Tagged unions (aka variant) in C++ with the same type multiple times

Question

I need to create an union, but 2 members of the union would have the same type, thus I need a way to identify them. For example in OCaml :

type A = 
  | B of int
  | C of float
  | D of float

Boost.Variant doesn't seem to support this case, is there a known library which supports that ?

Answer 1

If you want to do this, I think your best option is to wrap the same-but-different-types into a struct which then lets the boost variant visit the proper one:

struct Speed
{
    float val_;
};

struct Darkness
{
    float val_;
};

You might be able to use BOOST_STRONG_TYPEDEF to do this automatically but I'm not sure it's guaranteed to generate types legal for use in a union (although it would probably be fine in a variant).