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Typo fix in overload docs
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1package overload;
2
98225a64 3our $VERSION = '1.04';
b75c8c73 4
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5sub nil {}
6
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7sub OVERLOAD {
8 $package = shift;
9 my %arg = @_;
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10 my ($sub, $fb);
11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
4633a7c4 13 for (keys %arg) {
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14 if ($_ eq 'fallback') {
15 $fb = $arg{$_};
16 } else {
17 $sub = $arg{$_};
18 if (not ref $sub and $sub !~ /::/) {
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19 $ {$package . "::(" . $_} = $sub;
20 $sub = \&nil;
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21 }
22 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
23 *{$package . "::(" . $_} = \&{ $sub };
24 }
4633a7c4 25 }
a6006777 26 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
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27}
28
29sub import {
30 $package = (caller())[0];
31 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
32 shift;
33 $package->overload::OVERLOAD(@_);
34}
35
36sub unimport {
37 $package = (caller())[0];
a6006777 38 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
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39 shift;
40 for (@_) {
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41 if ($_ eq 'fallback') {
42 undef $ {$package . "::()"};
43 } else {
44 delete $ {$package . "::"}{"(" . $_};
45 }
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46 }
47}
48
49sub Overloaded {
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50 my $package = shift;
51 $package = ref $package if ref $package;
52 $package->can('()');
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53}
54
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55sub ov_method {
56 my $globref = shift;
57 return undef unless $globref;
58 my $sub = \&{*$globref};
59 return $sub if $sub ne \&nil;
60 return shift->can($ {*$globref});
61}
62
4633a7c4 63sub OverloadedStringify {
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64 my $package = shift;
65 $package = ref $package if ref $package;
44a8e56a 66 #$package->can('(""')
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67 ov_method mycan($package, '(""'), $package
68 or ov_method mycan($package, '(0+'), $package
69 or ov_method mycan($package, '(bool'), $package
70 or ov_method mycan($package, '(nomethod'), $package;
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71}
72
73sub Method {
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74 my $package = shift;
75 $package = ref $package if ref $package;
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76 #my $meth = $package->can('(' . shift);
77 ov_method mycan($package, '(' . shift), $package;
78 #return $meth if $meth ne \&nil;
79 #return $ {*{$meth}};
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80}
81
82sub AddrRef {
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83 my $package = ref $_[0];
84 return "$_[0]" unless $package;
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85
86 require Scalar::Util;
87 my $class = Scalar::Util::blessed($_[0]);
88 my $class_prefix = defined($class) ? "$class=" : "";
89 my $type = Scalar::Util::reftype($_[0]);
90 my $addr = Scalar::Util::refaddr($_[0]);
91 return sprintf("$class_prefix$type(0x%x)", $addr);
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92}
93
1b1d102f 94*StrVal = *AddrRef;
4633a7c4 95
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96sub mycan { # Real can would leave stubs.
97 my ($package, $meth) = @_;
98 return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
99 my $p;
100 foreach $p (@{$package . "::ISA"}) {
101 my $out = mycan($p, $meth);
102 return $out if $out;
103 }
104 return undef;
105}
106
b3ac6de7 107%constants = (
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108 'integer' => 0x1000, # HINT_NEW_INTEGER
109 'float' => 0x2000, # HINT_NEW_FLOAT
110 'binary' => 0x4000, # HINT_NEW_BINARY
111 'q' => 0x8000, # HINT_NEW_STRING
112 'qr' => 0x10000, # HINT_NEW_RE
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113 );
114
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115%ops = ( with_assign => "+ - * / % ** << >> x .",
116 assign => "+= -= *= /= %= **= <<= >>= x= .=",
2877bd81 117 num_comparison => "< <= > >= == !=",
ee239bfe 118 '3way_comparison'=> "<=> cmp",
2877bd81 119 str_comparison => "lt le gt ge eq ne",
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120 binary => "& | ^",
121 unary => "neg ! ~",
122 mutators => '++ --',
f216259d 123 func => "atan2 cos sin exp abs log sqrt int",
ee239bfe 124 conversion => 'bool "" 0+',
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125 iterators => '<>',
126 dereferencing => '${} @{} %{} &{} *{}',
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127 special => 'nomethod fallback =');
128
6b82e2f5 129use warnings::register;
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130sub constant {
131 # Arguments: what, sub
132 while (@_) {
6b82e2f5 133 if (@_ == 1) {
4498a751 134 warnings::warnif ("Odd number of arguments for overload::constant");
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135 last;
136 }
137 elsif (!exists $constants {$_ [0]}) {
4498a751 138 warnings::warnif ("`$_[0]' is not an overloadable type");
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139 }
140 elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
141 # Can't use C<ref $_[1] eq "CODE"> above as code references can be
142 # blessed, and C<ref> would return the package the ref is blessed into.
143 if (warnings::enabled) {
6b82e2f5 144 $_ [1] = "undef" unless defined $_ [1];
4498a751 145 warnings::warn ("`$_[1]' is not a code reference");
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146 }
147 }
148 else {
149 $^H{$_[0]} = $_[1];
f22a2069 150 $^H |= $constants{$_[0]};
6b82e2f5 151 }
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152 shift, shift;
153 }
154}
155
156sub remove_constant {
157 # Arguments: what, sub
158 while (@_) {
159 delete $^H{$_[0]};
160 $^H &= ~ $constants{$_[0]};
161 shift, shift;
162 }
163}
164
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1651;
166
167__END__
168
b267980d 169=head1 NAME
4633a7c4 170
7adf7a02 171overload - Package for overloading Perl operations
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172
173=head1 SYNOPSIS
174
175 package SomeThing;
176
b267980d 177 use overload
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178 '+' => \&myadd,
179 '-' => \&mysub;
180 # etc
181 ...
182
183 package main;
184 $a = new SomeThing 57;
185 $b=5+$a;
186 ...
187 if (overload::Overloaded $b) {...}
188 ...
189 $strval = overload::StrVal $b;
190
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191=head1 DESCRIPTION
192
193=head2 Declaration of overloaded functions
194
195The compilation directive
196
197 package Number;
198 use overload
b267980d 199 "+" => \&add,
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200 "*=" => "muas";
201
202declares function Number::add() for addition, and method muas() in
203the "class" C<Number> (or one of its base classes)
b267980d 204for the assignment form C<*=> of multiplication.
