为什么主编译器对stdint.h使用typedef,而对stdbool.h使用#define? [英] Why do major compilers use typedef for stdint.h but use #define for stdbool.h?
问题描述
我只是注意到gcc和clang似乎都对stdint.h使用typedef,但对stdbool.h使用#define。
I just noticed that gcc and clang both appear to use typedefs for stdint.h but #define for stdbool.h.
示例: clang的stdint.h
#ifdef __INT8_TYPE__
#ifndef __int8_t_defined /* glibc sys/types.h also defines int8_t*/
typedef __INT8_TYPE__ int8_t;
#endif /* __int8_t_defined */
typedef __UINT8_TYPE__ uint8_t;
# define __int_least8_t int8_t
# define __uint_least8_t uint8_t
#endif /* __INT8_TYPE__ */
#ifndef __cplusplus
#define bool _Bool
#define true 1
#define false 0
#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
/* Define _Bool, bool, false, true as a GNU extension. */
#define _Bool bool
#define bool bool
#define false false
#define true true
#endif
为什么不是 typedef _Bool bool;
?
推荐答案
stdbool.h
定义 bool
作为宏,因为C标准(第7.18节)说 bool
应定义为宏, stdint.h
定义 intN_t
等作为typedef,因为C标准(第7.20节) )说 intN_t
等等应定义为typedef。
stdbool.h
defines bool
as a macro because the C standard (section 7.18) says bool
shall be defined as a macro, and stdint.h
defines intN_t
etc as typedefs because the C standard (section 7.20) says intN_t
etc shall be defined as typedefs.
好的,为什么C标准说这些东西?我无法确定,但有一条线索在第7.18节第4段:
Okay, why does the C standard say these things? I cannot tell you for sure, but a clue is in section 7.18 paragraph 4:
尽管 7.1.3 ,一个程序可能会取消定义,然后重新定义宏bool,true和false。
Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the macros bool, true, and false.
如果 bool
是typedef和是
和 false
是,我不知道,枚举
常数,他们不能允许你这样做,因为没有办法撤消这些声明。
If bool
were a typedef and true
and false
were, I dunno, enum
constants, they couldn't have allowed you to do that, as there is no way to undo those kinds of declarations.
好的,为什么C委员会想让你这样做?这更具有推测性,但可能出于同样的原因,他们添加了 stdbool.h
和 _Bool
而不是 bool
, true
,以及 false
关键字,因为它们在C ++中:他们希望保持与定义 bool
, true
和 false <>的旧程序的兼容性code>他们自己,即使这些程序使用包含
stdbool.h的第三方标题
...
Okay, why does the C committee want to allow you to do that? This is even more speculative, but probably for the same reason they added stdbool.h
and _Bool
instead of making bool
, true
, and false
keywords as they are in C++: they wanted to preserve compatibility with old programs that defined bool
, true
, and false
themselves, even if those programs use third-party headers that include stdbool.h
...
没有这样的向后兼容性问题适用于 stdint.h
定义的类型;一些系统提供(一些)作为扩展,但它们总是typedef。
No such backward compatibility concerns apply to the types defined by stdint.h
; some systems provided (some) of them as extensions, but they were always typedefs.
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