Profile of /usr/local/lib/perl5/5.8.8/overload.pm

File	/usr/local/lib/perl5/5.8.8/overload.pm
Statements Executed	197
Statement Execution Time	2.90ms

Subroutines — ordered by exclusive time
Calls	P	F	Exclusive Time	Inclusive Time	Subroutine
6	1	1	1.80ms	1.89ms	overload::::OVERLOADoverload::OVERLOAD
6	6	4	310µs	2.20ms	overload::::importoverload::import
17	1	2	90µs	90µs	overload::::CORE:matchoverload::CORE:match (opcode)
0	0	0	0s	0s	overload::::AddrRefoverload::AddrRef
0	0	0	0s	0s	overload::::BEGINoverload::BEGIN
0	0	0	0s	0s	overload::::Methodoverload::Method
0	0	0	0s	0s	overload::::Overloadedoverload::Overloaded
0	0	0	0s	0s	overload::::OverloadedStringifyoverload::OverloadedStringify
0	0	0	0s	0s	overload::::constantoverload::constant
0	0	0	0s	0s	overload::::mycanoverload::mycan
0	0	0	0s	0s	overload::::niloverload::nil
0	0	0	0s	0s	overload::::ov_methodoverload::ov_method
0	0	0	0s	0s	overload::::remove_constantoverload::remove_constant
0	0	0	0s	0s	overload::::unimportoverload::unimport

Call graph for these subroutines as a Graphviz dot language file.

Line	State ments	Time on line	Calls	Time in subs	Code
1					package overload;
2
3	1	6µs			our $VERSION = '1.04';
4
5	1	5µs			$overload::hint_bits = 0x20000; # HINT_LOCALIZE_HH
6
7					sub nil {}
8
9					# spent 1.89ms (1.80+90µs) within overload::OVERLOAD which was called 6 times, avg 316µs/call: # 6 times (1.80ms+90µs) by overload::import at line 35, avg 316µs/call sub OVERLOAD {
10	6	30µs			$package = shift;
11	6	83µs			my %arg = @_;
12	6	32µs			my ($sub, $fb);
13	6	103µs			$ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
14	6	73µs			*{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
15	6	93µs			for (keys %arg) {
16	28	356µs			if ($_ eq 'fallback') {
17					$fb = $arg{$_};
18					} else {
19	22	131µs			$sub = $arg{$_};
20	22	429µs	17	90µs	if (not ref $sub and $sub !~ /::/) { # spent 90µs making 17 calls to overload::CORE:match, avg 5µs/call
21	17	177µs			$ {$package . "::(" . $_} = $sub;
22	17	110µs			$sub = \&nil;
23					}
24					#print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
25	22	176µs			*{$package . "::(" . $_} = \&{ $sub };
26					}
27					}
28	6	137µs			${$package . "::()"} = $fb; # Make it findable too (fallback only).
29					}
30
31					# spent 2.20ms (310µs+1.89) within overload::import which was called 6 times, avg 367µs/call: # once (49µs+522µs) by Class::DBI::_require_class at line 598 of Class/Date.pm # once (52µs+499µs) by Class::DBI::_require_class at line 728 of Class/Date.pm # once (45µs+431µs) by Class::DBI::_require_class at line 97 of Class/Date.pm # once (56µs+156µs) by URI::BEGIN at line 28 of URI.pm # once (53µs+156µs) by base::import at line 40 of Class/DBI.pm # once (55µs+130µs) by base::import at line 38 of Class/DBI/Column.pm sub import {
32	6	55µs			$package = (caller())[0];
33					# *{$package . "::OVERLOAD"} = \&OVERLOAD;
34	6	29µs			shift;
35	6	202µs	6	1.89ms	$package->overload::OVERLOAD(@_); # spent 1.89ms making 6 calls to overload::OVERLOAD, avg 316µs/call
36					}
37
38					sub unimport {
39					$package = (caller())[0];
40					${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
41					shift;
42					for (@_) {
43					if ($_ eq 'fallback') {
44					undef $ {$package . "::()"};
45					} else {
46					delete $ {$package . "::"}{"(" . $_};
47					}
48					}
49					}
50
51					sub Overloaded {
52					my $package = shift;
53					$package = ref $package if ref $package;
54					$package->can('()');
55					}
56
57					sub ov_method {
58					my $globref = shift;
59					return undef unless $globref;
60					my $sub = \&{*$globref};
61					return $sub if $sub ne \&nil;
62					return shift->can($ {*$globref});
63					}
64
65					sub OverloadedStringify {
66					my $package = shift;
67					$package = ref $package if ref $package;
68					#$package->can('(""')
69					ov_method mycan($package, '(""'), $package
70					or ov_method mycan($package, '(0+'), $package
71					or ov_method mycan($package, '(bool'), $package
72					or ov_method mycan($package, '(nomethod'), $package;
73					}
74
75					sub Method {
76					my $package = shift;
77					$package = ref $package if ref $package;
78					#my $meth = $package->can('(' . shift);
79					ov_method mycan($package, '(' . shift), $package;
80					#return $meth if $meth ne \&nil;
81					#return $ {*{$meth}};
82					}
83
84					sub AddrRef {
85					my $package = ref $_[0];
86					return "$_[0]" unless $package;
87
88					require Scalar::Util;
89					my $class = Scalar::Util::blessed($_[0]);
90					my $class_prefix = defined($class) ? "$class=" : "";
91					my $type = Scalar::Util::reftype($_[0]);
92					my $addr = Scalar::Util::refaddr($_[0]);
93					return sprintf("$class_prefix$type(0x%x)", $addr);
94					}
95
96	1	7µs			StrVal = AddrRef;
97
98					sub mycan { # Real can would leave stubs.
