Get nearest base class

Question

I have a class hierarchy:

struct B {};
struct D1 : B {};
struct D2 : B {};

Is it possible using some sort of std:: helper functions/classes (likely from <type_traits>) get common base class (i.e. B) for two variables:

D1 d1;
D2 d2;

I had tried to use std::common_type_t<decltype(d1), decltype(d2)>, but no luck.

Answer 1

http://www.cplusplus.com/reference/type_traits/common_type/

If you look at the code that implements common_type, it appears that it selects between the types that are inputted into the list. This is the reference behavior:

template <class... Types> struct common_type;

template <class T> struct common_type<T> {
  typedef T type;
};

template <class T, class U> struct common_type<T,U> {
  typedef decltype(true?declval<T>():declval<U>()) type;
};

template <class T, class U, class... V> struct common_type<T,U,V...> {
  typedef typename common_type<typename common_type<T,U>::type,V...>::type     type;
};

So if you only input one type "T", then it returns a typedef of that type. If you input two types, the "magic" happens in this line:

true?declval<T>():declval<U>()

What this does is it uses a ternary instruction (X?Y:Z) with an always-true argument to pick between two possible outcomes. Since a ternary can only have one return type, then the normal conversion rules of C++ apply: the standard conversion rules are applied to ensure this instruction has only one return type. That is the "trick" here. That process fails when you plug a D1 and a D2 into it, because the ternary instruction doesn't know what to turn both a D1 and a D2 into. So common_type relies on already-existing conversion logic inside C++ and can't actually find class "B".

You can in fact check B against D1, or B against D2, or B / D1 / D2 all at the same time (it uses template recursion to solve three or more), and each of these common_type calls will correctly tell you that B is the common type, but without including B in the check, the code implementation cannot return B for you.

If you want what you're asking for you probably need to build it specifically for the class heirarchy that you're after.