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For ddd2.pl
  Run on Tue May 25 16:52:24 2010
Reported on Tue May 25 16:56:59 2010

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
6111.80ms1.89msoverload::::OVERLOADoverload::OVERLOAD
664310µs2.20msoverload::::importoverload::import
171290µs90µsoverload::::CORE:matchoverload::CORE:match (opcode)
0000s0soverload::::AddrRefoverload::AddrRef
0000s0soverload::::BEGINoverload::BEGIN
0000s0soverload::::Methodoverload::Method
0000s0soverload::::Overloadedoverload::Overloaded
0000s0soverload::::OverloadedStringifyoverload::OverloadedStringify
0000s0soverload::::constantoverload::constant
0000s0soverload::::mycanoverload::mycan
0000s0soverload::::niloverload::nil
0000s0soverload::::ov_methodoverload::ov_method
0000s0soverload::::remove_constantoverload::remove_constant
0000s0soverload::::unimportoverload::unimport
Call graph for these subroutines as a Graphviz dot language file.
Line State
ments
Time
on line
Calls Time
in subs
Code
1package overload;
2
316µsour $VERSION = '1.04';
4
515µs$overload::hint_bits = 0x20000; # HINT_LOCALIZE_HH
6
7sub 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 {
10630µs $package = shift;
11683µs my %arg = @_;
12632µs my ($sub, $fb);
136103µs $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
14673µs *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
15693µs for (keys %arg) {
1628356µs if ($_ eq 'fallback') {
17 $fb = $arg{$_};
18 } else {
1922131µs $sub = $arg{$_};
2022429µs1790µs if (not ref $sub and $sub !~ /::/) {
# spent 90µs making 17 calls to overload::CORE:match, avg 5µs/call
2117177µs $ {$package . "::(" . $_} = $sub;
2217110µs $sub = \&nil;
23 }
24 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
2522176µs *{$package . "::(" . $_} = \&{ $sub };
26 }
27 }
286137µs ${$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 {
32655µs $package = (caller())[0];
33 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
34629µs shift;
356202µs61.89ms $package->overload::OVERLOAD(@_);
# spent 1.89ms making 6 calls to overload::OVERLOAD, avg 316µs/call
36}
37
38sub 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
51sub Overloaded {
52 my $package = shift;
53 $package = ref $package if ref $package;
54 $package->can('()');
55}
56
57sub 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
65sub 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
75sub 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
84sub 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)", $addr);
94}
95
9617µs*StrVal = *AddrRef;
97
98sub mycan { # Real can would leave stubs.
99 my ($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 $out;
105 }
106 return undef;
107}
108
109111µs%constants = (
110 'integer' => 0x1000, # HINT_NEW_INTEGER
111 'float' => 0x2000, # HINT_NEW_FLOAT
112 'binary' => 0x4000, # HINT_NEW_BINARY
113 'q' => 0x8000, # HINT_NEW_STRING
114 'qr' => 0x10000, # HINT_NEW_RE
115 );
116
117121µs%ops = ( with_assign => "+ - * / % ** << >> x .",
118 assign => "+= -= *= /= %= **= <<= >>= x= .=",
119 num_comparison => "< <= > >= == !=",
120 '3way_comparison'=> "<=> cmp",
121 str_comparison => "lt le gt ge eq ne",
122 binary => "& | ^",
123 unary => "neg ! ~",
124 mutators => '++ --',
125 func => "atan2 cos sin exp abs log sqrt int",
126 conversion => 'bool "" 0+',
127 iterators => '<>',
128 dereferencing => '${} @{} %{} &{} *{}',
129 special => 'nomethod fallback =');
130
1313606µs1703µsuse warnings::register;
# spent 703µs making 1 call to warnings::register::import
132sub 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
158sub remove_constant {
159 # Arguments: what, sub
160 while (@_) {
161 delete $^H{$_[0]};
162 $^H &= ~ $constants{$_[0]};
163 shift, shift;
164 }
165}
166
167123µs1;
168
169__END__
170
171=head1 NAME
172
173overload - 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 $b;
192
193=head1 DESCRIPTION
194
195=head2 Declaration of overloaded functions
196
197The compilation directive
198
199 package Number;
200 use overload
201 "+" => \&add,
202 "*=" => "muas";
203
204declares function Number::add() for addition, and method muas() in
205the "class" C<Number> (or one of its base classes)
206for the assignment form C<*=> of multiplication.
