C宏_Generic产生了意外的编译器错误 [英] C macro _Generic gives unexpected compiler error

问题描述

使用gcc.exe(Rev3，由MSYS2项目构建)8.2.0.

Using gcc.exe (Rev3, Built by MSYS2 project) 8.2.0.

我试图建立一个宏来自动进行两种类型之间的类型转换，其中两个参数绝不能是同一类型.我的问题是，如果我也不包含相同类型的大小写，则编译器将引发错误.我想要的是:

I was trying to build a macro to automatically do type conversions between two types, where the two parameters should never be the same type. My problem is the compiler throws an error if I don't also include the same type case. What I wanted:

#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, \

这个例子是我发现失败的最简单的例子.

This example is the simplest that I have found that will fail.

如果我这样添加相同类型"的转换行:

If I add in the "same type" conversion lines like this:

#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")  \
)

它可以生成并以预期的结果运行:

It build and runs with the expected result:

$ ./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的类型条件.我了解_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，则在使用未知类型(或不支持的类型组合)时，它将正确引发编译错误，这是我想要的行为)就像预处理器将两个宏参数混合在一起.

verifying that I am not using the macro to expand any same type conditions. I understand _Generic is not a string substitution macro but I also thought that if you could use it without a default case it would correctly throw a compile error if you used an unknown type (or an unsupported combination of types, which is the behaviour I wanted) It is like the pre-processor is getting the two macro parameters mixed up.

所以我有一个更好的理解，(请参阅下面的答案)，但是如果两个参数是同一类型，则仍希望获取该宏以引发编译错误.到目前为止，我有一个技巧来强制执行链接错误，该错误仍然比运行时错误好.

So I have a better understanding, (See my answer below) but still looking to get the macro to throw a compile error if the two parameters are the same type. So far I have a trick to force a link error which is still better than a runtime error.

推荐答案

问题在于，即使未评估泛型选择的每个分支，这些分支也必须有效.

The problem is that every branch of a generic selection must be valid, even if they are not evaluated.

例如，您的第一个宏:

bob(i1, s1)

扩展为(为清楚起见添加了类型):

Expands to (types added for clarity):

_Generic( ((int32_t) i1),
  int32_t: _Generic( ((int16_t) s1),
    int16_t: "s-l",
    int8_t:  "c-l" ),
  int16_t: _Generic( ((int16_t) s1),  // The error is here
    int32_t: "l-s",
    int8_t:  "c-s"),
  int8_t:_Generic( ((int16_t) s1),
    int32_t: "l-c",
    int16_t: "s-c")
)

显然， uint32_t 分支是有效的:它只是选择"s-l" .但是 int16_t 分支无效有效，因为 from ( int16_t 本身)没有相应的分支.

Obviously the uint32_t branch is valid: It just selects "s-l". But the int16_t branch is not valid, as from (An int16_t itself) does not have a corresponding branch.

在这种特殊情况下，添加不执行任何操作的自转换运算符不会有什么坏处.

In this particular scenario, it wouldn't hurt to add a self-conversion operator that does nothing.

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