Protected/Private Inheritance Casting

Question

I came across this issue accidentally when I was going over inheritance and up/down casting. Why is this not allowed (code is commented to show sections that are not allowed)? Now I can guess as to why it is not allowed but a factual answer would be great.

As for the code that is allowed, I know it is because (Base*) is a C-style cast which is essentially a reinterpret_cast in C++ which in turn means that in this case it will result in undefined behavior. Please correct me if I am wrong.

class Base
{
};

class Derived : public Base
{
};

class DerivedProt : protected Base
{
};

class DerivedPriv : private Base
{
};

int main()
{
  Base* a = new Derived();
  Base* b = new DerivedProt();  // Not allowed
  Base* c = new DerivedPriv();  // Not allowed

  Base* d = (Base*) new DerivedProt(); // Allowed but undefined behavior...?
  Base* e = (Base*) new DerivedPriv(); // Allowed but undefined behavior...?
}

Answer 1

The Standard explicitly specifies that C-style casts may perform this conversion. It's the only cast which C-style casts can do but no C++ cast can do. The results are not undefined a far as I know; it's just not allowed by any other cast.

Answer 2

Sounds like you are correct.

One thing to remember is that traditional OO principles such as the LSP only describe public inheritance. Non-public inheritance falls in-between inheritance and composition, the base subobject is non-public like composition, but you can also take advantage of features which rely on inheritance, such as virtual functions.

Just like a composed subobject, however, only the class (or its descendants, in case of protected inheritance), can get the address of the subobject.