207
208Arguments of this directive come in (key, value) pairs. Legal values
209are values legal inside a C<&{ ... }> call, so the name of a
210subroutine, a reference to a subroutine, or an anonymous subroutine
211will all work. Note that values specified as strings are
212interpreted as methods, not subroutines. Legal keys are listed below.
213
214The subroutine C<add> will be called to execute C<$a+$b> if $a
215is a reference to an object blessed into the package C<Number>, or if $a is
216not an object from a package with defined mathemagic addition, but $b is a
217reference to a C<Number>. It can also be called in other situations, like
218C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
219methods refer to methods triggered by an overloaded mathematical
220operator.)
221
222Since overloading respects inheritance via the @ISA hierarchy, the
223above declaration would also trigger overloading of C<+> and C<*=> in
224all the packages which inherit from C<Number>.
225
226=head2 Calling Conventions for Binary Operations
227
228The functions specified in the C<use overload ...> directive are called
229with three (in one particular case with four, see L<Last Resort>)
230arguments. If the corresponding operation is binary, then the first
231two arguments are the two arguments of the operation. However, due to
232general object calling conventions, the first argument should always be
233an object in the package, so in the situation of C<7+$a>, the
234order of the arguments is interchanged. It probably does not matter
235when implementing the addition method, but whether the arguments
236are reversed is vital to the subtraction method. The method can
237query this information by examining the third argument, which can take
238three different values:
239
240=over 7
241
242=item FALSE
243
244the order of arguments is as in the current operation.
245
246=item TRUE
247
248the arguments are reversed.
249
250=item C<undef>
251
252the current operation is an assignment variant (as in
253C<$a+=7>), but the usual function is called instead. This additional
254information can be used to generate some optimizations. Compare
255L<Calling Conventions for Mutators>.
256
257=back
258
259=head2 Calling Conventions for Unary Operations
260
261Unary operation are considered binary operations with the second
262argument being C<undef>. Thus the functions that overloads C<{"++"}>
263is called with arguments C<($a,undef,'')> when $a++ is executed.
264
265=head2 Calling Conventions for Mutators
266
267Two types of mutators have different calling conventions:
268
269=over
270
271=item C<++> and C<-->
272
273The routines which implement these operators are expected to actually
274I<mutate> their arguments. So, assuming that $obj is a reference to a
275number,
276
277 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
278
279is an appropriate implementation of overloaded C<++>. Note that
280
281 sub incr { ++$ {$_[0]} ; shift }
282
283is OK if used with preincrement and with postincrement. (In the case
284of postincrement a copying will be performed, see L<Copy Constructor>.)
285
286=item C<x=> and other assignment versions
287
288There is nothing special about these methods. They may change the
289value of their arguments, and may leave it as is. The result is going
290to be assigned to the value in the left-hand-side if different from
291this value.
292
293This allows for the same method to be used as overloaded C<+=> and
294C<+>. Note that this is I<allowed>, but not recommended, since by the
295semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
296if C<+=> is not overloaded.
297
298=back
299
300B<Warning.> Due to the presence of assignment versions of operations,
301routines which may be called in assignment context may create
302self-referential structures. Currently Perl will not free self-referential
303structures until cycles are C<explicitly> broken. You may get problems
304when traversing your structures too.
305
306Say,
307
308 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
309
310is asking for trouble, since for code C<$obj += $foo> the subroutine
311is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
312\$foo]>. If using such a subroutine is an important optimization, one
313can overload C<+=> explicitly by a non-"optimized" version, or switch
314to non-optimized version if C<not defined $_[2]> (see
315L<Calling Conventions for Binary Operations>).
316
317Even if no I<explicit> assignment-variants of operators are present in
318the script, they may be generated by the optimizer. Say, C<",$obj,"> or
319C<',' . $obj . ','> may be both optimized to
320
321 my $tmp = ',' . $obj; $tmp .= ',';
322
323=head2 Overloadable Operations
324
325The 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
334For these operations a substituted non-assignment variant can be called if
335the assignment variant is not available. Methods for operations C<+>,
336C<->, C<+=>, and C<-=> can be called to automatically generate
337increment and decrement methods. The operation C<-> can be used to
338autogenerate missing methods for unary minus or C<abs>.
