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=head1 TITLE
Objects : Mandatory and enhanced second argument to C<bless>
=head1 VERSION
Maintainer: Damian Conway <[EMAIL PROTECTED]>
Date: 1 September 2000
Mailing List: [EMAIL PROTECTED]
Version: 1
Number: 187
Status: Developing
=head1 ABSTRACT
This RFC proposes that the second argument to C<bless> be made
mandatory, and that its semantics be enhanced slightly to cover a
common, ugly, and frequently buggy usage.
=head1 DESCRIPTION
=head2 Mandatory second argument
When omitted, the second argument to C<bless> currently defaults to the
name of the package from which the call is made. This default behaviour
leads to unexpected and undesirable behaviour whenever a constructor
is inherited by a derived class:
package Base;
sub new { bless {} }
package Derived;
use base 'Base';
package main;
$obj = Derived->new(); # Creates an object blessed into Base!
To remove this nasty surprise is proposed that the second argument to
C<bless> be made mandatory.
=head2 Extended second argument semantics
A common idiom in OO Perl is to allow constructors to be called
as both class- and object- methods by writing:
sub new {
bless {}, ref($_[0]) || $_[0];
}
It is proposed that the above clutter be rendered unnecessary.
Specifically, if the second argument to C<bless> is a reference,
that it be stringified by calling C<ref>, rather than by the usual
stringification process.
This would allow the above example to be written as:
sub new {
bless {}, $_[0];
}
More importantly, it would allow calls to C<bless> which I<don't>
use the C<ref($_[0])||$_[0]> idiom to Do The Right Thing,
instead of the existing bizarre behaviour (i.e. blessing
into classes with names like C<'MyClass=HASH(0x5a7f590)'> [sic].
=head2 Alternative (restricted) second argument semantics
An alternative to extending the semantics of the second argument, would
be to cause C<bless> to issue a warning or exception when called with a
reference as its second argument. This would still require an explicit
C<ref($_[0])||$_[0]> where such behaviour was desired, but would catch
erroneous uses.
And anyone truly desiring the bizarre existing behaviour could
simply use:
sub new {
bless {}, "$_[0]";
}
=head1 MIGRATION
Minimal, since only the truly wicked ever invoke C<bless> with only
one argument.
Migration would consist of translating instances of:
bless REF;
to:
bless REF, __PACKAGE__; # Note: this may break inheritance!
Changes to second argument semantics would have no impact except in
pathological cases. In practice, the new semantics would silently fix a
large number of lurking bugs in existing code.
=head1 IMPLEMENTATION
Trivial.
=head1 REFERENCES
I<perltoot>
Conway, I<Object Oriented Perl>, p. 77 & 172
