package overload;
+our $VERSION = '1.18';
+
+%ops = (
+ with_assign => "+ - * / % ** << >> x .",
+ assign => "+= -= *= /= %= **= <<= >>= x= .=",
+ num_comparison => "< <= > >= == !=",
+ '3way_comparison' => "<=> cmp",
+ str_comparison => "lt le gt ge eq ne",
+ binary => '& &= | |= ^ ^=',
+ unary => "neg ! ~",
+ mutators => '++ --',
+ func => "atan2 cos sin exp abs log sqrt int",
+ conversion => 'bool "" 0+ qr',
+ iterators => '<>',
+ filetest => "-X",
+ dereferencing => '${} @{} %{} &{} *{}',
+ matching => '~~',
+ special => 'nomethod fallback =',
+);
+
+my %ops_seen;
+for $category (keys %ops) {
+ $ops_seen{$_}++ for (split /\s+/, $ops{$category});
+}
+
sub nil {}
sub OVERLOAD {
$package = shift;
my %arg = @_;
+ for (keys %arg) {
+ warn "overload arg '$_' is invalid" unless $ops_seen{$_};
+ }
my ($sub, $fb);
$ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
+ $fb = ${$package . "::()"}; # preserve old fallback value RT#68196
*{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
for (keys %arg) {
if ($_ eq 'fallback') {
$ {$package . "::(" . $_} = $sub;
$sub = \&nil;
}
- #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
+ #print STDERR "Setting '$ {'package'}::\cO$_' to \\&'$sub'.\n";
*{$package . "::(" . $_} = \&{ $sub };
}
}
sub Overloaded {
my $package = shift;
$package = ref $package if ref $package;
- $package->can('()');
+ mycan ($package, '()');
}
sub ov_method {
my $globref = shift;
return undef unless $globref;
my $sub = \&{*$globref};
- return $sub if $sub ne \&nil;
+ require Scalar::Util;
+ return $sub
+ if Scalar::Util::refaddr($sub) != Scalar::Util::refaddr(\&nil);
return shift->can($ {*$globref});
}
my $package = shift;
$package = ref $package if ref $package;
#$package->can('(""')
- ov_method mycan($package, '(""'), $package;
+ ov_method mycan($package, '(""'), $package
+ or ov_method mycan($package, '(0+'), $package
+ or ov_method mycan($package, '(bool'), $package
+ or ov_method mycan($package, '(nomethod'), $package;
}
sub Method {
my $package = shift;
- $package = ref $package if ref $package;
+ if(ref $package) {
+ local $@;
+ local $!;
+ require Scalar::Util;
+ $package = Scalar::Util::blessed($package);
+ return undef if !defined $package;
+ }
#my $meth = $package->can('(' . shift);
ov_method mycan($package, '(' . shift), $package;
#return $meth if $meth ne \&nil;
sub AddrRef {
my $package = ref $_[0];
return "$_[0]" unless $package;
- bless $_[0], overload::Fake; # Non-overloaded package
- my $str = "$_[0]";
- bless $_[0], $package; # Back
- $package . substr $str, index $str, '=';
-}
-sub StrVal {
- (OverloadedStringify($_[0])) ?
- (AddrRef(shift)) :
- "$_[0]";
+ local $@;
+ local $!;
+ require Scalar::Util;
+ my $class = Scalar::Util::blessed($_[0]);
+ my $class_prefix = defined($class) ? "$class=" : "";
+ my $type = Scalar::Util::reftype($_[0]);
+ my $addr = Scalar::Util::refaddr($_[0]);
+ return sprintf("%s%s(0x%x)", $class_prefix, $type, $addr);
}
+*StrVal = *AddrRef;
+
sub mycan { # Real can would leave stubs.
my ($package, $meth) = @_;
- return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
- my $p;
- foreach $p (@{$package . "::ISA"}) {
- my $out = mycan($p, $meth);
- return $out if $out;
+
+ local $@;
+ local $!;
+ require mro;
+
+ my $mro = mro::get_linear_isa($package);
+ foreach my $p (@$mro) {
+ my $fqmeth = $p . q{::} . $meth;
+ return \*{$fqmeth} if defined &{$fqmeth};
}
+
return undef;
}
+%constants = (
+ 'integer' => 0x1000, # HINT_NEW_INTEGER
+ 'float' => 0x2000, # HINT_NEW_FLOAT
+ 'binary' => 0x4000, # HINT_NEW_BINARY
+ 'q' => 0x8000, # HINT_NEW_STRING
+ 'qr' => 0x10000, # HINT_NEW_RE
+ );
+
+use warnings::register;
+sub constant {
+ # Arguments: what, sub
+ while (@_) {
+ if (@_ == 1) {
+ warnings::warnif ("Odd number of arguments for overload::constant");
+ last;
+ }
+ elsif (!exists $constants {$_ [0]}) {
+ warnings::warnif ("'$_[0]' is not an overloadable type");
+ }
+ elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
+ # Can't use C<ref $_[1] eq "CODE"> above as code references can be
+ # blessed, and C<ref> would return the package the ref is blessed into.
+ if (warnings::enabled) {
+ $_ [1] = "undef" unless defined $_ [1];
+ warnings::warn ("'$_[1]' is not a code reference");
+ }
+ }
+ else {
+ $^H{$_[0]} = $_[1];
+ $^H |= $constants{$_[0]};
+ }
+ shift, shift;
+ }
+}
+
+sub remove_constant {
+ # Arguments: what, sub
+ while (@_) {
+ delete $^H{$_[0]};
+ $^H &= ~ $constants{$_[0]};
+ shift, shift;
+ }
+}
+
1;
__END__
-=head1 NAME
+=head1 NAME
-overload - Package for overloading perl operations
+overload - Package for overloading Perl operations
=head1 SYNOPSIS
package SomeThing;
- use overload
+ use overload
'+' => \&myadd,
'-' => \&mysub;
# etc
...
package main;
- $a = new SomeThing 57;
- $b=5+$a;
+ $a = SomeThing->new( 57 );
+ $b = 5 + $a;
...
if (overload::Overloaded $b) {...}
...
$strval = overload::StrVal $b;
-=head1 CAVEAT SCRIPTOR
+=head1 DESCRIPTION
+
+This pragma allows overloading of Perl's operators for a class.
+To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
-Overloading of operators is a subject not to be taken lightly.
-Neither its precise implementation, syntax, nor semantics are
-100% endorsed by Larry Wall. So any of these may be changed
-at some point in the future.
+=head2 Fundamentals
-=head1 DESCRIPTION
+=head3 Declaration
-=head2 Declaration of overloaded functions
+Arguments of the C<use overload> directive are (key, value) pairs.
+For the full set of legal keys, see L<Overloadable Operations> below.
-The compilation directive
+Operator implementations (the values) can be subroutines,
+references to subroutines, or anonymous subroutines
+- in other words, anything legal inside a C<&{ ... }> call.
+Values specified as strings are interpreted as method names.
+Thus
package Number;
use overload
- "+" => \&add,
- "*=" => "muas";
+ "-" => "minus",
+ "*=" => \&muas,
+ '""' => sub { ...; };
-declares function Number::add() for addition, and method muas() in
-the "class" C<Number> (or one of its base classes)
-for the assignment form C<*=> of multiplication.
+declares that subtraction is to be implemented by method C<minus()>
+in the class C<Number> (or one of its base classes),
+and that the function C<Number::muas()> is to be used for the
+assignment form of multiplication, C<*=>.
+It also defines an anonymous subroutine to implement stringification:
+this is called whenever an object blessed into the package C<Number>
+is used in a string context (this subroutine might, for example,
+return the number as a Roman numeral).
