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From: David R Tribble <dtribble@technologist.com>
Subject: Re: nested functions
Date: 1999/06/17
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Christopher Eltschka wrote:
> 
> David R Tribble wrote:
> 
> [...]
> 
> > >> P.S.  I personally don't see any need for nested functions.
> > >
> > > I do. (It's true no single language feature is have-it-or-die,
> > > though.)
> >
> > Please provide a convincing example; no one's done that yet.
> 
> One from real life:
> 
> extern "C"
>  void fortran_integration_routine_(double(*)(double&),
>                                    int const& start, int& end,
>                                    double const& prec,
>                                    double& result);
> 
> double calc_something(double param1, double param2)
> {
>   // I wish I had this
>   double foo(double& pd)
>   {
>     return param1 + pd*param2;
>       // of course, the real function was more complex
>   }
>   double result;
>   fortran_integration_routine_(&foo, 1, 3, 1e-10, result);
>   return result;
> }
> 
> Of course, the real function was much more complex (and the
> integral was two-dimensional, so the integration was called
> recursively).

Ignoring for the moment the problems of integration with FORTRAN
subroutines, why not use a functor that stores the 'pd' parm as part
of its state?:

    double calc_something(double param1, double param2)
    {
        struct Functor
        {
            double      p1;
            double      p2;

                        Functor(double a, double b):
                            p1(a), p2(b)
                        { }

            double      operator ()(double pd)
                        { return p1 + pd*p2; }
        };

        result = integration_routine(
                     Functor(param1, param2), 1, 3, 1e-10);
        return result;
    }

Hooking this into FORTRAN adds another dimension to the problem,
of course.  But I don't think we should resort to adding nested
functions to C++ just to solve that particular problem.

-- David R. Tribble, dtribble@technologist.com --
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