Properly use Objective C++

Question

I'm coding an app for iOS and I recently #included a C++ header file in an Objective C implementation (.m) file. I changed the extension from .m to .mm and expected everything to run smoothly. Unexpectedly I got multiple compiler errors in the .h file of my C++ class. Such as: "C++ requires a type specifier for all declarations" and "Duplicate member...". Does anyone know what could be causing this? Edit - I've added the C++ header file for context: ``` #ifndef __CAAudioUnitOutputCapturer_h__ #define __CAAudioUnitOutputCapturer_h__ #include <AudioToolbox/ExtendedAudioFile.h> /* Class to capture output from an AudioUnit for analysis. example: CFURL fileurl = CFURLCreateWithFileSystemPath(NULL, CFSTR("/tmp/recording.caf"), kCFURLPOSIXPathStyle, false); CAAudioUnitOutputCapturer captor(someAU, fileurl, 'caff', anASBD); { captor.Start(); ... captor.Stop(); } // can repeat captor.Close(); // can be omitted; happens automatically from destructor */ class CAAudioUnitOutputCapturer { public:…

Accepted Answer

Unfortunately, if you just start making classes `.mm`, any class that uses that `.mm`'s header will also need to become `.mm`. If you continue to just change your class extensions, you will eventually make the whole project Objective-c++. If that is your intention, then you can just change your build settings to compile for Objective-c++ (which could be a house of pain for you).

However, if you use some header magic, you will avoid a lot of hassle. Just make sure to change your `Compile sources as` build property to `According to file type` before compiling.

Here's something I did with a wrapper class I wrote to isolate a c++ class from the rest of my Objective-c classes. The c++ class is `MyClass`.

MyClassWrapper.h

```
//declare c++ impl for Obj-C++
#ifdef __cplusplus
class CppPlanterModel;
namespace com{namespace company{namespace mypackage {class MyClass;}}}
typedef com::company::mypackage::MyClass CppMyClass;
#endif

//declare obj-c impl
#ifdef __OBJC__
#ifndef __cplusplus
typedef void CppMyClass;
#endif
#endif

@interface MyClassWrapper : NSObject {
    CppMyClass* _myClass;
}
//etc etc
@end
```

MyClassWrapper.mm

```
#include "MyClass.h"
using namespace com:company:mypackage;

class CppMyClass : public MyClass {
    CppMyClass() {};
    ~CppMyClass() {};
    //other stuff you might like to have
};

@implementation MyClassWrapper
    //etc etc
@end
```

Here's another thing I did with a different header to handle sharing `extern` stuff:

Something.h

```
#ifdef __cplusplus
#define FV_EXTERN       extern "C" __attribute__((visibility ("default")))
#else
#define FV_EXTERN       extern __attribute__((visibility ("default")))
#endif

FV_EXTERN const int kMyInt;
FV_EXTERN int GetAnotherInt(...);
```

I recommend reading this blog entry about wrapping c++ (which also has links to other blog entries of a similar topic): http://robnapier.net/blog/wrapping-c-take-2-1-486

Properly use Objective C++

Solution

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