-Arguments of this directive come in (key, value) pairs. Legal values
-are values legal inside a C<&{ ... }> call, so the name of a
-subroutine, a reference to a subroutine, or an anonymous subroutine
-will all work. Note that values specified as strings are
-interpreted as methods, not subroutines. Legal keys are listed below.
+=head3 Calling Conventions and Magic Autogeneration
-The subroutine C<add> will be called to execute C<$a+$b> if $a
-is a reference to an object blessed into the package C<Number>, or if $a is
-not an object from a package with defined mathemagic addition, but $b is a
-reference to a C<Number>. It can also be called in other situations, like
-C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
-methods refer to methods triggered by an overloaded mathematical
-operator.)
+The following sample implementation of C<minus()> (which assumes
+that C<Number> objects are simply blessed references to scalars)
+illustrates the calling conventions:
-Since overloading respects inheritance via the @ISA hierarchy, the
-above declaration would also trigger overloading of C<+> and C<*=> in
-all the packages which inherit from C<Number>.
+ package Number;
+ sub minus {
+ my ($self, $other, $swap) = @_;
+ my $result = $$self - $other; # *
+ $result = -$result if $swap;
+ ref $result ? $result : bless \$result;
+ }
+ # * may recurse once - see table below
+
+Three arguments are passed to all subroutines specified in the
+C<use overload> directive (with one exception - see L</nomethod>).
+The first of these is the operand providing the overloaded
+operator implementation -
+in this case, the object whose C<minus()> method is being called.
+
+The second argument is the other operand, or C<undef> in the
+case of a unary operator.
+
+The third argument is set to TRUE if (and only if) the two
+operands have been swapped. Perl may do this to ensure that the
+first argument (C<$self>) is an object implementing the overloaded
+operation, in line with general object calling conventions.
+For example, if C<$x> and C<$y> are C<Number>s:
+
+ operation | generates a call to
+ ============|======================
+ $x - $y | minus($x, $y, '')
+ $x - 7 | minus($x, 7, '')
+ 7 - $x | minus($x, 7, 1)
+
+Perl may also use C<minus()> to implement other operators which
+have not been specified in the C<use overload> directive,
+according to the rules for L<Magic Autogeneration> described later.
+For example, the C<use overload> above declared no subroutine
+for any of the operators C<-->, C<neg> (the overload key for
+unary minus), or C<-=>. Thus
+
+ operation | generates a call to
+ ============|======================
+ -$x | minus($x, 0, 1)
+ $x-- | minus($x, 1, undef)
+ $x -= 3 | minus($x, 3, undef)
+
+Note the C<undef>s:
+where autogeneration results in the method for a standard
+operator which does not change either of its operands, such
+as C<->, being used to implement an operator which changes
+the operand ("mutators": here, C<--> and C<-=>),
+Perl passes undef as the third argument.
+This still evaluates as FALSE, consistent with the fact that
+the operands have not been swapped, but gives the subroutine
+a chance to alter its behaviour in these cases.
+
+In all the above examples, C<minus()> is required
+only to return the result of the subtraction:
+Perl takes care of the assignment to $x.
+In fact, such methods should I<not> modify their operands,
+even if C<undef> is passed as the third argument
+(see L<Overloadable Operations>).
+
+The same is not true of implementations of C<++> and C<-->:
+these are expected to modify their operand.
+An appropriate implementation of C<--> might look like
+
+ use overload '--' => "decr",
+ # ...
+ sub decr { --${$_[0]}; }
+
+=head3 Mathemagic, Mutators, and Copy Constructors
+
+The term 'mathemagic' describes the overloaded implementation
+of mathematical operators.
+Mathemagical operations raise an issue.
+Consider the code:
+
+ $a = $b;
+ --$a;
+
+If C<$a> and C<$b> are scalars then after these statements
+
+ $a == $b - 1
+
+An object, however, is a reference to blessed data, so if
+C<$a> and C<$b> are objects then the assignment C<$a = $b>
+copies only the reference, leaving C<$a> and C<$b> referring
+to the same object data.
+One might therefore expect the operation C<--$a> to decrement
+C<$b> as well as C<$a>.
+However, this would not be consistent with how we expect the
+mathematical operators to work.
+
+Perl resolves this dilemma by transparently calling a copy
+constructor before calling a method defined to implement
+a mutator (C<-->, C<+=>, and so on.).
+In the above example, when Perl reaches the decrement
+statement, it makes a copy of the object data in C<$a> and
+assigns to C<$a> a reference to the copied data.
+Only then does it call C<decr()>, which alters the copied
+data, leaving C<$b> unchanged.
+Thus the object metaphor is preserved as far as possible,
+while mathemagical operations still work according to the
+arithmetic metaphor.
+
+Note: the preceding paragraph describes what happens when
+Perl autogenerates the copy constructor for an object based
+on a scalar.
+For other cases, see L<Copy Constructor>.
-=head2 Calling Conventions for Binary Operations
+=head2 Overloadable Operations
-The functions specified in the C<use overload ...> directive are called
-with three (in one particular case with four, see L<Last Resort>)
-arguments. If the corresponding operation is binary, then the first
-two arguments are the two arguments of the operation. However, due to
-general object calling conventions, the first argument should always be
-an object in the package, so in the situation of C<7+$a>, the
-order of the arguments is interchanged. It probably does not matter
-when implementing the addition method, but whether the arguments
-are reversed is vital to the subtraction method. The method can
-query this information by examining the third argument, which can take
-three different values:
+The complete list of keys that can be specified in the C<use overload>
+directive are given, separated by spaces, in the values of the
+hash C<%overload::ops>:
+
+ with_assign => '+ - * / % ** << >> x .',
+ assign => '+= -= *= /= %= **= <<= >>= x= .=',
+ num_comparison => '< <= > >= == !=',
+ '3way_comparison'=> '<=> cmp',
+ str_comparison => 'lt le gt ge eq ne',
+ binary => '& &= | |= ^ ^=',
+ unary => 'neg ! ~',
+ mutators => '++ --',
+ func => 'atan2 cos sin exp abs log sqrt int',
+ conversion => 'bool "" 0+ qr',
+ iterators => '<>',
+ filetest => '-X',
+ dereferencing => '${} @{} %{} &{} *{}',
+ matching => '~~',
+ special => 'nomethod fallback ='
+
+Most of the overloadable operators map one-to-one to these keys.
+Exceptions, including additional overloadable operations not
+apparent from this hash, are included in the notes which follow.
+
+A warning is issued if an attempt is made to register an operator not found
+above.
-=over 7
+=over 5
-=item FALSE
+=item * C<not>
-the order of arguments is as in the current operation.
+The operator C<not> is not a valid key for C<use overload>.
+However, if the operator C<!> is overloaded then the same
+implementation will be used for C<not>
+(since the two operators differ only in precedence).
-=item TRUE
+=item * C<neg>
-the arguments are reversed.
+The key C<neg> is used for unary minus to disambiguate it from
+binary C<->.
-=item C<undef>
+=item * C<++>, C<-->
-the current operation is an assignment variant (as in
-C<$a+=7>), but the usual function is called instead. This additional
-information can be used to generate some optimizations.
+Assuming they are to behave analogously to Perl's C<++> and C<-->,
+overloaded implementations of these operators are required to
+mutate their operands.
-=back
+No distinction is made between prefix and postfix forms of the
+increment and decrement operators: these differ only in the
+point at which Perl calls the associated subroutine when
+evaluating an expression.
-=head2 Calling Conventions for Unary Operations
+=item * I<Assignments>
-Unary operation are considered binary operations with the second
-argument being C<undef>. Thus the functions that overloads C<{"++"}>
-is called with arguments C<($a,undef,'')> when $a++ is executed.
