5.9. Overloading on const

5.9. Overloading on `const`
	Chapter 5. Functions

5.9. Overloading on `const`

[ fromfile: constoverloading.xml id: constoverloading ]

const changes the signature of a member function.
This means that functions can be overloaded on const-ness.

Example 5.16. src/const/overload/constoverload.h

#ifndef CONSTOVERLOAD_H
#define CONSTOVERLOAD_H
#include <iostream>
class Point3 {                                  
 public:
    friend std::ostream& operator<<(std::ostream& out, const Point3& v);
    Point3(double x = 0, double y = 0, double z = 0);
    double& operator[](int index);    
    const double& operator[](int index) const;  
    Point3 operator+(const Point3& v) const;
    Point3 operator-(const Point3& v) const;
    Point3 operator*(double s) const;           
 private:
    static const int cm_Dim = 3;
    double m_Coord[cm_Dim];
};
#endif

	3-dimension point (of double).
	Overloaded on const-ness.
	Scalar multiplication.

The operator function definitions are shown in Example 5.17.

Example 5.17. src/const/overload/constoverload.cpp

[ . . . . ]
const double& Point3::operator[](int index) const {
    if ((index >= 0) && (index < cm_Dim))
        return m_Coord[index];
    else
        return zero(index);
}

double& Point3::operator[](int index) {
    if ((index >= 0) && (index < cm_Dim))
        return m_Coord[index];
    else
        return zero(index);
 }
 
[ . . . . ]

If index is in range, each function returns m_Coord[index] - so what is the difference between them?


5.8. Returning References from Functions		5.9.1. Exercises: Overloading on const