为什么外部“C”是仅在要包含的文件内工作，而不是在 #include 指令周围工作

发布于 2024-10-19 10:08:27 字数 441 浏览 6 评论 0原文

我在 Objective C++ 类中包含了一个名为“AniUtils.h”的 C 文件。据我所知，如果我要在 C++ 文件中包含 C 文件，则需要使用“extern "C"”。让我感到困惑的是，将 include 指令包装在 extern C 中似乎不起作用，而将 extern C 放入文件正文中却可以。

因此，包装 include 指令不起作用。

extern "C" {
    #include "AniUtils.h"
}

然而，这样做效果很好：

// AniUtils.h
#ifdef __cplusplus
extern "C" {
#endif

// body of included file

#ifdef __cplusplus
}
#endif

为什么第二种方法有效，而第一种方法不行？

原文

I'm including a C file called "AniUtils.h" in an Objective C++ class. I understand that I need to use 'extern "C"' if I'm going to include a C file in a C++ file. What's confusing me is that wrapping the include directive in an extern C doesn't seem to work, while putting extern C in the body of the file does.

So, wrapping the include directive doesn't work.

extern "C" {
    #include "AniUtils.h"
}

However, doing it this way works fine:

// AniUtils.h
#ifdef __cplusplus
extern "C" {
#endif

// body of included file

#ifdef __cplusplus
}
#endif

Why does the second approach work but not the first?

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我家小可爱 2024-10-26 10:08:27

如果您的 AniUtils.h 文件被任何其他 Objective-C 类包含（例如，.m 文件；不是 Objective-C++、.mm 文件），那么第一种方法您实现的 extern C 语句将导致错误。

例如，下面是一个普通的 Objective C 类，它将导入有问题的 AniUtils.h Objective-C++ 类。请注意，因为这些文件是 WowClass.h & WowClass.m，这些文件将作为普通 Objective-C 进行处理：

WowClass.h：

#import <Foundation/Foundation.h>
#import "AniUtils.h"

@interface WowClass : NSObject {

}

@end

WowClass.m：

#import "WowClass.h"

@implementation
 .....
@end

现在，如果您的 AniUtils.h 是以第一种方式定义的，当您尝试像普通 Objective-C 一样处理包含 extern C 语句的 WowClass.m 文件时，您会收到错误。相反，您需要对 AniUtils.h 中的 extern C 语句进行条件化：

// AniUtils.h
#ifdef __cplusplus
extern "C" {
#endif

// body of included file

#ifdef __cplusplus
}
#endif

当然，另一种方法是将 WowClass.m 重命名为WowClass.mm 以便 WowClass 接口和实现文件始终作为 Objective-C++ 进行处理，但这种做法没有抓住重点。

If your AniUtils.h file is included by any other Objective-C classes (e.g., .m files; not Objective-C++, .mm files), then the first way you've implemented the extern C statement will cause errors.

For example, here is a plain Objective C class that will be importing the problematic AniUtils.h Objective-C++ class. Note that because these files are WowClass.h & WowClass.m, these files will be processed as plain Objective-C:

WowClass.h:

#import <Foundation/Foundation.h>
#import "AniUtils.h"

@interface WowClass : NSObject {

}

@end

WowClass.m:

#import "WowClass.h"

@implementation
 .....
@end

Now, if your AniUtils.h is defined in the first manner, you'll get errors when trying to process the WowClass.m file, which includes the extern C statement, as plain Objective-C. Instead, you need to conditionalize the extern C statement in AniUtils.h:

// AniUtils.h
#ifdef __cplusplus
extern "C" {
#endif

// body of included file

#ifdef __cplusplus
}
#endif

Of course, an alternative would be to simply rename WowClass.m to WowClass.mm so that the WowClass interface and implementation files are always processed as Objective-C++, but that kind of misses the point.

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