C预处理器:将宏迭代展开为逗号分隔的列表

在帖子中使用Paul Fultz II的解决方案 C-preprocessor recursive macro ,我想扩展无限个带括号的宏参数,例如。

#define MY_CHAIN (alpha) (beta) (gamma)

输入一个逗号分隔的列表,该列表可以传递给变量宏,例如。

CHAIN_COMMA(MY_CHAIN) // alpha, beta, gamma

我可以扩展成大括号 [alpha] [beta] [gamma] 并用我试过的所有东西来分隔列表,除了逗号, alpha :: beta :: gamma 在下面的示例中。

以下是我的完整(编译)代码:

#include <iostream>
using namespace std;

// unrelated macro utilities
#define SEE(expression) cout << #expression ": " << STR(expression) << endl;
#define CMD(function, ...) function(__VA_ARGS__)
#define STR(s) CMD(STR_, s)
#define STR_(s) #s

// concatenation
#define CAT(x, y) CAT_(x, y)
#define CAT_(x,y) x ## y // error from CHAIN_COMMA: passed 4 arguments

// surround each chain element with square brackets []
#define CHAIN_BRACE(chain) CAT(CHAIN_BRACE_1 chain, _END)
#define CHAIN_BRACE_1(x) [x] CHAIN_BRACE_2
#define CHAIN_BRACE_2(x) [x] CHAIN_BRACE_1
#define CHAIN_BRACE_1_END
#define CHAIN_BRACE_2_END

// separate each chain element with the scope operator ::
#define CHAIN_SCOPE(chain) CAT(CHAIN_SCOPE_0 chain, _END)
#define CHAIN_SCOPE_0(x) x CHAIN_SCOPE_1
#define CHAIN_SCOPE_1(x) :: x CHAIN_SCOPE_2
#define CHAIN_SCOPE_2(x) :: x CHAIN_SCOPE_1
#define CHAIN_SCOPE_0_END
#define CHAIN_SCOPE_1_END
#define CHAIN_SCOPE_2_END

// trouble here: can't separate chain elements with commas
#define CHAIN_COMMA(chain) CAT(CHAIN_COMMA_0 chain, _END) // error
#define CHAIN_COMMA_0(x) x CHAIN_COMMA_1
#define CHAIN_COMMA_1(x) , x CHAIN_COMMA_2
#define CHAIN_COMMA_2(x) , x CHAIN_COMMA_1
#define CHAIN_COMMA_0_END
#define CHAIN_COMMA_1_END
#define CHAIN_COMMA_2_END

// define a custom chain and save various forms of it
#define MY_CHAIN (alpha) (beta) (gamma)
#define MY_BRACES CHAIN_BRACE(MY_CHAIN) // [alpha] [beta] [gamma]
#define MY_SCOPES CHAIN_SCOPE(MY_CHAIN) // alpha :: beta :: gamma
#define MY_COMMAS CHAIN_COMMA(MY_CHAIN) // alpha , beta , gamma

int main() {
    SEE(MY_CHAIN);
    SEE(MY_BRACES);
    SEE(MY_SCOPES);
//  SEE(MY_COMMAS); // error: macro "CAT_" passed 4 arguments, but takes just 2
    return 0;
}

这将输出:

MY_CHAIN: (alpha) (beta) (gamma)
MY_BRACES: [alpha] [beta] [gamma]
MY_SCOPES: alpha :: beta :: gamma

我尝试用括号括起逗号分隔的列表,但CAT不会追加 ) 到 _END 。有什么聪明的想法可以扩展到 alpha, beta, gamma ?