Can't access Global Function from within Class

Question

I've created a global function, CallPrice(args). I have a class, EuropeanOption, and I have a class function called CallPrice, which should call the global function using variables from the EuropeanOption class, and return the CallPrice. I'm getting an error, "the global scope has no "CallPrice".

I think this is a common problem. I searched other threads, which said adding :: should solve the problem, but it's not working here for me. Could you identify the cause of the error? Do I need to make this a friend function or some other workaround?

Thanks!

Header:

#ifndef EuropeanOption_HPP
#define EuropeanOption_HPP

#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <boost/math/distributions/normal.hpp>

using namespace boost::math;
using namespace std;

namespace CLARK
{

struct EquityParms
{
    double T; // years until expiry
    double K; // strike price
    double sig; // vol
    double r; // risk free rate
    double b; // cost of carry  
};

// Global Call function
const double CallPrice(double T, double K, double sig, double r, double b, double EquityPrice);

class EuropeanOption
{
private:
    double T; // years until expiry
    double K; // strike price
    double sig; // vol
    double r; // risk free rate
    double b; // cost of carry
    double S; // current equity price   
    double ExactCallPrice;

public:
    EuropeanOption(); // default constructor (empty)
    EuropeanOption(const EquityParms& data, double EquityPrice); // constructor that sets parms
    void copy(const EuropeanOption& source);
    ~EuropeanOption();

    void init(const EquityParms& data, double EquityPrice); // initialize EquityParms

    const double CallPrice(); // trying to call global function in this function

};    
}

#endif

Source:

#include "EuropeanOption_H.hpp"

namespace CLARK
{

const double CallPrice(double T, double K, double sig, double r, double b, double EquityPrice)
{// Global Function
    double temp = sig * sqrt(T);
    double d1 = (log(EquityPrice / K) + (r + (sig*sig) * 0.5) * T) / temp;
    double d2 = d1 - temp;

    normal_distribution<> myNormal(0,1);
    return (EquityPrice * cdf(myNormal,d1)) - (K * exp((b - r) * T) * cdf(myNormal, d2));
}    

EuropeanOption::EuropeanOption()
{// default constructor
    cout << "Default constructor call" << endl;
}

EuropeanOption::EuropeanOption(const EquityParms& data, double EquityPrice)
{// constructor that sets parms
    init(data, EquityPrice);
}

void EuropeanOption::copy(const EuropeanOption& source)
{
    T = source.T;
    K = source.K;
    sig = source.sig;
    r = source.r;
    S = source.S;
    b = source.b;
}

EuropeanOption::~EuropeanOption()
{   
}

void EuropeanOption::init(const EquityParms& data, double EquityPrice)
{
    T = data.T;
    K = data.K;
    sig = data.sig;
    r = data.r;
    S = EquityPrice;
    b = data.b;     
}

const double EuropeanOption::CallPrice()
{ // trying to call global function in this function
    return ::CallPrice(T, K, sig, r, b, S); // the global scope has no "CallPrice" ???
}    

}

Answer 1

CallPrice is in namespace CLARK. So try

CLARK::CallPrice(/* ... */);

Answer 2

You have declared the global CallPrice in the namespace CLARK. The syntax ::CallPrice tries to use a function CallPrice defined in the global namespace, or an anonymous namespace. Instead, use CLARK::CallPrice.

Answer 3

You are in the namespace CLARK:

return CLARK::CallPrice(T, K, sig, r, b, S);