package overload;
+our $VERSION = '1.10';
+
sub nil {}
sub OVERLOAD {
my %arg = @_;
my ($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') {
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("$class_prefix$type(0x%x)", $addr);
}
+*StrVal = *AddrRef;
+
sub mycan { # Real can would leave stubs.
my ($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 &{$fqmeth};
}
+
return undef;
}
%constants = (
- 'integer' => 0x1000,
- 'float' => 0x2000,
- 'binary' => 0x4000,
- 'q' => 0x8000,
- 'qr' => 0x10000,
+ 'integer' => 0x1000, # HINT_NEW_INTEGER
+ 'float' => 0x2000, # HINT_NEW_FLOAT
+ 'binary' => 0x4000, # HINT_NEW_BINARY
+ 'q' => 0x8000, # HINT_NEW_STRING
+ 'qr' => 0x10000, # HINT_NEW_RE
);
%ops = ( with_assign => "+ - * / % ** << >> x .",
assign => "+= -= *= /= %= **= <<= >>= x= .=",
- str_comparison => "< <= > >= == !=",
+ num_comparison => "< <= > >= == !=",
'3way_comparison'=> "<=> cmp",
- num_comparison => "lt le gt ge eq ne",
- binary => "& | ^",
+ str_comparison => "lt le gt ge eq ne",
+ binary => '& &= | |= ^ ^=',
unary => "neg ! ~",
mutators => '++ --',
- func => "atan2 cos sin exp abs log sqrt",
- conversion => 'bool "" 0+',
+ func => "atan2 cos sin exp abs log sqrt int",
+ conversion => 'bool "" 0+ qr',
iterators => '<>',
+ filetest => "-X",
dereferencing => '${} @{} %{} &{} *{}',
+ matching => '~~',
special => 'nomethod fallback =');
+use warnings::register;
sub constant {
# Arguments: what, sub
while (@_) {
- $^H{$_[0]} = $_[1];
- $^H |= $constants{$_[0]} | 0x20000;
+ 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;
}
}
__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;
+ $a = SomeThing->new( 57 );
$b=5+$a;
...
if (overload::Overloaded $b) {...}
=head1 DESCRIPTION
+This pragma allows overloading of Perl's operators for a class.
+To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead.
+
=head2 Declaration of overloaded functions
The compilation directive
package Number;
use overload
- "+" => \&add,
+ "+" => \&add,
"*=" => "muas";
declares function Number::add() for addition, and method muas() in
the "class" C<Number> (or one of its base classes)
-for the assignment form C<*=> of multiplication.
+for the assignment form C<*=> of multiplication.
Arguments of this directive come in (key, value) pairs. Legal values
are values legal inside a C<&{ ... }> call, so the name of a
=back
-B<Warning.> Due to the presense of assignment versions of operations,
-routines which may be called in assignment context may create
-self-referential structures. Currently Perl will not free self-referential
+B<Warning.> Due to the presence of assignment versions of operations,
+routines which may be called in assignment context may create
+self-referential structures. Currently Perl will not free self-referential
structures until cycles are C<explicitly> broken. You may get problems
when traversing your structures too.
-Say,
+Say,
use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
is asking for trouble, since for code C<$obj += $foo> the subroutine
-is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
+is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
\$foo]>. If using such a subroutine is an important optimization, one
can overload C<+=> explicitly by a non-"optimized" version, or switch
-to non-optimized version if C<not defined $_[2]> (see
+to non-optimized version if C<not defined $_[2]> (see
L<Calling Conventions for Binary Operations>).
Even if no I<explicit> assignment-variants of operators are present in
"**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
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
+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>.
See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
=item * I<Bit operations>
- "&", "^", "|", "neg", "!", "~",
+ "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
-"C<neg>" stands for unary minus. If the method for C<neg> is not
+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+>".
