Hello Monks,

I would love to hear -yet again- your opinions. I have now decided to use SQLite for my program and want to use this step to also clean up my currently slightly messy data structure.

I have decided to create database source classes for every table, that do the database communication part, as well as object classes, that actually hold the data. All Object classes and source classes have a base class.
Finally there is a source access class, which returns arrays containing the objects, or can match them by my keys.

What I am looking for is a good idea on how to realize the singleton.
As demonstrated in http://c2.com/cgi/wiki?PerlSingleton, there are a couple of ways to create singleton by basically setting the "bless" function to a scalar and just returning this whenever it is required again. Having done a lot of work on updating all these data structures whenever some icky bits in the database changed on C++, I wanted to use the power of Perl to try and get the singleton definition out of the subclasses or any access classes and right into the base database source class.
I am now thinking of a good way to do this. The basic scheme I use for my objects is based on Damian Conway's OO Perl. I plan to use the the singleton technique 
package Singleton;
my $singleton;
sub new {
  my $class = shift;
  $singleton ||= bless {}, $class;
}

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within a subclass. Do I miss out any problems when I just replace $singleton by a hash which includes the classname: 
package SingletonBase;
my $singleton;
sub new {
  my $class = shift;
  $singleton{ $class } ||= bless {}, $class;
}

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The actual base class would look something like this: 
# encapsulated class data
{

  my %_singleton;

  my %_attr_data = 
    ( _dbd    => [ undef,        'r/w' ],
      _dbname    => [ undef,        'r/w' ],
      _dbpass    => [ undef,        'r/w' ],
      _dbserver => [ undef,        'r/w' ]
    );

  my $_count = 0;

  sub _get {
    my ( $class ) = @_;
    return $_singleton{ $class } if ( $defined( _singleton{ $class } )
+ );
    return 0;
  }

  sub _create {
    my ( $self ) = @_;
    my $_singleton{ ref($self) } = $self;
  }

  sub _accessible {

    my ( $self, $attr, $mode ) = @_;
    $_attr_data{$attr}[1] =~ /$mode/;
    
  }

  sub _default_for {

    my ( $self, $attr ) = @_;
    $_attr_data{$attr}[0];

  }

  sub _standard_keys {
    keys %_attr_data
  }

  sub _count {
    my ( $self ) = @_;
    ++$_count;
    $self->{ "_id" } = $_count;
  }
}

sub new {

  my ( $caller, %arg ) = @_;
  my $caller_is_obj = ref( $caller );
  my $class = $caller_is_obj || $caller;
  
  return _get( $class ) if ( _get( $class ) );
  my $self = bless {}, $class;
  $self->_create();

  foreach my $attrname ( $self->_standard_keys() ) {

    my ( $argname ) = ( $attrname =~ /^_(.*)/ );
    if ( exists $arg{ $argname } ) 
      { $self->{ $attrname } = $arg{ $argname } }
    elsif ( $caller_is_obj )
      { $self->{ $attrname } = $caller->{ $attrname } }
    else
      { $self->{ $attrname } = $self->_default_for( $attrname ) }

  }

  $self->_count();

  return $self;
  
}

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I have added the functions _get, _create and %_singleton in the closure as well as changed the lines above and below the bless statement. Do I miss anything out? Are there any big setbacks? It seems too easy to me. Would it for some reason be a lot better to go via getInstance methods? Thank you for all help. PerlingTheUK

In reply to Singletons and Inheritance by PerlingTheUK

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