Thread: My memory fails me, inheritance and casting

I read about this somewhere, but I completely forget where, so maybe someone could help me. This problem deals with inheritance and casting. As an example, I'll use, class A and class B which is a subclass of A.

Code:

class A {
     int num;
}

class B : public A {
     int num2;
}

You can't cast A into B, because an A is not a B, but a B is an A, so you can cast it, however you will lose certain data. For example:

Code:

A anA;
anA.num = 5;
B aB;
ab.num = 1;
ab.num2 = 2;
anA = (A)aB;

That should work, but anA is not a B so it can not hold the num2 value, so data would be lost in the conversion. But I read a way to get around this, using pointers or something. Anyone have any idea?

Code:

A anA;
anA.num = 5;
B aB;
ab.num = 1;
ab.num2 = 2;
anA = (A)aB;

this code creates a new instance of a new A object.
To keep a B object and change only a pointer yo have 2 choices.

Code:

A anA;
anA.num = 5;
B *aB = dynamic_cast<B*>(&anA);
ab->num = 1;
ab->num2 = 2;

//or
A anA;
anA.num = 5;
B &aB = dynamic_cast<B&>(anA);
ab.num = 1;
ab.num2 = 2;

This is how you keep the original object that use inheritance to handle different pointers.
To could also use static_cast, but it could cause a runtime-error if the class instance to be casted isn't the of the type of the cast. To prevent that kind of errors there is dynamic_cast which return NULL is the original object can«t be casted. Therefore you can only use dynamic_cast with pointers, not with objects os references.
Plus, dynamic_cast checks the type of the object at runtime. To do so the dynamic_cast need info which will help in determinig is the class is valid. I don't kno what the standard says about this, but for MS compilers, each class much have at least a virtual method non-pure. The vtable will be used for comparison at runtime.