Can a pointer of a derived class be type cast to the pointer of its base class?

Question

The pointer of derived class returned by new can be type cast to the pointer of its base class.

Is this true or false?

I know dynamic_cast can be used to cast downside. Generally, how to cast a pointer of derived class to a pointer of its base class?

Answer 1

Yes. Conversion from a pointer to a derived class to a pointer to a base class is implicit. Thus, the following is perfectly fine:

struct B { };
struct D : B { };

D* my_d_ptr = new D;
B* my_d_ptr_as_a_b_ptr = my_d_ptr;

Answer 2

Casting a pointer to a derived to a pointer to base should be implicit. This is the whole point of polymorphism: An instance of a derived class should always be safely usable as an instance of the base class. Therefore, no explicit cast is necessary.

Answer 3

That's true if derived class inherits 'publicly' and 'non-virtually' from base:

You can't convert Derived* to Base* neither implicitly nor using static_cast/dynamic_cast (C-cast will do the job, but you should think twice before use this hack!);

class Base { };

class Derived : protected Base { };

int main()
{
   Base* b = new Derived(); // compile error
}

Also don't work if base class is ambiguous:

class Base { };

class Derived1 : public Base { };
class Derived2 : public Base { };

class MostDerived : public Derived1, Derived2 { };

int main()
{
   Base* b = new MostDerived(); // won't work (but you could hint compiler
                                // which path to use for finding Base
}

Edit: added code samples, added ambiguous use case, removed virtual inheritance example.