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

Answer 1

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