+ += -= *= /= %= **= <<= >>= x= .=
+ &= |= ^=
-=head2 Overloadable Operations
+Simple assignment is not overloadable (the C<'='> key is used
+for the L<Copy Constructor>).
+Perl does have a way to make assignments to an object do whatever
+you want, but this involves using tie(), not overload -
+see L<perlfunc/tie> and the L</COOKBOOK> examples below.
-The following symbols can be specified in C<use overload>:
+The subroutine for the assignment variant of an operator is
+required only to return the result of the operation.
+It is permitted to change the value of its operand
+(this is safe because Perl calls the copy constructor first),
+but this is optional since Perl assigns the returned value to
+the left-hand operand anyway.
-=over 5
+An object that overloads an assignment operator does so only in
+respect of assignments to that object.
+In other words, Perl never calls the corresponding methods with
+the third argument (the "swap" argument) set to TRUE.
+For example, the operation
-=item * I<Arithmetic operations>
+ $a *= $b
- "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
- "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
+cannot lead to C<$b>'s implementation of C<*=> being called,
+even if C<$a> is a scalar.
+(It can, however, generate a call to C<$b>'s method for C<*>).
-For these operations a substituted non-assignment variant can be called if
-the assignment variant is not available. Methods for operations "C<+>",
-"C<->", "C<+=>", and "C<-=>" can be called to automatically generate
-increment and decrement methods. The operation "C<->" can be used to
-autogenerate missing methods for unary minus or C<abs>.
+=item * I<Non-mutators with a mutator variant>
-=item * I<Comparison operations>
+ + - * / % ** << >> x .
+ & | ^
- "<", "<=", ">", ">=", "==", "!=", "<=>",
- "lt", "le", "gt", "ge", "eq", "ne", "cmp",
+As described L<above|"Calling Conventions and Magic Autogeneration">,
+Perl may call methods for operators like C<+> and C<&> in the course
+of implementing missing operations like C<++>, C<+=>, and C<&=>.
+While these methods may detect this usage by testing the definedness
+of the third argument, they should in all cases avoid changing their
+operands.
+This is because Perl does not call the copy constructor before
+invoking these methods.
-If the corresponding "spaceship" variant is available, it can be
-used to substitute for the missing operation. During C<sort>ing
-arrays, C<cmp> is used to compare values subject to C<use overload>.
+=item * C<int>
-=item * I<Bit operations>
+Traditionally, the Perl function C<int> rounds to 0
+(see L<perlfunc/int>), and so for floating-point-like types one
+should follow the same semantic.
- "&", "^", "|", "neg", "!", "~",
+=item * I<String, numeric, boolean, and regexp conversions>
-"C<neg>" stands for unary minus. If the method for C<neg> is not
-specified, it can be autogenerated using the method for
-subtraction. If the method for "C<!>" is not specified, it can be
-autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>".
+ "" 0+ bool
-=item * I<Increment and decrement>
+These conversions are invoked according to context as necessary.
+For example, the subroutine for C<'""'> (stringify) may be used
+where the overloaded object is passed as an argument to C<print>,
+and that for C<'bool'> where it is tested in the condition of a flow
+control statement (like C<while>) or the ternary C<?:> operation.
- "++", "--",
+Of course, in contexts like, for example, C<$obj + 1>, Perl will
+invoke C<$obj>'s implementation of C<+> rather than (in this
+example) converting C<$obj> to a number using the numify method
+C<'0+'> (an exception to this is when no method has been provided
+for C<'+'> and L</fallback> is set to TRUE).
-If undefined, addition and subtraction methods can be
-used instead. These operations are called both in prefix and
-postfix form.
+The subroutines for C<'""'>, C<'0+'>, and C<'bool'> can return
+any arbitrary Perl value.
+If the corresponding operation for this value is overloaded too,
+the operation will be called again with this value.
-=item * I<Transcendental functions>
+As a special case if the overload returns the object itself then it will
+be used directly. An overloaded conversion returning the object is
+probably a bug, because you're likely to get something that looks like
+C<YourPackage=HASH(0x8172b34)>.
- "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
+ qr
-If C<abs> is unavailable, it can be autogenerated using methods
-for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
+The subroutine for C<'qr'> is used wherever the object is
+interpolated into or used as a regexp, including when it
+appears on the RHS of a C<=~> or C<!~> operator.
-=item * I<Boolean, string and numeric conversion>
+C<qr> must return a compiled regexp, or a ref to a compiled regexp
+(such as C<qr//> returns), and any further overloading on the return
+value will be ignored.
- "bool", "\"\"", "0+",
+=item * I<Iteration>
-If one or two of these operations are unavailable, the remaining ones can
-be used instead. C<bool> is used in the flow control operators
-(like C<while>) and for the ternary "C<?:>" operation. These functions can
-return any arbitrary Perl value. If the corresponding operation for this value
-is overloaded too, that operation will be called again with this value.
+If C<E<lt>E<gt>> is overloaded then the same implementation is used
+for both the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
+I<globbing> syntax C<E<lt>${var}E<gt>>.
-=item * I<Special>
+B<BUGS> Even in list context, the iterator is currently called only
+once and with scalar context.
- "nomethod", "fallback", "=",
+=item * I<File tests>
-see L<SPECIAL SYMBOLS FOR C<use overload>>.
+The key C<'-X'> is used to specify a subroutine to handle all the
+filetest operators (C<-f>, C<-x>, and so on: see L<perlfunc/-X> for
+the full list);
+it is not possible to overload any filetest operator individually.
+To distinguish them, the letter following the '-' is passed as the
+second argument (that is, in the slot that for binary operators
+is used to pass the second operand).
-=back
+Calling an overloaded filetest operator does not affect the stat value
+associated with the special filehandle C<_>. It still refers to the
+result of the last C<stat>, C<lstat> or unoverloaded filetest.
-See L<"Fallback"> for an explanation of when a missing method can be autogenerated.
+This overload was introduced in Perl 5.12.
-=head2 Inheritance and overloading
+=item * I<Matching>
-Inheritance interacts with overloading in two ways.
+The key C<"~~"> allows you to override the smart matching logic used by
+the C<~~> operator and the switch construct (C<given>/C<when>). See
+L<perlsyn/Switch Statements> and L<feature>.
-=over
+Unusually, the overloaded implementation of the smart match operator
+does not get full control of the smart match behaviour.
+In particular, in the following code:
-=item Strings as values of C<use overload> directive
+ package Foo;
+ use overload '~~' => 'match';
-If C<value> in
+ my $obj = Foo->new();
+ $obj ~~ [ 1,2,3 ];
- use overload key => value;
+the smart match does I<not> invoke the method call like this:
-is a string, it is interpreted as a method name.
+ $obj->match([1,2,3],0);
-=item Overloading of an operation is inherited by derived classes
+rather, the smart match distributive rule takes precedence, so $obj is
+smart matched against each array element in turn until a match is found,
+so you may see between one and three of these calls instead:
-Any class derived from an overloaded class is also overloaded. The
-set of overloaded methods is the union of overloaded methods of all
-the ancestors. If some method is overloaded in several ancestor, then
-which description will be used is decided by the usual inheritance
-rules:
+ $obj->match(1,0);
+ $obj->match(2,0);
+ $obj->match(3,0);
-If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
-C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
-then the subroutine C<D::plus_sub> will be called to implement
-operation C<+> for an object in package C<A>.
+Consult the match table in L<perlop/"Smartmatch Operator"> for
+details of when overloading is invoked.
-=back
+=item * I<Dereferencing>
-Note that since the value of the C<fallback> key is not a subroutine,
-its inheritance is not governed by the above rules. In the current
-implementation, the value of C<fallback> in the first overloaded
-ancestor is used, but this is accidental and subject to change.
