Question

I’m reading the book by Daoqi Yang “C++ and Object Oriented Numeric Computing for Scientists and Engineers”. He has a similar example to what I am showing below, but the exceptions are the class “P” I define and the second to last line (which doesn’t work). My question is: why does my compiler generate and error when I supply the function member f.integrand? What can I do to correct this? The errors being generated are C3867, C2440, and C2973.

Here is the code:

class P{
public:
double integrand(double x){
    return (exp(-x*x));
}
};

template<double F(double)>
double trapezoidal(double a, double b, int n)
{
    double h=(b-a)/n;
    double sum=F(a)*0.5;
    for(int i=1;i<n;i++)
    {
        sum+=F(a+i*h);
    }
    sum+=F(b)*0.5;

    return (sum*h);
}

double integrand2(double x){
    return (exp(-x*x));
}

int main(){
    P f;
    cout<< trapezoidal<integrand2>(0,1,100)<<endl;    // this works
    cout<< trapezoidal<f.integrand>(0,1,100)<<endl;   // this doesn't work
}

Answer 1

Template arguments must be compile-time constant expressions or types, and member functions require special handling anyway. Instead of doing this, use boost::function<> as an argument, and boost::bind to create the functor, e.g.

double trapezoidal(double, double, boost::function<double(double)>);

// ...

P f;
trapezoidal(0, 1, 100, integrand2);
trapezoidal(0, 1, 100, boost::bind(&P::integrand, boost::ref(f)));

If you have 0x-capable compiler, you can use std::function and std::bind instead.