339
340See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
341L<"Calling Conventions for Binary Operations">) for details of these
342substitutions.
343
344=item * I<Comparison operations>
345
346 "<", "<=", ">", ">=", "==", "!=", "<=>",
347 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
348
349If the corresponding "spaceship" variant is available, it can be
350used to substitute for the missing operation. During C<sort>ing
351arrays, C<cmp> is used to compare values subject to C<use overload>.
352
353=item * I<Bit operations>
354
355 "&", "^", "|", "neg", "!", "~",
356
357C<neg> stands for unary minus. If the method for C<neg> is not
358specified, it can be autogenerated using the method for
359subtraction. If the method for C<!> is not specified, it can be
360autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
361
362=item * I<Increment and decrement>
363
364 "++", "--",
365
366If undefined, addition and subtraction methods can be
367used instead. These operations are called both in prefix and
368postfix form.
369
370=item * I<Transcendental functions>
371
372 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
373
374If C<abs> is unavailable, it can be autogenerated using methods
375for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
376
377Note that traditionally the Perl function L<int> rounds to 0, thus for
378floating-point-like types one should follow the same semantic. If
379C<int> is unavailable, it can be autogenerated using the overloading of
380C<0+>.
381
382=item * I<Boolean, string and numeric conversion>
383
384 'bool', '""', '0+',
385
386If one or two of these operations are not overloaded, the remaining ones can
387be used instead. C<bool> is used in the flow control operators
388(like C<while>) and for the ternary C<?:> operation. These functions can
389return any arbitrary Perl value. If the corresponding operation for this value
390is overloaded too, that operation will be called again with this value.
391
392As a special case if the overload returns the object itself then it will
393be used directly. An overloaded conversion returning the object is
394probably a bug, because you're likely to get something that looks like
395C<YourPackage=HASH(0x8172b34)>.
396
397=item * I<Iteration>
398
399 "<>"
400
401If not overloaded, the argument will be converted to a filehandle or
402glob (which may require a stringification). The same overloading
403happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
404I<globbing> syntax C<E<lt>${var}E<gt>>.
405
406B<BUGS> Even in list context, the iterator is currently called only
407once and with scalar context.
408
409=item * I<Dereferencing>
410
411 '${}', '@{}', '%{}', '&{}', '*{}'.
412
413If not overloaded, the argument will be dereferenced I<as is>, thus
414should be of correct type. These functions should return a reference
415of correct type, or another object with overloaded dereferencing.
416
417As a special case if the overload returns the object itself then it
418will be used directly (provided it is the correct type).
419
420The dereference operators must be specified explicitly they will not be passed to
421"nomethod".
422
423=item * I<Special>
424
425 "nomethod", "fallback", "=",
426
427see L<SPECIAL SYMBOLS FOR C<use overload>>.
428
429=back
430
431See L<"Fallback"> for an explanation of when a missing method can be
432autogenerated.
433
434A 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 => '${} @{} %{} &{} *{}',
449 special => 'nomethod fallback ='
450
451=head2 Inheritance and overloading
452
453Inheritance interacts with overloading in two ways.
454
455=over
456
457=item Strings as values of C<use overload> directive
458
459If C<value> in
460
461 use overload key => value;
462
463is a string, it is interpreted as a method name.
464
465=item Overloading of an operation is inherited by derived classes
466
467Any class derived from an overloaded class is also overloaded. The
468set of overloaded methods is the union of overloaded methods of all
469the ancestors. If some method is overloaded in several ancestor, then
470which description will be used is decided by the usual inheritance
471rules:
472
473If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
474C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
475then the subroutine C<D::plus_sub> will be called to implement
476operation C<+> for an object in package C<A>.
477
478=back
479
480Note that since the value of the C<fallback> key is not a subroutine,
481its inheritance is not governed by the above rules. In the current
482implementation, the value of C<fallback> in the first overloaded
483ancestor is used, but this is accidental and subject to change.
484
485=head1 SPECIAL SYMBOLS FOR C<use overload>
486
487Three keys are recognized by Perl that are not covered by the above
488description.
489
490=head2 Last Resort
491
492C<"nomethod"> should be followed by a reference to a function of four
493parameters. If defined, it is called when the overloading mechanism
494cannot find a method for some operation. The first three arguments of
495this function coincide with the arguments for the corresponding method if
496it were found, the fourth argument is the symbol
497corresponding to the missing method. If several methods are tried,
498the last one is used. Say, C<1-$a> can be equivalent to
499
500 &nomethodMethod($a,1,1,"-")
501
502if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
503C<use overload> directive.