99					my ($package, $meth) = @_;
100					return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
101					my $p;
102					foreach $p (@{$package . "::ISA"}) {
103					my $out = mycan($p, $meth);
104					return $out if $out;
105					}
106					return undef;
107					}
108
109	1	11µs			%constants = (
110					'integer' => 0x1000, # HINT_NEW_INTEGER
111					'float' => 0x2000, # HINT_NEW_FLOAT
112					'binary' => 0x4000, # HINT_NEW_BINARY
113					'q' => 0x8000, # HINT_NEW_STRING
114					'qr' => 0x10000, # HINT_NEW_RE
115					);
116
117	1	21µs			%ops = ( with_assign => "+ - * / % ** << >> x .",
118					assign => "+= -= = /= %= *= <<= >>= x= .=",
119					num_comparison => "< <= > >= == !=",
120					'3way_comparison'=> "<=> cmp",
121					str_comparison => "lt le gt ge eq ne",
122					binary => "& \| ^",
123					unary => "neg ! ~",
124					mutators => '++ --',
125					func => "atan2 cos sin exp abs log sqrt int",
126					conversion => 'bool "" 0+',
127					iterators => '<>',
128					dereferencing => '${} @{} %{} &{} *{}',
129					special => 'nomethod fallback =');
130
131	3	606µs	1	703µs	use warnings::register; # spent 703µs making 1 call to warnings::register::import
132					sub constant {
133					# Arguments: what, sub
134					while (@_) {
135					if (@_ == 1) {
136					warnings::warnif ("Odd number of arguments for overload::constant");
137					last;
138					}
139					elsif (!exists $constants {$_ [0]}) {
140					warnings::warnif ("`$_[0]' is not an overloadable type");
141					}
142					elsif (!ref $_ [1] \|\| "$_[1]" !~ /CODE$0x[\da-f]+$$/) {
143					# Can't use C<ref $_[1] eq "CODE"> above as code references can be
144					# blessed, and C<ref> would return the package the ref is blessed into.
145					if (warnings::enabled) {
146					$_ [1] = "undef" unless defined $_ [1];
147					warnings::warn ("`$_[1]' is not a code reference");
148					}
149					}
150					else {
151					$^H{$_[0]} = $_[1];
152					$^H \|= $constants{$_[0]} \| $overload::hint_bits;
153					}
154					shift, shift;
155					}
156					}
157
158					sub remove_constant {
159					# Arguments: what, sub
160					while (@_) {
161					delete $^H{$_[0]};
162					$^H &= ~ $constants{$_[0]};
163					shift, shift;
164					}
165					}
166
167	1	23µs			1;
168
169					__END__
170
171					=head1 NAME
172
173					overload - Package for overloading Perl operations
174
175					=head1 SYNOPSIS
176
177					package SomeThing;
178
179					use overload
180					'+' => \&myadd,
181					'-' => \&mysub;
182					# etc
183					...
184
185					package main;
186					$a = new SomeThing 57;
187					$b=5+$a;
188					...
189					if (overload::Overloaded $b) {...}
190					...
191					$strval = overload::StrVal $b;
192
193					=head1 DESCRIPTION
194
195					=head2 Declaration of overloaded functions
196
197					The compilation directive
198
199					package Number;
200					use overload
201					"+" => \&add,
202					"*=" => "muas";
203
204					declares function Number::add() for addition, and method muas() in
205					the "class" C<Number> (or one of its base classes)
206					for the assignment form C<*=> of multiplication.
207
208					Arguments of this directive come in (key, value) pairs. Legal values
209					are values legal inside a C<&{ ... }> call, so the name of a
210					subroutine, a reference to a subroutine, or an anonymous subroutine
211					will all work. Note that values specified as strings are
212					interpreted as methods, not subroutines. Legal keys are listed below.
213
214					The subroutine C<add> will be called to execute C<$a+$b> if $a
215					is a reference to an object blessed into the package C<Number>, or if $a is
216					not an object from a package with defined mathemagic addition, but $b is a
217					reference to a C<Number>. It can also be called in other situations, like
218					C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
219					methods refer to methods triggered by an overloaded mathematical
220					operator.)
221
222					Since overloading respects inheritance via the @ISA hierarchy, the
223					above declaration would also trigger overloading of C<+> and C<*=> in
224					all the packages which inherit from C<Number>.
225
226					=head2 Calling Conventions for Binary Operations
227
228					The functions specified in the C<use overload ...> directive are called
229					with three (in one particular case with four, see L<Last Resort>)
230					arguments. If the corresponding operation is binary, then the first
231					two arguments are the two arguments of the operation. However, due to
232					general object calling conventions, the first argument should always be
233					an object in the package, so in the situation of C<7+$a>, the
234					order of the arguments is interchanged. It probably does not matter
235					when implementing the addition method, but whether the arguments
236					are reversed is vital to the subtraction method. The method can
237					query this information by examining the third argument, which can take
238					three different values:
239
240					=over 7
241
242					=item FALSE
243
244					the order of arguments is as in the current operation.
245
246					=item TRUE
247
248					the arguments are reversed.
249
250					=item C<undef>
251
252					the current operation is an assignment variant (as in
253					C<$a+=7>), but the usual function is called instead. This additional
254					information can be used to generate some optimizations. Compare
255					L<Calling Conventions for Mutators>.
256
257					=back
258
259					=head2 Calling Conventions for Unary Operations
260
261					Unary operation are considered binary operations with the second
262					argument being C<undef>. Thus the functions that overloads C<{"++"}>
263					is called with arguments C<($a,undef,'')> when $a++ is executed.
264
265					=head2 Calling Conventions for Mutators
266
267					Two types of mutators have different calling conventions:
268
269					=over
270
271					=item C<++> and C<-->
272
273					The routines which implement these operators are expected to actually
274					I<mutate> their arguments. So, assuming that $obj is a reference to a
275					number,
276
277					sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
278
279					is an appropriate implementation of overloaded C<++>. Note that
280
281					sub incr { ++$ {$_[0]} ; shift }
282
283					is OK if used with preincrement and with postincrement. (In the case
284					of postincrement a copying will be performed, see L<Copy Constructor>.)
285
286					=item C<x=> and other assignment versions
287
288					There is nothing special about these methods. They may change the
289					value of their arguments, and may leave it as is. The result is going
290					to be assigned to the value in the left-hand-side if different from
291					this value.
292
293					This allows for the same method to be used as overloaded C<+=> and
294					C<+>. Note that this is I<allowed>, but not recommended, since by the
295					semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
296					if C<+=> is not overloaded.
297
298					=back
299
300					B<Warning.> Due to the presence of assignment versions of operations,
301					routines which may be called in assignment context may create
302					self-referential structures. Currently Perl will not free self-referential
303					structures until cycles are C<explicitly> broken. You may get problems
304					when traversing your structures too.
