pointer and new

It won't work. You need a terminating semicolon after the class definitions, and a definition of the addData method.

EDIT: wow, I didn't even notice that. I must be completely stupid.

[Edited by - ToohrVyk on January 22, 2006 4:45:37 AM]

You cannot treat arrays polymorphically. A pointer to A cannot be treated as a pointer to the first element in an array of Bs. If you want this behaviour you should be using a pointer-to-pointer:

A * * a = new B *[5];for (int i = 0; i < 5; ++i){	a = new B();	a->addData();}

Or better yet use a container like std::vector with a smart pointer or boost::ptr_vector.

The problem occurs because array indexing is based off of the static type of the variable, not the dynamic type. So when you do a where a is of type A * the compiler translates this the same as *(a + (i * sizeof(A))), whereas an array of B objects has its i^th member at *(a + (i * sizeof(B))). An extra level of indirection solves the problem because sizeof(A *) is always the same as sizeof(B *) (providing both are pointer-to-objects).

Enigma

No, it won't work. Think about how that array is laid out in memory.

class A{public:    virtual addData() {}};class B : public A{public:    B() { a = 1; }    virtual addData() {a*=2;}private:    int a;};A* a = (A*)new B[5];

This is the memory layout that the compiler assumes your array has

 0 [vtable-ptr] 1st A 4 [vtable-ptr] 2nd A 8 [vtable-ptr] 3rd A12 [vtable-ptr] 4th A16 [vtable-ptr] 5th A

This is the actual memory layout of the allocated array, provided the classes I wrote above.

 0 [vtable-ptr, int] 1st B 8 [vtable-ptr, int] 2nd B16 [vtable-ptr, int] 3rd B24 [vtable-ptr, int] 4th B32 [vtable-ptr, int] 5th B

As you can see, they don't match, because sizeof(B) is larger than sizeof(A).

You want to have an array of pointers instead.

A **a = new B*[5];for (int i=0;i<5;i++)   a = new B;for (int i=0;i<5;i++)a->addData();

Now the array is

 0 [ptr] 1st ptr to A 4 [ptr] 2nd ptr to A 8 [ptr] 3rd ptr to A12 [ptr] 4th ptr to A16 [ptr] 5th ptr to A

And each ptr in the array could point to an instance of any type that inherits from A