504
505The C<"nomethod"> mechanism is I<not> used for the dereference operators
506( ${} @{} %{} &{} *{} ).
507
508
509If some operation cannot be resolved, and there is no function
510assigned to C<"nomethod">, then an exception will be raised via die()--
511unless C<"fallback"> was specified as a key in C<use overload> directive.
512
513
514=head2 Fallback
515
516The key C<"fallback"> governs what to do if a method for a particular
517operation is not found. Three different cases are possible depending on
518the value of C<"fallback">:
519
520=over 16
521
522=item * C<undef>
523
524Perl tries to use a
525substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
526then tries to calls C<"nomethod"> value; if missing, an exception
527will be raised.
528
529=item * TRUE
530
531The same as for the C<undef> value, but no exception is raised. Instead,
532it silently reverts to what it would have done were there no C<use overload>
533present.
534
535=item * defined, but FALSE
536
537No autogeneration is tried. Perl tries to call
538C<"nomethod"> value, and if this is missing, raises an exception.
539
540=back
541
542B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
543yet, see L<"Inheritance and overloading">.
544
545=head2 Copy Constructor
546
547The value for C<"="> is a reference to a function with three
548arguments, i.e., it looks like the other values in C<use
549overload>. However, it does not overload the Perl assignment
550operator. This would go against Camel hair.
551
552This operation is called in the situations when a mutator is applied
553to a reference that shares its object with some other reference, such
554as
555
556 $a=$b;
557 ++$a;
558
559To make this change $a and not change $b, a copy of C<$$a> is made,
560and $a is assigned a reference to this new object. This operation is
561done during execution of the C<++$a>, and not during the assignment,
562(so before the increment C<$$a> coincides with C<$$b>). This is only
563done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
564C<nomethod>). Note that if this operation is expressed via C<'+'>
565a nonmutator, i.e., as in
566
567 $a=$b;
568 $a=$a+1;
569
570then C<$a> does not reference a new copy of C<$$a>, since $$a does not
571appear as lvalue when the above code is executed.
572
573If the copy constructor is required during the execution of some mutator,
574but a method for C<'='> was not specified, it can be autogenerated as a
575string copy if the object is a plain scalar.
576
577=over 5
578
579=item B<Example>
580
581The actually executed code for
582
583 $a=$b;
584 Something else which does not modify $a or $b....
585 ++$a;
586
587may 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
594if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
595C<'='> was overloaded with C<\&clone>.
596
597=back
598
599Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
600C<$b = $a; ++$a>.
601
602=head1 MAGIC AUTOGENERATION
603
604If a method for an operation is not found, and the value for C<"fallback"> is
605TRUE or undefined, Perl tries to autogenerate a substitute method for
606the missing operation based on the defined operations. Autogenerated method
607substitutions are possible for the following operations:
608
609=over 16
610
611=item I<Assignment forms of arithmetic operations>
612
613C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
614is not defined.
615
616=item I<Conversion operations>
617
618String, numeric, and boolean conversion are calculated in terms of one
619another if not all of them are defined.
620
621=item I<Increment and decrement>
622
623The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
624and C<$a--> in terms of C<$a-=1> and C<$a-1>.
625
626=item C<abs($a)>
627
628can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
629
630=item I<Unary minus>
631
632can be expressed in terms of subtraction.
633
634=item I<Negation>
635
636C<!> and C<not> can be expressed in terms of boolean conversion, or
637string or numerical conversion.
638
639=item I<Concatenation>
640
641can be expressed in terms of string conversion.
642
643=item I<Comparison operations>
644
645can be expressed in terms of its "spaceship" counterpart: either
646C<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
661can be expressed in terms of an assignment to the dereferenced value, if this
662value is a scalar and not a reference.
663
664=back
665
666=head1 Losing overloading
667
668The restriction for the comparison operation is that even if, for example,
669`C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
670function will produce only a standard logical value based on the
671numerical value of the result of `C<cmp>'. In particular, a working
672numeric conversion is needed in this case (possibly expressed in terms of
673other conversions).
674
675Similarly, C<.=> and C<x=> operators lose their mathemagical properties
676if the string conversion substitution is applied.
