Can I cast a derived class to a private base class, using C-style cast?

Question

Can I do this?

class A { ... };

class B : private A
{
    const A &foo() const
    {
        return *((const A *)this);
    }
};

Can I take a subclass that inherits privately from a base class and cast it to a public version of its base class? Can I do this without it having virtual methods?

My guess is yes, but I wanted to make sure it's safe / portable.

Answer 1

Yes you can: §5.4/7 of the standard:

... the following static_cast and reinterpret_cast operations (optionally followed by a const_cast operation) may be performed using the cast notation of explicit type conversion, even if the base class type is not accessible:

a pointer to an object of derived class type or an lvalue of derived class type may be explicitly converted to a pointer or reference to an unambiguous base class type, respectively;

But try not to as it defeats the purpose of private inheritance.

Answer 2

Yes, that is explicitly allowed. Alexandrescu uses this extensively in Modern C++ Design for his approach of policy based design:

template <typename Policy>
class foo : Policy
{
    public:
        void do_something()
        {
            Policy & p = *this;
            p.do_something();
        }
};

So while the use cases may be limited, there are some out there.