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205
206Arguments of this directive come in (key, value) pairs. Legal values
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207are values legal inside a C<&{ ... }> call, so the name of a
208subroutine, a reference to a subroutine, or an anonymous subroutine
209will all work. Note that values specified as strings are
210interpreted as methods, not subroutines. Legal keys are listed below.
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211
212The subroutine C<add> will be called to execute C<$a+$b> if $a
213is a reference to an object blessed into the package C<Number>, or if $a is
214not an object from a package with defined mathemagic addition, but $b is a
215reference to a C<Number>. It can also be called in other situations, like
216C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
217methods refer to methods triggered by an overloaded mathematical
218operator.)
219
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220Since overloading respects inheritance via the @ISA hierarchy, the
221above declaration would also trigger overloading of C<+> and C<*=> in
222all the packages which inherit from C<Number>.
e7ea3e70 223
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224=head2 Calling Conventions for Binary Operations
225
226The functions specified in the C<use overload ...> directive are called
227with three (in one particular case with four, see L<Last Resort>)
228arguments. If the corresponding operation is binary, then the first
229two arguments are the two arguments of the operation. However, due to
230general object calling conventions, the first argument should always be
231an object in the package, so in the situation of C<7+$a>, the
232order of the arguments is interchanged. It probably does not matter
233when implementing the addition method, but whether the arguments
234are reversed is vital to the subtraction method. The method can
235query this information by examining the third argument, which can take
236three different values:
237
238=over 7
239
240=item FALSE
241
242the order of arguments is as in the current operation.
243
244=item TRUE
245
246the arguments are reversed.
247
248=item C<undef>
249
250the current operation is an assignment variant (as in
251C<$a+=7>), but the usual function is called instead. This additional
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252information can be used to generate some optimizations. Compare
253L<Calling Conventions for Mutators>.
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254
255=back
256
257=head2 Calling Conventions for Unary Operations
258
259Unary operation are considered binary operations with the second
260argument being C<undef>. Thus the functions that overloads C<{"++"}>
261is called with arguments C<($a,undef,'')> when $a++ is executed.
262
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263=head2 Calling Conventions for Mutators
264
265Two types of mutators have different calling conventions:
266
88c28ceb 267=over
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268
269=item C<++> and C<-->
270
271The routines which implement these operators are expected to actually
272I<mutate> their arguments. So, assuming that $obj is a reference to a
273number,
274
275 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
276
277is an appropriate implementation of overloaded C<++>. Note that
278
279 sub incr { ++$ {$_[0]} ; shift }
280
281is OK if used with preincrement and with postincrement. (In the case
282of postincrement a copying will be performed, see L<Copy Constructor>.)
283
284=item C<x=> and other assignment versions
285
286There is nothing special about these methods. They may change the
287value of their arguments, and may leave it as is. The result is going
288to be assigned to the value in the left-hand-side if different from
289this value.
290
f610777f 291This allows for the same method to be used as overloaded C<+=> and
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292C<+>. Note that this is I<allowed>, but not recommended, since by the
293semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
294if C<+=> is not overloaded.
295
296=back
297
d1be9408 298B<Warning.> Due to the presence of assignment versions of operations,
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299routines which may be called in assignment context may create
300self-referential structures. Currently Perl will not free self-referential
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301structures until cycles are C<explicitly> broken. You may get problems
302when traversing your structures too.
303
b267980d 304Say,
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305
306 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
307
308is asking for trouble, since for code C<$obj += $foo> the subroutine
b267980d 309is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
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310\$foo]>. If using such a subroutine is an important optimization, one
311can overload C<+=> explicitly by a non-"optimized" version, or switch
b267980d 312to non-optimized version if C<not defined $_[2]> (see
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313L<Calling Conventions for Binary Operations>).
314
315Even if no I<explicit> assignment-variants of operators are present in
316the script, they may be generated by the optimizer. Say, C<",$obj,"> or
317C<',' . $obj . ','> may be both optimized to
318
319 my $tmp = ',' . $obj; $tmp .= ',';
320
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321=head2 Overloadable Operations
322
ee239bfe 323The following symbols can be specified in C<use overload> directive:
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324
325=over 5
326
327=item * I<Arithmetic operations>
328
329 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
330 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
331
332For these operations a substituted non-assignment variant can be called if
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333the assignment variant is not available. Methods for operations C<+>,
334C<->, C<+=>, and C<-=> can be called to automatically generate
335increment and decrement methods. The operation C<-> can be used to
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336autogenerate missing methods for unary minus or C<abs>.
337
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338See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
339L<"Calling Conventions for Binary Operations">) for details of these
340substitutions.
341
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342=item * I<Comparison operations>
343
344 "<", "<=", ">", ">=", "==", "!=", "<=>",
345 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
346
347If the corresponding "spaceship" variant is available, it can be
348used to substitute for the missing operation. During C<sort>ing
349arrays, C<cmp> is used to compare values subject to C<use overload>.
350
351=item * I<Bit operations>
352
353 "&", "^", "|", "neg", "!", "~",
354
fa8a6580 355C<neg> stands for unary minus. If the method for C<neg> is not
3bc6ec80 356specified, it can be autogenerated using the method for
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357subtraction. If the method for C<!> is not specified, it can be
358autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
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359
360=item * I<Increment and decrement>
361
362 "++", "--",
363
364If undefined, addition and subtraction methods can be
365used instead. These operations are called both in prefix and
366postfix form.
367
368=item * I<Transcendental functions>
369
f216259d 370 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
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371
372If C<abs> is unavailable, it can be autogenerated using methods
1fef88e7 373for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
4633a7c4 374
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375Note that traditionally the Perl function L<int> rounds to 0, thus for
376floating-point-like types one should follow the same semantic. If
377C<int> is unavailable, it can be autogenerated using the overloading of
378C<0+>.
379
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380=item * I<Boolean, string and numeric conversion>
381
fa8a6580 382 'bool', '""', '0+',
4633a7c4 383
f5284f61 384If one or two of these operations are not overloaded, the remaining ones can
4633a7c4 385be used instead. C<bool> is used in the flow control operators
fa8a6580 386(like C<while>) and for the ternary C<?:> operation. These functions can
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387return any arbitrary Perl value. If the corresponding operation for this value
388is overloaded too, that operation will be called again with this value.
