CPP Pointers to Class Methods

Pointers to Class Methods

• For a pointer to a method we use the same syntax as with a pointer to function, but include the class name ending with the scoping operator (::).

  double (Shape::*pointerFunction) (double, double);

  To set the pointer to a method to the address of a method in a class we must use the address-of operator (&) in front of class name ending with the scoping operator(::) in the front of the method name:

  double (Shape::*pointerFunction) (double, double);
  pointerFunction=&Shape::getArea;

• To execute the method using the pointer to method .

  double (Shape::*pointerFunction) (double, double);
  double area=(shapeInstance.*pointerFunction)(5.0, 3.0);
  // and if the shape object is on the free store
  double area=(shapePointer->*pointerFunction)(5.0, 3.0);

• The pointer to methods is in a way polymorphic and we normally solve this though use of virtual methods in a class inheritance is-a relationship.
• The pointer to methods can also be combined with polymorphism.
• A pointer to a class method can be created inside a class or outside a class.
  Here is an example how you can do that:

  #include <iostream>
  using namespace std;
  
  class Shape {
  protected:
    int width, height;
    int (Shape:: * pointerArea) ();
    void (Shape:: * pointerSetValues)(int, int);
  public:
    Shape () {
      pointerArea=&Shape::area;
      pointerSetValues=&Shape::set_values;
    }
    virtual int area ()=0;
    void set_values (int a, int b)
    { width=a; height=b;}
    int getPointerArea(){
      return (this->*pointerArea)();
    }
    void setPointerValues(int width, int height){
      (this->*pointerSetValues)(width, height);
    }
  };
  
  class Rectangle: public Shape {
  public:
    int area ()
    { return (width * height); }
  };
  
  class Triangle: public Shape {
  public:
    int area ()
    { return (width * height / 2); }
  };
  
  int main () {
    Rectangle rect;
    Triangle  trgl;
    int width=6;
    int height=5;
    rect.set_values (width,height);
    trgl.set_values (width,height);
    cout << "For width=" << width << " and height=" << height
         << " the Rectangle area=" << rect.area() << endl;
    cout << "For width=" << width << " and height=" << height
         << " the Triangle  area=" << trgl.area() << endl;
  
    cout << "\nUsing pointers to methods:\n";
    // Creates a pointer to method Area() (which is virual)
    // in the Shape class 
    int (Shape:: * pArea)()= pArea=&Shape::area;
    void (Shape:: * pSetValues)(int, int)=&Shape::set_values;
  
    // Set values using pointer to methods
    width=4;
    height=7;
    (rect.*pSetValues)(width,height);
    (trgl.*pSetValues)(width,height);
    // Get Area using pointer to methods
    // As area() is virtual the area() method for the right object is executed
    int rectArea=(rect.*pArea)();
    int trglArea=(trgl.*pArea)();
  
    cout << "For width=" << width << " and height=" << height
         << " the Rectangle area=" << rectArea << endl;
    cout << "For width=" << width << " and height=" << height
         << " the Triangle  area=" << trglArea << endl;
  
    cout << "\nUsing methods that use pointer to method in the Shape class:\n";
    // Set values using methods that use pointer to methods in the class
    width=5;
    height=8;
    rect.setPointerValues(width,height);
    trgl.setPointerValues(width,height);
    // Get Area using methods that use pointer to methods in the class
    // As area() is virtual the area() method for the right object is executed
    rectArea=rect.getPointerArea();
    trglArea=trgl.getPointerArea();
  
    cout << "For width=" << width << " and height=" << height
         << " the Rectangle area=" << rectArea << endl;
    cout << "For width=" << width << " and height=" << height
         << " the Triangle  area=" << trglArea << endl;
  
    return 0;
  }

  When we run this application, the result will be:

  For width=6 and height=5 the Rectangle area=30
  For width=6 and height=5 the Triangle  area=15
  
  Using pointers to methods:
  For width=4 and height=7 the Rectangle area=28
  For width=4 and height=7 the Triangle  area=14
  
  Using methods that use pointer to method in the Shape class:
  For width=5 and height=8 the Rectangle area=40
  For width=5 and height=8 the Triangle  area=20

  You can download this example here (needed tools can be found in the right menu on this page).