[% setvar title Objects : Mandatory and enhanced second argument to C %]

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Objects : Mandatory and enhanced second argument to bless


  Maintainer: Damian Conway <damian@conway.org>
  Date: 1 Sep 2000
  Last Modified: 18 Sep 2000
  Mailing List: perl6-language-objects@perl.org
  Number: 187
  Version: 2
  Status: Frozen


This RFC proposes that the second argument to bless be made mandatory, and that its semantics be enhanced slightly to cover a common, ugly, and frequently buggy usage.


Mandatory second argument

When omitted, the second argument to bless currently defaults to the name of the package from which the call to 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 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 use invocant.

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 bless is a reference, that it be stringified by calling 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 bless which don't use the ref($_[0])||$_[0] idiom to Do The Right Thing, instead of the existing bizarre behaviour (i.e. blessing into classes with names like 'MyClass=HASH(0x5a7f590)' [sic].

Alternative (restricted) second argument semantics

An alternative to extending the semantics of the second argument, would be to cause bless to issue a warning or exception when called with a reference as its second argument. This would still require an explicit 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]";


Minimal, since only the truly wicked ever invoke bless with only one argument.

Migration would consist of translating instances of:

        bless REF;


        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.





Conway, Object Oriented Perl, p. 77 & 172

Forthcoming RFC on use invocant