struct A
{
    int a;
    A() { }
    virtual ~A() { }
};

struct B: public A
{
    int b;
    B() { }
    ~B() { }
};

int main (void) 
{
    A *ptr;
    ptr = new B[2];
    delete[] ptr;
}

Arrays are ill-adjusted: Treating arrays polymorphically is a gross type error that your compiler will probably remain silent about. Don't fall into the trap.

> struct A
> {
>     int a;
>     A() { }
>     virtual ~A() { }
> };
>
> struct B: public A
> {
>     int b;
>     B() { }
>     ~B() { }
> };
>
> int main (void)
> {
>     A *ptr;
>     ptr = new B[2];
>     delete[] ptr;
> }
>

#include <iostream>

class A
{
public:
     int a;

     A(){ a=10; }
     virtual ~A(){ std::cout << "A::~A" << std::endl; }

     virtual void foo() = 0;
};

class B
:
    public A
{
public:
     int b;

     B(){ b=11; }
     virtual ~B(){ std::cout << "B::~B" << std::endl; }

     virtual void foo(){std::cout << "B::foo" << std::endl;}
};

int main (void)
{
     A *ptr[2];
     B *ptr_b = new B[2];

    for(int i = 0; i < 2; ++i)
    {
        ptr[i] = ptr_b+i;
    }

     ptr[0]->foo();
     ptr[1]->foo();

     delete[] ptr_b;
}