Re: Is this bad coding ?

From:

"Martin T." <0xCDCDCDCD@gmx.at>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 15 May 2009 06:26:19 CST

Message-ID:

<guj775$pnr$1@news.motzarella.org>

mast4as wrote:

Thanks Martin

I have made the changes you suggest (just to learn) but I do have this
compilation error:

error: cannot dynamic_cast 'data' (of type 'class ShaderParameters*')
to type 'class PlasticParams*' (source type is not polymorphic)

(... snipped ...)

Google is your friend: Entering "error: cannot dynamic_cast source type
is not polymorphic" into google will give you the answer.

You can only use dynamic_cast on types that have at least one virtual
function. (On vf, see:
http://www.parashift.com/c++-faq-lite/virtual-functions.html)

Of course this involves some kind of overhead, that may or may not be
acceptable to you.
If you do not wish for the parameter type to use vf, you will have to
resort to a static_cast and some hand coded switching on what the actual
target type should be.

...
   color surface(const ShadingInfo &shadingInfo) const
   {
     color c = { 0, 0, 0 };
     ShaderParameters params;
     //params.Ks = .4f;
     //params.Kd = .6f;
     illuminate(rc, this, &params);
     return c;
   }
   void evaluate(ShaderParameters* data) const
   {
     PlasticParams *params = dynamic_cast<PlasticParams*>(data);
     fprintf(stderr, "Kd : %f\n", params->Kd);
   }
...

Error! You create a params of type ShaderParameters and then you expect
PlasticParams ... I'll just assume you meant for Plastic::surface to
always pass a PlasticParams parameter to evaluate.

Why is evaluate not just declared with a parameter type of PlasticParams
   ... ?

br,
Martin

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