389
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390As a special case if the overload returns the object itself then it will
391be used directly. An overloaded conversion returning the object is
392probably a bug, because you're likely to get something that looks like
393C<YourPackage=HASH(0x8172b34)>.
394
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395=item * I<Iteration>
396
397 "<>"
398
399If not overloaded, the argument will be converted to a filehandle or
400glob (which may require a stringification). The same overloading
401happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
402I<globbing> syntax C<E<lt>${var}E<gt>>.
403
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404B<BUGS> Even in list context, the iterator is currently called only
405once and with scalar context.
406
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407=item * I<Dereferencing>
408
409 '${}', '@{}', '%{}', '&{}', '*{}'.
410
411If not overloaded, the argument will be dereferenced I<as is>, thus
412should be of correct type. These functions should return a reference
413of correct type, or another object with overloaded dereferencing.
414
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415As a special case if the overload returns the object itself then it
416will be used directly (provided it is the correct type).
417
418The dereference operators must be specified explicitly they will not be passed to
419"nomethod".
420
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421=item * I<Special>
422
0d863452 423 "nomethod", "fallback", "=", "~~",
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424
425see L<SPECIAL SYMBOLS FOR C<use overload>>.
426
427=back
428
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429See L<"Fallback"> for an explanation of when a missing method can be
430autogenerated.
431
432A computer-readable form of the above table is available in the hash
433%overload::ops, with values being space-separated lists of names:
434
435 with_assign => '+ - * / % ** << >> x .',
436 assign => '+= -= *= /= %= **= <<= >>= x= .=',
2877bd81 437 num_comparison => '< <= > >= == !=',
ee239bfe 438 '3way_comparison'=> '<=> cmp',
2877bd81 439 str_comparison => 'lt le gt ge eq ne',
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440 binary => '& | ^',
441 unary => 'neg ! ~',
442 mutators => '++ --',
443 func => 'atan2 cos sin exp abs log sqrt',
444 conversion => 'bool "" 0+',
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445 iterators => '<>',
446 dereferencing => '${} @{} %{} &{} *{}',
ee239bfe 447 special => 'nomethod fallback ='
4633a7c4 448
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449=head2 Inheritance and overloading
450
774d564b 451Inheritance interacts with overloading in two ways.
e7ea3e70 452
88c28ceb 453=over
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454
455=item Strings as values of C<use overload> directive
456
774d564b 457If C<value> in
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458
459 use overload key => value;
460
774d564b 461is a string, it is interpreted as a method name.
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462
463=item Overloading of an operation is inherited by derived classes
464
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465Any class derived from an overloaded class is also overloaded. The
466set of overloaded methods is the union of overloaded methods of all
467the ancestors. If some method is overloaded in several ancestor, then
e7ea3e70 468which description will be used is decided by the usual inheritance
774d564b 469rules:
e7ea3e70 470
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471If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
472C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
473then the subroutine C<D::plus_sub> will be called to implement
474operation C<+> for an object in package C<A>.
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475
476=back
477
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478Note that since the value of the C<fallback> key is not a subroutine,
479its inheritance is not governed by the above rules. In the current
480implementation, the value of C<fallback> in the first overloaded
481ancestor is used, but this is accidental and subject to change.
e7ea3e70 482
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483=head1 SPECIAL SYMBOLS FOR C<use overload>
484
485Three keys are recognized by Perl that are not covered by the above
486description.
487
774d564b 488=head2 Last Resort
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489
490C<"nomethod"> should be followed by a reference to a function of four
491parameters. If defined, it is called when the overloading mechanism
492cannot find a method for some operation. The first three arguments of
493this function coincide with the arguments for the corresponding method if
494it were found, the fourth argument is the symbol
495corresponding to the missing method. If several methods are tried,
496the last one is used. Say, C<1-$a> can be equivalent to
497
498 &nomethodMethod($a,1,1,"-")
499
500if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
501C<use overload> directive.
502
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503The C<"nomethod"> mechanism is I<not> used for the dereference operators
504( ${} @{} %{} &{} *{} ).
505
506
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507If some operation cannot be resolved, and there is no function
508assigned to C<"nomethod">, then an exception will be raised via die()--
509unless C<"fallback"> was specified as a key in C<use overload> directive.
510
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511
512=head2 Fallback
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513
514The key C<"fallback"> governs what to do if a method for a particular
515operation is not found. Three different cases are possible depending on
516the value of C<"fallback">:
517
518=over 16
519
520=item * C<undef>
521
522Perl tries to use a
523substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
524then tries to calls C<"nomethod"> value; if missing, an exception
525will be raised.
526
527=item * TRUE
528
529The same as for the C<undef> value, but no exception is raised. Instead,
530it silently reverts to what it would have done were there no C<use overload>
531present.
532
533=item * defined, but FALSE
534
535No autogeneration is tried. Perl tries to call
b267980d 536C<"nomethod"> value, and if this is missing, raises an exception.
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537
538=back
539
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540B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
541yet, see L<"Inheritance and overloading">.
542
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543=head2 Smart Match
544
545The key C<"~~"> allows you to override the smart matching used by
546the switch construct. See L<feature>.
547
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548=head2 Copy Constructor
549
550The value for C<"="> is a reference to a function with three
551arguments, i.e., it looks like the other values in C<use
552overload>. However, it does not overload the Perl assignment
553operator. This would go against Camel hair.
554
555This operation is called in the situations when a mutator is applied
556to a reference that shares its object with some other reference, such
557as
558
b267980d 559 $a=$b;
ee239bfe 560 ++$a;
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561
562To make this change $a and not change $b, a copy of C<$$a> is made,
563and $a is assigned a reference to this new object. This operation is
ee239bfe 564done during execution of the C<++$a>, and not during the assignment,
4633a7c4 565(so before the increment C<$$a> coincides with C<$$b>). This is only
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566done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
567C<nomethod>). Note that if this operation is expressed via C<'+'>
568a nonmutator, i.e., as in
4633a7c4 569
b267980d 570 $a=$b;
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571 $a=$a+1;
572
573then C<$a> does not reference a new copy of C<$$a>, since $$a does not
574appear as lvalue when the above code is executed.