305
306					Say,
307
308					use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
309
310					is asking for trouble, since for code C<$obj += $foo> the subroutine
311					is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
312					\$foo]>. If using such a subroutine is an important optimization, one
313					can overload C<+=> explicitly by a non-"optimized" version, or switch
314					to non-optimized version if C<not defined $_[2]> (see
315					L<Calling Conventions for Binary Operations>).
316
317					Even if no I<explicit> assignment-variants of operators are present in
318					the script, they may be generated by the optimizer. Say, C<",$obj,"> or
319					C<',' . $obj . ','> may be both optimized to
320
321					my $tmp = ',' . $obj; $tmp .= ',';
322
323					=head2 Overloadable Operations
324
325					The following symbols can be specified in C<use overload> directive:
326
327					=over 5
328
329					=item * I<Arithmetic operations>
330
331					"+", "+=", "-", "-=", "", "=", "/", "/=", "%", "%=",
332					"", "=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
333
334					For these operations a substituted non-assignment variant can be called if
335					the assignment variant is not available. Methods for operations C<+>,
336					C<->, C<+=>, and C<-=> can be called to automatically generate
337					increment and decrement methods. The operation C<-> can be used to
338					autogenerate missing methods for unary minus or C<abs>.
339
340					See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
341					L<"Calling Conventions for Binary Operations">) for details of these
342					substitutions.
343
344					=item * I<Comparison operations>
345
346					"<", "<=", ">", ">=", "==", "!=", "<=>",
347					"lt", "le", "gt", "ge", "eq", "ne", "cmp",
348
349					If the corresponding "spaceship" variant is available, it can be
350					used to substitute for the missing operation. During C<sort>ing
351					arrays, C<cmp> is used to compare values subject to C<use overload>.
352
353					=item * I<Bit operations>
354
355					"&", "^", "\|", "neg", "!", "~",
356
357					C<neg> stands for unary minus. If the method for C<neg> is not
358					specified, it can be autogenerated using the method for
359					subtraction. If the method for C<!> is not specified, it can be
360					autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
361
362					=item * I<Increment and decrement>
363
364					"++", "--",
365
366					If undefined, addition and subtraction methods can be
367					used instead. These operations are called both in prefix and
368					postfix form.
369
370					=item * I<Transcendental functions>
371
372					"atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
373
374					If C<abs> is unavailable, it can be autogenerated using methods
375					for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
376
377					Note that traditionally the Perl function L<int> rounds to 0, thus for
378					floating-point-like types one should follow the same semantic. If
379					C<int> is unavailable, it can be autogenerated using the overloading of
380					C<0+>.
381
382					=item * I<Boolean, string and numeric conversion>
383
384					'bool', '""', '0+',
385
386					If one or two of these operations are not overloaded, the remaining ones can
387					be used instead. C<bool> is used in the flow control operators
388					(like C<while>) and for the ternary C<?:> operation. These functions can
389					return any arbitrary Perl value. If the corresponding operation for this value
390					is overloaded too, that operation will be called again with this value.
391
392					As a special case if the overload returns the object itself then it will
393					be used directly. An overloaded conversion returning the object is
394					probably a bug, because you're likely to get something that looks like
395					C<YourPackage=HASH(0x8172b34)>.
396
397					=item * I<Iteration>
398
399					"<>"
400
401					If not overloaded, the argument will be converted to a filehandle or
402					glob (which may require a stringification). The same overloading
403					happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
404					I<globbing> syntax C<E<lt>${var}E<gt>>.
405
406					B<BUGS> Even in list context, the iterator is currently called only
407					once and with scalar context.
408
409					=item * I<Dereferencing>
410
411					'${}', '@{}', '%{}', '&{}', '*{}'.
412
413					If not overloaded, the argument will be dereferenced I<as is>, thus
414					should be of correct type. These functions should return a reference
415					of correct type, or another object with overloaded dereferencing.
416
417					As a special case if the overload returns the object itself then it
418					will be used directly (provided it is the correct type).
419
420					The dereference operators must be specified explicitly they will not be passed to
421					"nomethod".
422
423					=item * I<Special>
424
425					"nomethod", "fallback", "=",
426
427					see L<SPECIAL SYMBOLS FOR C<use overload>>.
428
429					=back
430
431					See L<"Fallback"> for an explanation of when a missing method can be
432					autogenerated.
433
434					A computer-readable form of the above table is available in the hash
435					%overload::ops, with values being space-separated lists of names:
436
437					with_assign => '+ - * / % ** << >> x .',
438					assign => '+= -= = /= %= *= <<= >>= x= .=',
439					num_comparison => '< <= > >= == !=',
440					'3way_comparison'=> '<=> cmp',
441					str_comparison => 'lt le gt ge eq ne',
442					binary => '& \| ^',
443					unary => 'neg ! ~',
444					mutators => '++ --',
445					func => 'atan2 cos sin exp abs log sqrt',
446					conversion => 'bool "" 0+',
447					iterators => '<>',
448					dereferencing => '${} @{} %{} &{} *{}',
449					special => 'nomethod fallback ='
450
451					=head2 Inheritance and overloading
452
453					Inheritance interacts with overloading in two ways.
454
455					=over
456
457					=item Strings as values of C<use overload> directive
458
459					If C<value> in
460
461					use overload key => value;
462
463					is a string, it is interpreted as a method name.
464
465					=item Overloading of an operation is inherited by derived classes
466
467					Any class derived from an overloaded class is also overloaded. The
468					set of overloaded methods is the union of overloaded methods of all
469					the ancestors. If some method is overloaded in several ancestor, then
470					which description will be used is decided by the usual inheritance
471					rules:
472
473					If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
474					C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
475					then the subroutine C<D::plus_sub> will be called to implement
476					operation C<+> for an object in package C<A>.
477
478					=back
479
480					Note that since the value of the C<fallback> key is not a subroutine,
481					its inheritance is not governed by the above rules. In the current
482					implementation, the value of C<fallback> in the first overloaded
483					ancestor is used, but this is accidental and subject to change.
484
485					=head1 SPECIAL SYMBOLS FOR C<use overload>
486
487					Three keys are recognized by Perl that are not covered by the above
488					description.
489
490					=head2 Last Resort
491
492					C<"nomethod"> should be followed by a reference to a function of four
493					parameters. If defined, it is called when the overloading mechanism
494					cannot find a method for some operation. The first three arguments of
495					this function coincide with the arguments for the corresponding method if
496					it were found, the fourth argument is the symbol
497					corresponding to the missing method. If several methods are tried,
498					the last one is used. Say, C<1-$a> can be equivalent to
499
500					&nomethodMethod($a,1,1,"-")
501
502					if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
503					C<use overload> directive.
504
505					The C<"nomethod"> mechanism is I<not> used for the dereference operators
506					( ${} @{} %{} &{} *{} ).
