C++ type registration at compile time trick

There is a way to register types one by one and then retrieve all of them in the form of mpl::vector or similar. I’ve learned this trick on the boost mailing lists (perhaps from Dave Abrahams, although I can’t recall for sure).

Edit: I learned it from slide 28 on https://github.com/boostcon/2011_presentations/raw/master/thu/Boost.Generic.pdf.

I won’t use MPL in the code to make it self contained.

// The maximum number of types that can be registered with the same tag.
enum { kMaxRegisteredTypes = 10 };

template <int N>
struct Rank : Rank<N - 1> {};

template <>
struct Rank<0> {};

// Poor man's MPL vector.
template <class... Ts>
struct TypeList {
  static const int size = sizeof...(Ts);
};

template <class List, class T>
struct Append;

template <class... Ts, class T>
struct Append<TypeList<Ts...>, T> {
  typedef TypeList<Ts..., T> type;
};

template <class Tag>
TypeList<> GetTypes(Tag*, Rank<0>) { return {}; }

// Evaluates to TypeList of all types previously registered with
// REGISTER_TYPE macro with the same tag.
#define GET_REGISTERED_TYPES(Tag) \
  decltype(GetTypes(static_cast<Tag*>(nullptr), Rank<kMaxRegisteredTypes>()))

// Appends Type to GET_REGISTERED_TYPES(Tag).
#define REGISTER_TYPE(Tag, Type)                              \
  inline Append<GET_REGISTERED_TYPES(Tag), Type>::type        \
  GetTypes(Tag*, Rank<GET_REGISTERED_TYPES(Tag)::size + 1>) { \
    return {};                                                \
  }                                                           \
  static_assert(true, "")

Usage example:

struct IntegralTypes;
struct FloatingPointTypes;

// Initially both type lists are empty.
static_assert(std::is_same<GET_REGISTERED_TYPES(IntegralTypes), TypeList<>>::value, "");
static_assert(std::is_same<GET_REGISTERED_TYPES(FloatingPointTypes), TypeList<>>::value, "");

// Add something to both lists.
REGISTER_TYPE(IntegralTypes, int);
REGISTER_TYPE(FloatingPointTypes, float);
static_assert(std::is_same<GET_REGISTERED_TYPES(IntegralTypes), TypeList<int>>::value, "");
static_assert(std::is_same<GET_REGISTERED_TYPES(FloatingPointTypes), TypeList<float>>::value, "");

// Add more types.
REGISTER_TYPE(IntegralTypes, long);
REGISTER_TYPE(FloatingPointTypes, double);
static_assert(std::is_same<GET_REGISTERED_TYPES(IntegralTypes), TypeList<int, long>>::value, "");
static_assert(std::is_same<GET_REGISTERED_TYPES(FloatingPointTypes), TypeList<float, double>>::value, "");