677
678When you chop() a mathemagical object it is promoted to a string and its
679mathemagical properties are lost. The same can happen with other
680operations as well.
681
682=head1 Run-time Overloading
683
684Since all C<use> directives are executed at compile-time, the only way to
685change overloading during run-time is to
686
687 eval 'use overload "+" => \&addmethod';
688
689You can also use
690
691 eval 'no overload "+", "--", "<="';
692
693though the use of these constructs during run-time is questionable.
694
695=head1 Public functions
696
697Package C<overload.pm> provides the following public functions:
698
699=over 5
700
701=item overload::StrVal(arg)
702
703Gives string value of C<arg> as in absence of stringify overloading. If you
704are using this to get the address of a reference (useful for checking if two
705references point to the same thing) then you may be better off using
706C<Scalar::Util::refaddr()>, which is faster.
707
708=item overload::Overloaded(arg)
709
710Returns true if C<arg> is subject to overloading of some operations.
711
712=item overload::Method(obj,op)
713
714Returns C<undef> or a reference to the method that implements C<op>.
715
716=back
717
718=head1 Overloading constants
719
720For some applications, the Perl parser mangles constants too much.
721It is possible to hook into this process via C<overload::constant()>
722and C<overload::remove_constant()> functions.
723
724These functions take a hash as an argument. The recognized keys of this hash
725are:
726
727=over 8
728
729=item integer
730
731to overload integer constants,
732
733=item float
734
735to overload floating point constants,
736
737=item binary
738
739to overload octal and hexadecimal constants,
740
741=item q
742
743to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
744strings and here-documents,
745
746=item qr
747
748to overload constant pieces of regular expressions.
749
750=back
751
752The corresponding values are references to functions which take three arguments:
753the first one is the I<initial> string form of the constant, the second one
754is how Perl interprets this constant, the third one is how the constant is used.
755Note that the initial string form does not
756contain string delimiters, and has backslashes in backslash-delimiter
757combinations stripped (thus the value of delimiter is not relevant for
758processing of this string). The return value of this function is how this
759constant is going to be interpreted by Perl. The third argument is undefined
760unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
761context (comes from strings, regular expressions, and single-quote HERE
762documents), it is C<tr> for arguments of C<tr>/C<y> operators,
763it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
764
765Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
766it is expected that overloaded constant strings are equipped with reasonable
767overloaded catenation operator, otherwise absurd results will result.
768Similarly, negative numbers are considered as negations of positive constants.
769
770Note that it is probably meaningless to call the functions overload::constant()
771and overload::remove_constant() from anywhere but import() and unimport() methods.
772From 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
781B<BUGS> Currently overloaded-ness of constants does not propagate
782into C<eval '...'>.
783
784=head1 IMPLEMENTATION
785
786What follows is subject to change RSN.
787
788The table of methods for all operations is cached in magic for the
789symbol table hash for the package. The cache is invalidated during
790processing of C<use overload>, C<no overload>, new function
791definitions, and changes in @ISA. However, this invalidation remains
792unprocessed until the next C<bless>ing into the package. Hence if you
793want to change overloading structure dynamically, you'll need an
794additional (fake) C<bless>ing to update the table.
795
796(Every SVish thing has a magic queue, and magic is an entry in that
797queue. This is how a single variable may participate in multiple
798forms of magic simultaneously. For instance, environment variables
799regularly have two forms at once: their %ENV magic and their taint
800magic. However, the magic which implements overloading is applied to
801the stashes, which are rarely used directly, thus should not slow down
802Perl.)
803
804If an object belongs to a package using overload, it carries a special
805flag. Thus the only speed penalty during arithmetic operations without
806overloading is the checking of this flag.
807
808In fact, if C<use overload> is not present, there is almost no overhead
809for overloadable operations, so most programs should not suffer
810measurable performance penalties. A considerable effort was made to
811minimize the overhead when overload is used in some package, but the
812arguments in question do not belong to packages using overload. When
813in doubt, test your speed with C<use overload> and without it. So far
814there have been no reports of substantial speed degradation if Perl is
815compiled with optimization turned on.
816
817There is no size penalty for data if overload is not used. The only
818size penalty if overload is used in some package is that I<all> the
819packages acquire a magic during the next C<bless>ing into the
820package. This magic is three-words-long for packages without
821overloading, and carries the cache table if the package is overloaded.
