Issue with C++ constructor

Question

EDIT: This question came up and I think I aced it! Go StackOverflow!! :D

I have exams coming up, and one of the questions on last year's exams was to spot the problem with implementation of the following constructor and to write a corrected one.

Rectangle::Rectangle(string col, int len, int br) {     setColour(col);     length =len;     breadth=br; } 

The class definitions are as follows:

class Polygon { public:     Polygon(string col="red");     void printDetails(); // prints colour only     virtual double getArea()=0;     void setColour(string col); private:     string colour; };   class Rectangle : public Polygon { public:     Rectangle(string, int, int);     void printDetails(); // prints colour and area     // for part 3, delete the line below     double getArea(); private:     int length;     int breadth; }; 

I've written the code into the compiler and it runs fine. I'm guessing the question is relating to inheritance, since string colour; is private, but setColour is public so I cant figure it out. Unless its Rectangle::Rectangle(string col, int len, int br):length(len), breadth(br) and then set the colour inside the construcor or something.

Its only worth 3 marks so its not that big a deal if nobody wants to answer, but I figure if I'm going to have a career as a programmer, its in my interest to know as much as possible. ;)

Thanks for any help.

PS, see getArea() in Rectangle. When I remove that it tells me it "cannot instantiate the abstract class". What does that mean?

EDIT: Here's the main:

void main (void) {     Rectangle rect1 ("blue",5,6);     Rectangle *prect2 = new Rectangle("red",5,6);     rect1.setColour("red");     rect1.printDetails();     prect2->printDetails(); } 

Answer 1

I don't see anything wrong, though you could make it more efficient by using an initialization list (otherwise your private members of both classes get initialized twice):

Rectangle::Rectangle(string col, int len, int br)  : Polygon(col), length(len), breadth(br) {  } 

Notice that the initialization list can call the constructor of Polygon as well.

See Why should I prefer to use member initialization list? for a complete description of the advantages of using initialization lists.