+ ${} @{} %{} &{} *{}
+
+If these operators are not explicitly overloaded then they
+work in the normal way, yielding the underlying scalar,
+array, or whatever stores the object data (or the appropriate
+error message if the dereference operator doesn't match it).
+Defining a catch-all C<'nomethod'> (see L<below|/nomethod>)
+makes no difference to this as the catch-all function will
+not be called to implement a missing dereference operator.
+
+If a dereference operator is overloaded then it must return a
+I<reference> of the appropriate type (for example, the
+subroutine for key C<'${}'> should return a reference to a
+scalar, not a scalar), or another object which overloads the
+operator: that is, the subroutine only determines what is
+dereferenced and the actual dereferencing is left to Perl.
+As a special case, if the subroutine returns the object itself
+then it will not be called again - avoiding infinite recursion.
-=head1 SPECIAL SYMBOLS FOR C<use overload>
+=item * I<Special>
-Three keys are recognized by Perl that are not covered by the above
-description.
+ nomethod fallback =
-=head2 Last Resort
+See L<Special Keys for C<use overload>>.
-C<"nomethod"> should be followed by a reference to a function of four
-parameters. If defined, it is called when the overloading mechanism
-cannot find a method for some operation. The first three arguments of
-this function coincide with the arguments for the corresponding method if
-it were found, the fourth argument is the symbol
-corresponding to the missing method. If several methods are tried,
-the last one is used. Say, C<1-$a> can be equivalent to
+=back
- &nomethodMethod($a,1,1,"-")
+=head2 Magic Autogeneration
+
+If a method for an operation is not found then Perl tries to
+autogenerate a substitute implementation from the operations
+that have been defined.
+
+Note: the behaviour described in this section can be disabled
+by setting C<fallback> to FALSE (see L</fallback>).
+
+In the following tables, numbers indicate priority.
+For example, the table below states that,
+if no implementation for C<'!'> has been defined then Perl will
+implement it using C<'bool'> (that is, by inverting the value
+returned by the method for C<'bool'>);
+if boolean conversion is also unimplemented then Perl will
+use C<'0+'> or, failing that, C<'""'>.
+
+ operator | can be autogenerated from
+ |
+ | 0+ "" bool . x
+ =========|==========================
+ 0+ | 1 2
+ "" | 1 2
+ bool | 1 2
+ int | 1 2 3
+ ! | 2 3 1
+ qr | 2 1 3
+ . | 2 1 3
+ x | 2 1 3
+ .= | 3 2 4 1
+ x= | 3 2 4 1
+ <> | 2 1 3
+ -X | 2 1 3
+
+Note: The iterator (C<'E<lt>E<gt>'>) and file test (C<'-X'>)
+operators work as normal: if the operand is not a blessed glob or
+IO reference then it is converted to a string (using the method
+for C<'""'>, C<'0+'>, or C<'bool'>) to be interpreted as a glob
+or filename.
+
+ operator | can be autogenerated from
+ |
+ | < <=> neg -= -
+ =========|==========================
+ neg | 1
+ -= | 1
+ -- | 1 2
+ abs | a1 a2 b1 b2 [*]
+ < | 1
+ <= | 1
+ > | 1
+ >= | 1
+ == | 1
+ != | 1
+
+ * one from [a1, a2] and one from [b1, b2]
+
+Just as numeric comparisons can be autogenerated from the method
+for C<< '<=>' >>, string comparisons can be autogenerated from
+that for C<'cmp'>:
+
+ operators | can be autogenerated from
+ ====================|===========================
+ lt gt le ge eq ne | cmp
+
+Similarly, autogeneration for keys C<'+='> and C<'++'> is analogous
+to C<'-='> and C<'--'> above:
+
+ operator | can be autogenerated from
+ |
+ | += +
+ =========|==========================
+ += | 1
+ ++ | 1 2
+
+And other assignment variations are analogous to
+C<'+='> and C<'-='> (and similar to C<'.='> and C<'x='> above):
+
+ operator || *= /= %= **= <<= >>= &= ^= |=
+ -------------------||--------------------------------
+ autogenerated from || * / % ** << >> & ^ |
+
+Note also that the copy constructor (key C<'='>) may be
+autogenerated, but only for objects based on scalars.
+See L<Copy Constructor>.
+
+=head3 Minimal Set of Overloaded Operations
+
+Since some operations can be automatically generated from others, there is
+a minimal set of operations that need to be overloaded in order to have
+the complete set of overloaded operations at one's disposal.
+Of course, the autogenerated operations may not do exactly what the user
+expects. The minimal set is:
+
+ + - * / % ** << >> x
+ <=> cmp
+ & | ^ ~
+ atan2 cos sin exp log sqrt int
+ "" 0+ bool
+ ~~
+
+Of the conversions, only one of string, boolean or numeric is
+needed because each can be generated from either of the other two.
+
+=head2 Special Keys for C<use overload>
+
+=head3 C<nomethod>
+
+The C<'nomethod'> key is used to specify a catch-all function to
+be called for any operator that is not individually overloaded.
+The specified function will be passed four parameters.
+The first three arguments coincide with those that would have been
+passed to the corresponding method if it had been defined.
+The fourth argument is the C<use overload> key for that missing
+method.
+
+For example, if C<$a> is an object blessed into a package declaring
+
+ use overload 'nomethod' => 'catch_all', # ...
+
+then the operation
+
+ 3 + $a
+
+could (unless a method is specifically declared for the key
+C<'+'>) result in a call
-if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
-C<use overload> directive.
+ catch_all($a, 3, 1, '+')
-If some operation cannot be resolved, and there is no function
-assigned to C<"nomethod">, then an exception will be raised via die()--
-unless C<"fallback"> was specified as a key in C<use overload> directive.
+See L<How Perl Chooses an Operator Implementation>.
-=head2 Fallback
+=head3 C<fallback>
-The key C<"fallback"> governs what to do if a method for a particular
-operation is not found. Three different cases are possible depending on
-the value of C<"fallback">:
+The value assigned to the key C<'fallback'> tells Perl how hard
+it should try to find an alternative way to implement a missing
+operator.
-=over 16
+=over
+
+=item * defined, but FALSE
+
+ use overload "fallback" => 0, # ... ;
+
+This disables L<Magic Autogeneration>.
=item * C<undef>
-Perl tries to use a
-substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
-then tries to calls C<"nomethod"> value; if missing, an exception
-will be raised.
+In the default case where no value is explicitly assigned to
+C<fallback>, magic autogeneration is enabled.
=item * TRUE
-The same as for the C<undef> value, but no exception is raised. Instead,
-it silently reverts to what it would have done were there no C<use overload>
-present.
-
-=item * defined, but FALSE
+The same as for C<undef>, but if a missing operator cannot be
+autogenerated then, instead of issuing an error message, Perl
+is allowed to revert to what it would have done for that
+operator if there had been no C<use overload> directive.
-No autogeneration is tried. Perl tries to call
-C<"nomethod"> value, and if this is missing, raises an exception.
+Note: in most cases, particularly the L<Copy Constructor>,
+this is unlikely to be appropriate behaviour.
=back
-B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
-yet, see L<"Inheritance and overloading">.
+See L<How Perl Chooses an Operator Implementation>.
-=head2 Copy Constructor
+=head3 Copy Constructor
-The value for C<"="> is a reference to a function with three
-arguments, i.e., it looks like the other values in C<use
-overload>. However, it does not overload the Perl assignment
-operator. This would go against Camel hair.
