Re: I need help understanding inheritance and virtual functions

From:

tpl@eng.cam.ac.uk (Tim Love)

Newsgroups:

comp.lang.c++

Date:

7 May 2009 06:05:01 GMT

Message-ID:

<gtttmd$72v$1@gemini.csx.cam.ac.uk>

dwightarmyofchampions@hotmail.com writes:

Could someone please point me to a lengthy book
and/or website that goes really in-depth into it, and preferably with
lots and lots of examples?

I can offer a standard example. The program has a list of shapes which need
redrawing. Each type of object has its own draw routine.
Compile the code below as is, run it, then add the word "virtual" as
indicated and compile+run again.

#include <iostream>
using namespace std;

class Point {
public:
  float x;
  float y;
};

class Shape {
public:
  // Try adding "virtual" to the front of the next line.
  void draw() { cout << "Shape's draw routine\n";} ;
  // Note that replacing the above line by
  // "virtual void draw()=0;" should force derived classes to
  // have draw() routines. See if this is true.
  float area;
};

class Triangle: public Shape {
  Point points[3];
  void draw() { cout << "Triangle's draw routine\n";} ;
};

class Rectangle: public Shape {
  Point points[4];
  void draw() { cout << "Rectangle's draw routine\n";} ;
};

int main() {

Triangle t1, t2;
Rectangle r1, r2;

Shape* shapes[4];
shapes[0]=&t1;
shapes[1]=&r1;
shapes[2]=&t2;
shapes[3]=&r2;

for (int i=0;i<4; i++)
   shapes[i]->draw();
}