507
508
509					If some operation cannot be resolved, and there is no function
510					assigned to C<"nomethod">, then an exception will be raised via die()--
511					unless C<"fallback"> was specified as a key in C<use overload> directive.
512
513
514					=head2 Fallback
515
516					The key C<"fallback"> governs what to do if a method for a particular
517					operation is not found. Three different cases are possible depending on
518					the value of C<"fallback">:
519
520					=over 16
521
522					=item * C<undef>
523
524					Perl tries to use a
525					substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
526					then tries to calls C<"nomethod"> value; if missing, an exception
527					will be raised.
528
529					=item * TRUE
530
531					The same as for the C<undef> value, but no exception is raised. Instead,
532					it silently reverts to what it would have done were there no C<use overload>
533					present.
534
535					=item * defined, but FALSE
536
537					No autogeneration is tried. Perl tries to call
538					C<"nomethod"> value, and if this is missing, raises an exception.
539
540					=back
541
542					B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
543					yet, see L<"Inheritance and overloading">.
544
545					=head2 Copy Constructor
546
547					The value for C<"="> is a reference to a function with three
548					arguments, i.e., it looks like the other values in C<use
549					overload>. However, it does not overload the Perl assignment
550					operator. This would go against Camel hair.
551
552					This operation is called in the situations when a mutator is applied
553					to a reference that shares its object with some other reference, such
554					as
555
556					$a=$b;
557					++$a;
558
559					To make this change $a and not change $b, a copy of C<$$a> is made,
560					and $a is assigned a reference to this new object. This operation is
561					done during execution of the C<++$a>, and not during the assignment,
562					(so before the increment C<$$a> coincides with C<$$b>). This is only
563					done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
564					C<nomethod>). Note that if this operation is expressed via C<'+'>
565					a nonmutator, i.e., as in
566
567					$a=$b;
568					$a=$a+1;
569
570					then C<$a> does not reference a new copy of C<$$a>, since $$a does not
571					appear as lvalue when the above code is executed.
572
573					If the copy constructor is required during the execution of some mutator,
574					but a method for C<'='> was not specified, it can be autogenerated as a
575					string copy if the object is a plain scalar.
576
577					=over 5
578
579					=item B<Example>
580
581					The actually executed code for
582
583					$a=$b;
584					Something else which does not modify $a or $b....
585					++$a;
586
587					may be
588
589					$a=$b;
590					Something else which does not modify $a or $b....
591					$a = $a->clone(undef,"");
592					$a->incr(undef,"");
593
594					if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
595					C<'='> was overloaded with C<\&clone>.
596
597					=back
598
599					Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
600					C<$b = $a; ++$a>.
601
602					=head1 MAGIC AUTOGENERATION
603
604					If a method for an operation is not found, and the value for C<"fallback"> is
605					TRUE or undefined, Perl tries to autogenerate a substitute method for
606					the missing operation based on the defined operations. Autogenerated method
607					substitutions are possible for the following operations:
608
609					=over 16
610
611					=item I<Assignment forms of arithmetic operations>
612
613					C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
614					is not defined.
615
616					=item I<Conversion operations>
617
618					String, numeric, and boolean conversion are calculated in terms of one
619					another if not all of them are defined.
620
621					=item I<Increment and decrement>
622
623					The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
624					and C<$a--> in terms of C<$a-=1> and C<$a-1>.
625
626					=item C<abs($a)>
627
628					can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
629
630					=item I<Unary minus>
631
632					can be expressed in terms of subtraction.
633
634					=item I<Negation>
635
636					C<!> and C<not> can be expressed in terms of boolean conversion, or
637					string or numerical conversion.
638
639					=item I<Concatenation>
640
641					can be expressed in terms of string conversion.
642
643					=item I<Comparison operations>
644
645					can be expressed in terms of its "spaceship" counterpart: either
646					C<E<lt>=E<gt>> or C<cmp>:
647
648					<, >, <=, >=, ==, != in terms of <=>
649					lt, gt, le, ge, eq, ne in terms of cmp
650
651					=item I<Iterator>
652
653					<> in terms of builtin operations
654
655					=item I<Dereferencing>
656
657					${} @{} %{} &{} *{} in terms of builtin operations
658
659					=item I<Copy operator>
660
661					can be expressed in terms of an assignment to the dereferenced value, if this
662					value is a scalar and not a reference.
663
664					=back
665
666					=head1 Losing overloading
667
668					The restriction for the comparison operation is that even if, for example,
669					`C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
670					function will produce only a standard logical value based on the
671					numerical value of the result of `C<cmp>'. In particular, a working
672					numeric conversion is needed in this case (possibly expressed in terms of
673					other conversions).
674
675					Similarly, C<.=> and C<x=> operators lose their mathemagical properties
676					if the string conversion substitution is applied.
677
678					When you chop() a mathemagical object it is promoted to a string and its
679					mathemagical properties are lost. The same can happen with other
680					operations as well.
681
682					=head1 Run-time Overloading
683
684					Since all C<use> directives are executed at compile-time, the only way to
685					change overloading during run-time is to
686
687					eval 'use overload "+" => \&addmethod';
688
689					You can also use
690
691					eval 'no overload "+", "--", "<="';
692
693					though the use of these constructs during run-time is questionable.
694
695					=head1 Public functions
696
697					Package C<overload.pm> provides the following public functions:
698
699					=over 5
700
701					=item overload::StrVal(arg)
702
703					Gives string value of C<arg> as in absence of stringify overloading. If you
704					are using this to get the address of a reference (useful for checking if two
705					references point to the same thing) then you may be better off using
706					C<Scalar::Util::refaddr()>, which is faster.
707
708					=item overload::Overloaded(arg)
709
710					Returns true if C<arg> is subject to overloading of some operations.
711
712					=item overload::Method(obj,op)
713
714					Returns C<undef> or a reference to the method that implements C<op>.
715
716					=back
717
718					=head1 Overloading constants
719
720					For some applications, the Perl parser mangles constants too much.
721					It is possible to hook into this process via C<overload::constant()>
722					and C<overload::remove_constant()> functions.