822
823Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
824carried out before any operation that can imply an assignment to the
825object $a (or $b) refers to, like C<$a++>. You can override this
826behavior by defining your own copy constructor (see L<"Copy Constructor">).
827
828It is expected that arguments to methods that are not explicitly supposed
829to be changed are constant (but this is not enforced).
830
831=head1 Metaphor clash
832
833One may wonder why the semantic of overloaded C<=> is so counter intuitive.
834If it I<looks> counter intuitive to you, you are subject to a metaphor
835clash.
836
837Here is a Perl object metaphor:
838
839I< object is a reference to blessed data>
840
841and an arithmetic metaphor:
842
843I< object is a thing by itself>.
844
845The I<main> problem of overloading C<=> is the fact that these metaphors
846imply different actions on the assignment C<$a = $b> if $a and $b are
847objects. 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
850that $a and $b are separate entities.
851
852The difference is not relevant in the absence of mutators. After
853a Perl-way assignment an operation which mutates the data referenced by $a
854would change the data referenced by $b too. Effectively, after
855C<$a = $b> values of $a and $b become I<indistinguishable>.
856
857On the other hand, anyone who has used algebraic notation knows the
858expressive power of the arithmetic metaphor. Overloading works hard
859to enable this metaphor while preserving the Perlian way as far as
860possible. Since it is not possible to freely mix two contradicting
861metaphors, overloading allows the arithmetic way to write things I<as
862far as all the mutators are called via overloaded access only>. The
863way it is done is described in L<Copy Constructor>.
864
865If some mutator methods are directly applied to the overloaded values,
866one may need to I<explicitly unlink> other values which references the
867same 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
876Note 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
882However, it would not make
883
884 $a = new Data 23;
885 $a = 4; # Now $a is a plain 4, not 'Data'
886
887preserve "objectness" of $a. But Perl I<has> a way to make assignments
888to an object do whatever you want. It is just not the overload, but
889tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
890which returns the object itself, and STORE() method which changes the
891value of the object, one can reproduce the arithmetic metaphor in its
892completeness, 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
898Please add examples to what follows!
899
900=head2 Two-face scalars
901
902Put 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
911Use 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
919numeric value.) This prints:
920
921 seven=vii, seven=7, eight=8
922 seven contains `i'
923
924=head2 Two-face references
925
926Suppose you want to create an object which is accessible as both an
927array reference and a hash reference, similar to the
928L<pseudo-hash|perlref/"Pseudo-hashes: Using an array as a hash">
929builtin Perl type. Let's make it better than a pseudo-hash by
930allowing 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 $$self->[$key];
960 }
961
962Now one can access an object using both the array and hash syntax:
963
964 my $bar = new two_refs 3,4,5,6;
965 $bar->[2] = 11;
966 $bar->{two} == 11 or die 'bad hash fetch';
967
968Note several important features of this example. First of all, the
969I<actual> type of $bar is a scalar reference, and we do not overload
970the scalar dereference. Thus we can get the I<actual> non-overloaded
971contents of $bar by just using C<$$bar> (what we do in functions which
972overload dereference). Similarly, the object returned by the
973TIEHASH() method is a scalar reference.
974
975Second, we create a new tied hash each time the hash syntax is used.
976This allows us not to worry about a possibility of a reference loop,
977which would lead to a memory leak.
978
979Both these problems can be cured. Say, if we want to overload hash
980dereference on a reference to an object which is I<implemented> as a
981hash itself, the only problem one has to circumvent is how to access
982this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
983overloaded 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
994To remove creation of the tied hash on each access, one may an extra
995level 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
1031Now if $baz is overloaded like this, then C<$baz> is a reference to a
1032reference to the intermediate array, which keeps a reference to an
1033actual array, and the access hash. The tie()ing object for the access
1034hash is a reference to a reference to the actual array, so
1035
1036=over
1037
1038=item *
1039
1040There are no loops of references.
1041
1042=item *
1043
1044Both "objects" which are blessed into the class C<two_refs1> are
1045references to a reference to an array, thus references to a I<scalar>.
1046Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1047overloaded operations.