575
576If the copy constructor is required during the execution of some mutator,
577but a method for C<'='> was not specified, it can be autogenerated as a
578string copy if the object is a plain scalar.
579
580=over 5
581
582=item B<Example>
583
b267980d 584The actually executed code for
4633a7c4 585
b267980d 586 $a=$b;
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587 Something else which does not modify $a or $b....
588 ++$a;
589
590may be
591
b267980d 592 $a=$b;
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593 Something else which does not modify $a or $b....
594 $a = $a->clone(undef,"");
595 $a->incr(undef,"");
596
597if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
598C<'='> was overloaded with C<\&clone>.
599
600=back
601
f610777f 602Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
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603C<$b = $a; ++$a>.
604
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605=head1 MAGIC AUTOGENERATION
606
607If a method for an operation is not found, and the value for C<"fallback"> is
608TRUE or undefined, Perl tries to autogenerate a substitute method for
609the missing operation based on the defined operations. Autogenerated method
610substitutions are possible for the following operations:
611
612=over 16
613
614=item I<Assignment forms of arithmetic operations>
615
616C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
617is not defined.
618
b267980d 619=item I<Conversion operations>
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620
621String, numeric, and boolean conversion are calculated in terms of one
622another if not all of them are defined.
623
624=item I<Increment and decrement>
625
626The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
627and C<$a--> in terms of C<$a-=1> and C<$a-1>.
628
629=item C<abs($a)>
630
631can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
632
633=item I<Unary minus>
634
635can be expressed in terms of subtraction.
636
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637=item I<Negation>
638
639C<!> and C<not> can be expressed in terms of boolean conversion, or
640string or numerical conversion.
641
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642=item I<Concatenation>
643
644can be expressed in terms of string conversion.
645
b267980d 646=item I<Comparison operations>
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647
648can be expressed in terms of its "spaceship" counterpart: either
649C<E<lt>=E<gt>> or C<cmp>:
1fef88e7 650
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651 <, >, <=, >=, ==, != in terms of <=>
652 lt, gt, le, ge, eq, ne in terms of cmp
653
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654=item I<Iterator>
655
656 <> in terms of builtin operations
657
658=item I<Dereferencing>
659
660 ${} @{} %{} &{} *{} in terms of builtin operations
661
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662=item I<Copy operator>
663
664can be expressed in terms of an assignment to the dereferenced value, if this
665value is a scalar and not a reference.
666
667=back
668
84fc275b
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669=head1 Minimal set of overloaded operations
670
671Since some operations can be automatically generated from others, there is
672a minimal set of operations that need to be overloaded in order to have
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S
673the complete set of overloaded operations at one's disposal.
674Of course, the autogenerated operations may not do exactly what the user
675expects. See L<MAGIC AUTOGENERATION> above. The minimal set is:
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676
677 + - * / % ** << >> x
678 <=> cmp
679 & | ^ ~
680 atan2 cos sin exp log sqrt int
681
682Additionally, you need to define at least one of string, boolean or
299476e0
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683numeric conversions because any one can be used to emulate the others.
684The string conversion can also be used to emulate concatenation.
84fc275b 685
ee239bfe 686=head1 Losing overloading
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687
688The restriction for the comparison operation is that even if, for example,
689`C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
690function will produce only a standard logical value based on the
691numerical value of the result of `C<cmp>'. In particular, a working
692numeric conversion is needed in this case (possibly expressed in terms of
693other conversions).
694
695Similarly, C<.=> and C<x=> operators lose their mathemagical properties
696if the string conversion substitution is applied.
697
698When you chop() a mathemagical object it is promoted to a string and its
699mathemagical properties are lost. The same can happen with other
700operations as well.
701
702=head1 Run-time Overloading
703
704Since all C<use> directives are executed at compile-time, the only way to
705change overloading during run-time is to
706
707 eval 'use overload "+" => \&addmethod';
708
709You can also use
710
711 eval 'no overload "+", "--", "<="';
712
713though the use of these constructs during run-time is questionable.
714
715=head1 Public functions
716
717Package C<overload.pm> provides the following public functions:
718
719=over 5
720
721=item overload::StrVal(arg)
722
6a0e9e72
FD
723Gives string value of C<arg> as in absence of stringify overloading. If you
724are using this to get the address of a reference (useful for checking if two
725references point to the same thing) then you may be better off using
726C<Scalar::Util::refaddr()>, which is faster.
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727
728=item overload::Overloaded(arg)
729
730Returns true if C<arg> is subject to overloading of some operations.
731
732=item overload::Method(obj,op)
733
734Returns C<undef> or a reference to the method that implements C<op>.
735
736=back
737
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738=head1 Overloading constants
739
7adf7a02 740For some applications, the Perl parser mangles constants too much.
bfce84ec 741It is possible to hook into this process via C<overload::constant()>
7adf7a02 742and C<overload::remove_constant()> functions.
b3ac6de7
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743
744These functions take a hash as an argument. The recognized keys of this hash
7adf7a02 745are:
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746
747=over 8
748
749=item integer
750
751to overload integer constants,
752
753=item float
754
755to overload floating point constants,
756
757=item binary
758
759to overload octal and hexadecimal constants,
760
761=item q
762
763to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
764strings and here-documents,
765
766=item qr
767
768to overload constant pieces of regular expressions.
769
770=back
771
772The corresponding values are references to functions which take three arguments:
773the first one is the I<initial> string form of the constant, the second one
b267980d 774is how Perl interprets this constant, the third one is how the constant is used.
b3ac6de7 775Note that the initial string form does not
b267980d 776contain string delimiters, and has backslashes in backslash-delimiter
b3ac6de7 777combinations stripped (thus the value of delimiter is not relevant for
b267980d 778processing of this string). The return value of this function is how this
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779constant is going to be interpreted by Perl. The third argument is undefined
780unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
781context (comes from strings, regular expressions, and single-quote HERE
b267980d 782documents), it is C<tr> for arguments of C<tr>/C<y> operators,
b3ac6de7
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783it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
784
785Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
786it is expected that overloaded constant strings are equipped with reasonable
b267980d 787overloaded catenation operator, otherwise absurd results will result.
b3ac6de7
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788Similarly, negative numbers are considered as negations of positive constants.
