C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 19:
[19.4] Is it OK to convert a pointer from a derived class to its base class?

Yes.

An object of a derived class is a kind of the base class. Therefore the conversion from a derived class pointer to a base class pointer is perfectly safe, and happens all the time. For example, if I am pointing at a car, I am in fact pointing at a vehicle, so converting a Car* to a Vehicle* is perfectly safe and normal:

void f(Vehicle* v);
void g(Car* c) { f(c); }  // Perfectly safe; no cast
(Note: this FAQ has to do with public inheritance; private and protected inheritance are different.)