+As mentioned L<above|"Mathemagic, Mutators, and Copy Constructors">,
+this operation is called when a mutator is applied to a reference
+that shares its object with some other reference.
+For example, if C<$b> is mathemagical, and C<'++'> is overloaded
+with C<'incr'>, and C<'='> is overloaded with C<'clone'>, then the
+code
-This operation is called in the situations when a mutator is applied
-to a reference that shares its object with some other reference, such
-as
+ $a = $b;
+ # ... (other code which does not modify $a or $b) ...
+ ++$b;
- $a=$b;
- $a++;
+would be executed in a manner equivalent to
-To make this change $a and not change $b, a copy of C<$$a> is made,
-and $a is assigned a reference to this new object. This operation is
-done during execution of the C<$a++>, and not during the assignment,
-(so before the increment C<$$a> coincides with C<$$b>). This is only
-done if C<++> is expressed via a method for C<'++'> or C<'+='>. Note
-that if this operation is expressed via C<'+'> a nonmutator, i.e., as
-in
+ $a = $b;
+ # ...
+ $b = $b->clone(undef, "");
+ $b->incr(undef, "");
- $a=$b;
- $a=$a+1;
+Note:
-then C<$a> does not reference a new copy of C<$$a>, since $$a does not
-appear as lvalue when the above code is executed.
+=over
-If the copy constructor is required during the execution of some mutator,
-but a method for C<'='> was not specified, it can be autogenerated as a
-string copy if the object is a plain scalar.
+=item *
-=over 5
+The subroutine for C<'='> does not overload the Perl assignment
+operator: it is used only to allow mutators to work as described
+here. (See L</Assignments> above.)
-=item B<Example>
+=item *
-The actually executed code for
+As for other operations, the subroutine implementing '=' is passed
+three arguments, though the last two are always C<undef> and C<''>.
- $a=$b;
- Something else which does not modify $a or $b....
- ++$a;
+=item *
-may be
+The copy constructor is called only before a call to a function
+declared to implement a mutator, for example, if C<++$b;> in the
+code above is effected via a method declared for key C<'++'>
+(or 'nomethod', passed C<'++'> as the fourth argument) or, by
+autogeneration, C<'+='>.
+It is not called if the increment operation is effected by a call
+to the method for C<'+'> since, in the equivalent code,
- $a=$b;
- Something else which does not modify $a or $b....
- $a = $a->clone(undef,"");
- $a->incr(undef,"");
+ $a = $b;
+ $b = $b + 1;
-if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
-C<'='> was overloaded with C<\&clone>.
+the data referred to by C<$a> is unchanged by the assignment to
+C<$b> of a reference to new object data.
-=back
+=item *
-=head1 MAGIC AUTOGENERATION
+The copy constructor is not called if Perl determines that it is
+unnecessary because there is no other reference to the data being
+modified.
-If a method for an operation is not found, and the value for C<"fallback"> is
-TRUE or undefined, Perl tries to autogenerate a substitute method for
-the missing operation based on the defined operations. Autogenerated method
-substitutions are possible for the following operations:
+=item *
-=over 16
+If C<'fallback'> is undefined or TRUE then a copy constructor
+can be autogenerated, but only for objects based on scalars.
+In other cases it needs to be defined explicitly.
+Where an object's data is stored as, for example, an array of
+scalars, the following might be appropriate:
-=item I<Assignment forms of arithmetic operations>
+ use overload '=' => sub { bless [ @{$_[0]} ] }, # ...
-C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
-is not defined.
+=item *
-=item I<Conversion operations>
+If C<'fallback'> is TRUE and no copy constructor is defined then,
+for objects not based on scalars, Perl may silently fall back on
+simple assignment - that is, assignment of the object reference.
+In effect, this disables the copy constructor mechanism since
+no new copy of the object data is created.
+This is almost certainly not what you want.
+(It is, however, consistent: for example, Perl's fallback for the
+C<++> operator is to increment the reference itself.)
-String, numeric, and boolean conversion are calculated in terms of one
-another if not all of them are defined.
+=back
+
+=head2 How Perl Chooses an Operator Implementation
+
+Which is checked first, C<nomethod> or C<fallback>?
+If the two operands of an operator are of different types and
+both overload the operator, which implementation is used?
+The following are the precedence rules:
+
+=over
+
+=item 1.
-=item I<Increment and decrement>
+If the first operand has declared a subroutine to overload the
+operator then use that implementation.
-The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
-and C<$a--> in terms of C<$a-=1> and C<$a-1>.
+=item 2.
-=item C<abs($a)>
+Otherwise, if fallback is TRUE or undefined for the
+first operand then see if the
+L<rules for autogeneration|"Magic Autogeneration">
+allows another of its operators to be used instead.
-can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
+=item 3.
-=item I<Unary minus>
+Unless the operator is an assignment (C<+=>, C<-=>, etc.),
+repeat step (1) in respect of the second operand.
-can be expressed in terms of subtraction.
+=item 4.
-=item I<Negation>
+Repeat Step (2) in respect of the second operand.
-C<!> and C<not> can be expressed in terms of boolean conversion, or
-string or numerical conversion.
+=item 5.
-=item I<Concatenation>
+If the first operand has a "nomethod" method then use that.
-can be expressed in terms of string conversion.
+=item 6.
-=item I<Comparison operations>
+If the second operand has a "nomethod" method then use that.
-can be expressed in terms of its "spaceship" counterpart: either
-C<E<lt>=E<gt>> or C<cmp>:
+=item 7.
- <, >, <=, >=, ==, != in terms of <=>
- lt, gt, le, ge, eq, ne in terms of cmp
+If C<fallback> is TRUE for both operands
+then perform the usual operation for the operator,
+treating the operands as numbers, strings, or booleans
+as appropriate for the operator (see note).
-=item I<Copy operator>
+=item 8.
-can be expressed in terms of an assignment to the dereferenced value, if this
-value is a scalar and not a reference.
+Nothing worked - die.
=back
-=head1 WARNING
+Where there is only one operand (or only one operand with
+overloading) the checks in respect of the other operand above are
+skipped.
+
+There are exceptions to the above rules for dereference operations
+(which, if Step 1 fails, always fall back to the normal, built-in
+implementations - see Dereferencing), and for C<~~> (which has its
+own set of rules - see C<Matching> under L</Overloadable Operations>
+above).
+
+Note on Step 7: some operators have a different semantic depending
+on the type of their operands.
+As there is no way to instruct Perl to treat the operands as, e.g.,
+numbers instead of strings, the result here may not be what you
+expect.
+See L<BUGS AND PITFALLS>.
+
+=head2 Losing Overloading
The restriction for the comparison operation is that even if, for example,
-`C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
+C<cmp> should return a blessed reference, the autogenerated C<lt>
function will produce only a standard logical value based on the
-numerical value of the result of `C<cmp>'. In particular, a working
+numerical value of the result of C<cmp>. In particular, a working
numeric conversion is needed in this case (possibly expressed in terms of
other conversions).
mathemagical properties are lost. The same can happen with other
operations as well.
-=head1 Run-time Overloading
+=head2 Inheritance and Overloading
+
+Overloading respects inheritance via the @ISA hierarchy.
+Inheritance interacts with overloading in two ways.
+
+=over
+
+=item Method names in the C<use overload> directive
+
+If C<value> in
+
+ use overload key => value;
+
+is a string, it is interpreted as a method name - which may
+(in the usual way) be inherited from another class.
+
+=item Overloading of an operation is inherited by derived classes
+
+Any class derived from an overloaded class is also overloaded
+and inherits its operator implementations.
+If the same operator is overloaded in more than one ancestor
+then the implementation is determined by the usual inheritance
+rules.