789
790Note that it is probably meaningless to call the functions overload::constant()
791and overload::remove_constant() from anywhere but import() and unimport() methods.
792From these methods they may be called as
793
794 sub import {
795 shift;
796 return unless @_;
797 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
798 overload::constant integer => sub {Math::BigInt->new(shift)};
799 }
800
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801=head1 IMPLEMENTATION
802
803What follows is subject to change RSN.
804
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805The table of methods for all operations is cached in magic for the
806symbol table hash for the package. The cache is invalidated during
807processing of C<use overload>, C<no overload>, new function
808definitions, and changes in @ISA. However, this invalidation remains
809unprocessed until the next C<bless>ing into the package. Hence if you
810want to change overloading structure dynamically, you'll need an
811additional (fake) C<bless>ing to update the table.
812
813(Every SVish thing has a magic queue, and magic is an entry in that
814queue. This is how a single variable may participate in multiple
815forms of magic simultaneously. For instance, environment variables
816regularly have two forms at once: their %ENV magic and their taint
817magic. However, the magic which implements overloading is applied to
818the stashes, which are rarely used directly, thus should not slow down
819Perl.)
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820
821If an object belongs to a package using overload, it carries a special
822flag. Thus the only speed penalty during arithmetic operations without
823overloading is the checking of this flag.
824
774d564b
PP
825In fact, if C<use overload> is not present, there is almost no overhead
826for overloadable operations, so most programs should not suffer
827measurable performance penalties. A considerable effort was made to
828minimize the overhead when overload is used in some package, but the
829arguments in question do not belong to packages using overload. When
830in doubt, test your speed with C<use overload> and without it. So far
831there have been no reports of substantial speed degradation if Perl is
832compiled with optimization turned on.
4633a7c4 833
e7ea3e70
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834There is no size penalty for data if overload is not used. The only
835size penalty if overload is used in some package is that I<all> the
836packages acquire a magic during the next C<bless>ing into the
837package. This magic is three-words-long for packages without
f610777f 838overloading, and carries the cache table if the package is overloaded.
4633a7c4 839
b267980d 840Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
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LW
841carried out before any operation that can imply an assignment to the
842object $a (or $b) refers to, like C<$a++>. You can override this
843behavior by defining your own copy constructor (see L<"Copy Constructor">).
844
845It is expected that arguments to methods that are not explicitly supposed
846to be changed are constant (but this is not enforced).
847
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848=head1 Metaphor clash
849
f610777f 850One may wonder why the semantic of overloaded C<=> is so counter intuitive.
b267980d
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851If it I<looks> counter intuitive to you, you are subject to a metaphor
852clash.
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853
854Here is a Perl object metaphor:
855
856I< object is a reference to blessed data>
857
858and an arithmetic metaphor:
859
860I< object is a thing by itself>.
861
862The I<main> problem of overloading C<=> is the fact that these metaphors
863imply different actions on the assignment C<$a = $b> if $a and $b are
864objects. Perl-think implies that $a becomes a reference to whatever
865$b was referencing. Arithmetic-think implies that the value of "object"
866$a is changed to become the value of the object $b, preserving the fact
867that $a and $b are separate entities.
868
869The difference is not relevant in the absence of mutators. After
870a Perl-way assignment an operation which mutates the data referenced by $a
b267980d 871would change the data referenced by $b too. Effectively, after
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872C<$a = $b> values of $a and $b become I<indistinguishable>.
873
b267980d 874On the other hand, anyone who has used algebraic notation knows the
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875expressive power of the arithmetic metaphor. Overloading works hard
876to enable this metaphor while preserving the Perlian way as far as
d1be9408 877possible. Since it is not possible to freely mix two contradicting
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878metaphors, overloading allows the arithmetic way to write things I<as
879far as all the mutators are called via overloaded access only>. The
880way it is done is described in L<Copy Constructor>.
881
882If some mutator methods are directly applied to the overloaded values,
b267980d 883one may need to I<explicitly unlink> other values which references the
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884same value:
885
886 $a = new Data 23;
887 ...
888 $b = $a; # $b is "linked" to $a
889 ...
890 $a = $a->clone; # Unlink $b from $a
891 $a->increment_by(4);
892
893Note that overloaded access makes this transparent:
894
895 $a = new Data 23;
896 $b = $a; # $b is "linked" to $a
897 $a += 4; # would unlink $b automagically
898
899However, it would not make
900
901 $a = new Data 23;
902 $a = 4; # Now $a is a plain 4, not 'Data'
903
904preserve "objectness" of $a. But Perl I<has> a way to make assignments
905to an object do whatever you want. It is just not the overload, but
906tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
b267980d 907which returns the object itself, and STORE() method which changes the
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908value of the object, one can reproduce the arithmetic metaphor in its
909completeness, at least for variables which were tie()d from the start.
910
911(Note that a workaround for a bug may be needed, see L<"BUGS">.)
912
913=head1 Cookbook
914
915Please add examples to what follows!
916
917=head2 Two-face scalars
918
919Put this in F<two_face.pm> in your Perl library directory:
920
921 package two_face; # Scalars with separate string and
922 # numeric values.
