RFC 187 (v2) Objects : Mandatory and enhanced second argument to C

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=head1 TITLE

Objects : Mandatory and enhanced second argument to C<bless>

=head1 VERSION

  Maintainer: Damian Conway <[EMAIL PROTECTED]>
  Date: 1 Sep 2000
  Last Modified: 18 Sep 2000
  Mailing List: [EMAIL PROTECTED]
  Number: 187
  Version: 2
  Status: Frozen

=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 to C<bless> is being 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.

Alternatively, if the second argument is to remain optional, the default
value should be the invocant: normally $_[0], but possibly whatever
access mechanism has been specified under C<use invocant>.


=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 ISSUES

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

Forthcoming RFC on C<use invocant>