+
+For example, if C<A> inherits from C<B> and C<C> (in that order),
+C<B> overloads C<+> with C<\&D::plus_sub>, and C<C> overloads
+C<+> by C<"plus_meth">, then the subroutine C<D::plus_sub> will
+be called to implement operation C<+> for an object in package C<A>.
+
+=back
+
+Note that since the value of the C<fallback> key is not a subroutine,
+its inheritance is not governed by the above rules. In the current
+implementation, the value of C<fallback> in the first overloaded
+ancestor is used, but this is accidental and subject to change.
+
+=head2 Run-time Overloading
Since all C<use> directives are executed at compile-time, the only way to
change overloading during run-time is to
though the use of these constructs during run-time is questionable.
-=head1 Public functions
+=head2 Public Functions
Package C<overload.pm> provides the following public functions:
=item overload::StrVal(arg)
-Gives string value of C<arg> as in absence of stringify overloading.
+Gives string value of C<arg> as in absence of stringify overloading. If you
+are using this to get the address of a reference (useful for checking if two
+references point to the same thing) then you may be better off using
+C<Scalar::Util::refaddr()>, which is faster.
=item overload::Overloaded(arg)
=back
+=head2 Overloading Constants
+
+For some applications, the Perl parser mangles constants too much.
+It is possible to hook into this process via C<overload::constant()>
+and C<overload::remove_constant()> functions.
+
+These functions take a hash as an argument. The recognized keys of this hash
+are:
+
+=over 8
+
+=item integer
+
+to overload integer constants,
+
+=item float
+
+to overload floating point constants,
+
+=item binary
+
+to overload octal and hexadecimal constants,
+
+=item q
+
+to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
+strings and here-documents,
+
+=item qr
+
+to overload constant pieces of regular expressions.
+
+=back
+
+The corresponding values are references to functions which take three arguments:
+the first one is the I<initial> string form of the constant, the second one
+is how Perl interprets this constant, the third one is how the constant is used.
+Note that the initial string form does not
+contain string delimiters, and has backslashes in backslash-delimiter
+combinations stripped (thus the value of delimiter is not relevant for
+processing of this string). The return value of this function is how this
+constant is going to be interpreted by Perl. The third argument is undefined
+unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
+context (comes from strings, regular expressions, and single-quote HERE
+documents), it is C<tr> for arguments of C<tr>/C<y> operators,
+it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
+
+Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
+it is expected that overloaded constant strings are equipped with reasonable
+overloaded catenation operator, otherwise absurd results will result.
+Similarly, negative numbers are considered as negations of positive constants.
+
+Note that it is probably meaningless to call the functions overload::constant()
+and overload::remove_constant() from anywhere but import() and unimport() methods.
+From these methods they may be called as
+
+ sub import {
+ shift;
+ return unless @_;
+ die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
+ overload::constant integer => sub {Math::BigInt->new(shift)};
+ }
+
=head1 IMPLEMENTATION
What follows is subject to change RSN.
size penalty if overload is used in some package is that I<all> the
packages acquire a magic during the next C<bless>ing into the
package. This magic is three-words-long for packages without
-overloading, and carries the cache tabel if the package is overloaded.
-
-Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
-carried out before any operation that can imply an assignment to the
-object $a (or $b) refers to, like C<$a++>. You can override this
-behavior by defining your own copy constructor (see L<"Copy Constructor">).
+overloading, and carries the cache table if the package is overloaded.
It is expected that arguments to methods that are not explicitly supposed
to be changed are constant (but this is not enforced).
+=head1 COOKBOOK
+
+Please add examples to what follows!
+
+=head2 Two-face Scalars
+
+Put this in F<two_face.pm> in your Perl library directory:
+
+ package two_face; # Scalars with separate string and
+ # numeric values.
+ sub new { my $p = shift; bless [@_], $p }
+ use overload '""' => \&str, '0+' => \&num, fallback => 1;
+ sub num {shift->[1]}
+ sub str {shift->[0]}
+
+Use it as follows:
+
+ require two_face;
+ my $seven = two_face->new("vii", 7);
+ printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
+ print "seven contains 'i'\n" if $seven =~ /i/;
+
+(The second line creates a scalar which has both a string value, and a
+numeric value.) This prints:
+
+ seven=vii, seven=7, eight=8
+ seven contains 'i'
+
+=head2 Two-face References
+
+Suppose you want to create an object which is accessible as both an
+array reference and a hash reference.
+
+ package two_refs;
+ use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
+ sub new {
+ my $p = shift;
+ bless \ [@_], $p;
+ }
+ sub gethash {
+ my %h;
+ my $self = shift;
+ tie %h, ref $self, $self;
+ \%h;
+ }
+
+ sub TIEHASH { my $p = shift; bless \ shift, $p }
+ my %fields;
+ my $i = 0;
+ $fields{$_} = $i++ foreach qw{zero one two three};
+ sub STORE {
+ my $self = ${shift()};
+ my $key = $fields{shift()};
+ defined $key or die "Out of band access";
+ $$self->[$key] = shift;
+ }
+ sub FETCH {
+ my $self = ${shift()};
+ my $key = $fields{shift()};
+ defined $key or die "Out of band access";
+ $$self->[$key];
+ }
+
+Now one can access an object using both the array and hash syntax:
+
+ my $bar = two_refs->new(3,4,5,6);
+ $bar->[2] = 11;
+ $bar->{two} == 11 or die 'bad hash fetch';
+
+Note several important features of this example. First of all, the
+I<actual> type of $bar is a scalar reference, and we do not overload
+the scalar dereference. Thus we can get the I<actual> non-overloaded
+contents of $bar by just using C<$$bar> (what we do in functions which
+overload dereference). Similarly, the object returned by the
+TIEHASH() method is a scalar reference.
+
+Second, we create a new tied hash each time the hash syntax is used.
+This allows us not to worry about a possibility of a reference loop,
+which would lead to a memory leak.
+
+Both these problems can be cured. Say, if we want to overload hash
+dereference on a reference to an object which is I<implemented> as a
+hash itself, the only problem one has to circumvent is how to access
+this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
+overloaded dereference operator). Here is one possible fetching routine:
+
+ sub access_hash {
+ my ($self, $key) = (shift, shift);
+ my $class = ref $self;
+ bless $self, 'overload::dummy'; # Disable overloading of %{}
+ my $out = $self->{$key};
+ bless $self, $class; # Restore overloading
+ $out;
+ }
+
+To remove creation of the tied hash on each access, one may an extra
+level of indirection which allows a non-circular structure of references:
+
+ package two_refs1;
+ use overload '%{}' => sub { ${shift()}->[1] },
+ '@{}' => sub { ${shift()}->[0] };
+ sub new {
+ my $p = shift;
+ my $a = [@_];
+ my %h;
+ tie %h, $p, $a;
+ bless \ [$a, \%h], $p;
+ }
+ sub gethash {
+ my %h;
+ my $self = shift;
+ tie %h, ref $self, $self;
+ \%h;
+ }
+
+ sub TIEHASH { my $p = shift; bless \ shift, $p }
+ my %fields;
+ my $i = 0;
+ $fields{$_} = $i++ foreach qw{zero one two three};
+ sub STORE {
+ my $a = ${shift()};
+ my $key = $fields{shift()};
+ defined $key or die "Out of band access";
+ $a->[$key] = shift;
+ }
+ sub FETCH {
+ my $a = ${shift()};
+ my $key = $fields{shift()};
+ defined $key or die "Out of band access";
+ $a->[$key];
+ }
+
+Now if $baz is overloaded like this, then C<$baz> is a reference to a
+reference to the intermediate array, which keeps a reference to an
+actual array, and the access hash. The tie()ing object for the access
+hash is a reference to a reference to the actual array, so
+
+=over
+
+=item *
+
+There are no loops of references.