+subtraction. If the method for C<!> is not specified, it can be
+autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
+
+The same remarks in L<"Arithmetic operations"> about
+assignment-variants and autogeneration apply for
+bit operations C<"&">, C<"^">, and C<"|"> as well.
=item * I<Increment and decrement>
=item * I<Transcendental functions>
- "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
+ "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
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.
-=item * I<Boolean, string and numeric conversion>
+Note that traditionally the Perl function L<int> rounds to 0, thus for
+floating-point-like types one should follow the same semantic. If
+C<int> is unavailable, it can be autogenerated using the overloading of
+C<0+>.
+
+=item * I<Boolean, string, numeric and regexp conversions>
- "bool", "\"\"", "0+",
+ 'bool', '""', '0+', 'qr'
-If one or two of these operations are not overloaded, 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 one or two of these operations are not overloaded, 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; C<qr> is used for
+the RHS of C<=~> and when an object is interpolated into a regexp.
+
+C<bool>, C<"">, and C<0+> 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. 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.
+
+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)>.
=item * I<Iteration>
happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
I<globbing> syntax C<E<lt>${var}E<gt>>.
+B<BUGS> Even in list context, the iterator is currently called only
+once and with scalar context.
+
+=item * I<File tests>
+
+ "-X"
+
+This overload is used for all the filetest operators (C<-f>, C<-x> and
+so on: see L<perlfunc/-X> for the full list). Even though these are
+unary operators, the method will be called with a second argument which
+is a single letter indicating which test was performed. Note that the
+overload key is the literal string C<"-X">: you can't provide separate
+overloads for the different tests.
+
+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.
+
+If not overloaded, these operators will fall back to the default
+behaviour even without C<< fallback => 1 >>. This means that if the
+object is a blessed glob or blessed IO ref it will be treated as a
+filehandle, otherwise string overloading will be invoked and the result
+treated as a filename.
+
+This overload was introduced in perl 5.12.
+
+=item * I<Matching>
+
+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> and L<feature>.
+
+Unusually, overloading of the smart match operator does not automatically
+take precedence over normal smart match behaviour. In particular, in the
+following code:
+
+ package Foo;
+ use overload '~~' => 'match';
+
+ my $obj = Foo->new();
+ $obj ~~ [ 1,2,3 ];
+
+the smart match does I<not> invoke the method call like this:
+
+ $obj->match([1,2,3],0);
+
+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:
+
+ $obj->match(1,0);
+ $obj->match(2,0);
+ $obj->match(3,0);
+
+Consult the match table in L<perlsyn/"Smart matching in detail"> for
+details of when overloading is invoked.
+
=item * I<Dereferencing>
'${}', '@{}', '%{}', '&{}', '*{}'.
should be of correct type. These functions should return a reference
of correct type, or another object with overloaded dereferencing.
+As a special case if the overload returns the object itself then it
+will be used directly (provided it is the correct type).
+
+The dereference operators must be specified explicitly they will not be passed to
+"nomethod".
+
=item * I<Special>
- "nomethod", "fallback", "=",
+ "nomethod", "fallback", "=".
see L<SPECIAL SYMBOLS FOR C<use overload>>.
with_assign => '+ - * / % ** << >> x .',
assign => '+= -= *= /= %= **= <<= >>= x= .=',
- str_comparison => '< <= > >= == !=',
+ num_comparison => '< <= > >= == !=',
'3way_comparison'=> '<=> cmp',
- num_comparison => 'lt le gt ge eq ne',
- binary => '& | ^',
+ str_comparison => 'lt le gt ge eq ne',
+ binary => '& &= | |= ^ ^=',
unary => 'neg ! ~',
mutators => '++ --',
func => 'atan2 cos sin exp abs log sqrt',
- conversion => 'bool "" 0+',
+ conversion => 'bool "" 0+ qr',
iterators => '<>',
+ filetest => '-X',
dereferencing => '${} @{} %{} &{} *{}',
+ matching => '~~',
special => 'nomethod fallback ='
=head2 Inheritance and overloading
if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
C<use overload> directive.
