Polymorphism & Pointers to arrays [duplicate]

You can't treat arrays polymorphically, so while new B[100] creates an array of B objects and returns a pointer to the array - or equivalently the first element of the array - and while it is valid to assign this pointer to a pointer to a base class, it is not valid to treat this as a pointer into an array of A objects.

The principal reason that you can't is that (typically) derived objects are a different size to their base classes, so attempting to access the array as an array of base class objects will not use the correct offset to get a pointer to the next base class subobject of the next member of the derived class array.

There is no problem with the polymrphism but with the way you are dealing with memory. The [] operator will advance you through the array by the sizeof(A) bytes in the first case and the sizeof(B) bytes in the second case. Because the objects are of type B the A* is not pointing to the correct location in memory.

Here is another way of looking at it

char * var;var = (char*) new B[100];std::cout << ((A*)var[0]).getValue(); //This works finestd::cout << ((A*)var[1]).getValue(); //This will failstd::cout << ((A*)var[sizeof(B)]).getValue(); // should work

You didn't allocate the objects in the array:

for (int i=0;i<100;i++)  var[i] = new B;

(although I may be mixing up C++ and C#)