+
+=item *
+
+Both "objects" which are blessed into the class C<two_refs1> are
+references to a reference to an array, thus references to a I<scalar>.
+Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
+overloaded operations.
+
+=back
+
+=head2 Symbolic Calculator
+
+Put this in F<symbolic.pm> in your Perl library directory:
+
+ package symbolic; # Primitive symbolic calculator
+ use overload nomethod => \&wrap;
+
+ sub new { shift; bless ['n', @_] }
+ sub wrap {
+ my ($obj, $other, $inv, $meth) = @_;
+ ($obj, $other) = ($other, $obj) if $inv;
+ bless [$meth, $obj, $other];
+ }
+
+This module is very unusual as overloaded modules go: it does not
+provide any usual overloaded operators, instead it provides an
+implementation for L<C<nomethod>>. In this example the C<nomethod>
+subroutine returns an object which encapsulates operations done over
+the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 +
+symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>.
+
+Here is an example of the script which "calculates" the side of
+circumscribed octagon using the above package:
+
+ require symbolic;
+ my $iter = 1; # 2**($iter+2) = 8
+ my $side = symbolic->new(1);
+ my $cnt = $iter;
+
+ while ($cnt--) {
+ $side = (sqrt(1 + $side**2) - 1)/$side;
+ }
+ print "OK\n";
+
+The value of $side is
+
+ ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
+ undef], 1], ['n', 1]]
+
+Note that while we obtained this value using a nice little script,
+there is no simple way to I<use> this value. In fact this value may
+be inspected in debugger (see L<perldebug>), but only if
+C<bareStringify> B<O>ption is set, and not via C<p> command.
+
+If one attempts to print this value, then the overloaded operator
+C<""> will be called, which will call C<nomethod> operator. The
+result of this operator will be stringified again, but this result is
+again of type C<symbolic>, which will lead to an infinite loop.
+
+Add a pretty-printer method to the module F<symbolic.pm>:
+
+ sub pretty {
+ my ($meth, $a, $b) = @{+shift};
+ $a = 'u' unless defined $a;
+ $b = 'u' unless defined $b;
+ $a = $a->pretty if ref $a;
+ $b = $b->pretty if ref $b;
+ "[$meth $a $b]";
+ }
+
+Now one can finish the script by
+
+ print "side = ", $side->pretty, "\n";
+
+The method C<pretty> is doing object-to-string conversion, so it
+is natural to overload the operator C<""> using this method. However,
+inside such a method it is not necessary to pretty-print the
+I<components> $a and $b of an object. In the above subroutine
+C<"[$meth $a $b]"> is a catenation of some strings and components $a
+and $b. If these components use overloading, the catenation operator
+will look for an overloaded operator C<.>; if not present, it will
+look for an overloaded operator C<"">. Thus it is enough to use
+
+ use overload nomethod => \&wrap, '""' => \&str;
+ sub str {
+ my ($meth, $a, $b) = @{+shift};
+ $a = 'u' unless defined $a;
+ $b = 'u' unless defined $b;
+ "[$meth $a $b]";
+ }
+
+Now one can change the last line of the script to
+
+ print "side = $side\n";
+
+which outputs
+
+ side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
+
+and one can inspect the value in debugger using all the possible
+methods.
+
+Something is still amiss: consider the loop variable $cnt of the
+script. It was a number, not an object. We cannot make this value of
+type C<symbolic>, since then the loop will not terminate.
+
+Indeed, to terminate the cycle, the $cnt should become false.
+However, the operator C<bool> for checking falsity is overloaded (this
+time via overloaded C<"">), and returns a long string, thus any object
+of type C<symbolic> is true. To overcome this, we need a way to
+compare an object to 0. In fact, it is easier to write a numeric
+conversion routine.
+
+Here is the text of F<symbolic.pm> with such a routine added (and
+slightly modified str()):
+
+ package symbolic; # Primitive symbolic calculator
+ use overload
+ nomethod => \&wrap, '""' => \&str, '0+' => \#
+
+ sub new { shift; bless ['n', @_] }
+ sub wrap {
+ my ($obj, $other, $inv, $meth) = @_;
+ ($obj, $other) = ($other, $obj) if $inv;
+ bless [$meth, $obj, $other];
+ }
+ sub str {
+ my ($meth, $a, $b) = @{+shift};
+ $a = 'u' unless defined $a;
+ if (defined $b) {
+ "[$meth $a $b]";
+ } else {
+ "[$meth $a]";
+ }
+ }
+ my %subr = ( n => sub {$_[0]},
+ sqrt => sub {sqrt $_[0]},
+ '-' => sub {shift() - shift()},
+ '+' => sub {shift() + shift()},
+ '/' => sub {shift() / shift()},
+ '*' => sub {shift() * shift()},
+ '**' => sub {shift() ** shift()},
+ );
+ sub num {
+ my ($meth, $a, $b) = @{+shift};
+ my $subr = $subr{$meth}
+ or die "Do not know how to ($meth) in symbolic";
+ $a = $a->num if ref $a eq __PACKAGE__;
+ $b = $b->num if ref $b eq __PACKAGE__;
+ $subr->($a,$b);
+ }
+
+All the work of numeric conversion is done in %subr and num(). Of
+course, %subr is not complete, it contains only operators used in the
+example below. Here is the extra-credit question: why do we need an
+explicit recursion in num()? (Answer is at the end of this section.)
+
+Use this module like this:
+
+ require symbolic;
+ my $iter = symbolic->new(2); # 16-gon
+ my $side = symbolic->new(1);
+ my $cnt = $iter;
+
+ while ($cnt) {
+ $cnt = $cnt - 1; # Mutator '--' not implemented
+ $side = (sqrt(1 + $side**2) - 1)/$side;
+ }
+ printf "%s=%f\n", $side, $side;
+ printf "pi=%f\n", $side*(2**($iter+2));
+
+It prints (without so many line breaks)
+
+ [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
+ [n 1]] 2]]] 1]
+ [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
+ pi=3.182598
+
+The above module is very primitive. It does not implement
+mutator methods (C<++>, C<-=> and so on), does not do deep copying
+(not required without mutators!), and implements only those arithmetic
+operations which are used in the example.
+
+To implement most arithmetic operations is easy; one should just use
+the tables of operations, and change the code which fills %subr to
+
+ my %subr = ( 'n' => sub {$_[0]} );
+ foreach my $op (split " ", $overload::ops{with_assign}) {
+ $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
+ }
+ my @bins = qw(binary 3way_comparison num_comparison str_comparison);
+ foreach my $op (split " ", "@overload::ops{ @bins }") {
+ $subr{$op} = eval "sub {shift() $op shift()}";
+ }
+ foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
+ print "defining '$op'\n";
+ $subr{$op} = eval "sub {$op shift()}";
+ }
+
+Since subroutines implementing assignment operators are not required
+to modify their operands (see L<Overloadable Operations> above),
+we do not need anything special to make C<+=> and friends work,
+besides adding these operators to %subr and defining a copy
+constructor (needed since Perl has no way to know that the
+implementation of C<'+='> does not mutate the argument -
+see L<Copy Constructor>).
+
+To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
+line, and code (this code assumes that mutators change things one level
+deep only, so recursive copying is not needed):
+
+ sub cpy {
+ my $self = shift;
+ bless [@$self], ref $self;
+ }
+
+To make C<++> and C<--> work, we need to implement actual mutators,
+either directly, or in C<nomethod>. We continue to do things inside
+C<nomethod>, thus add
+
+ if ($meth eq '++' or $meth eq '--') {
+ @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
+ return $obj;
+ }
+
+after the first line of wrap(). This is not a most effective
+implementation, one may consider
+
+ sub inc { $_[0] = bless ['++', shift, 1]; }
+
+instead.
