Virtual functions in subclass
So I was looking through some code for a project trying to debug it, and I ran into a gap in my understanding. The basic question is, what is the purpose of the "virtual" keyword before overridden functions in the sub class.
For example(stolen from Microsoft's website:
Code:
#include <iostream>
#include <string>
using namespace std;
class BasePoint {
public:
BasePoint(int px, int py):x(px),y(py) {}
int x, y; //position
virtual string type() = 0;
virtual void info() {
cout << endl << "figure: " << type() << endl;
cout << "position: x=" << x << ", y=" << y << endl;
}
};
class Figure1P : public BasePoint {
public:
Figure1P(int px, int py, int r):p1(r),BasePoint(px, py) {}
int p1;
virtual void info() {
BasePoint::info();
cout << "property 1: p=" << p1 << endl;
}
};
class Square : public Figure1P {
public:
Square(int px, int py, int r):Figure1P(px, py, r) {};
virtual string type() {
return "square";
}
};
class Circle : public Figure1P {
public:
Circle(int px, int py, int r):Figure1P(px, py, r) {}
virtual string type() {
return "circle";
}
};
class Figure2P : public Figure1P {
public:
Figure2P(int px, int py, int w, int h):p2(h),Figure1P(px, py, w) {}
int p2;
virtual void info() {
Figure1P::info();
cout << "property 2: p=" << p2 << endl;
}
};
class Rectangle : public Figure2P {
public:
Rectangle(int px, int py, int w, int h):Figure2P(px, py, w, h) {}
virtual string type() {
return "rectangle";
}
};
class Oval : public Figure2P {
public:
Oval(int px, int py, int w, int h):Figure2P(px, py, w, h) {};
virtual string type() {
return "oval";
}
};
int main(void) {
BasePoint **objs = new BasePoint*[5];
// allocate space for 10 BasePoint pointers,
// they may be used to store derived classes
// creating objects
objs[0] = new Circle(7, 6, 55);
objs[1] = new Rectangle(12, 54, 21, 14);
objs[2] = new Square(19, 32, 10);
objs[3] = new Oval(43, 10, 4, 3);
objs[4] = new Square(3, 41, 3);
bool flag=false;
do {
cout << endl << "We have 5 objects with numbers 0..4" << endl;
cout << "Enter object number to view information about it " << endl;
cout << "Enter any other number to quit " << endl;
char onum; // in fact, this is a character, not a number
// this allows user to enter letter and quit...
cin >> onum;
// flag -- user have entered number 0..4
flag = ((onum >= '0')&&(onum <= '4'));
if (flag)
objs[onum-'0']->info();
} while(flag);
// freeing memory
for(int i=0;i<5;i++)
delete objs[i];
delete [] objs;
}
In all the base classes, such as circle, the type() function is overridden. why is the virtual keyword used?
Code:
Circle(int px, int py, int r):Figure1P(px, py, r) {}
virtual string type() {
return "circle";
}
I removed the virtual keywords and it compiled and ran fine.