923 sub new { my $p = shift; bless [@_], $p }
924 use overload '""' => \&str, '0+' => \&num, fallback => 1;
925 sub num {shift->[1]}
926 sub str {shift->[0]}
927
928Use it as follows:
929
930 require two_face;
931 my $seven = new two_face ("vii", 7);
932 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
933 print "seven contains `i'\n" if $seven =~ /i/;
934
935(The second line creates a scalar which has both a string value, and a
936numeric value.) This prints:
937
938 seven=vii, seven=7, eight=8
939 seven contains `i'
940
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941=head2 Two-face references
942
943Suppose you want to create an object which is accessible as both an
6d822dc4 944array reference and a hash reference.
f5284f61
IZ
945
946 package two_refs;
947 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
b267980d
NIS
948 sub new {
949 my $p = shift;
f5284f61
IZ
950 bless \ [@_], $p;
951 }
952 sub gethash {
953 my %h;
954 my $self = shift;
955 tie %h, ref $self, $self;
956 \%h;
957 }
958
959 sub TIEHASH { my $p = shift; bless \ shift, $p }
960 my %fields;
961 my $i = 0;
962 $fields{$_} = $i++ foreach qw{zero one two three};
b267980d 963 sub STORE {
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964 my $self = ${shift()};
965 my $key = $fields{shift()};
966 defined $key or die "Out of band access";
967 $$self->[$key] = shift;
968 }
b267980d 969 sub FETCH {
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970 my $self = ${shift()};
971 my $key = $fields{shift()};
972 defined $key or die "Out of band access";
973 $$self->[$key];
974 }
975
976Now one can access an object using both the array and hash syntax:
977
978 my $bar = new two_refs 3,4,5,6;
979 $bar->[2] = 11;
980 $bar->{two} == 11 or die 'bad hash fetch';
981
982Note several important features of this example. First of all, the
983I<actual> type of $bar is a scalar reference, and we do not overload
984the scalar dereference. Thus we can get the I<actual> non-overloaded
985contents of $bar by just using C<$$bar> (what we do in functions which
986overload dereference). Similarly, the object returned by the
987TIEHASH() method is a scalar reference.
988
989Second, we create a new tied hash each time the hash syntax is used.
990This allows us not to worry about a possibility of a reference loop,
d1be9408 991which would lead to a memory leak.
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992
993Both these problems can be cured. Say, if we want to overload hash
994dereference on a reference to an object which is I<implemented> as a
995hash itself, the only problem one has to circumvent is how to access
1fd16925 996this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
f5284f61
IZ
997overloaded dereference operator). Here is one possible fetching routine:
998
999 sub access_hash {
1000 my ($self, $key) = (shift, shift);
1001 my $class = ref $self;
b267980d 1002 bless $self, 'overload::dummy'; # Disable overloading of %{}
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1003 my $out = $self->{$key};
1004 bless $self, $class; # Restore overloading
1005 $out;
1006 }
1007
1fd16925 1008To remove creation of the tied hash on each access, one may an extra
f5284f61
IZ
1009level of indirection which allows a non-circular structure of references:
1010
1011 package two_refs1;
1012 use overload '%{}' => sub { ${shift()}->[1] },
1013 '@{}' => sub { ${shift()}->[0] };
b267980d
NIS
1014 sub new {
1015 my $p = shift;
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1016 my $a = [@_];
1017 my %h;
1018 tie %h, $p, $a;
1019 bless \ [$a, \%h], $p;
1020 }
1021 sub gethash {
1022 my %h;
1023 my $self = shift;
1024 tie %h, ref $self, $self;
1025 \%h;
1026 }
1027
1028 sub TIEHASH { my $p = shift; bless \ shift, $p }
1029 my %fields;
1030 my $i = 0;
1031 $fields{$_} = $i++ foreach qw{zero one two three};
b267980d 1032 sub STORE {
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1033 my $a = ${shift()};
1034 my $key = $fields{shift()};
1035 defined $key or die "Out of band access";
1036 $a->[$key] = shift;
1037 }
b267980d 1038 sub FETCH {
f5284f61
IZ
1039 my $a = ${shift()};
1040 my $key = $fields{shift()};
1041 defined $key or die "Out of band access";
1042 $a->[$key];
1043 }
1044
1fd16925 1045Now if $baz is overloaded like this, then C<$baz> is a reference to a
f5284f61
IZ
1046reference to the intermediate array, which keeps a reference to an
1047actual array, and the access hash. The tie()ing object for the access
1fd16925 1048hash is a reference to a reference to the actual array, so
f5284f61 1049
88c28ceb 1050=over
f5284f61
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1051
1052=item *
1053
1054There are no loops of references.
1055
1056=item *
1057
1058Both "objects" which are blessed into the class C<two_refs1> are
1059references to a reference to an array, thus references to a I<scalar>.
1060Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1061overloaded operations.
1062
1063=back
1064
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1065=head2 Symbolic calculator
1066
1067Put this in F<symbolic.pm> in your Perl library directory:
1068
1069 package symbolic; # Primitive symbolic calculator
1070 use overload nomethod => \&wrap;
1071
1072 sub new { shift; bless ['n', @_] }
1073 sub wrap {
1074 my ($obj, $other, $inv, $meth) = @_;
1075 ($obj, $other) = ($other, $obj) if $inv;
1076 bless [$meth, $obj, $other];
1077 }
1078
1079This module is very unusual as overloaded modules go: it does not
88c28ceb
JH
1080provide any usual overloaded operators, instead it provides the L<Last
1081Resort> operator C<nomethod>. In this example the corresponding
f610777f 1082subroutine returns an object which encapsulates operations done over
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1083the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1084symbolic 3> contains C<['+', 2, ['n', 3]]>.
1085
1086Here is an example of the script which "calculates" the side of
1087circumscribed octagon using the above package:
1088
1089 require symbolic;
1090 my $iter = 1; # 2**($iter+2) = 8
1091 my $side = new symbolic 1;
1092 my $cnt = $iter;
3cb6de81 1093
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1094 while ($cnt--) {
1095 $side = (sqrt(1 + $side**2) - 1)/$side;
1096 }
1097 print "OK\n";
1098
1099The value of $side is
1100
1101 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1102 undef], 1], ['n', 1]]
1103
1104Note that while we obtained this value using a nice little script,
1105there is no simple way to I<use> this value. In fact this value may
2d3232d7 1106be inspected in debugger (see L<perldebug>), but only if
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1107C<bareStringify> B<O>ption is set, and not via C<p> command.
1108
1109If one attempts to print this value, then the overloaded operator
1110C<""> will be called, which will call C<nomethod> operator. The
1111result of this operator will be stringified again, but this result is
1112again of type C<symbolic>, which will lead to an infinite loop.
