Why and when is cast to char volatile& needed?

问题

In boost::detail::addressof_impl::f() a series of reinterpret_casts is done to obtain the actual address of the object in case class T has overloaded operator&():

template<class T> struct addressof_impl
{
    static inline T* f( T& v, long )
    {
        return reinterpret_cast<T*>(
            &const_cast<char&>(reinterpret_cast<const volatile char&>(v)));
    }
}

What's the purpose of cast to const volatile char& instead of just casting to char&?

回答1:

A cast straight to char& would fail if T has const or volatile qualifiers - reinterpret_cast can't remove these (but can add them), and const_cast can't make arbitrary type changes.

回答2:

The object may be const or volatile, or both (as oxymoronic as that may be), in which case it is probably illegal to reinterpret_cast it to a type that lacks these attributes. (Going in the opposite direction is of course never a problem).

来源：https://stackoverflow.com/questions/2333960/why-and-when-is-cast-to-char-volatile-needed