Detect common base class

Question

Suppose one has a class hierarchy, without multiple inheritance:

struct TOP{};
struct L : TOP{}; 
struct R : TOP{};
struct LL : L{};
struct LR : L{};
struct RL : R{};
struct RR : R{};

Is it possible to write a metafunction that will return the type of the common base of two types? (it could return void if not common base class exists.) For example

common_base<RR, R>::type == R
common_base<RL, RR>::type == R
common_base<LL, RR>::type == TOP
common_base<LL, std::string>::type == void

Obviously this wouldn't work with multiple inhertance, but I am focused in the the single inheritance case.

First, it doesn't seem to be possible without some introspection of the base class. So, I have this easier problem, do it in such a way that each clase knows its base (by an internal base type), for example:

struct LR : L{using base = L;};

Even in this way, I cannot seem to get the metaprogramming right.

Also I read somewhere (I can't find it now) that GCC has some extensions to detect common base class. Is that the case?

Answer 1

There was at some point bases and direct_bases in std::tr2 but that wasn't included. Some versions of gcc have it. Using these perhaps you can get what you want.