Why does protected inheritance cause dynamic_cast to fail?
c++, dynamic-cast, protected, public
Solution
You as an outside user cannot access the protected or private members of a class. The same applies to protected or private inheritance. The authors of a class don't want outside users to access protected/private parent classes any more than they want outside users to access protected/private members.
One reason: Suppose the parent class has a non-virtual destructor. Deleting from an instance derived class from a base class pointer would result in undefined behavior. Making the parent class protected/private means you can't do this (see footnote).
Another reason: Suppose the authors of the class in question don't want outside users to have access to the public members of the parent class. One could use public inheritance (is-a) and demote those public interfaces to protected or private, but this would violate the Liskov substitution principle. Protected or private inheritance is not an is-a relationship. Those public methods become protected or private with protected or private inheritance. There's no problem with Liskov substitution because protected/private inheritance is not is-a.
Footnote: There is an ugly way around this: Use C-style casts. Outside users can cast a derived class pointer to a base class pointer, even if the base class is not accessible. To me, that's yet another reason to compile with `-Wold-style-cast -Werror`.
Problem
I changed my C++ base class to be `protected` inheritance and my `dynamic_cast`(s) stopped working. Why should changing the inheritance to `protected` change the behavior of `dynamic_cast`? ``` struct Base { static Base *lookupDerived(); // Actually returns a Derived * object. }; struct Derived : protected /* Switch this to public to get it working */ Base { static void test() { Base *base = lookupDerived(); if (dynamic_cast<Derived *>(base)) { std::cout << "It worked (we must be using public inheritance)." << std::endl; } else { std::cout << "It failed (we must be using protected inheritance)." << std::endl; } }; ```