723
724					These functions take a hash as an argument. The recognized keys of this hash
725					are:
726
727					=over 8
728
729					=item integer
730
731					to overload integer constants,
732
733					=item float
734
735					to overload floating point constants,
736
737					=item binary
738
739					to overload octal and hexadecimal constants,
740
741					=item q
742
743					to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
744					strings and here-documents,
745
746					=item qr
747
748					to overload constant pieces of regular expressions.
749
750					=back
751
752					The corresponding values are references to functions which take three arguments:
753					the first one is the I<initial> string form of the constant, the second one
754					is how Perl interprets this constant, the third one is how the constant is used.
755					Note that the initial string form does not
756					contain string delimiters, and has backslashes in backslash-delimiter
757					combinations stripped (thus the value of delimiter is not relevant for
758					processing of this string). The return value of this function is how this
759					constant is going to be interpreted by Perl. The third argument is undefined
760					unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
761					context (comes from strings, regular expressions, and single-quote HERE
762					documents), it is C<tr> for arguments of C<tr>/C<y> operators,
763					it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
764
765					Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
766					it is expected that overloaded constant strings are equipped with reasonable
767					overloaded catenation operator, otherwise absurd results will result.
768					Similarly, negative numbers are considered as negations of positive constants.
769
770					Note that it is probably meaningless to call the functions overload::constant()
771					and overload::remove_constant() from anywhere but import() and unimport() methods.
772					From these methods they may be called as
773
774					sub import {
775					shift;
776					return unless @_;
777					die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
778					overload::constant integer => sub {Math::BigInt->new(shift)};
779					}
780
781					B<BUGS> Currently overloaded-ness of constants does not propagate
782					into C<eval '...'>.
783
784					=head1 IMPLEMENTATION
785
786					What follows is subject to change RSN.
787
788					The table of methods for all operations is cached in magic for the
789					symbol table hash for the package. The cache is invalidated during
790					processing of C<use overload>, C<no overload>, new function
791					definitions, and changes in @ISA. However, this invalidation remains
792					unprocessed until the next C<bless>ing into the package. Hence if you
793					want to change overloading structure dynamically, you'll need an
794					additional (fake) C<bless>ing to update the table.
795
796					(Every SVish thing has a magic queue, and magic is an entry in that
797					queue. This is how a single variable may participate in multiple
798					forms of magic simultaneously. For instance, environment variables
799					regularly have two forms at once: their %ENV magic and their taint
800					magic. However, the magic which implements overloading is applied to
801					the stashes, which are rarely used directly, thus should not slow down
802					Perl.)
803
804					If an object belongs to a package using overload, it carries a special
805					flag. Thus the only speed penalty during arithmetic operations without
806					overloading is the checking of this flag.
807
808					In fact, if C<use overload> is not present, there is almost no overhead
809					for overloadable operations, so most programs should not suffer
810					measurable performance penalties. A considerable effort was made to
811					minimize the overhead when overload is used in some package, but the
812					arguments in question do not belong to packages using overload. When
813					in doubt, test your speed with C<use overload> and without it. So far
814					there have been no reports of substantial speed degradation if Perl is
815					compiled with optimization turned on.
816
817					There is no size penalty for data if overload is not used. The only
818					size penalty if overload is used in some package is that I<all> the
819					packages acquire a magic during the next C<bless>ing into the
820					package. This magic is three-words-long for packages without
821					overloading, and carries the cache table if the package is overloaded.
822
823					Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
824					carried out before any operation that can imply an assignment to the
825					object $a (or $b) refers to, like C<$a++>. You can override this
826					behavior by defining your own copy constructor (see L<"Copy Constructor">).
827
828					It is expected that arguments to methods that are not explicitly supposed
829					to be changed are constant (but this is not enforced).
830
831					=head1 Metaphor clash
832
833					One may wonder why the semantic of overloaded C<=> is so counter intuitive.
834					If it I<looks> counter intuitive to you, you are subject to a metaphor
835					clash.
836
837					Here is a Perl object metaphor:
838
839					I< object is a reference to blessed data>
840
841					and an arithmetic metaphor:
842
843					I< object is a thing by itself>.
844
845					The I<main> problem of overloading C<=> is the fact that these metaphors
846					imply different actions on the assignment C<$a = $b> if $a and $b are
847					objects. Perl-think implies that $a becomes a reference to whatever
848					$b was referencing. Arithmetic-think implies that the value of "object"
849					$a is changed to become the value of the object $b, preserving the fact
850					that $a and $b are separate entities.
851
852					The difference is not relevant in the absence of mutators. After
853					a Perl-way assignment an operation which mutates the data referenced by $a
854					would change the data referenced by $b too. Effectively, after
855					C<$a = $b> values of $a and $b become I<indistinguishable>.
856
857					On the other hand, anyone who has used algebraic notation knows the
858					expressive power of the arithmetic metaphor. Overloading works hard
859					to enable this metaphor while preserving the Perlian way as far as
860					possible. Since it is not possible to freely mix two contradicting
861					metaphors, overloading allows the arithmetic way to write things I<as
862					far as all the mutators are called via overloaded access only>. The
863					way it is done is described in L<Copy Constructor>.
864
865					If some mutator methods are directly applied to the overloaded values,
866					one may need to I<explicitly unlink> other values which references the
867					same value:
868
869					$a = new Data 23;
870					...
871					$b = $a; # $b is "linked" to $a
872					...
873					$a = $a->clone; # Unlink $b from $a
874					$a->increment_by(4);
875
876					Note that overloaded access makes this transparent:
877
878					$a = new Data 23;
879					$b = $a; # $b is "linked" to $a
880					$a += 4; # would unlink $b automagically
881
882					However, it would not make
883
884					$a = new Data 23;
885					$a = 4; # Now $a is a plain 4, not 'Data'
886
887					preserve "objectness" of $a. But Perl I<has> a way to make assignments
888					to an object do whatever you want. It is just not the overload, but
889					tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
890					which returns the object itself, and STORE() method which changes the
891					value of the object, one can reproduce the arithmetic metaphor in its
892					completeness, at least for variables which were tie()d from the start.
893
894					(Note that a workaround for a bug may be needed, see L<"BUGS">.)
895
896					=head1 Cookbook
897
898					Please add examples to what follows!
899
900					=head2 Two-face scalars
901
902					Put this in F<two_face.pm> in your Perl library directory:
903
904					package two_face; # Scalars with separate string and
905					# numeric values.
