为什么我们将 C++ 类型定义为 LLVM 中的 Objective-C 编译器的 void（而 C++ 编译器可以看到 Objective-C 接口）？

Question

I was going through the source of an iOS project that uses a C++ library when I stumbled upon this few rather interesting lines of code in the header file of a Objective-C wrapper for C++:

#ifdef __OBJC__
#ifndef __cplusplus
typedef void VisageTracker;
typedef void FaceData;
typedef void DemoFrameGrabber;
typedef void FDP;
typedef void VsImage;
#endif
#endif

@interface TrackerWrapper : NSObject {
CustomGLView *glView;
#ifdef __cplusplus
    VisageSDK::VisageTracker* tracker;
    VisageSDK::FaceData trackingData;
    DemoFrameGrabber *demoFrameGrabber;
    DemoObserver *demoObserver;
    int glWidth;
    int glHeight;
    bool inGetTrackingResults;
#endif
}

A question immediately hit me: Why are all the C++ types such as VisageTracker, FaceData, DemoFrameGrabber, FDP, VsImage being defined as void for the Objective-C compiler?

I'm quite curious as how LLVM does the compilation here. It appears that the header file would be gone through twice, once by a C++ compiler and once by a Objective-C compiler.

Question: why must we hide the C++ types from the Objective-C compiler, but the C++ compiler is okay seeing an objective-C interface (and not only that but we can also "inject" C++ stuff into the interface)?

Update:

I tried removing these lines

#ifdef __OBJC__
#ifndef __cplusplus
typedef void VisageTracker;
typedef void FaceData;
typedef void DemoFrameGrabber;
typedef void FDP;
typedef void VsImage;
#endif
#endif

after which the code didn't fail to compile: it appears to work just fine without these lines. Therefore I conclude that these lines are probably there because

1. In an older version of Clang there's a bug in the type system which can be fixed by adding these lines.

or/and

2. It is an old tradition passed down from generation to generation with the belief that blinding the Objective-C compiler this way it can save some memories perhaps.

Answer 1

Because C++ and Objective-C is different languages. They can be used both only in Objective-C++ .