1113
1114Add a pretty-printer method to the module F<symbolic.pm>:
1115
1116 sub pretty {
1117 my ($meth, $a, $b) = @{+shift};
1118 $a = 'u' unless defined $a;
1119 $b = 'u' unless defined $b;
1120 $a = $a->pretty if ref $a;
1121 $b = $b->pretty if ref $b;
1122 "[$meth $a $b]";
b267980d 1123 }
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1124
1125Now one can finish the script by
1126
1127 print "side = ", $side->pretty, "\n";
1128
1129The method C<pretty> is doing object-to-string conversion, so it
1130is natural to overload the operator C<""> using this method. However,
1131inside such a method it is not necessary to pretty-print the
1132I<components> $a and $b of an object. In the above subroutine
1133C<"[$meth $a $b]"> is a catenation of some strings and components $a
1134and $b. If these components use overloading, the catenation operator
1fd16925 1135will look for an overloaded operator C<.>; if not present, it will
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IZ
1136look for an overloaded operator C<"">. Thus it is enough to use
1137
1138 use overload nomethod => \&wrap, '""' => \&str;
1139 sub str {
1140 my ($meth, $a, $b) = @{+shift};
1141 $a = 'u' unless defined $a;
1142 $b = 'u' unless defined $b;
1143 "[$meth $a $b]";
b267980d 1144 }
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1145
1146Now one can change the last line of the script to
1147
1148 print "side = $side\n";
1149
1150which outputs
1151
1152 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1153
1154and one can inspect the value in debugger using all the possible
b267980d 1155methods.
ee239bfe 1156
d1be9408 1157Something is still amiss: consider the loop variable $cnt of the
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1158script. It was a number, not an object. We cannot make this value of
1159type C<symbolic>, since then the loop will not terminate.
1160
1161Indeed, to terminate the cycle, the $cnt should become false.
1162However, the operator C<bool> for checking falsity is overloaded (this
1163time via overloaded C<"">), and returns a long string, thus any object
1164of type C<symbolic> is true. To overcome this, we need a way to
1165compare an object to 0. In fact, it is easier to write a numeric
1166conversion routine.
1167
1168Here is the text of F<symbolic.pm> with such a routine added (and
f610777f 1169slightly modified str()):
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IZ
1170
1171 package symbolic; # Primitive symbolic calculator
1172 use overload
1173 nomethod => \&wrap, '""' => \&str, '0+' => \&num;
1174
1175 sub new { shift; bless ['n', @_] }
1176 sub wrap {
1177 my ($obj, $other, $inv, $meth) = @_;
1178 ($obj, $other) = ($other, $obj) if $inv;
1179 bless [$meth, $obj, $other];
1180 }
1181 sub str {
1182 my ($meth, $a, $b) = @{+shift};
1183 $a = 'u' unless defined $a;
1184 if (defined $b) {
1185 "[$meth $a $b]";
1186 } else {
1187 "[$meth $a]";
1188 }
b267980d
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1189 }
1190 my %subr = ( n => sub {$_[0]},
1191 sqrt => sub {sqrt $_[0]},
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1192 '-' => sub {shift() - shift()},
1193 '+' => sub {shift() + shift()},
1194 '/' => sub {shift() / shift()},
1195 '*' => sub {shift() * shift()},
1196 '**' => sub {shift() ** shift()},
1197 );
1198 sub num {
1199 my ($meth, $a, $b) = @{+shift};
b267980d 1200 my $subr = $subr{$meth}
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1201 or die "Do not know how to ($meth) in symbolic";
1202 $a = $a->num if ref $a eq __PACKAGE__;
1203 $b = $b->num if ref $b eq __PACKAGE__;
1204 $subr->($a,$b);
1205 }
1206
1207All the work of numeric conversion is done in %subr and num(). Of
f610777f 1208course, %subr is not complete, it contains only operators used in the
ee239bfe
IZ
1209example below. Here is the extra-credit question: why do we need an
1210explicit recursion in num()? (Answer is at the end of this section.)
1211
1212Use this module like this:
1213
1214 require symbolic;
1215 my $iter = new symbolic 2; # 16-gon
1216 my $side = new symbolic 1;
1217 my $cnt = $iter;
3cb6de81 1218
ee239bfe
IZ
1219 while ($cnt) {
1220 $cnt = $cnt - 1; # Mutator `--' not implemented
1221 $side = (sqrt(1 + $side**2) - 1)/$side;
1222 }
1223 printf "%s=%f\n", $side, $side;
1224 printf "pi=%f\n", $side*(2**($iter+2));
1225
1226It prints (without so many line breaks)
1227
1228 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1229 [n 1]] 2]]] 1]
1230 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1231 pi=3.182598
1232
1233The above module is very primitive. It does not implement
1234mutator methods (C<++>, C<-=> and so on), does not do deep copying
1235(not required without mutators!), and implements only those arithmetic
1236operations which are used in the example.
1237
1fd16925 1238To implement most arithmetic operations is easy; one should just use
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IZ
1239the tables of operations, and change the code which fills %subr to
1240
1241 my %subr = ( 'n' => sub {$_[0]} );
1242 foreach my $op (split " ", $overload::ops{with_assign}) {
1243 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1244 }
1245 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1246 foreach my $op (split " ", "@overload::ops{ @bins }") {
1247 $subr{$op} = eval "sub {shift() $op shift()}";
1248 }
1249 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1250 print "defining `$op'\n";
1251 $subr{$op} = eval "sub {$op shift()}";
1252 }
1253
1254Due to L<Calling Conventions for Mutators>, we do not need anything
1255special to make C<+=> and friends work, except filling C<+=> entry of
1256%subr, and defining a copy constructor (needed since Perl has no
1257way to know that the implementation of C<'+='> does not mutate
1258the argument, compare L<Copy Constructor>).
1259
1fd16925 1260To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
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IZ
1261line, and code (this code assumes that mutators change things one level
1262deep only, so recursive copying is not needed):
1263
1264 sub cpy {
1265 my $self = shift;
1266 bless [@$self], ref $self;
1267 }
1268
b267980d 1269To make C<++> and C<--> work, we need to implement actual mutators,
ee239bfe
IZ
1270either directly, or in C<nomethod>. We continue to do things inside
1271C<nomethod>, thus add
1272
1273 if ($meth eq '++' or $meth eq '--') {
1274 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1275 return $obj;
1276 }
1277
b267980d 1278after the first line of wrap(). This is not a most effective
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IZ
1279implementation, one may consider
1280
1281 sub inc { $_[0] = bless ['++', shift, 1]; }
1282
1283instead.