906					sub new { my $p = shift; bless [@_], $p }
907					use overload '""' => \&str, '0+' => \&num, fallback => 1;
908					sub num {shift->[1]}
909					sub str {shift->[0]}
910
911					Use it as follows:
912
913					require two_face;
914					my $seven = new two_face ("vii", 7);
915					printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
916					print "seven contains `i'\n" if $seven =~ /i/;
917
918					(The second line creates a scalar which has both a string value, and a
919					numeric value.) This prints:
920
921					seven=vii, seven=7, eight=8
922					seven contains `i'
923
924					=head2 Two-face references
925
926					Suppose you want to create an object which is accessible as both an
927					array reference and a hash reference, similar to the
928					L<pseudo-hash\|perlref/"Pseudo-hashes: Using an array as a hash">
929					builtin Perl type. Let's make it better than a pseudo-hash by
930					allowing index 0 to be treated as a normal element.
931
932					package two_refs;
933					use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
934					sub new {
935					my $p = shift;
936					bless \ [@_], $p;
937					}
938					sub gethash {
939					my %h;
940					my $self = shift;
941					tie %h, ref $self, $self;
942					\%h;
943					}
944
945					sub TIEHASH { my $p = shift; bless \ shift, $p }
946					my %fields;
947					my $i = 0;
948					$fields{$_} = $i++ foreach qw{zero one two three};
949					sub STORE {
950					my $self = ${shift()};
951					my $key = $fields{shift()};
952					defined $key or die "Out of band access";
953					$$self->[$key] = shift;
954					}
955					sub FETCH {
956					my $self = ${shift()};
957					my $key = $fields{shift()};
958					defined $key or die "Out of band access";
959					$$self->[$key];
960					}
961
962					Now one can access an object using both the array and hash syntax:
963
964					my $bar = new two_refs 3,4,5,6;
965					$bar->[2] = 11;
966					$bar->{two} == 11 or die 'bad hash fetch';
967
968					Note several important features of this example. First of all, the
969					I<actual> type of $bar is a scalar reference, and we do not overload
970					the scalar dereference. Thus we can get the I<actual> non-overloaded
971					contents of $bar by just using C<$$bar> (what we do in functions which
972					overload dereference). Similarly, the object returned by the
973					TIEHASH() method is a scalar reference.
974
975					Second, we create a new tied hash each time the hash syntax is used.
976					This allows us not to worry about a possibility of a reference loop,
977					which would lead to a memory leak.
978
979					Both these problems can be cured. Say, if we want to overload hash
980					dereference on a reference to an object which is I<implemented> as a
981					hash itself, the only problem one has to circumvent is how to access
982					this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
983					overloaded dereference operator). Here is one possible fetching routine:
984
985					sub access_hash {
986					my ($self, $key) = (shift, shift);
987					my $class = ref $self;
988					bless $self, 'overload::dummy'; # Disable overloading of %{}
989					my $out = $self->{$key};
990					bless $self, $class; # Restore overloading
991					$out;
992					}
993
994					To remove creation of the tied hash on each access, one may an extra
995					level of indirection which allows a non-circular structure of references:
996
997					package two_refs1;
998					use overload '%{}' => sub { ${shift()}->[1] },
999					'@{}' => sub { ${shift()}->[0] };
1000					sub new {
1001					my $p = shift;
1002					my $a = [@_];
1003					my %h;
1004					tie %h, $p, $a;
1005					bless \ [$a, \%h], $p;
1006					}
1007					sub gethash {
1008					my %h;
1009					my $self = shift;
1010					tie %h, ref $self, $self;
1011					\%h;
1012					}
1013
1014					sub TIEHASH { my $p = shift; bless \ shift, $p }
1015					my %fields;
1016					my $i = 0;
1017					$fields{$_} = $i++ foreach qw{zero one two three};
1018					sub STORE {
1019					my $a = ${shift()};
1020					my $key = $fields{shift()};
1021					defined $key or die "Out of band access";
1022					$a->[$key] = shift;
1023					}
1024					sub FETCH {
1025					my $a = ${shift()};
1026					my $key = $fields{shift()};
1027					defined $key or die "Out of band access";
1028					$a->[$key];
1029					}
1030
1031					Now if $baz is overloaded like this, then C<$baz> is a reference to a
1032					reference to the intermediate array, which keeps a reference to an
1033					actual array, and the access hash. The tie()ing object for the access
1034					hash is a reference to a reference to the actual array, so
1035
1036					=over
1037
1038					=item *
1039
1040					There are no loops of references.
1041
1042					=item *
1043
1044					Both "objects" which are blessed into the class C<two_refs1> are
1045					references to a reference to an array, thus references to a I<scalar>.
1046					Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1047					overloaded operations.
1048
1049					=back
1050
1051					=head2 Symbolic calculator
1052
1053					Put this in F<symbolic.pm> in your Perl library directory:
1054
1055					package symbolic; # Primitive symbolic calculator
1056					use overload nomethod => \&wrap;
1057
1058					sub new { shift; bless ['n', @_] }
1059					sub wrap {
1060					my ($obj, $other, $inv, $meth) = @_;
1061					($obj, $other) = ($other, $obj) if $inv;
1062					bless [$meth, $obj, $other];
1063					}
1064
1065					This module is very unusual as overloaded modules go: it does not
1066					provide any usual overloaded operators, instead it provides the L<Last
1067					Resort> operator C<nomethod>. In this example the corresponding
1068					subroutine returns an object which encapsulates operations done over
1069					the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1070					symbolic 3> contains C<['+', 2, ['n', 3]]>.
1071
1072					Here is an example of the script which "calculates" the side of
1073					circumscribed octagon using the above package:
1074
1075					require symbolic;
1076					my $iter = 1; # 2**($iter+2) = 8
1077					my $side = new symbolic 1;
1078					my $cnt = $iter;
1079
1080					while ($cnt--) {
1081					$side = (sqrt(1 + $side**2) - 1)/$side;
1082					}
1083					print "OK\n";
1084
1085					The value of $side is
1086
1087					['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1088					undef], 1], ['n', 1]]
1089
1090					Note that while we obtained this value using a nice little script,
1091					there is no simple way to I<use> this value. In fact this value may
1092					be inspected in debugger (see L<perldebug>), but ony if
1093					C<bareStringify> B<O>ption is set, and not via C<p> command.
1094
1095					If one attempts to print this value, then the overloaded operator
1096					C<""> will be called, which will call C<nomethod> operator. The
1097					result of this operator will be stringified again, but this result is
1098					again of type C<symbolic>, which will lead to an infinite loop.
