C++ Polymorphism: from parent class to child [duplicate]

Answer 1

int main() { 
   Parent parent;
   Child child;

   // upcast - implicit type cast allowed
   Parent *pParent = &child; 

   // downcast - explicit type case required 
   Child *pChild = (Child *) &parent;
}

You should use the dynamic_cast to do this safely:

Child *p = dynamic_cast(pParent)

EDIT

With dynamic_cast returns a null pointer if the type is not apart of the base class, also casting to a reference throws a bad_cast exception. dynamic_cast is particularly useful if you do not know what the object type will be.

On the other hand static_cast:

Child *p = static_cast(pParent)

This assumes that you want to reverse an explicit conversion and perform no runtime checks. This allows for flexibility but must be used with caution.

The regular downcast shown above:

Child *pChild = (Child *) &parent;

Is a C-style down cast (like a static_cast), which may also cast to a private base class (not sure about, multiple inheritance), while static_cast would cause a compile-time error. Things like numeric conversions is a good example to use this on.