+
+As a final remark, note that one can fill %subr by
+
+ my %subr = ( 'n' => sub {$_[0]} );
+ foreach my $op (split " ", $overload::ops{with_assign}) {
+ $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
+ }
+ my @bins = qw(binary 3way_comparison num_comparison str_comparison);
+ foreach my $op (split " ", "@overload::ops{ @bins }") {
+ $subr{$op} = eval "sub {shift() $op shift()}";
+ }
+ foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
+ $subr{$op} = eval "sub {$op shift()}";
+ }
+ $subr{'++'} = $subr{'+'};
+ $subr{'--'} = $subr{'-'};
+
+This finishes implementation of a primitive symbolic calculator in
+50 lines of Perl code. Since the numeric values of subexpressions
+are not cached, the calculator is very slow.
+
+Here is the answer for the exercise: In the case of str(), we need no
+explicit recursion since the overloaded C<.>-operator will fall back
+to an existing overloaded operator C<"">. Overloaded arithmetic
+operators I<do not> fall back to numeric conversion if C<fallback> is
+not explicitly requested. Thus without an explicit recursion num()
+would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
+the argument of num().
+
+If you wonder why defaults for conversion are different for str() and
+num(), note how easy it was to write the symbolic calculator. This
+simplicity is due to an appropriate choice of defaults. One extra
+note: due to the explicit recursion num() is more fragile than sym():
+we need to explicitly check for the type of $a and $b. If components
+$a and $b happen to be of some related type, this may lead to problems.
+
+=head2 I<Really> Symbolic Calculator
+
+One may wonder why we call the above calculator symbolic. The reason
+is that the actual calculation of the value of expression is postponed
+until the value is I<used>.
+
+To see it in action, add a method
+
+ sub STORE {
+ my $obj = shift;
+ $#$obj = 1;
+ @$obj->[0,1] = ('=', shift);
+ }
+
+to the package C<symbolic>. After this change one can do
+
+ my $a = symbolic->new(3);
+ my $b = symbolic->new(4);
+ my $c = sqrt($a**2 + $b**2);
+
+and the numeric value of $c becomes 5. However, after calling
+
+ $a->STORE(12); $b->STORE(5);
+
+the numeric value of $c becomes 13. There is no doubt now that the module
+symbolic provides a I<symbolic> calculator indeed.
+
+To hide the rough edges under the hood, provide a tie()d interface to the
+package C<symbolic>. Add methods
+
+ sub TIESCALAR { my $pack = shift; $pack->new(@_) }
+ sub FETCH { shift }
+ sub nop { } # Around a bug
+
+(the bug, fixed in Perl 5.14, is described in L<"BUGS">). One can use this
+new interface as
+
+ tie $a, 'symbolic', 3;
+ tie $b, 'symbolic', 4;
+ $a->nop; $b->nop; # Around a bug
+
+ my $c = sqrt($a**2 + $b**2);
+
+Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
+of $c becomes 13. To insulate the user of the module add a method
+
+ sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
+
+Now
+
+ my ($a, $b);
+ symbolic->vars($a, $b);
+ my $c = sqrt($a**2 + $b**2);
+
+ $a = 3; $b = 4;
+ printf "c5 %s=%f\n", $c, $c;
+
+ $a = 12; $b = 5;
+ printf "c13 %s=%f\n", $c, $c;
+
+shows that the numeric value of $c follows changes to the values of $a
+and $b.
+
=head1 AUTHOR
Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
+=head1 SEE ALSO
+
+The C<overloading> pragma can be used to enable or disable overloaded
+operations within a lexical scope - see L<overloading>.
+
=head1 DIAGNOSTICS
When Perl is run with the B<-Do> switch or its equivalent, overloading
is shown by debugger. The method C<()> corresponds to the C<fallback>
key (in fact a presence of this method shows that this package has
overloading enabled, and it is what is used by the C<Overloaded>
-function).
+function of module C<overload>).
+
+The module might issue the following warnings:
+
+=over 4
-=head1 BUGS
+=item Odd number of arguments for overload::constant
-Because it is used for overloading, the per-package associative array
-%OVERLOAD now has a special meaning in Perl. The symbol table is
-filled with names looking like line-noise.
+(W) The call to overload::constant contained an odd number of arguments.
+The arguments should come in pairs.
+
+=item '%s' is not an overloadable type
+
+(W) You tried to overload a constant type the overload package is unaware of.
+
+=item '%s' is not a code reference
+
+(W) The second (fourth, sixth, ...) argument of overload::constant needs
+to be a code reference. Either an anonymous subroutine, or a reference
+to a subroutine.
+
+=back
+
+=head1 BUGS AND PITFALLS
+
+=over
+
+=item *
+
+No warning is issued for invalid C<use overload> keys.
+Such errors are not always obvious:
+
+ use overload "+0" => sub { ...; }, # should be "0+"
+ "not" => sub { ...; }; # should be "!"
+
+(Bug #74098)
+
+=item *
+
+A pitfall when fallback is TRUE and Perl resorts to a built-in
+implementation of an operator is that some operators have more
+than one semantic, for example C<|>:
+
+ use overload '0+' => sub { $_[0]->{n}; },
+ fallback => 1;
+ my $x = bless { n => 4 }, "main";
+ my $y = bless { n => 8 }, "main";
+ print $x | $y, "\n";
+
+You might expect this to output "12".
+In fact, it prints "<": the ASCII result of treating "|"
+as a bitwise string operator - that is, the result of treating
+the operands as the strings "4" and "8" rather than numbers.
+The fact that numify (C<0+>) is implemented but stringify
+(C<"">) isn't makes no difference since the latter is simply
+autogenerated from the former.
+
+The only way to change this is to provide your own subroutine
+for C<'|'>.
+
+=item *
+
+Magic autogeneration increases the potential for inadvertently
+creating self-referential structures.
+Currently Perl will not free self-referential
+structures until cycles are explicitly broken.
+For example,
+
+ use overload '+' => 'add';
+ sub add { bless [ \$_[0], \$_[1] ] };
+
+is asking for trouble, since
+
+ $obj += $y;
+
+will effectively become
+
+ $obj = add($obj, $y, undef);
+
+with the same result as
+
+ $obj = [\$obj, \$foo];
+
+Even if no I<explicit> assignment-variants of operators are present in
+the script, they may be generated by the optimizer.
+For example,
+
+ "obj = $obj\n"
+
+may be optimized to
+
+ my $tmp = 'obj = ' . $obj; $tmp .= "\n";
+
+=item *
+
+Because it is used for overloading, the per-package hash
+C<%OVERLOAD> now has a special meaning in Perl.
+The symbol table is filled with names looking like line-noise.
+
+=item *
For the purpose of inheritance every overloaded package behaves as if
C<fallback> is present (possibly undefined). This may create
interesting effects if some package is not overloaded, but inherits
from two overloaded packages.
-This document is confusing.
+=item *
+
+Before Perl 5.14, the relation between overloading and tie()ing was broken.
+Overloading is triggered or not basing on the I<previous> class of the
+tie()d variable.
+
+This happened because the presence of overloading was checked
+too early, before any tie()d access was attempted. If the
+class of the value FETCH()ed from the tied variable does not
+change, a simple workaround for code that is to run on older Perl
+versions is to access the value (via C<() = $foo> or some such)
+immediately after tie()ing, so that after this call the I<previous> class
+coincides with the current one.
+
+=item *
+
+Barewords are not covered by overloaded string constants.
+
+=back
=cut
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