将宏的可变参数 arguments 拆分成对

Question

Suppose I want to treat the arguments of a macro as pairs and then pass it to something else (expansively)

I imagine it looking something like this (pseudocode):

template<unsigned index,
         typename type>
struct my_pair
{
    static constexpr auto i = index;
    using t = type;
};
#define pair_off(...) my_pair<__VA_ARGS__[0], __VA_ARGS__[1]>, ...

Answer 1

With a variant of code from Is it possible to iterate over arguments in variadic macros? , you might do:

#define CONCATENATE(arg1, arg2)   CONCATENATE1(arg1, arg2)
#define CONCATENATE1(arg1, arg2)  CONCATENATE2(arg1, arg2)
#define CONCATENATE2(arg1, arg2)  arg1##arg2

#define FOR_EACH_BY_PAIR_2(what, x, y)\
  what(x, y)
#define FOR_EACH_BY_PAIR_4(what, x, y, ...)\
  what(x, y)\
  FOR_EACH_BY_PAIR_2(what,  __VA_ARGS__)
#define FOR_EACH_BY_PAIR_6(what, x, y, ...)\
  what(x, y)\
  FOR_EACH_BY_PAIR_4(what,  __VA_ARGS__)
#define FOR_EACH_BY_PAIR_8(what, x, y, ...)\
  what(x, y)\
  FOR_EACH_BY_PAIR_6(what,  __VA_ARGS__)

#define NARG(...) NARG_(__VA_ARGS__, RSEQ_N())
#define NARG_(...) ARG_N(__VA_ARGS__) 
#define ARG_N(_1, _2, _3, _4, _5, _6, _7, _8, N, ...) N 
#define RSEQ_N() 8, 7, 6, 5, 4, 3, 2, 1, 0

#define FOR_EACH_BY_PAIR_(N, what, ...) CONCATENATE(FOR_EACH_BY_PAIR_, N)(what, __VA_ARGS__)
#define FOR_EACH_BY_PAIR(what, ...) FOR_EACH_BY_PAIR_(NARG(__VA_ARGS__), what, __VA_ARGS__)

Limit is currently 8, up to you to increase that.

and then, you specific part:

#define MAKE_PAIR(x, y) COMMA my_pair<x, y>{}
#define COMMA ,
#define pair_off(p1, p2, ...) my_pair<p1, p2>{} FOR_EACH_BY_PAIR(MAKE_PAIR, __VA_ARGS__)

Demo