Re: Problems with custom converter

On Thu, Jun 24, 2010 at 01:53:14PM +0200, Szymon Gatner wrote:
> Hi,
> 
> recently I am having very rough time with code using luabind so I
> finally decided to ask here.
> 
> In my code I have a class like this, color.h:
> 
> struct Color3b
> {
>   const static Color3b White;
>   const static Color3b Black;
> 
>   Color3b() {}
> 
>   Color3b(Byte r_, Byte g_, Byte b_) : r(r_), g(g_), b(b_) {}
> 
>   Byte r, g, b;
> };
> 
> then in some cpp file:
> 
> namespace luabind
> {
>   template <>
>   struct default_converter<Color3b> : native_converter_base<Color3b>
>   {
>     static int compute_score(lua_State* L, int index)
>     {
>       return lua_type(L, index) == LUA_TTABLE ? 0 : -1;
>     }
> 
>     Color3b from(lua_State* L, int index)
>     {
>       Color3b ret;
> 
>      //snip
> 
>       return ret;
>     }
> 
>     void to(lua_State* L, Color3b const& color)
>     {
>       object obj = newtable(L);
> 
>       obj[1] = color.r;
>       obj[2] = color.g;
>       obj[3] = color.b;
> 
>       obj.push(L);
>     }
>   };
> 
>   template <>
>   struct default_converter<Color3b const&>
>     : default_converter<Color3b>
>   {};
> }
> 
> then later in same file:
> 
> Color3b color3b = luabind::object_cast<Color3b>(params["normal"]);
> 
> Problem is that this cast always results with exception "unable to
> make cast". Program never enters compute_score() for provided
> converter, in fact break point is marked invalid because this code
> seem to be unused at all. I know that params["normal'] is a valid lua
> object having needed 3 values inside
> (in lua it is like: color = {1123, 123, 123}).
> 
> What is the possible reason for object_cast not to see this converter?

If the specialization is indeed visible when object_cast<> is
instantiated, I don't know. Can you reduce to a complete failing
example?

> Why is compile-time expression object_cast<Color3b> not a compile
> error if no proper converter/conversion trait is found?

Because the user might have exported the Color3b class using class_<>,
and you can't tell until runtime.

-- 
Daniel Wallin
BoostPro Computing
http://www.boostpro.com

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