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=head1 TITLE
Deep Copying, aka, cloning around.
=head1 VERSION
Maintainer: Peter Scott <[EMAIL PROTECTED]>
Date: 8 Aug 2000
Version: 1
Mailing List: [EMAIL PROTECTED]
Number: 67
=head1 ABSTRACT
Perl should have a C<clone> method for deep copying of hierarchical data
structures.
Damian Conway mooted a C<clone> function for deep copying of objects as
part of an earlier discussion. This RFC expands upon that and gives it
special attention. I expect several iterations of this RFC from feedback.
=head1 DESCRIPTION
The language interface is the C<clone> method which takes an object as
argument and returns a copy of it as result:
=over 4
=item clone SCALAR
=item clone SCALAR, CALLBACK
$copy = clone($obj)
$copy = clone($obj, sub { die 'Filehandle copying not allowed' })
The C<CALLBACK> is an optional function to be called if C<clone> encounters
a filehandle. It will either throw an exception or return a value to be
used as the cloned filehandle.
=back
=head1 IMPLEMENTATION
=head1 Default
The default functionality will switch on C<ref SCALAR> and recursively
create new data structures in the way many people have written themselves:
(/^$/ || /^CODE$/) && return $obj;
/^SCALAR|REF$/ && return \clone $$obj;
/^ARRAY$/ && return [ map clone($_), @$obj ];
/^HASH$/ && return { map {$_, clone $obj->{$_}} keys %$obj };
TODO: behavior for C<GLOB>, C<REGEX>, and C<LVALUE>.
=head2 Blessed objects
If C<clone> encounters a blessed object C<$obj> say, it will call
C<$obj->CLONE(CALLBACK)>. If the C<CLONE> method is not defined in
C<$obj>'s class or any of its superclasses, C<UNIVERSAL::CLONE> will carry
out the default functionality on the internal representation of C<$obj>.
Classes (e.g., DBI) may well choose to throw an exception in their C<CLONE>
methods.
=head2 Filehandles
If C<clone> encounters an C<IO::Handle>, its default behavior will be to
make a copy of the filehandle (debatable: perhaps the default should be to
throw an exception) unless a C<CALLBACK> function was specified
=head2 Tied variables
If C<clone> encounters a tied variable, it will call the C<CLONE> method in
that class or fall back to C<UNIVERSAL::CLONE>. (Note: perhaps all missing
functions from tied classes should punt to C<UNIVERSAL>. But that is
outside the scope of this RFC.)
=head2 Cyclic references
The C<clone> function should detect circular references and replicate the
same structure in the copy. One implementation that suggests itself is to
keep a hash of input references with output references as values. It has
been suggested that the code to do this will already be available in the
garbage collector.
=head2 Exceptions
If an exception is thrown anywhere during the copying, it needs to be
trapped so that C<clone> can clean up any cyclic references it has created,
then rethrown.
=head1 REFERENCES
None.
