macOS Calling Objective-C Code from C++

[cpsmusic]

Hi,

I'm currently helping out with an open-source cross-platform C++ graphics application. Is it possible to use OS X Cocoa APIs with C++? For example, I'd like to use Image I/O to read and write image formats but I need to call it from within C++.

Cheers,

Chris

 

[lee1210]

There are a few problems here. For starters, the cross-platform piece. Once you start using APIs that are OS X-only, it's not cross-platform. Secondly, once you add a single message pass to a C++ file, you have to make it an Objective-C++ file (.mm), so it will compile properly. This means it's Objectve-C(++) now, not C++. It sounds like you are a new contributor to a project with good intentions, but what you're suggesting is totally changing the architecture of the project. I would say call a C function that has some #ifdef code to determine the platform, which calls a C function in your Objective-C library, which can then go to town calling Objective-C methods. This would allow for different per-platform implementations of this behavior. However, it would probably really be best to just use a C or C++ library instead.

-Lee

 

[cpsmusic]

There are a few problems here. For starters, the cross-platform piece. Once you start using APIs that are OS X-only, it's not cross-platform. Secondly, once you add a single message pass to a C++ file, you have to make it an Objective-C++ file (.mm), so it will compile properly. This means it's Objectve-C(++) now, not C++. It sounds like you are a new contributor to a project with good intentions, but what you're suggesting is totally changing the architecture of the project. I would say call a C function that has some #ifdef code to determine the platform, which calls a C function in your Objective-C library, which can then go to town calling Objective-C methods. This would allow for different per-platform implementations of this behavior. However, it would probably really be best to just use a C or C++ library instead.

-Lee

Thanks for the reply.

My initial thought was to go with a cross-platform library (like you suggest). However, the app already has different capabilities on different platforms (for example, it's using Qt's Phonon for audio I/O which has different capabilities depending on the platform). Also, developers are adding features for particular platforms but not others.

Cheers,

Chris

 

[Sander]

To minimize dependencies, the normal way of doing things like this is to break them out into platform-specific implementations returned from a class factory. So you'd have something like this:

// Generic code
void GenericClass::LoadImage(const GenericPath& path)
{
    GenericFileIOInterface* itf = CreateSpecificLoader();
    itf->LoadFile(path);
}

The GenericFileIOInterface is a C++ interface, and the CreateSpecificLoader creates a platform-specific implementation. On the Mac, you create an Objective-C++ class in that factory. You select which version to build in the Makefile. That way, the other platforms will never see the .mm file.

If the current cross-platformness is achieved with preprocessor tricks, you'll have to refactor a bit:

// Generic code
void GenericClass::LoadImage(const GenericPath& path)
{
#ifdef WIN32
    SomeWin32APIForLoadingFilesEx(ConvertPathToLPSZWSTR(path));
#else if LINUX
    some_library_call_for_reading_images(path_to_str(path));
#else
    #error Unknown platform
#endif
}

because, like lee1210 explained, to use Objective-C inside a new #if MACOSX section, the entire file will have to become Objective-C++ which will likely mess it up for the other platforms.

 

[NickK1066]

// Generic code
void GenericClass::LoadImage(const GenericPath& path)
{
#ifdef WIN32
    SomeWin32APIForLoadingFilesEx(ConvertPathToLPSZWSTR(path));
#else if LINUX
    some_library_call_for_reading_images(path_to_str(path));
#else
    #error Unknown platform
#endif
}

Eww.. sorry but #defs like this really gets my goat.

Use a mechanism to provide specialist implementations for a cross platform interface. Done correctly there should be little repetition but as others have said it would require refactoring.

 

[Sander]

That's what I meant. Usually, cross-platform compatibility starts out with #ifdefs here and there (especially if it wasn't designed in from the start). If the current state of the code is like that, he'll need to refactor.

 

[mfram]

You don't necessarily need #ifdefs. Create different classes that have the same interface to the rest of your program. Compile the one that's appropriate for your project. No #ifdefs will be needed.

graphics_lib_mac.mm
graphics_lib_linux.cpp
graphics_lib_win.cpp

or whatever.