+The C<"nomethod"> mechanism is I<not> used for the dereference operators
+( ${} @{} %{} &{} *{} ).
+
+
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.
-=head2 Fallback
+
+=head2 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
=item * defined, but FALSE
No autogeneration is tried. Perl tries to call
-C<"nomethod"> value, and if this is missing, raises an exception.
+C<"nomethod"> value, and if this is missing, raises an exception.
=back
to a reference that shares its object with some other reference, such
as
- $a=$b;
+ $a=$b;
++$a;
To make this change $a and not change $b, a copy of C<$$a> is made,
C<nomethod>). Note that if this operation is expressed via C<'+'>
a nonmutator, i.e., as in
- $a=$b;
+ $a=$b;
$a=$a+1;
then C<$a> does not reference a new copy of C<$$a>, since $$a does not
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.
+string copy if the object is a plain scalar or a simple assignment if it
+is not.
=over 5
=item B<Example>
-The actually executed code for
+The actually executed code for
- $a=$b;
+ $a=$b;
Something else which does not modify $a or $b....
++$a;
may be
- $a=$b;
+ $a=$b;
Something else which does not modify $a or $b....
$a = $a->clone(undef,"");
$a->incr(undef,"");
C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
is not defined.
-=item I<Conversion operations>
+=item I<Conversion operations>
-String, numeric, and boolean conversion are calculated in terms of one
-another if not all of them are defined.
+String, numeric, boolean and regexp conversions are calculated in terms
+of one another if not all of them are defined.
=item I<Increment and decrement>
can be expressed in terms of string conversion.
-=item I<Comparison operations>
+=item I<Comparison operations>
can be expressed in terms of its "spaceship" counterpart: either
C<E<lt>=E<gt>> or C<cmp>:
=item I<Copy operator>
can be expressed in terms of an assignment to the dereferenced value, if this
-value is a scalar and not a reference.
+value is a scalar and not a reference, or simply a reference assignment
+otherwise.
=back
+=head1 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. See L<MAGIC AUTOGENERATION> above. The minimal set is:
+
+ + - * / % ** << >> x
+ <=> cmp
+ & | ^ ~
+ atan2 cos sin exp log sqrt int
+
+Additionally, you need to define at least one of string, boolean or
+numeric conversions because any one can be used to emulate the others.
+The string conversion can also be used to emulate concatenation.
+
=head1 Losing overloading
The restriction for the comparison operation is that even if, for example,
=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)
=head1 Overloading constants
-For some application Perl parser mangles constants too much. It is possible
-to hook into this process via overload::constant() and overload::remove_constant()
-functions.
+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
+are:
=over 8
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.
+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
+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
+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,
+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.
+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()
overload::constant integer => sub {Math::BigInt->new(shift)};
}
-B<BUGS> Currently overloaded-ness of constants does not propagate
-into C<eval '...'>.
-
=head1 IMPLEMENTATION
What follows is subject to change RSN.
package. This magic is three-words-long for packages without
overloading, and carries the cache table if the package is overloaded.
-Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
+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">).
=head1 Metaphor clash
One may wonder why the semantic of overloaded C<=> is so counter intuitive.
-If it I<looks> counter intuitive to you, you are subject to a metaphor
-clash.
+If it I<looks> counter intuitive to you, you are subject to a metaphor
+clash.
Here is a Perl object metaphor:
The difference is not relevant in the absence of mutators. After
a Perl-way assignment an operation which mutates the data referenced by $a
-would change the data referenced by $b too. Effectively, after
+would change the data referenced by $b too. Effectively, after
C<$a = $b> values of $a and $b become I<indistinguishable>.
-On the other hand, anyone who has used algebraic notation knows the
+On the other hand, anyone who has used algebraic notation knows the
expressive power of the arithmetic metaphor. Overloading works hard
to enable this metaphor while preserving the Perlian way as far as
-possible. Since it is not not possible to freely mix two contradicting
+possible. Since it is not possible to freely mix two contradicting
metaphors, overloading allows the arithmetic way to write things I<as
far as all the mutators are called via overloaded access only>. The
way it is done is described in L<Copy Constructor>.
