Exam 15: Polymorphism and Virtual Functions

Correct Answer:

Verified

The requested code and further discussion follow.
#include<iostream>
using namespace std;
class B
{
public:
B():bPtr( new int[5]){ cout << "allocates 5 ints\n";}
virtual ~B()
{ delete[] bPtr;cout << "deallocates 5 ints\n";}
private:
int * bPtr;
};
class D:public B
{
public:
D():B(),dPtr(new int[1000]){cout << "allocates 1000 ints\n";}
~D(){ delete[] dPtr;cout << "deallocates 1000 ints\n";}
private:
int * dPtr;
};
int main()
{
for (int i = 0;i < 100;i++)
{
B* bPtr = new D;
delete bPtr;
}
cout << "\nWithout virtual destructor,\n";
cout << "100 * 1000 + 500 ints allocated\n"
<< "but only 500 ints deallocated.\n";
cout << "\nWith virtual destructor,\n";
cout << "100 * 1000 + 500 ints allocated,and "
<< "100 * 1000 + 500 ints deallocated.\n";
}
In this code,if the base class destructor is not virtual,the system must bind the pointer to the destructor at compile time.The pointer in main,bPtr,is of type B*,i.e. ,pointer to B,so it is bound to B's member functions.The destructor for D,~D fails to run.
If the keyword virtual is removed from the destructor in the base class,class B,the derived class destructor ~D is not called.Approximately 400K of memory would then be leaked.