如果嵌套，C11 _Generic() 表达式会失败

Question

我试图找出为什么嵌套 _Generic() 表达式对我来说失败。如果我不嵌套它们，它们就可以正常工作。

当然，这在一个较长的程序中发生在我身上，但我将其总结为一个最小的工作示例，它触发了与较大程序中完全相同的错误。

我尝试一遍又一遍地检查代码，但没有找到确凿的证据...问题的原因是什么？

#include <stdio.h>

typedef struct sta
    {
    char a;
    }ta;

typedef struct stb
    {
    short b;
    }tb;

typedef struct stc
    {
    int c;
    }tc;

#define MAGICNUMBER(X) _Generic((X), \
ta: 1, \
tb: 2, \
tc: 3 \
)

#define NESTEDMAGICNUMBER3(X) _Generic((X), \
ta: 1 \
)

#define NESTEDMAGICNUMBER2(X) _Generic((X), \
tb: 2, \
default: NESTEDMAGICNUMBER3(X) \
)

#define NESTEDMAGICNUMBER(X) _Generic((X), \
tc: 3, \
default: NESTEDMAGICNUMBER2(X) \
)

int     main(int argc, char *argv[])
    {
    ta a;
    tb b;
    tc c;
    
    printf("direct macro: ta has # %d\n",MAGICNUMBER(a));
    printf("direct macro: tb has # %d\n",MAGICNUMBER(b));
    printf("direct macro: tc has # %d\n",MAGICNUMBER(c));
    printf("nested macro: ta has # %d\n",NESTEDMAGICNUMBER(a)); /* Compile-time error why? */
    printf("nested macro: tb has # %d\n",NESTEDMAGICNUMBER(b)); /* Compile-time error why? */
    printf("nested macro: tc has # %d\n",NESTEDMAGICNUMBER(c)); /* Compile-time error why? */
    
    return 0;
    }

编译时，我得到这个:

cc -o testgeneric testgeneric.c
testgeneric.c:49:39: error: controlling expression type 'tb' (aka 'struct stb') not compatible with any generic association type
        printf("nested macro: tb has # %d\n",NESTEDMAGICNUMBER(b)); /* Compile-time error why? */
                                             ^~~~~~~~~~~~~~~~~~~~
testgeneric.c:36:10: note: expanded from macro 'NESTEDMAGICNUMBER'
default: NESTEDMAGICNUMBER2(X) \
         ^~~~~~~~~~~~~~~~~~~~~
testgeneric.c:31:10: note: expanded from macro 'NESTEDMAGICNUMBER2'
default: NESTEDMAGICNUMBER3(X) \
         ^~~~~~~~~~~~~~~~~~~~~
testgeneric.c:25:40: note: expanded from macro 'NESTEDMAGICNUMBER3'
#define NESTEDMAGICNUMBER3(X) _Generic((X), \
                                       ^~~
testgeneric.c:50:39: error: controlling expression type 'tc' (aka 'struct stc') not compatible with any generic association type
        printf("nested macro: tc has # %d\n",NESTEDMAGICNUMBER(c)); /* Compile-time error why? */
                                             ^~~~~~~~~~~~~~~~~~~~
testgeneric.c:36:10: note: expanded from macro 'NESTEDMAGICNUMBER'
default: NESTEDMAGICNUMBER2(X) \
         ^~~~~~~~~~~~~~~~~~~~~
testgeneric.c:31:10: note: expanded from macro 'NESTEDMAGICNUMBER2'
default: NESTEDMAGICNUMBER3(X) \
         ^~~~~~~~~~~~~~~~~~~~~
testgeneric.c:25:40: note: expanded from macro 'NESTEDMAGICNUMBER3'
#define NESTEDMAGICNUMBER3(X) _Generic((X), \
                                       ^~~
2 errors generated.

为什么嵌套版本会失败，而非嵌套版本却可以正常工作？

谢谢!

Answer 1

Why does the nested version fail, while the non-nested version works fine?

考虑NESTEDMAGICNUMBER(b))的扩展:

_Generic((b), tc: 3, default: _Generic((b), tb: 2, default: _Generic((b), ta: 1 ) ) )

编译器拒绝此表达式，因为子表达式 _Generic((b), ta: 1 ) 由于没有 b 类型的情况而无效(tb，又名struct stb)。事实上，这不是将要选择的通用替代方案的一部分，这一事实是无关紧要的——程序源代码中的每个表达式和子表达式都必须有效，无论它是否有任何效果。您可以通过添加 default 替代 NESTEDMAGICNUMBER3 来解决该问题:

#define NESTEDMAGICNUMBER3(X) _Generic((X), \
ta: 1, \
default: 0 \
)