C11_Generic将true和false推断为整数

在C11中,有_Generic宏可以允许很酷的泛型函数.但是,使用true和false会导致在正常情况下不正确的推论：

#include <stdio.h>
#include <stdbool.h>

#define TypeName(x) \
  _Generic((x),\
    bool: "bool",\
    int: "int",\
    default: "unkNown")

#if defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L) && (__bool_true_false_are_defined)
# undef true
# define true ((bool)(1))
# undef false
# define false ((bool)(0))
#endif

int main(void)
{
    printf("1: %s\n",TypeName(1));
    printf("true: %s\n",TypeName(true));
    printf("false: %s\n",TypeName(false));
}

这打印：

1: int
true: bool
false: bool

但是没有重新定义真假的中间位：

1: int
true: int
false: int

这意味着您无法执行_Generic功能,例如：

struct Variant * const int32 = variant_create(1);
struct Variant * const boolean = variant_create(true);

所以我的问题是：

>重新定义片段是安全的吗？
>这是C11标准中的疏忽还是GCC和Clang中的错误？

解决方法

这两种类型都是宏：

7.18 Boolean type and values

3. The remaining three macros are suitable for use in #if preprocessing directives.
   They are:
   true which expands to the integer constant 1,
   false which expands to the integer constant 0,
   and
   __bool_true_false_are_defined which expands to the integer constant 1.

最后一条规则说您可以重新定义宏：

4. Notwithstanding the provisions of 7.1.3,a program may undefine and perhaps then
   redefine the macros bool,true,and false. ²⁵⁹⁾

   ^{259) See ‘‘future library directions’’ (7.31.9)}

尽管引用了规则：

7.1.3 Reserved identifiers

3. If the program removes (with #undef) any macro deFinition of an identifier in the first
   group listed above,the behavior is undefined.

规则7.31.9说重新定义可能不是一个好主意：

7.31.9 Boolean type and values

1. The ability to undefine and perhaps then redefine the macros bool,and false is
   an obsolescent feature.

所以我建议你创建自己的my_true和my_false宏,这些宏被转换为_Bool.

原文地址：https://www.jb51.cc/c/116175.html