c++ 递归宏无法在 MSVC 上编译？

Question

I got this source code from [https://www.scs.stanford.edu/~dm/blog/va-opt.html]. Using MSVC with C++20 it doesn't compile, but it does compile on other compilers. Why? And how can I fix that?

`/* compile with:
   c++ -std=c++20 -Wall -Werror make_enum.cc -o make_enum
 */

#include <iostream>

#define PARENS ()

// Rescan macro tokens 256 times
#define EXPAND(arg) EXPAND1(EXPAND1(EXPAND1(EXPAND1(arg))))
#define EXPAND1(arg) EXPAND2(EXPAND2(EXPAND2(EXPAND2(arg))))
#define EXPAND2(arg) EXPAND3(EXPAND3(EXPAND3(EXPAND3(arg))))
#define EXPAND3(arg) EXPAND4(EXPAND4(EXPAND4(EXPAND4(arg))))
#define EXPAND4(arg) arg

#define FOR_EACH(macro, ...)                                    \
  __VA_OPT__(EXPAND(FOR_EACH_HELPER(macro, __VA_ARGS__)))
#define FOR_EACH_HELPER(macro, a1, ...)                         \
  macro(a1)                                                     \
  __VA_OPT__(FOR_EACH_AGAIN PARENS (macro, __VA_ARGS__))
#define FOR_EACH_AGAIN() FOR_EACH_HELPER

#define ENUM_CASE(name) case name: return #name;

#define MAKE_ENUM(type, ...)                    \
enum type {                                     \
  __VA_ARGS__                                   \
};                                              \
constexpr const char *                          \
to_cstring(type _e)                             \
{                                               \
  using enum type;                              \
  switch (_e) {                                 \
  FOR_EACH(ENUM_CASE, __VA_ARGS__)              \
  default:                                      \
    return "unknown";                           \
  }                                             \
}

MAKE_ENUM(MyType, ZERO, ONE, TWO, THREE);

void
test(MyType e)
{
  std::cout << to_cstring(e) << " = " << e << std::endl;
}

int
main()
{
  test(ZERO);
  test(ONE);
  test(TWO);
  test(THREE);
}

/*
  Local Variables:
  c-macro-pre-processor: "c++ -x c++ -std=c++20 -E -"
  End:
*/`

I cant figure out for the life of me what the problem is. I've tried modifying the code, to me it seems that __VA_ARGS__ isn't accepted when it should be, or that it can't accept the way the macros are ordered. Here are the errors:

1>make_enum.cc
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): warning C4003: not enough arguments for function-like macro invocation 'FOR_EACH_HELPER'
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): warning C4003: not enough arguments for function-like macro invocation 'ENUM_CASE'
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): error C2059: syntax error: 'case'
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): error C2065: 'ENUM_CASE': undeclared identifier
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): error C3861: 'FOR_EACH_HELPER': identifier not found
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): error C2059: syntax error: ')'
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): error C2146: syntax error: missing ';' before identifier 'default'
1>C:\Users\Thomas\source\repos\outoftouch\outoftouch\make_enum.cc(40,1): warning C4065: switch statement contains 'default' but no 'case' labels
1>Done building project "outoftouch.vcxproj" -- FAILED.

Answer 1

The old preprocessor in MSVC is known to contain many problems, see eg this article by Microsoft. Their new preprocessor can be used via the /Zc:preprocessor flag, causing the preprocessor to be more standard conform and also closer to what other major compilers do.

In this specific case, I think the problem is that the old preprocessor incorrectly expands FOR_EACH_AGAIN in FOR_EACH_HELPER early; it shouldn't, because it is not followed by parentheses. Compare the corresponding section "Rescanning replacement list for macros" in the above mentioned article . The warning "not enough arguments for function-like macro invocation 'FOR_EACH_HELPER'" also hints at that.

With /Zc:preprocessor , the code compiles without issues. Example on godbolt .