template based d pointer class

[Ravikiran Rajagopal]

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Hello,
  This seems like a fairly complicated way to store extra data members. As far 
as I can see, what you have is a Singleton to hold instances of the private 
data for the class with no d-pointer.
  One simpler(?) solution would be to define a generic singleton pattern 
template class [1] that would be usable much more widely in KDE. Then, derive 
another template class from the singleton which acts as a registrar (sorry, I 
forgot the name for this kind of pattern) which is similar to your proposed 
template here but would be safer as it cannot be instantiated multiple times. 
If you don't like singletons, you can add a static member (to the class 
lacking the d-pointer). Either way, you would have to remember to register 
your instantiations in each constructor, though.
  A nitpick: instead of the #define, it would be better to add a private const 
member function. See http://www.gotw.ca/gotw/012.htm for a similarly flavored 
example.
  Anyway, are there any classes with publicly available API without 
d-pointers?

[1] Is Simon Hausmann the person who typically ports Loki code to kdelibs? I 
have been wanting to add a couple of Alexei Andrescu's pattern 
implementations as well as a little more typelist functionality to kdelibs. 
It would not increase binary code size (unless you instantiate the templates 
elsewhere).

Regards,
Ravi

> USAGE:
> ------
>
> class KBookmarkBarPrivate : public Private<KBookmarkBar,
> KBookmarkBarPrivate> { public:
>     QPtrList<KAction> m_actions;
> };
> QPtrDict<KBookmarkBarPrivate>* Private<KBookmarkBar,
> KBookmarkBarPrivate>::d_ptr = 0;
>
> #define d() KBookmarkBarPrivate::d(this)
>
> IMPLEMENTATION: (most code from seli's developer.kde.org page on bic)
> ---------------
>
> template<class KBookmarkBar, class KBookmarkBarPrivate>
> class Private {
> public:
>     static KBookmarkBarPrivate* d( const KBookmarkBar* instance )
>     {
>         if ( !d_ptr ) {
>             d_ptr = new QPtrDict<KBookmarkBarPrivate>;
>             qAddPostRoutine( cleanup_d_ptr );
>         }
>         KBookmarkBarPrivate* ret = d_ptr->find( (void*) instance );
>         if ( ! ret ) {
>             ret = new KBookmarkBarPrivate;
>             d_ptr->replace( (void*) instance, ret );
>         }
>         return ret;
>     }
>     static void delete_d( const KBookmarkBar* instance )
>     {
>         if ( d_ptr )
>             d_ptr->remove( (void*) instance );
>     }
> private:
>     static void cleanup_d_ptr()
>     {
>         delete d_ptr;
>     }
>     static QPtrDict<KBookmarkBarPrivate>* d_ptr;
> };
>
> (completely untested and the above won't
>  say KBookmarkBar everywhere of course)
>
> Alex
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