Question

In the Bird class, there is one virtual function canFly() which is implemented in two classes: Parrot and Crow. G is a global class which remembers the no. of birds (i.e. either Crow or parrot) and then printBirds() prints the birds’ ability to fly.

But i am getting some error due to undefined references. Can somebody please explain this. Why are these errors occuring and how to rectify the program to remove errors.

#include <iostream>
using namespace std;
class Bird
{
    bool abilityToFly;
    public:
    Bird()
    {
        abilityToFly=0;
    }
    bool getAbility()
    {
        return abilityToFly;
    }
    void setAbility(bool x)
    {
        abilityToFly=x;
    }
    virtual void canFly()
    {
        abilityToFly=0;
    }
};

class Crow: public Bird
{
     public:
    void canFly()
    {
        setAbility(1);
    }
};

class Parrot: public Bird
{
    public:
    void canFly()
    {
         setAbility(1);
    }
};

class G
{
    public:
    static int numBirds;
    static Bird *b[10];
    static void addBird(Bird bird)
    {
         b[numBirds]= &bird;
         numBirds++;
         if (numBirds>10)
         cout<<"Error in program";
    }
    static void printBirds()
    {
         for(int i=0;i<numBirds;i++)
         {
         cout<<"Bird "<<i<<"'s ability to fly"<<b[i]->getAbility();
         }
    }
 };
 int G::numBirds=0;

 int main()
 {
     Parrot p;
     p.canFly();
     Crow c;
     c.canFly();
     G::addBird(p);
     G::addBird(c);
     G::printBirds();
     return 0;
 }

The errors are:

 In function `main':
 undefined reference to `G::b'
 undefined reference to `G::b'
 undefined reference to `G::b'

The link to the code is:
http://codepad.org/Mjpu4wFv

Answer 1

G::b is a static data member, you should initialize it.

int G::numBirds=0;
Bird * G::b[10];

Another error in your code, you should modify as follows:

static void addBird(Bird* bird)
{
    b[numBirds] = bird;
    numBirds++;
    if (numBirds>10)
    cout<<"Error in program";
}

and in main():

G::addBird(&p);
G::addBird(&c);

Otherwise the parameter you transfer in addBird is a temporary one, but you give the address of the temporary parameter to G::b[]