perl 5.000

[p5sagit/p5-mst-13.2.git] / pod / perlovl.pod

=head1 NAME 

perlovl - perl overloading semantics

=head1 SYNOPSIS

    package SomeThing;

    %OVERLOAD = (
	'+' => \&myadd,
	'-' => \&mysub,
	# etc
    );
    ...

    package main;
    $a = new SomeThing 57;
    $b=5+$a;

=head1 CAVEAT SCRIPTOR

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.

=head1 DESCRIPTION

=head2 Declaration of overloaded functions

    package Number;
    %OVERLOAD = ( 
	"+" => \&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.  Legal values of this
hash array are values legal inside C<&{ ... }> call, so the name of a
subroutine, a reference to a subroutine, or an anonymous subroutine 
will all work.

The subroutine C<$OVERLOAD{"+"}> will be called to execute C<$a+$b> if $a
is a reference to an object blessed into the package C<Number>, or $a is
not an object from a package with defined mathemagic addition, but $b is a
reference to a C<Number>.  It can be called also in other situations, like
C<$a+=7>, or C<$a++>.  See L<MAGIC AUTOGENERATION>.  (Mathemagical
methods refer to methods triggered by an overloaded mathematical
operator.)

=head2 Calling Conventions for Binary Operations

The functions in C<values %OVERLOAD> 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 be always an object in the package,
so in the situation of C<7+$a>, the order of arguments is interchanged.
Most probably it does not matter for implementation of the addition
method, but whether the arguments are reversed is vital for the
subtraction method.  The subroutine can query this information by
examining the third argument, which can take three different values:

=over 7

=item FALSE

the order of arguments is as in the current operation.

=item TRUE

the arguments are reversed.

=item C<undef>

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.

=back

=head2 Calling Conventions for Unary Operations

Unary operation are considered binary operations with the second
argument being C<undef>.  Thus C<$OVERLOAD{"++"}> is called with
arguments C<($a,undef,'')> when $a++ is executed.

=head2 Overloadable Operations

The following keys of %OVERLOAD are recognized:

=over 5

=item * I<Arithmetic operations>

    "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
    "**", "**=", "<<", "<<=", ">>", ">>=", "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 operations "C<->" can be used to
autogenerate missing methods for unary minus or C<abs>.

=item * I<Comparison operations>

    "<",  "<=", ">",  ">=", "==", "!=", "<=>",
    "lt", "le", "gt", "ge", "eq", "ne", "cmp",

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 %OVERLOAD.

=item * I<Bit operations>

    "&", "^", "|", "neg", "!", "~",

"C<neg>" stands for unary minus.  If the method for C<neg> is not
specified, it can be autogenerated using on the method for subtraction.

=item * I<Increment and decrement>

    "++", "--",

If undefined, addition and subtraction methods can be
used instead.  These operations are called both in prefix and
postfix form.

=item * I<Transcendental functions>

    "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",

If C<abs> is unavailable, it can be autogenerated using methods
for "<" or "<=>" combined with either unary minus or subtraction.

=item * I<Boolean, string and numeric conversion>

    "bool", "\"\"", "0+",

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.

=item * I<Special>

    "nomethod", "fallback", "=",

see L<SPECIAL KEYS OF %OVERLOAD>.

=back

See L<"Fallback"> for an explanation of when a missing method can be autogenerated.

=head1 SPECIAL KEYS OF %OVERLOAD

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

=head2  Last Resort

C<$OVERLOAD{"nomethod"}> is a reference to a function of four parameters.
If defined, it is called when the overloading mechanism cannot find a
method for some operation.  The first three arguments of this function
coincide with arguments for the corresponding method if it were found, the
fourth argument is the key of %OVERLOAD corresponding to the missing
method.  If several methods are tried, the last one is used.  Say, C<1-$a>
can be equivalent to

	&{ $Pack::OVERLOAD{"nomethod"} }($a,1,1,"-").

If some operation cannot be resolved, and there is no
C<$OVERLOAD{"nomethod"}>, then an exception will be raised 
via die() -- unless C<$OVERLOAD{"fallback"}> is true. 

=head2 Fallback 

C<$OVERLOAD{"fallback"}> governs what to do if a method for a particular
operation is not found.  Three different cases are possible depending on
value of C<$OVERLOAD{"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<$OVERLOAD{"nomethod"}>; 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 %OVERLOAD is
present.

=item * defined, but FALSE

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

=back

=head2 Copy Constructor

C<$OVERLOAD{"="}> is a reference to a function with three arguments,
i.e., it looks like a usual value of %OVERLOAD.  What is special about
this subroutine is that it should not return a blessed reference into
a package (as most other methods are expected to), but rather a freshly made
copy of its dereferenced argument (see L<"BUGS">, though).  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 to $a and not to change $b, a freshly made copy of
C<$$a> is made, and $a is assigned a reference to this object.  This
operation is executed during C<$a++>, (so before this C<$$a> coincides
with C<$$b>), and only if C<++> is expressed via  C<$OPERATOR{'++'}> or
C<$OPERATOR{'+='}>.  Note that if this operation is expressed via 'C<+>',
i.e., as

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

then C<$$a> and C<$$b> do not appear as lvalues.

If the copy constructor is required during execution of some mutator, but
C<$OPERATOR{'='}> is missing, it can be autogenerated as a string
copy if an object of
the package is a plain scalar.

=head1 MAGIC AUTOGENERATION

If a method for an operation is not found, and C<$OVERLOAD{"fallback"}> is
TRUE or undefined, Perl tries to to autogenerate a substitute method for
the missing operation based on 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 C<$OVERLOAD{"+"}> method if C<$OVERLOAD{"+="}>
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<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<Copy operator>

can be expressed in terms of assignment to the dereferenced value, if this
value is scalar but not a reference.

=back

=head1 WARNING

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 becomes promoted to a string
first, and its mathemagical qualities is lost.  The same can happen with other
operations as well.

=head1 IMPLEMENTATION

The table of methods for all operations is cached as a magic for the
symbol table hash of the package.  It is rechecked for changes of
%OVERLOAD and @ISA only during C<bless>ing; so if it is changed
dynamically, you'll need an additional fake C<bless>ing to update the
table.

(Every SVish thing has a magic queue, and a 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.)

If an object belongs to a package with %OVERLOAD, it carries a special
flag.  Thus the only speed penalty during arithmetic operations without
overload is the check of this flag.

In fact, if no %OVERLOAD is ever accessed, there is almost no overhead for
overloadable operations, so most programs should not suffer measurable
performance penalties.  Considerable effort was made minimize overhead
when %OVERLOAD is accessed and the current operation is overloadable but
the arguments in question do not belong to packages with %OVERLOAD.  When
in doubt, test your speed with %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 there is no %OVERLOAD. 

The copying like 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 $b (or $a) refers to, like C<$b++>.  You can override this
behavior by defining your 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 AUTHOR

Ilya Zakharevich <F<ilya@math.mps.ohio-state.edu>>.

=head1 DIAGNOSTICS

When Perl is run with the B<-Do> switch or its equivalent, overloading
induces diagnostic messages.

=head1 BUGS

Because it's used for overloading, the per-package associative array
%OVERLOAD now has a special meaning in Perl.

Although the copy constructor is specially designed to make overloading
operations with references to an array simpler, as it now works it's
useless for this because a subroutine cannot return an array in the same
way as it returns a scalar (from the point of view of Perl
internals).  Expect a change of interface for the copy constructor.

As shipped, %OVERLOAD is not inherited via the @ISA tree.  A patch for
this is available from the author.

This document is confusing.