1048
1049=back
1050
1051=head2 Symbolic calculator
1052
1053Put this in F<symbolic.pm> in your Perl library directory:
1054
1055 package symbolic; # Primitive symbolic calculator
1056 use overload nomethod => \&wrap;
1057
1058 sub new { shift; bless ['n', @_] }
1059 sub wrap {
1060 my ($obj, $other, $inv, $meth) = @_;
1061 ($obj, $other) = ($other, $obj) if $inv;
1062 bless [$meth, $obj, $other];
1063 }
1064
1065This module is very unusual as overloaded modules go: it does not
1066provide any usual overloaded operators, instead it provides the L<Last
1067Resort> operator C<nomethod>. In this example the corresponding
1068subroutine returns an object which encapsulates operations done over
1069the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1070symbolic 3> contains C<['+', 2, ['n', 3]]>.
1071
1072Here is an example of the script which "calculates" the side of
1073circumscribed 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
1085The value of $side is
1086
1087 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1088 undef], 1], ['n', 1]]
1089
1090Note that while we obtained this value using a nice little script,
1091there is no simple way to I<use> this value. In fact this value may
1092be inspected in debugger (see L<perldebug>), but ony if
1093C<bareStringify> B<O>ption is set, and not via C<p> command.
1094
1095If one attempts to print this value, then the overloaded operator
1096C<""> will be called, which will call C<nomethod> operator. The
1097result of this operator will be stringified again, but this result is
1098again of type C<symbolic>, which will lead to an infinite loop.
1099
1100Add a pretty-printer method to the module F<symbolic.pm>:
1101
1102 sub pretty {
1103 my ($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
1111Now one can finish the script by
1112
1113 print "side = ", $side->pretty, "\n";
1114
1115The method C<pretty> is doing object-to-string conversion, so it
1116is natural to overload the operator C<""> using this method. However,
1117inside such a method it is not necessary to pretty-print the
1118I<components> $a and $b of an object. In the above subroutine
1119C<"[$meth $a $b]"> is a catenation of some strings and components $a
1120and $b. If these components use overloading, the catenation operator
1121will look for an overloaded operator C<.>; if not present, it will
1122look for an overloaded operator C<"">. Thus it is enough to use
1123
1124 use overload nomethod => \&wrap, '""' => \&str;
1125 sub str {
1126 my ($meth, $a, $b) = @{+shift};
1127 $a = 'u' unless defined $a;
1128 $b = 'u' unless defined $b;
1129 "[$meth $a $b]";
1130 }
1131
1132Now one can change the last line of the script to
1133
1134 print "side = $side\n";
1135
1136which outputs
1137
1138 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1139
1140and one can inspect the value in debugger using all the possible
1141methods.
1142
1143Something is still amiss: consider the loop variable $cnt of the
1144script. It was a number, not an object. We cannot make this value of
1145type C<symbolic>, since then the loop will not terminate.
1146
1147Indeed, to terminate the cycle, the $cnt should become false.
1148However, the operator C<bool> for checking falsity is overloaded (this
1149time via overloaded C<"">), and returns a long string, thus any object
1150of type C<symbolic> is true. To overcome this, we need a way to
1151compare an object to 0. In fact, it is easier to write a numeric
1152conversion routine.
1153
1154Here is the text of F<symbolic.pm> with such a routine added (and
1155slightly modified str()):
1156
1157 package symbolic; # Primitive symbolic calculator
1158 use overload
1159 nomethod => \&wrap, '""' => \&str, '0+' => \&num;
1160
1161 sub new { shift; bless ['n', @_] }
1162 sub wrap {
1163 my ($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
1193All the work of numeric conversion is done in %subr and num(). Of
1194course, %subr is not complete, it contains only operators used in the
1195example below. Here is the extra-credit question: why do we need an
1196explicit recursion in num()? (Answer is at the end of this section.)
1197
1198Use 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
1212It 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
1219The above module is very primitive. It does not implement
1220mutator methods (C<++>, C<-=> and so on), does not do deep copying
1221(not required without mutators!), and implements only those arithmetic
1222operations which are used in the example.
1223
1224To implement most arithmetic operations is easy; one should just use
1225the 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
1240Due to L<Calling Conventions for Mutators>, we do not need anything
1241special to make C<+=> and friends work, except filling C<+=> entry of
1242%subr, and defining a copy constructor (needed since Perl has no
1243way to know that the implementation of C<'+='> does not mutate
1244the argument, compare L<Copy Constructor>).
