Re: generic imports? void* -> generics?

[Matthew Heaney <matthewjheaney@earthlink.net>]

chris <spamoff.danx@ntlworld.com> writes:

> void *lua_touserdata (Lua_State* L, int pos)

I prefer thin bindings, so you just do it this way:

   function lua_touserdata
     (L   : Lua_State_Access;
      Pos : C.int) return System.Address;

   pragma Import (C, lua_touserdata);
   pragma Convention (C, lua_touserdata);

There's no binding to C type void* in Interfaces.C, but type
System.Address should work OK.  That way you can say:

   Get_MyUserData:
   declare
      Data : MyUserData_Type;
      for Data'Address use lua_touserdata(L, Pos);
   begin
      ... -- manipulate Data as appropriate
   end Get_MyUserData;

I suppose you could wrap this in a generic, which is implemented using
the function above.


> I was thinking of something like
> 
> generic
>     type Userdata is private;
> function to_userdata (L   : in Lua_State_Access;
>                        Pos : in Index) return Userdata;
> 
> but am unsure how to complete it...  I tried
> 
>     function To_Userdata (L   : in Lua_State_Access;
>                           Pos : in Index) return Userdata is
>        function Lua_ToUserData (L   : in Lua_State_Access;
>                                 Pos : in Index) return Userdata;
>        pragma Import (C, Lua_ToUserData);
>     begin
>        return Lua_Touserdata (L, Pos);
>     end To_Userdata;
> 
> and it compiles ok, but is it right?  It looks odd to me.  Will the
> function Lua_ToUserData be imported upon instantiation of the
> To_Userdata function?


This is wrong, because your Ada function to_userdata returns an object,
not a pointer to object, as the C function does.  You need to bind to an
access type, like this:

   function To_Userdata
     (L : lua_state_access;
      Pos : Index) return Userdata is

      type Userdata_Access is access all Userdata;
      for Userdata_Access'Storage_Size use 0;
      pragma Convention (C, Userdata_Access);

      function lua_touserdata
        (L   : Lua_State_Access;
         Pos : C.int) return Userdata_Access;  --ok now

      pragma Import (C, lua_touserdata);
      pragma Convention (C, lua_touserdata);

      Result : constant Userdata_Access :=
         lua_touserdata(L, Pos);
   begin
      return Result.all;
   end;