Help needed with scope resolution

[beagleears]

I have a piece of code in which I pass a function to a function and it works fine in stand alone code but when I encorporate it in a class my compiler tells me there is no matching function. The error message is:

test.C: In member function ‘void Field::Evaluate ()’:
test.C:29: error: no matching function for call to ‘Field:: Diff(double&, <unknown type>)’
test.C:18: note: candidates are: double Field:: Diff(double, double (*)(double))

Why does it not recogize the type? What can I do to correct the situation? I think it is because the function MyMathFunction is not defined in the scope of the function Diff but I don't know how to fix that. I wrote a small sample of code that exhibits the behavior I am trying to correct. My compiler version is
g++ (GCC) 4.0.2 20051125 (Red Hat 4.0.2-8)
Thanks in advance.

#include <iostream.h>
#include <math.h>
 
class Field{
 public:
 
double MyMathFunction1( double );
// In the function below I pass x and f(x) separately
double Diff( double, double(*)(double) ); 
 
 void Evaluate();
};
 
double Field::MyMathFunction1(double x){  return sin(x);}
double Field::Diff( double x, double(*func)(double) )
{
  // The idea is that func(x) becomes  MyMathFunction1(x) 
 
  double h=1e-6;
  return (func(x+h)-func(x-h))/2./h;
}
void Field::Evaluate(){
  // Use some dummy value of x for testing
  double x=1.5707963;
  // pass the function MyMathfunction1(x) to Diff(double,double(*)() )
  cout << "Dsin(x)|x="<<x<<":\t"<<Diff( x, MyMathFunction1 )<<endl;
}
 
int main()
{
 
  Field * DoAnalysis = new Field;
  DoAnalysis->Evaluate();
 
  return 0;
}
 

[tyreld]

Why are you using a function pointer when the function is already a member of the class? Why not just call the function directly?