[Insight-developers] Re: [Vxl-users] Optimizers : CostFunction and Const-Correctness : ITK + VNL

[Luis Ibanez]

Andrew,

Thanks for your rapid answer.

I agree with your statement regarding the possiblity of
using non-const cost_function methods inside the const
method of a different class.

Following your example I have modified the VNL CostFunction
Adaptors in ITK in such a way that they will provide safe
const-correctness face to ITK Optimizers and still use the
non-const calls of internal vnl_cost_functions.

The ITK wrappers around VNL optimizers have been modified
accordingly.


    Regards,


       Luis



--------------------------
Andrew Fitzgibbon wrote:

> Quick answer: those were designed pretty early, and predated widespread
> "mutable" support.
> 
> Other answer: I'm surprosed you ever find yourself with an optimizer
> object whose constness propagates.  If I want to fit an "Ellipse" to 
> "Data", there are a variety of cost functions I might implement.  One
> might be called "Sampson", and would be implemented thus:
> 
> struct Sampson: public vnl_cost_function {
>   const Data* thedata;
>   CachedStuff thestuff;
> 
>   double f(vnl_vector<double> const& x) {
>     Ellipse e(x);
>     double tot(0);
>     for k = 0 to data->n
>       tot +=  e.distance(data->point[k]);
>     cachedstuff.tot = tot;
>     return tot;
>   }
> };
> 
> 
> Then it might be used in a const method on Data:
> 
> Ellipse Data::fitellipse() const
> {
>    Sampson costfn;
>    costfn.data = this;
>    return Ellipse(amoeba(costfn));
> }
> 
> 
> I cannot see where constness will propagate, unless you try to
> multiply inherit Sampson from cost_function and Data, but why would
> one do that?
> 
> 
> 
> 
>