使用 gcc.exe(Rev3,由 MSYS2 项目构建)8.2.0。
我试图构建一个宏来自动在两种类型之间进行类型转换,其中两个参数永远不应该是相同的类型。我的问题是,如果我不包含相同类型的情况,编译器会抛出错误。
我想要什么:
#include <stdio.h>
#include <stdint.h>
// Macro to return string based on two different types
#define bob( to, from ) \
_Generic( to , \
int32_t: _Generic(from, \
int16_t: "s-l", \
int8_t: "c-l" ) , \
int16_t: _Generic(from, \
int32_t: "l-s", \
int8_t: "c-s") , \
int8_t:_Generic(from, \
int32_t: "l-c", \
int16_t: "s-c") \
)
void main(void)
{
int32_t i1;
int16_t s1;
int8_t c1;
printf("%s\n", bob(i1,s1));
printf("%s\n", bob(i1,c1));
printf("%s\n", bob(s1,c1));
printf("%s\n", bob(s1,i1));
printf("%s\n", bob(c1,s1));
printf("%s\n", bob(c1,s1));
}
$ gcc gbug.c -o gbug.exe
gbug.c: In function 'main':
gbug.c:23:27: error: '_Generic' selector of type 'short int' is not compatible with any association
printf("%s\n", bob(i1,s1));
^~
gbug.c:9:19: note: in definition of macro 'bob'
int16_t: _Generic(from, \
^~~~
gbug.c:24:27: error: '_Generic' selector of type 'signed char' is not compatible with any association
printf("%s\n", bob(i1,c1));
^~
gbug.c:12:17: note: in definition of macro 'bob'
int8_t:_Generic(from, \
^~~~
gbug.c:25:27: error: '_Generic' selector of type 'signed char' is not compatible with any association
printf("%s\n", bob(s1,c1));
^~
gbug.c:12:17: note: in definition of macro 'bob'
int8_t:_Generic(from, \
^~~~
gbug.c:26:27: error: '_Generic' selector of type 'int' is not compatible with any association
printf("%s\n", bob(s1,i1));
^~
gbug.c:6:19: note: in definition of macro 'bob'
int32_t: _Generic(from, \
^~~~
gbug.c:27:27: error: '_Generic' selector of type 'short int' is not compatible with any association
printf("%s\n", bob(c1,s1));
^~
gbug.c:9:19: note: in definition of macro 'bob'
int16_t: _Generic(from, \
^~~~
gbug.c:28:27: error: '_Generic' selector of type 'short int' is not compatible with any association
printf("%s\n", bob(c1,s1));
^~
gbug.c:9:19: note: in definition of macro 'bob'
int16_t: _Generic(from, \
这个例子是我发现的最简单的例子,但会失败。
如果我添加“相同类型”转换行,如下所示:
#define bob( to, from ) \
_Generic( to , \
int32_t: _Generic(from, \
int16_t: "s-l", \
int32_t: "bug", \
int8_t: "c-l" ) , \
int16_t: _Generic(from, \
int32_t: "l-s", \
int16_t: "bug", \
int8_t: "c-s") , \
int8_t:_Generic(from, \
int32_t: "l-c", \
int8_t: "bug", \
int16_t: "s-c") \
)
它构建并运行并达到预期结果:
$ ./gbug.exe
s-l
c-l
c-s
l-s
s-c
s-c
验证我没有使用宏来扩展任何相同类型的条件。
我知道 _Generic 不是字符串替换宏,但我也认为,如果您可以在没有默认情况下使用它,那么如果您使用未知类型(或不受支持的类型组合,这是我想要的行为),它会正确抛出编译错误)
这就像预处理器将两个宏参数混合在一起。
编辑:所以我有了更好的理解,(请参阅下面的答案),但仍然希望在两个参数类型相同时让宏抛出编译错误。到目前为止,我有一个技巧来强制出现链接错误,这仍然比运行时错误更好。