1099
1100					Add a pretty-printer method to the module F<symbolic.pm>:
1101
1102					sub pretty {
1103					my ($meth, $a, $b) = @{+shift};
1104					$a = 'u' unless defined $a;
1105					$b = 'u' unless defined $b;
1106					$a = $a->pretty if ref $a;
1107					$b = $b->pretty if ref $b;
1108					"[$meth $a $b]";
1109					}
1110
1111					Now one can finish the script by
1112
1113					print "side = ", $side->pretty, "\n";
1114
1115					The method C<pretty> is doing object-to-string conversion, so it
1116					is natural to overload the operator C<""> using this method. However,
1117					inside such a method it is not necessary to pretty-print the
1118					I<components> $a and $b of an object. In the above subroutine
1119					C<"[$meth $a $b]"> is a catenation of some strings and components $a
1120					and $b. If these components use overloading, the catenation operator
1121					will look for an overloaded operator C<.>; if not present, it will
1122					look for an overloaded operator C<"">. Thus it is enough to use
1123
1124					use overload nomethod => \&wrap, '""' => \&str;
1125					sub str {
1126					my ($meth, $a, $b) = @{+shift};
1127					$a = 'u' unless defined $a;
1128					$b = 'u' unless defined $b;
1129					"[$meth $a $b]";
1130					}
1131
1132					Now one can change the last line of the script to
1133
1134					print "side = $side\n";
1135
1136					which outputs
1137
1138					side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1139
1140					and one can inspect the value in debugger using all the possible
1141					methods.
1142
1143					Something is still amiss: consider the loop variable $cnt of the
1144					script. It was a number, not an object. We cannot make this value of
1145					type C<symbolic>, since then the loop will not terminate.
1146
1147					Indeed, to terminate the cycle, the $cnt should become false.
1148					However, the operator C<bool> for checking falsity is overloaded (this
1149					time via overloaded C<"">), and returns a long string, thus any object
1150					of type C<symbolic> is true. To overcome this, we need a way to
1151					compare an object to 0. In fact, it is easier to write a numeric
1152					conversion routine.
1153
1154					Here is the text of F<symbolic.pm> with such a routine added (and
1155					slightly modified str()):
1156
1157					package symbolic; # Primitive symbolic calculator
1158					use overload
1159					nomethod => \&wrap, '""' => \&str, '0+' => \&num;
1160
1161					sub new { shift; bless ['n', @_] }
1162					sub wrap {
1163					my ($obj, $other, $inv, $meth) = @_;
1164					($obj, $other) = ($other, $obj) if $inv;
1165					bless [$meth, $obj, $other];
1166					}
1167					sub str {
1168					my ($meth, $a, $b) = @{+shift};
1169					$a = 'u' unless defined $a;
1170					if (defined $b) {
1171					"[$meth $a $b]";
1172					} else {
1173					"[$meth $a]";
1174					}
1175					}
1176					my %subr = ( n => sub {$_[0]},
1177					sqrt => sub {sqrt $_[0]},
1178					'-' => sub {shift() - shift()},
1179					'+' => sub {shift() + shift()},
1180					'/' => sub {shift() / shift()},
1181					'' => sub {shift() shift()},
1182					'' => sub {shift() shift()},
1183					);
1184					sub num {
1185					my ($meth, $a, $b) = @{+shift};
1186					my $subr = $subr{$meth}
1187					or die "Do not know how to ($meth) in symbolic";
1188					$a = $a->num if ref $a eq __PACKAGE__;
1189					$b = $b->num if ref $b eq __PACKAGE__;
1190					$subr->($a,$b);
1191					}
1192
1193					All the work of numeric conversion is done in %subr and num(). Of
1194					course, %subr is not complete, it contains only operators used in the
1195					example below. Here is the extra-credit question: why do we need an
1196					explicit recursion in num()? (Answer is at the end of this section.)
1197
1198					Use this module like this:
1199
1200					require symbolic;
1201					my $iter = new symbolic 2; # 16-gon
1202					my $side = new symbolic 1;
1203					my $cnt = $iter;
1204
1205					while ($cnt) {
1206					$cnt = $cnt - 1; # Mutator `--' not implemented
1207					$side = (sqrt(1 + $side**2) - 1)/$side;
1208					}
1209					printf "%s=%f\n", $side, $side;
1210					printf "pi=%f\n", $side(2*($iter+2));
1211
1212					It prints (without so many line breaks)
1213
1214					[/ [- [sqrt [+ 1 [ [/ [- [sqrt [+ 1 [ [n 1] 2]]] 1]
1215					[n 1]] 2]]] 1]
1216					[/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1217					pi=3.182598
1218
1219					The above module is very primitive. It does not implement
1220					mutator methods (C<++>, C<-=> and so on), does not do deep copying
1221					(not required without mutators!), and implements only those arithmetic
1222					operations which are used in the example.
1223
1224					To implement most arithmetic operations is easy; one should just use
1225					the tables of operations, and change the code which fills %subr to
1226
1227					my %subr = ( 'n' => sub {$_[0]} );
1228					foreach my $op (split " ", $overload::ops{with_assign}) {
1229					$subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1230					}
1231					my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1232					foreach my $op (split " ", "@overload::ops{ @bins }") {
1233					$subr{$op} = eval "sub {shift() $op shift()}";
1234					}
1235					foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1236					print "defining `$op'\n";
1237					$subr{$op} = eval "sub {$op shift()}";
1238					}
1239
1240					Due to L<Calling Conventions for Mutators>, we do not need anything
1241					special to make C<+=> and friends work, except filling C<+=> entry of
1242					%subr, and defining a copy constructor (needed since Perl has no
1243					way to know that the implementation of C<'+='> does not mutate
1244					the argument, compare L<Copy Constructor>).
1245
1246					To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1247					line, and code (this code assumes that mutators change things one level
1248					deep only, so recursive copying is not needed):
1249
1250					sub cpy {
1251					my $self = shift;
1252					bless [@$self], ref $self;
1253					}
1254
1255					To make C<++> and C<--> work, we need to implement actual mutators,
1256					either directly, or in C<nomethod>. We continue to do things inside
1257					C<nomethod>, thus add
1258
1259					if ($meth eq '++' or $meth eq '--') {
1260					@$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1261					return $obj;
1262					}
1263
1264					after the first line of wrap(). This is not a most effective
1265					implementation, one may consider
1266
1267					sub inc { $_[0] = bless ['++', shift, 1]; }
1268
1269					instead.
