嵌套宏调用

Question

尝试嵌套宏调用，如下所示：

#include <stdint.h>

#define INT
#define LONG

#define AS(t) AS_##t
#define AS_INT as_int
#define AS_LONG as_long

#define LET(v, t) v. AS(t)

typedef union
{
    int32_t as_int;
    int64_t as_long;
} mytype_t;

int main()
{
    mytype_t s;

    s.AS(INT) = 10;    /* This is OK */

    LET(s, INT) = 10;  /* This makes error */

    return 0;
}

它产生错误：

main.c:xx:yy: error: ‘mytype_t {aka union <anonymous>}’ has no member named ‘AS_’
 #define LET(v, t) v. AS(t)
                    ^
main.c:zz:ww: note: in expansion of macro ‘LET’
     LET(s, INT) = 10;
     ^~~

有什么解决方法LET(s, INT) = 10;吗？

Answer 1

这是两个空的宏

#define INT
#define LONG

INT传递给AS(t)via 时，将LET进行中间扩展，并在扩展AS()自身之前将其扩展为空令牌序列。这样，您将连接AS_一个空的令牌序列。只需删除这两个宏，对于您的示例，具有AS_INT和AS_LONG定义就足够了。