Is it possible to use X-Macro with std::variant (or with template in general)?

Question

I hope to do the following using X-macro with c++17, but since template parameter does not support trailing comma, it does not work for the std::variant part. Is there someway around it?

#define LIST_OF_TYPES(X) \
  X(Type1)               \
  X(Type2)               \
  X(Type3)

#define MAKE_TYPE(name) class name {};
LIST_OF_TYPES(MAKE_TYPE)
#undef MAKE_TYPE

std::variant<
#define MAKE_VARIANT(name) name,
LIST_OF_TYPES(MAKE_VARIANT)
#undef MAKE_VARIANT
>

Answer 1

Yes, there's a workaround:

#define EMPTY(...)
#define IDENTITY(...) __VA_ARGS__
#define IDENTITY2(...) __VA_ARGS__

std::variant<
#define MAKE_VARIANT(name) (,) name IDENTITY
IDENTITY2(EMPTY LIST_OF_TYPES(MAKE_VARIANT) () )
#undef MAKE_VARIANT
>

Without IDENTITY2(...) this expands to EMPTY(,) Type1 IDENTITY(,) Type2 IDENTITY(,) Type3 IDENTITY(). IDENTITY2 forces it to expand again, this time to Type1, Type2, Type3.

---

Or, with my own macro looping library:

run on gcc.godbolt.org

#include <macro_sequence_for.h>

#define LIST_OF_TYPES (Type1)(Type2)(Type3)

#define DECLARE_CLASSES(seq) SF_FOR_EACH(DECLARE_CLASSES_BODY, SF_NULL, SF_NULL,, seq)
#define DECLARE_CLASSES_BODY(n, d, x) class x {};

#define MAKE_VARIANT(seq) std::variant<SF_FOR_EACH(MAKE_VARIANT_BODY, USE_COMMA, SF_NULL, EMPTY, seq)>
#define MAKE_VARIANT_BODY(n, d, x) d() x
#define EMPTY(...)
#define COMMA(...) ,
#define USE_COMMA(n, d, x) COMMA

DECLARE_CLASSES(LIST_OF_TYPES) // class Type1 {}; class Type2 {}; class Type3 {};
MAKE_VARIANT(LIST_OF_TYPES) // std::variant<Type1, Type2, Type3>

Slightly more verbose, but more readable in my taste.

Here, #define MAKE_VARIANT_BODY(n, d, x) d() x is called for each element, with x being the element, and d initially set to EMPTY (4th argument of SF_FOR_EACH()). After the first (and any subsequent) iteration, d is reassigned to USE_COMMA(...) (aka COMMA), so starting from the second iteration d() expands to , instead of .

Answer 2

Put the comma at the beginning and add a first element of some kind, whether it is relevant or not. One possibility:

template<template<typename...> class Tmpl, typename... Ts>
using specialization = Tmpl<Ts...>;

using my_variant = specialization<std::variant
    #define MAKE_VARIANT(name) ,name
    LIST_OF_TYPES(MAKE_VARIANT)
    #undef MAKE_VARIANT
>;