Array of polymorphic base class objects initialized with child class objects

Question

Sorry for the complicated title. I have something like this:

class Base
{
public:
  int SomeMember;
  Base() : SomeMember(42) {}
  virtual int Get() { return SomeMember; }
};

class ChildA : public Base
{
public:
  virtual int Get() { return SomeMember*2; }
};

class ChildB : public Base
{
public:
  virtual int Get() { return SomeMember/2; }
};

class ChildC : public Base
{
public:
  virtual int Get() { return SomeMember+2; }
};

Base ar[] = { ChildA(), ChildB(), ChildC() };

for (int i=0; i<sizeof(ar)/sizeof(Base); i++)
{
  Base* ptr = &ar[i];
  printf("El %i: %i\n", i, ptr->Get());
}

Which outputs:

El 0: 42
El 1: 42
El 2: 42

Is this correct behavior (in VC++ 2005)? To be perfectly honest, I expected this code not to compile, but it did, however it does not give me the results I need. Is this at all possible?

Answer 1

Yes, this is correct behavior. The reason is

Base ar[] = { ChildA(), ChildB(), ChildC() };

initializes array elements by copying objects of three different classes onto objects of class Base and that yields objects of class Base and therefore you observe behavior of class Base from each element of the array.

If you want to store objects of different classes you have to allocate them with new and store pointers to them.

Answer 2

To achieve polymorphic behaviour you expected, you should use array of pointers to Base and create objects via new:

Base* ar[] = { new ChildA(), new ChildB(), new ChildC() };