Overloading on const-ness
const changes the signature of a member function. This means that functions can be overloaded on const-ness. Example 5.16 shows a homemade vector class with member functions overloaded in this way.
Example 5.16. src/const/overload/constoverload.h
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#ifndef CONSTOVERLOAD_H #define CONSTOVERLOAD_H #include class Point3 { <-- 1 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; <-- 2 Point3 operator+(const Point3& v) const; Point3 operator-(const Point3& v) const; Point3 operator*(double s) const; <-- 3 private: static const int cm_Dim = 3; double m_Coord[cm_Dim]; }; #endif (1)a 3D point (of double) (2)overloaded on const-ness (3)scalar multiplication |
Exercises: Overloading on const-ness
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In Example 5.17, the compiler can tell the difference between calls to the const and to the non-const versions of operator[] based on the const-ness of the object. |
Example 5.17. src/const/overload/constoverload-client.cpp
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#include "constoverload.h" #include int main( ) { using namespace std; Point3 pt1(1.2, 3.4, 5.6); const Point3 pt2(7.8, 9.1, 6.4); double d ; d = pt2[2]; <-- 1 cout << d << endl; d = pt1[0]; <-- 2 cout << d << endl; d = pt1[3]; <-- 3 cout << d << endl; pt1[2] = 8.7; <-- 4 cout << pt1 << endl; // pt2[2] = 'd'; cout << pt2 << endl; return 0; } (1)__________ (2)__________ (3)__________ (4)__________ |
- Which operator is called for each of the notes?
- Why is the last assignment commented out?
- The operator function definitions are shown in Example 5.18. The fact that the two function bodies are identical is worth pondering. If index is in range, each function returns m_Coord[index], so what is the difference between them?
Example 5.18. src/const/overload/constoverload.cpp
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[ . . . . ] 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); } [ . . . . ] |
Inline Functions
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