1284
1285As a final remark, note that one can fill %subr by
1286
1287 my %subr = ( 'n' => sub {$_[0]} );
1288 foreach my $op (split " ", $overload::ops{with_assign}) {
1289 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1290 }
1291 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1292 foreach my $op (split " ", "@overload::ops{ @bins }") {
1293 $subr{$op} = eval "sub {shift() $op shift()}";
1294 }
1295 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1296 $subr{$op} = eval "sub {$op shift()}";
1297 }
1298 $subr{'++'} = $subr{'+'};
1299 $subr{'--'} = $subr{'-'};
1300
b267980d
NIS
1301This finishes implementation of a primitive symbolic calculator in
130250 lines of Perl code. Since the numeric values of subexpressions
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IZ
1303are not cached, the calculator is very slow.
1304
1305Here is the answer for the exercise: In the case of str(), we need no
1306explicit recursion since the overloaded C<.>-operator will fall back
1307to an existing overloaded operator C<"">. Overloaded arithmetic
1308operators I<do not> fall back to numeric conversion if C<fallback> is
1309not explicitly requested. Thus without an explicit recursion num()
1310would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1311the argument of num().
1312
1313If you wonder why defaults for conversion are different for str() and
1314num(), note how easy it was to write the symbolic calculator. This
1315simplicity is due to an appropriate choice of defaults. One extra
f610777f
A
1316note: due to the explicit recursion num() is more fragile than sym():
1317we need to explicitly check for the type of $a and $b. If components
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IZ
1318$a and $b happen to be of some related type, this may lead to problems.
1319
1320=head2 I<Really> symbolic calculator
1321
1322One may wonder why we call the above calculator symbolic. The reason
1323is that the actual calculation of the value of expression is postponed
1324until the value is I<used>.
1325
1326To see it in action, add a method
1327
b267980d
NIS
1328 sub STORE {
1329 my $obj = shift;
1330 $#$obj = 1;
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IZ
1331 @$obj->[0,1] = ('=', shift);
1332 }
1333
1334to the package C<symbolic>. After this change one can do
1335
1336 my $a = new symbolic 3;
1337 my $b = new symbolic 4;
1338 my $c = sqrt($a**2 + $b**2);
1339
1340and the numeric value of $c becomes 5. However, after calling
1341
1342 $a->STORE(12); $b->STORE(5);
1343
1344the numeric value of $c becomes 13. There is no doubt now that the module
1345symbolic provides a I<symbolic> calculator indeed.
1346
1347To hide the rough edges under the hood, provide a tie()d interface to the
1348package C<symbolic> (compare with L<Metaphor clash>). Add methods
1349
1350 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1351 sub FETCH { shift }
1352 sub nop { } # Around a bug
1353
1354(the bug is described in L<"BUGS">). One can use this new interface as
1355
1356 tie $a, 'symbolic', 3;
1357 tie $b, 'symbolic', 4;
1358 $a->nop; $b->nop; # Around a bug
1359
1360 my $c = sqrt($a**2 + $b**2);
1361
1362Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1363of $c becomes 13. To insulate the user of the module add a method
1364
1365 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1366
1367Now
1368
1369 my ($a, $b);
1370 symbolic->vars($a, $b);
1371 my $c = sqrt($a**2 + $b**2);
1372
1373 $a = 3; $b = 4;
1374 printf "c5 %s=%f\n", $c, $c;
1375
1376 $a = 12; $b = 5;
1377 printf "c13 %s=%f\n", $c, $c;
1378
1379shows that the numeric value of $c follows changes to the values of $a
1380and $b.
1381
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LW
1382=head1 AUTHOR
1383
1fef88e7 1384Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
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LW
1385
1386=head1 DIAGNOSTICS
1387
1388When Perl is run with the B<-Do> switch or its equivalent, overloading
1389induces diagnostic messages.
1390
e7ea3e70
IZ
1391Using the C<m> command of Perl debugger (see L<perldebug>) one can
1392deduce which operations are overloaded (and which ancestor triggers
1393this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1394is shown by debugger. The method C<()> corresponds to the C<fallback>
1395key (in fact a presence of this method shows that this package has
1396overloading enabled, and it is what is used by the C<Overloaded>
ee239bfe 1397function of module C<overload>).
e7ea3e70 1398
6ad11d81 1399The module might issue the following warnings:
6b82e2f5
A
1400
1401=over 4
1402
1403=item Odd number of arguments for overload::constant
1404
1405(W) The call to overload::constant contained an odd number of arguments.
1406The arguments should come in pairs.
1407
1408=item `%s' is not an overloadable type
1409
1410(W) You tried to overload a constant type the overload package is unaware of.
1411
1412=item `%s' is not a code reference
1413
1414(W) The second (fourth, sixth, ...) argument of overload::constant needs
1415to be a code reference. Either an anonymous subroutine, or a reference
1416to a subroutine.
1417
1418=back
1419
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LW
1420=head1 BUGS
1421
aa689395
PP
1422Because it is used for overloading, the per-package hash %OVERLOAD now
1423has a special meaning in Perl. The symbol table is filled with names
1424looking like line-noise.
4633a7c4 1425
a6006777
PP
1426For the purpose of inheritance every overloaded package behaves as if
1427C<fallback> is present (possibly undefined). This may create
1428interesting effects if some package is not overloaded, but inherits
1429from two overloaded packages.
4633a7c4 1430
b267980d 1431Relation between overloading and tie()ing is broken. Overloading is
ee239bfe
IZ
1432triggered or not basing on the I<previous> class of tie()d value.
1433
b267980d 1434This happens because the presence of overloading is checked too early,
ee239bfe 1435before any tie()d access is attempted. If the FETCH()ed class of the
b267980d 1436tie()d value does not change, a simple workaround is to access the value
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IZ
1437immediately after tie()ing, so that after this call the I<previous> class
1438coincides with the current one.
1439
1440B<Needed:> a way to fix this without a speed penalty.
1441
b3ac6de7
IZ
1442Barewords are not covered by overloaded string constants.
1443
ee239bfe
IZ
1444This document is confusing. There are grammos and misleading language
1445used in places. It would seem a total rewrite is needed.
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LW
1446
1447=cut
1448