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

Answer 2

following the code at Rob Napier's blog I did it for CAAudioUnitOutputCapturer. Thought I would share it to save other folks time.

RNWrap.h

//
//  RNWrap.h
//
//  ObjC wrapper for Wrap.cpp
//

#import <Foundation/Foundation.h>
#import <CoreFoundation/CoreFoundation.h>
#import <AudioUnit/AudioUnit.h>
#import <AudioToolbox/AudioToolbox.h>
#import <AVFoundation/AVFoundation.h>
#import <CoreAudio/CoreAudioTypes.h>

struct RNWrapOpaque;

@interface RNWrap : NSObject {
struct RNWrapOpaque *_cpp;
}

- (id) initWithAudioUnit:(AudioUnit) au andURL:(CFURLRef) outputFileURL andAudioFileTypeID:(AudioFileTypeID) fileType andAudioStreamBasicDescription: (const AudioStreamBasicDescription) asbd;
- (void) Start;
- (void) Close;
- (void) Stop;

@end

RNWrap.mm

//
//  RNWrap.mm
//  Objective C++ Wrapper Class for CAAudioUnitOutputCapturer.h
//
//  Created by Pier 23/10/12
//  Copyright 2012 DreamUpApps. All rights reserved.
//

#import "RNWrap.h"
#import "CAAudioUnitOutputCapturer.h"

@interface RNWrap ()
@property (nonatomic, readwrite, assign) RNWrapOpaque *cpp;
@end

@implementation RNWrap
@synthesize cpp = _cpp;

struct RNWrapOpaque
{
public:
    RNWrapOpaque(AudioUnit au, CFURLRef outputFileURL, AudioFileTypeID fileType, const AudioStreamBasicDescription format) : outputCapturer(au, outputFileURL, fileType,  format, 0) {}; // note added bus number = 0 at the end
CAAudioUnitOutputCapturer outputCapturer;
};

- (id)initWithAudioUnit:(AudioUnit) au andURL:(CFURLRef) outputFileURL andAudioFileTypeID:(AudioFileTypeID) fileType andAudioStreamBasicDescription: (const AudioStreamBasicDescription) format
{
self = [super init];
if (self != nil)
{
    self.cpp = new RNWrapOpaque(au, outputFileURL, fileType, format);
}
return self;
}

- (void)dealloc
{
delete _cpp;
_cpp = NULL;

//[super dealloc];
}

- (void) Start
{
self.cpp->outputCapturer.Start();
}

- (void) Stop
{
self.cpp->outputCapturer.Stop();
}

- (void) Close
{
self.cpp->outputCapturer.Close();
}

@end

You use it like this in your class.

- (void) captureInAppAudio:(AudioUnit) auToCapture
{
AudioStreamBasicDescription destFormat; 
memset( &destFormat, 0, sizeof(AudioStreamBasicDescription) );
destFormat.mSampleRate = 44100;
destFormat.mFormatID = kAudioFormatLinearPCM;
destFormat.mFormatFlags = ( kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked | kAudioFormatFlagIsBigEndian );
destFormat.mBytesPerPacket = 2;
destFormat.mFramesPerPacket = 1;
destFormat.mBytesPerFrame = 2;
destFormat.mChannelsPerFrame = 1;
destFormat.mBitsPerChannel = 16;

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];

soundPath = [documentsDirectory stringByAppendingString:@"/recording.caf"] ;
CFURLRef fileurl = CFURLCreateWithFileSystemPath(NULL, (CFStringRef)soundPath, kCFURLPOSIXPathStyle, false);
captor = [[RNWrap alloc] initWithAudioUnit:auToCapture andURL:fileurl andAudioFileTypeID:'caff' andAudioStreamBasicDescription:destFormat];

[captor Start];
}

Hope this helps someone else out there!

Pier.