Help with templates and code generalization

From:

StephQ <askmeofit@mailinator.com>

Newsgroups:

comp.lang.c++

Date:

5 May 2007 04:07:59 -0700

Message-ID:

<1178363279.904860.88190@p77g2000hsh.googlegroups.com>

I face the following problem.
I wrote a poor's man plotting function: computePlot. This function
append some x-values belonging to [xmin,xmax] step and the
correseponding f( x ) (for a given const member function f that takes
and return a double) values to a reference stringstream, let's call
this ss.
Then I usually use
ss.str()
to transfer the results in a file and plot these results using an
external software like gnuplot.
Because I'm using this plotting function in a lot of situations I
decided to write it in template form. Here is a simplifyed version:

    template <class T, double (T::*F)(double) const>
    class Display
    {
    private:

        static double resolution;

    public:

        static void computePlot(std::stringstream& ss, const T& t, const
ClosedInterval& interval);
    };

    template <class T, double (T::*F)(double) const>
    double Display<T,F>::resolution = 0.01;

    template <class T, double (T::*F)(double) const>
    void Display<T,F>::computePlot(std::stringstream& ss, const T& t,
const ClosedInterval& interval)
    {
        double xMin = interval.getLower();
                double xMax = interval.getUpper();

        double lenght = xMax - xMin;

        int n = static_cast<int> (lenght/resolution);

        if (n < 2)
        {
            n = 2;
        }

        double delta = lenght/(n-1);

        /* We print the first and last value separately to avoid numerical
issues. */
        ss << xMin << " " << (t.*F)( xMin ) << std::endl;

        double x;
        int i;
        for (i = 1 ; i < n-1 ; i++)
        {
            x = xMin + delta*i;

            ss << x << " " << (t.*F)( x ) << std::endl;
        }

        ss << xMax << " " << (t.*F)( xMax ) << std::endl;

    }

The problem is that this template expect a second template parameter
of the form:
double (T::*F)(double) const
a pointer to const member function that returns a double and takes a
double.
double (T::*F)(double) already requires a rewriting of the whole
Display class.

As efficency is not a concern with this part of the code maybe I sould
use a generic second template parameter:
class F
but then I have no idea of how to procede.

To sum up, the problem I'm trying to solve is to find a way that
minimize code rewriting while allowing me to apply this poor's man
plotting to different types of member functions of a generic class.
For example I would like to be able to apply computePlot to a member
function that returns a dobule but takes two doubles: f( xvalue,
parameter ) applying the alorithm in computePlot for a fixed parameter
value.
It would also be nice if I could generalize Display in shuch a way
that I could apply it even for nonmember functions.

Is there a way to write templates of this kinds, that apply to
"generic" functions without too much effort?

Thank you.

Cheers
StephQ