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();
}

question from:https://stackoverflow.com/questions/5967412/issue-with-c-constructor

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.