If some mutator methods are directly applied to the overloaded values,
-one may need to I<explicitly unlink> other values which references the
+one may need to I<explicitly unlink> other values which references the
same value:
- $a = new Data 23;
+ $a = Data->new(23);
...
$b = $a; # $b is "linked" to $a
...
Note that overloaded access makes this transparent:
- $a = new Data 23;
+ $a = Data->new(23);
$b = $a; # $b is "linked" to $a
$a += 4; # would unlink $b automagically
However, it would not make
- $a = new Data 23;
+ $a = Data->new(23);
$a = 4; # Now $a is a plain 4, not 'Data'
preserve "objectness" of $a. But Perl I<has> a way to make assignments
to an object do whatever you want. It is just not the overload, but
tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
-which returns the object itself, and STORE() method which changes the
+which returns the object itself, and STORE() method which changes the
value of the object, one can reproduce the arithmetic metaphor in its
completeness, at least for variables which were tie()d from the start.
Use it as follows:
require two_face;
- my $seven = new two_face ("vii", 7);
+ 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/;
=head2 Two-face references
Suppose you want to create an object which is accessible as both an
-array reference, and a hash reference, similar to the builtin
-L<array-accessible-as-a-hash|perlref/"Pseudo-hashes: Using an array as
-a hash"> builtin Perl type. Let us make it better than the builtin
-type, there will be no restriction that you cannot use the index 0 of
-your array.
+array reference and a hash reference.
package two_refs;
use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
- sub new {
- my $p = shift;
+ sub new {
+ my $p = shift;
bless \ [@_], $p;
}
sub gethash {
my %fields;
my $i = 0;
$fields{$_} = $i++ foreach qw{zero one two three};
- sub STORE {
+ sub STORE {
my $self = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$$self->[$key] = shift;
}
- sub FETCH {
+ sub FETCH {
my $self = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
Now one can access an object using both the array and hash syntax:
- my $bar = new two_refs 3,4,5,6;
+ my $bar = two_refs->new(3,4,5,6);
$bar->[2] = 11;
$bar->{two} == 11 or die 'bad hash fetch';
Second, we create a new tied hash each time the hash syntax is used.
This allows us not to worry about a possibility of a reference loop,
-would would lead to a memory leak.
+which would lead to a memory leak.
Both these problems can be cured. Say, if we want to overload hash
dereference on a reference to an object which is I<implemented> as a
hash itself, the only problem one has to circumvent is how to access
-this I<actual> hash (as opposed to the I<virtual> exhibited by
+this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
overloaded dereference operator). Here is one possible fetching routine:
sub access_hash {
my ($self, $key) = (shift, shift);
my $class = ref $self;
- bless $self, 'overload::dummy'; # Disable overloading of %{}
+ bless $self, 'overload::dummy'; # Disable overloading of %{}
my $out = $self->{$key};
bless $self, $class; # Restore overloading
$out;
}
-To move creation of the tied hash on each access, one may an extra
+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;
+ sub new {
+ my $p = shift;
my $a = [@_];
my %h;
tie %h, $p, $a;
my %fields;
my $i = 0;
$fields{$_} = $i++ foreach qw{zero one two three};
- sub STORE {
+ sub STORE {
my $a = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$a->[$key] = shift;
}
- sub FETCH {
+ sub FETCH {
my $a = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$a->[$key];
}
-Now if $baz is overloaded like this, then C<$bar> is a reference to a
+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 also a reference to a reference to the actual array, so
+hash is a reference to a reference to the actual array, so
=over
provide any usual overloaded operators, instead it provides the L<Last
Resort> operator C<nomethod>. In this example the corresponding
subroutine returns an object which encapsulates operations done over
-the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
-symbolic 3> contains C<['+', 2, ['n', 3]]>.
