Reinterpreting a union to a different union

Question

I have a standard-layout union that has a whole bunch of types in it:

union Big {
    Hdr h;

    A a;
    B b;
    C c;
    D d;
    E e;
    F f;
};
         


        

Answer 1

I'm not sure, if this really applies here. In the reinterpret_cast - Notes section they talk about pointer-interconvertible objects.

And from [basic.compound]/4:

Two objects a and b are pointer-interconvertible if:

• they are the same object, or
• one is a union object and the other is a non-static data member of that object, or
• one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, the first base class subobject of that object, or
• there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

In this case, we have Hdr h; (c) as a non-static data member in both unions, which should allow to (because of the second and last bullet point)

Big* (a) -> Hdr* (c) -> Little* (b)