Re: [boost] Re: Anyone interested in a virtual constructor lib?

From: Roland Schwarz (roland.schwarz_at_[hidden])
Date: 2004-12-06 04:10:03

Vladimir Prus wrote:

>Hi Roland,
>  
>
>What if I write:
>
>     shape* ps2 = dynamic_new<shape>("circle", 20);
>
>?
>
Since I am using a template function as a type generator and 
tipeid().name() in turn
this will deduce a function requiring an int for the ctor. Since only 
double has been
registered in the example program your example will not work. (It will 
give a 0 -pointer
at runtime.)
Please note that my implementation is not very smart and could be 
improved by someone
more knowledgeable than me. Perhaps it is not so hard to add support for 
automatic
type conversion too?
But then my implementation fits entirely (almost) on two sheets of paper.

> 
>I tried to create virtual construct functionality for a future Boost.Plugin
>library and it worked like this:
>
>   namespace boost { namespace plugin {
>   template<>
>   struct virtual_constructors<Weapon> {
>       typedef mpl::list<mpl::list<std::string>,
>                         mpl::list<std::string, int>
>       > type;
>   };
>   }}
>
>Given those declaration, instantiaton of boost::plugin_factory<Weapon> will
>have a 'get' method for each listed constructor signature, and usual
>overload resolution will work.
>
Altough I have not a very good knowledge of the mpl lib right now, this 
looks
very interseting to me.

> The disadvantage is that you need to
>explicitly instantiate 'virtual_construct',
>
Hmm, I cannot see how I ever could do away with this instatiation. Or do
you mean it should better be done implicitely?

> but then you get static
>checking. For example, a plugin can't forget to define a required
>constructor.
>
>I also considered this syntax:
>   struct virtual_constructors<Weapon> {
>       typedef mpl::list<ctor (std::string),
>                         ctor (std::string, int)> .....
>
>where 'ctor' is auxillary struct, but maybe it's too smart, haven't decided
>yet.
>  
>
I am not entirely sure, if you intend this struct as external to your 
class or as a
member? Most likely I did not yet get the full picture and my question 
might seem
somewhat blurry. Could you post a little example that exhibits the user 
interface?

I have the vague feeling that you have found a more pleasing way to 
specify the
virtual constructors. Having said this I should mention another design 
criterion of
mine: I wanted the mechansim to be non-intrusive and appliable to any 
class (as
I think yours is too, isn't it?)

Hidden behind my macros are objects whose sole purpose is that their 
(global) ctors
are to be called before program start to register all the ctor types. E.g.:
#define TYPE_REGISTER(B, D) dynanew::type_entry<B, D>     type_entry_##D(#D)
If there is anything smarter possible I would prefer it!

BTW.: I am not so much interested in getting my proposal into boost as 
seeing this
kind of functionality within boost. If you already are on the way to 
preparation of
such a proposal I would in no way contend, rather support your efforts. 
If you
think my implementation could be of interest to you I am pleased to mail 
you a
copy for further study.

Roland