overload.pm

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=head1 SPECIAL SYMBOLS FOR C<use overload>

Three keys are recognized by Perl that are not covered by the above
description.

=head2 Last Resort

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

	&nomethodMethod($a,1,1,"-")

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

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">:

=over 16

=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.

=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

No autogeneration is tried.  Perl tries to call
C<"nomethod"> value, and if this is missing, raises an exception.

=back

B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
yet, see L<"Inheritance and overloading">.

=head2 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.

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;
	++$a;

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<'+='> (or
C<nomethod>).  Note that if this operation is expressed via C<'+'>
a nonmutator, i.e., as in

	$a=$b;
	$a=$a+1;

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.

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.

=over 5

=item B<Example>

The actually executed code for

	$a=$b;
        Something else which does not modify $a or $b....
	++$a;

may be

	$a=$b;
        Something else which does not modify $a or $b....
	$a = $a->clone(undef,"");
        $a->incr(undef,"");

if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
C<'='> was overloaded with C<\&clone>.

=back

Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
C<$b = $a; ++$a>.

=head1 MAGIC AUTOGENERATION

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:

=over 16

=item I<Assignment forms of arithmetic operations>

C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
is not defined.

=item I<Conversion operations>

String, numeric, and boolean conversion are calculated in terms of one
another if not all of them are defined.

=item I<Increment and decrement>

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 C<abs($a)>

can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).

=item I<Unary minus>

can be expressed in terms of subtraction.

=item I<Negation>

C<!> and C<not> can be expressed in terms of boolean conversion, or
string or numerical conversion.

=item I<Concatenation>

can be expressed in terms of string conversion.

=item I<Comparison operations>

can be expressed in terms of its "spaceship" counterpart: either
C<E<lt>=E<gt>> or C<cmp>:

    <, >, <=, >=, ==, != 	in terms of <=>
    lt, gt, le, ge, eq, ne 	in terms of cmp

=item I<Iterator>

    <>				in terms of builtin operations

=item I<Dereferencing>

    ${} @{} %{} &{} *{}		in terms of builtin operations

=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.

=back

=head1 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>'
function will produce only a standard logical value based on the
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).

Similarly, C<.=>  and C<x=> operators lose their mathemagical properties
if the string conversion substitution is applied.

When you chop() a mathemagical object it is promoted to a string and its
mathemagical properties are lost.  The same can happen with other
operations as well.

=head1 Run-time Overloading

Since all C<use> directives are executed at compile-time, the only way to
change overloading during run-time is to

    eval 'use overload "+" => \&addmethod';

You can also use

    eval 'no overload "+", "--", "<="';

though the use of these constructs during run-time is questionable.

=head1 Public functions

Package C<overload.pm> provides the following public functions:

=over 5

=item overload::StrVal(arg)

Gives string value of C<arg> as in absence of stringify overloading.

=item overload::Overloaded(arg)

Returns true if C<arg> is subject to overloading of some operations.

=item overload::Method(obj,op)

Returns C<undef> or a reference to the method that implements C<op>.

=back

=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.

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)};
	}

B<BUGS> Currently overloaded-ness of constants does not propagate
into C<eval '...'>.

=head1 IMPLEMENTATION

What follows is subject to change RSN.

The table of methods for all operations is cached in magic for the
symbol table hash for the package.  The cache is invalidated during
processing of C<use overload>, C<no overload>, new function
definitions, and changes in @ISA. However, this invalidation remains
unprocessed until the next C<bless>ing into the package. Hence if you
want to change overloading structure dynamically, you'll need an
additional (fake) C<bless>ing to update the table.

(Every SVish thing has a magic queue, and magic is an entry in that
queue.  This is how a single variable may participate in multiple
forms of magic simultaneously.  For instance, environment variables
regularly have two forms at once: their %ENV magic and their taint
magic. However, the magic which implements overloading is applied to
the stashes, which are rarely used directly, thus should not slow down
Perl.)

If an object belongs to a package using overload, it carries a special
flag.  Thus the only speed penalty during arithmetic operations without
overloading is the checking of this flag.

In fact, if C<use overload> is not present, there is almost no overhead
for overloadable operations, so most programs should not suffer
measurable performance penalties.  A considerable effort was made to
minimize the overhead when overload is used in some package, but the
arguments in question do not belong to packages using overload.  When
in doubt, test your speed with C<use overload> and without it.  So far
there have been no reports of substantial speed degradation if Perl is
compiled with optimization turned on.

There is no size penalty for data if overload is not used. The only
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 table 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">).

It is expected that arguments to methods that are not explicitly supposed
to be changed are constant (but this is not enforced).

=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.

Here is a Perl object metaphor:

I<  object is a reference to blessed data>

and an arithmetic metaphor:

I<  object is a thing by itself>.

The I<main> problem of overloading C<=> is the fact that these metaphors
imply different actions on the assignment C<$a = $b> if $a and $b are
objects.  Perl-think implies that $a becomes a reference to whatever
$b was referencing.  Arithmetic-think implies that the value of "object"
$a is changed to become the value of the object $b, preserving the fact
that $a and $b are separate entities.

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 $b too.  Effectively, after
C<$a = $b> values of $a and $b become I<indistinguishable>.

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
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
same value:

    $a = new Data 23;
    ...
    $b = $a;		# $b is "linked" to $a
    ...
    $a = $a->clone;	# Unlink $b from $a
    $a->increment_by(4);

Note that overloaded access makes this transparent:

    $a = new Data 23;
    $b = $a;		# $b is "linked" to $a
    $a += 4;		# would unlink $b automagically

However, it would not make

    $a = new Data 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
value of the object, one can reproduce the arithmetic metaphor in its
completeness, at least for variables which were tie()d from the start.

(Note that a workaround for a bug may be needed, see L<"BUGS">.)

=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 $seven = new two_face ("vii", 7);
  printf "seven=$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, similar to the
L<pseudo-hash|perlref/"Pseudo-hashes: Using an array as a hash">
builtin Perl type.  Let's make it better than a pseudo-hash by
allowing index 0 to be treated as a normal element.

  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: