Pointers to Class Members

[emaghero]

I have the following class defined in code

 
//header file
 
class froot{
public:
    void calculate_root(int i);
    double F(double x);
    double G(double x);
 
    double find_root(double (*g)(double),double x1,double x2,int &depth);
    double root(double x1,double x2,double (*g)(double),double f1,double f2,int &depth);
 
private:
 
};
 
//Source File
 
double froot::find_root(double (*g)(double), double x1, double x2, int &depth)
{
    double f1=(*g)(x1);
    double f2=(*g)(x2);
 
    if(f1*f2>0.0){
        cout<<"ERROR in find_root(): values at end-points have same sign"<<endl;
        exit(EXIT_FAILURE);
        return 0;
    }
    else if(x2-x1>0.0)
        return root(x1,x2,*g,f1,f2,depth);
    else
        return root(x2,x1,*g,f2,f1,depth);
}
 
double froot::root(double x1, double x2, double (*g)(double), double f1, double f2, int &depth)
{
    //This function calculates the root of a function using the bisection method
    const int max_depth=1000;
    const double x_limit=1e-12;
    double estimated_root;
    double x_mid=(x1+x2)*0.5;
 
    if(x2-x1<=x_limit)
        estimated_root=x_mid;
    else if(++depth>max_depth){
        cout<<"WARNING: Maximum limit of "<<max_depth<<" bisections reached in root().\n";
        estimated_root=x_mid;
    }
    else{
        double f_mid=(*g)(x_mid);
        if(f_mid==0.0){
            //Zero at x_mid
            estimated_root=x_mid;
        }
        else if((*g)(x1)*f_mid<0.0){
            //Zero in first segment
            estimated_root=root(x1,x_mid,*g,f1,f_mid,depth);
        }
        else{
            //Zero in second segment
            estimated_root=root(x_mid,x2,*g,f_mid,f2,depth);
        }
    }
    return estimated_root;
}
 
double froot::F(double x)
{
    return exp(x)+pow(2.0,-x)+2.0*cos(x)-6.0;    
}
 
double froot::G(double x)
{
    return x-cos(x);    
}
 
void froot::calculate_root(int i)
{
    int depth;
    double sol;
 
    if(i==1){
        depth=0;
        sol=find_root(F,1.0,2.0,depth);
    }
    else{
        depth=0;
        sol=find_root(G,0.0,PI/2,depth);
    }
}
 
 

When I try to build the project I get an error in the calculate_root function.

It's telling me to replace F with &froot::F and to replace G with &froot::G, but this doesn't work.

I can kind of see why there is a problem, in that I'm trying to call find_root by passing a class member when it is expecting a pointer to a function

How should I change the syntax of the find_root, and root, declaration so that it will accept functions which are declared inside the same class?

Thanks for your help.

Any sugestions greatly appreciated.

[Savage]

Member functions cannot be used like that to represent callback functions.You must make them either static functions,or create wrapper function outside the class which will call the objects method.That's one of the reasons why functors(objects with overloaded function call operators,so that they behave just like a functions) are a better choice than callback functions.

[emaghero]

@Savage
So, is it the case that I can't do what I was trying to do unless I use functors?

I guess I better get reading then.

Thanks very much for your help.