preprocessor的#ifndef [英] Preprocessor #ifndef

问题描述

假设我有 A.H ，其中包括以下内容：

Assume I have a.h which includes the following:

<stdbool.h>
<stddef.h>
<stdin.h>

假设我也有 b.h 其中还包括＆LT; stdbool.h＆GT; 。如果啊的的#ifndef preprocessor定义语句在它和 BH 没有。将 A.H 只包括什么也没有被列入 b.h ？因此，当 BH 包括啊，将啊包括 STDDEF.H 和 stein.h ，而不是再包括 stdbool.h 或者是那些preprocessor定义功能仅用于看到全班同学是否进行了重新定义，而不是具体的职能范围内呢？

Assume I also have b.h which also includes <stdbool.h>. If a.h has the #ifndef preprocessor definition statement in it and b.h doesn't. Will a.h include only what hasn't been included in b.h? So when b.h includes a.h, will a.h include stddef.h and stein.h and not re-include stdbool.h or are those preprocessor definition functions only used to see whether this whole class is redefined, and not specific functions within it?

编辑：

另外，假设 BH 包含另一个头文件，其中包括 stdbool.h - 即品牌 BH 有 stdbool.h 无论从那个类和啊。会导致错误的？

Also, assume b.h includes another header file which includes stdbool.h -that makes b.h have stdbool.h both from that class and a.h. Will that cause errors?

推荐答案

所有的C头文件标准必须作出这样它们可以包含多次，以任意顺序：

All C standard headers must be made such that they can be included several times and in any order:

标准报头可以被包括在任何命令;每个可包括
  作为多于在给定范围内一次，没有效果不同
  包括只有一次

Standard headers may be included in any order; each may be included more than once in a given scope, with no effect different from being included only once

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