1270
1271					As a final remark, note that one can fill %subr by
1272
1273					my %subr = ( 'n' => sub {$_[0]} );
1274					foreach my $op (split " ", $overload::ops{with_assign}) {
1275					$subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1276					}
1277					my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1278					foreach my $op (split " ", "@overload::ops{ @bins }") {
1279					$subr{$op} = eval "sub {shift() $op shift()}";
1280					}
1281					foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1282					$subr{$op} = eval "sub {$op shift()}";
1283					}
1284					$subr{'++'} = $subr{'+'};
1285					$subr{'--'} = $subr{'-'};
1286
1287					This finishes implementation of a primitive symbolic calculator in
1288					50 lines of Perl code. Since the numeric values of subexpressions
1289					are not cached, the calculator is very slow.
1290
1291					Here is the answer for the exercise: In the case of str(), we need no
1292					explicit recursion since the overloaded C<.>-operator will fall back
1293					to an existing overloaded operator C<"">. Overloaded arithmetic
1294					operators I<do not> fall back to numeric conversion if C<fallback> is
1295					not explicitly requested. Thus without an explicit recursion num()
1296					would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1297					the argument of num().
1298
1299					If you wonder why defaults for conversion are different for str() and
1300					num(), note how easy it was to write the symbolic calculator. This
1301					simplicity is due to an appropriate choice of defaults. One extra
1302					note: due to the explicit recursion num() is more fragile than sym():
1303					we need to explicitly check for the type of $a and $b. If components
1304					$a and $b happen to be of some related type, this may lead to problems.
1305
1306					=head2 I<Really> symbolic calculator
1307
1308					One may wonder why we call the above calculator symbolic. The reason
1309					is that the actual calculation of the value of expression is postponed
1310					until the value is I<used>.
1311
1312					To see it in action, add a method
1313
1314					sub STORE {
1315					my $obj = shift;
1316					$#$obj = 1;
1317					@$obj->[0,1] = ('=', shift);
1318					}
1319
1320					to the package C<symbolic>. After this change one can do
1321
1322					my $a = new symbolic 3;
1323					my $b = new symbolic 4;
1324					my $c = sqrt($a2 + $b2);
1325
1326					and the numeric value of $c becomes 5. However, after calling
1327
1328					$a->STORE(12); $b->STORE(5);
1329
1330					the numeric value of $c becomes 13. There is no doubt now that the module
1331					symbolic provides a I<symbolic> calculator indeed.
1332
1333					To hide the rough edges under the hood, provide a tie()d interface to the
1334					package C<symbolic> (compare with L<Metaphor clash>). Add methods
1335
1336					sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1337					sub FETCH { shift }
1338					sub nop { } # Around a bug
1339
1340					(the bug is described in L<"BUGS">). One can use this new interface as
1341
1342					tie $a, 'symbolic', 3;
1343					tie $b, 'symbolic', 4;
1344					$a->nop; $b->nop; # Around a bug
1345
1346					my $c = sqrt($a2 + $b2);
1347
1348					Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1349					of $c becomes 13. To insulate the user of the module add a method
1350
1351					sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1352
1353					Now
1354
1355					my ($a, $b);
1356					symbolic->vars($a, $b);
1357					my $c = sqrt($a2 + $b2);
1358
1359					$a = 3; $b = 4;
1360					printf "c5 %s=%f\n", $c, $c;
1361
1362					$a = 12; $b = 5;
1363					printf "c13 %s=%f\n", $c, $c;
1364
1365					shows that the numeric value of $c follows changes to the values of $a
1366					and $b.
1367
1368					=head1 AUTHOR
1369
1370					Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1371
1372					=head1 DIAGNOSTICS
1373
1374					When Perl is run with the B<-Do> switch or its equivalent, overloading
1375					induces diagnostic messages.
1376
1377					Using the C<m> command of Perl debugger (see L<perldebug>) one can
1378					deduce which operations are overloaded (and which ancestor triggers
1379					this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1380					is shown by debugger. The method C<()> corresponds to the C<fallback>
1381					key (in fact a presence of this method shows that this package has
1382					overloading enabled, and it is what is used by the C<Overloaded>
1383					function of module C<overload>).
1384
1385					The module might issue the following warnings:
1386
1387					=over 4
1388
1389					=item Odd number of arguments for overload::constant
1390
1391					(W) The call to overload::constant contained an odd number of arguments.
1392					The arguments should come in pairs.
1393
1394					=item `%s' is not an overloadable type
1395
1396					(W) You tried to overload a constant type the overload package is unaware of.
1397
1398					=item `%s' is not a code reference
1399
1400					(W) The second (fourth, sixth, ...) argument of overload::constant needs
1401					to be a code reference. Either an anonymous subroutine, or a reference
1402					to a subroutine.
1403
1404					=back
1405
1406					=head1 BUGS
1407
1408					Because it is used for overloading, the per-package hash %OVERLOAD now
1409					has a special meaning in Perl. The symbol table is filled with names
1410					looking like line-noise.
1411
1412					For the purpose of inheritance every overloaded package behaves as if
1413					C<fallback> is present (possibly undefined). This may create
1414					interesting effects if some package is not overloaded, but inherits
1415					from two overloaded packages.
1416
1417					Relation between overloading and tie()ing is broken. Overloading is
1418					triggered or not basing on the I<previous> class of tie()d value.
1419
1420					This happens because the presence of overloading is checked too early,
1421					before any tie()d access is attempted. If the FETCH()ed class of the
1422					tie()d value does not change, a simple workaround is to access the value
1423					immediately after tie()ing, so that after this call the I<previous> class
1424					coincides with the current one.
1425
1426					B<Needed:> a way to fix this without a speed penalty.
1427
1428					Barewords are not covered by overloaded string constants.
1429
1430					This document is confusing. There are grammos and misleading language
1431					used in places. It would seem a total rewrite is needed.
1432
1433					=cut
1434
					# spent 90µs within overload::CORE:match which was called 17 times, avg 5µs/call: # 17 times (90µs+0s) by overload::OVERLOAD at line 20 of overload.pm, avg 5µs/call sub overload::CORE:match; # xsub