1245
1246To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
1247line, and code (this code assumes that mutators change things one level
1248deep only, so recursive copying is not needed):
1249
1250 sub cpy {
1251 my $self = shift;
1252 bless [@$self], ref $self;
1253 }
1254
1255To make C<++> and C<--> work, we need to implement actual mutators,
1256either directly, or in C<nomethod>. We continue to do things inside
1257C<nomethod>, thus add
1258
1259 if ($meth eq '++' or $meth eq '--') {
1260 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1261 return $obj;
1262 }
1263
1264after the first line of wrap(). This is not a most effective
1265implementation, one may consider
1266
1267 sub inc { $_[0] = bless ['++', shift, 1]; }
1268
1269instead.
1270
1271As a final remark, note that one can fill %subr by
1272
1273 my %subr = ( 'n' => sub {$_[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
1287This finishes implementation of a primitive symbolic calculator in
128850 lines of Perl code. Since the numeric values of subexpressions
1289are not cached, the calculator is very slow.
1290
1291Here is the answer for the exercise: In the case of str(), we need no
1292explicit recursion since the overloaded C<.>-operator will fall back
1293to an existing overloaded operator C<"">. Overloaded arithmetic
1294operators I<do not> fall back to numeric conversion if C<fallback> is
1295not explicitly requested. Thus without an explicit recursion num()
1296would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1297the argument of num().
1298
1299If you wonder why defaults for conversion are different for str() and
1300num(), note how easy it was to write the symbolic calculator. This
1301simplicity is due to an appropriate choice of defaults. One extra
1302note: due to the explicit recursion num() is more fragile than sym():
1303we 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
1308One may wonder why we call the above calculator symbolic. The reason
1309is that the actual calculation of the value of expression is postponed
1310until the value is I<used>.
1311
1312To 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
1320to 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 $c = sqrt($a**2 + $b**2);
1325
1326and the numeric value of $c becomes 5. However, after calling
1327
1328 $a->STORE(12); $b->STORE(5);
1329
1330the numeric value of $c becomes 13. There is no doubt now that the module
1331symbolic provides a I<symbolic> calculator indeed.
1332
1333To hide the rough edges under the hood, provide a tie()d interface to the
1334package 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 $c = sqrt($a**2 + $b**2);
1347
1348Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1349of $c becomes 13. To insulate the user of the module add a method
1350
1351 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1352
1353Now
1354
1355 my ($a, $b);
1356 symbolic->vars($a, $b);
1357 my $c = sqrt($a**2 + $b**2);
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
1365shows that the numeric value of $c follows changes to the values of $a
1366and $b.
1367
1368=head1 AUTHOR
1369
1370Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
1371
1372=head1 DIAGNOSTICS
1373
1374When Perl is run with the B<-Do> switch or its equivalent, overloading
1375induces diagnostic messages.
1376
1377Using the C<m> command of Perl debugger (see L<perldebug>) one can
1378deduce which operations are overloaded (and which ancestor triggers
1379this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1380is shown by debugger. The method C<()> corresponds to the C<fallback>
1381key (in fact a presence of this method shows that this package has
1382overloading enabled, and it is what is used by the C<Overloaded>
1383function of module C<overload>).
1384
1385The 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.
1392The 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
1401to be a code reference. Either an anonymous subroutine, or a reference
1402to a subroutine.
1403
1404=back
1405
1406=head1 BUGS
1407
1408Because it is used for overloading, the per-package hash %OVERLOAD now
1409has a special meaning in Perl. The symbol table is filled with names
1410looking like line-noise.
1411
1412For the purpose of inheritance every overloaded package behaves as if
1413C<fallback> is present (possibly undefined). This may create
1414interesting effects if some package is not overloaded, but inherits
1415from two overloaded packages.
1416
1417Relation between overloading and tie()ing is broken. Overloading is
1418triggered or not basing on the I<previous> class of tie()d value.
1419
1420This happens because the presence of overloading is checked too early,
1421before any tie()d access is attempted. If the FETCH()ed class of the
1422tie()d value does not change, a simple workaround is to access the value
1423immediately after tie()ing, so that after this call the I<previous> class
1424coincides with the current one.
1425
1426B<Needed:> a way to fix this without a speed penalty.
1427
1428Barewords are not covered by overloaded string constants.
1429
1430This document is confusing. There are grammos and misleading language
1431used 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