeric value of $c follows changes to the values of $a
1460 Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1464 The L<overloading> pragma can be used to enable or disable overloaded
1465 operations within a lexical scope.
1469 When Perl is run with the B<-Do> switch or its equivalent, overloading
1470 induces diagnostic messages.
1472 Using the C<m> command of Perl debugger (see L<perldebug>) one can
1473 deduce which operations are overloaded (and which ancestor triggers
1474 this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1475 is shown by debugger. The method C<()> corresponds to the C<fallback>
1476 key (in fact a presence of this method shows that this package has
1477 overloading enabled, and it is what is used by the C<Overloaded>
1478 function of module C<overload>).
1480 The module might issue the following warnings:
1484 =item Odd number of arguments for overload::constant
1486 (W) The call to overload::constant contained an odd number of arguments.
1487 The arguments should come in pairs.
1489 =item `%s' is not an overloadable type
1491 (W) You tried to overload a constant type the overload package is unaware of.
1493 =item `%s' is not a code reference
1495 (W) The second (fourth, sixth, ...) argument of overload::constant needs
1496 to be a code reference. Either an anonymous subroutine, or a reference
1503 Because it is used for overloading, the per-package hash %OVERLOAD now
1504 has a special meaning in Perl. The symbol table is filled with names
1505 looking like line-noise.
1507 For the purpose of inheritance every overloaded package behaves as if
1508 C<fallback> is present (possibly undefined). This may create
1509 interesting effects if some package is not overloaded, but inherits
1510 from two overloaded packages.
1512 Relation between overloading and tie()ing is broken. Overloading is
1513 triggered or not basing on the I<previous> class of tie()d value.
1515 This happens because the presence of overloading is checked too early,
1516 before any tie()d access is attempted. If the FETCH()ed class of the
1517 tie()d value does not change, a simple workaround is to access the value
1518 immediately after tie()ing, so that after this call the I<previous> class
1519 coincides with the current one.
1521 B<Needed:> a way to fix this without a speed penalty.
1523 Barewords are not covered by overloaded string constants.
1525 This document is confusing. There are grammos and misleading language
1526 used in places. It would seem a total rewrite is needed.