C++ -- Questions about "protected inheritance"

Question

I assume that I understand the meaning of "protected inheritance". However, after discussing this issue with one folk here, I feel a little confused now.

Here is my understanding of "protected inheritance" in c++

Assume the following class structure.

class Base {}
class SubClass : protected Base {}

1> If a subclass is defined as "protected BaseClass", then this subclass is no longer a subclass of the BaseClass. Instead, the BaseClass only serves as a utility tool for the subclass. In other word, if you cast SubClass* to Base*, SubClass& to Base&, or SubClass to Base, you should expect an error.

2> The major reason why people use protected inheritance is that the expected SubClass is NOT a subclass of the Base (For example, Car is not a subclass of Engine). While, in the same time, SubClass wants to call the functions defined in the Base class.

3> There is a good reason sometimes you prefer to using protected inheritance rather than define a member variable as an object of the Base. (but I don't remember in which case).

Please correct my comments if I am wrong.

thank you

Answer 1

For 1. - SubClass is still a subclass of Base. Protected inheritance is still inheritance. You are correct that automatic conversion from SubClass to Base is not possible, though.

SubClass sub;
Base* base(&sub);

gives

error C2243: 'type cast' : conversion from 'SubClass *' to 'Base *' exists, but is inaccessible

For 2. and 3. - the major reason people want this is to hide public/protected members of Base from clients of SubClass. Subclasses of SubClass CAN still see them. Contrast this with private inheritance which hides Base completely from subclasses and clients of SubClass - this would also meet your criteria in the last sentence of 2, but disallow any other external use of Base via SubClass.