+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 = new symbolic 1;
+ my $side = symbolic->new(1);
my $cnt = $iter;
-
+
while ($cnt--) {
$side = (sqrt(1 + $side**2) - 1)/$side;
}
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 ony if
+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
$a = $a->pretty if ref $a;
$b = $b->pretty if ref $b;
"[$meth $a $b]";
- }
+ }
Now one can finish the script by
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
+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;
$a = 'u' unless defined $a;
$b = 'u' unless defined $b;
"[$meth $a $b]";
- }
+ }
Now one can change the last line of the script to
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.
+methods.
-Something is is still amiss: consider the loop variable $cnt of the
+Something is still amiss: consider the loop variable $cnt of the
script. It was a number, not an object. We cannot make this value of
type C<symbolic>, since then the loop will not terminate.
} else {
"[$meth $a]";
}
- }
- my %subr = ( n => sub {$_[0]},
- sqrt => sub {sqrt $_[0]},
+ }
+ my %subr = ( n => sub {$_[0]},
+ sqrt => sub {sqrt $_[0]},
'-' => sub {shift() - shift()},
'+' => sub {shift() + shift()},
'/' => sub {shift() / shift()},
);
sub num {
my ($meth, $a, $b) = @{+shift};
- my $subr = $subr{$meth}
+ 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__;
Use this module like this:
require symbolic;
- my $iter = new symbolic 2; # 16-gon
- my $side = new symbolic 1;
+ 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;
(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
+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 {$_[0]} );
way to know that the implementation of C<'+='> does not mutate
the argument, compare L<Copy Constructor>).
-To implement a copy constructor, add C<'=' => \&cpy> to C<use overload>
+To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
line, and code (this code assumes that mutators change things one level
deep only, so recursive copying is not needed):
bless [@$self], ref $self;
}
-To make C<++> and C<--> work, we need to implement actual mutators,
+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
return $obj;
}
-after the first line of wrap(). This is not a most effective
+after the first line of wrap(). This is not a most effective
implementation, one may consider
sub inc { $_[0] = bless ['++', shift, 1]; }
$subr{'++'} = $subr{'+'};
$subr{'--'} = $subr{'-'};
-This finishes implementation of a primitive symbolic calculator in
-50 lines of Perl code. Since the numeric values of subexpressions
+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
To see it in action, add a method
- sub STORE {
- my $obj = shift;
- $#$obj = 1;
+ sub STORE {
+ my $obj = shift;
+ $#$obj = 1;
@$obj->[0,1] = ('=', shift);
}
to the package C<symbolic>. After this change one can do
- my $a = new symbolic 3;
- my $b = new symbolic 4;
+ 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
Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
+=head1 SEE ALSO
+
+The L<overloading> pragma can be used to enable or disable overloaded
+operations within a lexical scope.
+
=head1 DIAGNOSTICS
When Perl is run with the B<-Do> switch or its equivalent, overloading
overloading enabled, and it is what is used by the C<Overloaded>
function of module C<overload>).
+The module might issue the following warnings:
+
+=over 4
+
+=item Odd number of arguments for overload::constant
+
+(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
Because it is used for overloading, the per-package hash %OVERLOAD now
interesting effects if some package is not overloaded, but inherits
from two overloaded packages.
-Relation between overloading and tie()ing is broken. Overloading is
+Relation between overloading and tie()ing is broken. Overloading is
triggered or not basing on the I<previous> class of tie()d value.
-This happens because the presence of overloading is checked too early,
+This happens because the presence of overloading is checked too early,
before any tie()d access is attempted. If the FETCH()ed class of the
-tie()d value does not change, a simple workaround is to access the value
+tie()d value does not change, a simple workaround is to access the value
immediately after tie()ing, so that after this call the I<previous> class
coincides with the current one.
https://perl5.git.perl.org / perl5.git / blobdiff