The basic rules for dynamic_cast

The rules are rather complicated, especially when:

• There is multiple inheritance

• We cast to or from the type void*

We are just going to look at some basic cases.

Assume class B is a polymorphic type

Assume furthermore that class D is derived from B directly or indirectly, that is, through multiple layers of inheritance.

• Due to inheritance, D is also a polymorphic type.

The dynamic_cast from D* (up) to B* succeeds.

• This is known as an upcast.

The dynamic_cast from B* (down) to D* fails.

• This is known as an downcast.

The dynamic_cast from B* to B* succeeds, as does the dynamic_cast from D* to D*.

Assume Apple and Orange are unrelated polymorphic types

• Both have virtual functions, either declared directly or through inheritance.

Then:

• The dynamic_cast from Apple* to Orange* fails.

• The dynamic_cast from Orange* to Apple* fails.

Generally, casts involving unrelated types, other than void*, fail.