Use C++ with Cocoa Instead of Objective-C?

Question

I would like to write applications that use C++ and the Cocoa frameworks because Apple is not making Carbon 64-bit capable. C++ seems to be pretty vanilla in its implementation on Linux and Windows but on Mac OS X it seems like additional Apple specific pieces of code are required (like an Obj-C wrapper). It also seems that Apple is forcing developers to write in Objective-C rather than C++, although I could be wrong.

I am trying to find a path to write code on the Mac that would be easy to keep cross platform. Having to write code in C++ for Linux/Windows and then rewrite large portions in Objective-C would be very inefficient.

Is there a way to write code in C++ that will be supported for the future and supported in Xcode? Also, if this is possible, how would I mix C++ and Objective-C in Xcode? Thanks.

Answer 1

You cannot write a Cocoa application entirely in C++. Cocoa relies heavily on the late binding capabilities of Objective-C for many of its core technologies such as Key-Value Bindings, delegates (Cocoa style), and the target-action pattern. The late binding requirements make it very difficult to implement the Cocoa API in a compile-time bound, typed language like C++ⁱ. You can, of course, write a pure C++ app that runs on OS X. It just can't use the Cocoa APIs.

So, you have two options if you want to share code between C++ apps on other platforms and your Cocoa-based application. The first is to write the model layer in C++ and the GUI in Cocoa. This is a common approach used by some very large apps, including Mathematica. Your C++ code can be left unchanged (you do not need "funky" apple extensions to write or compile C++ on OS X). Your controller layer will likely make use of Objective-C++ (perhaps the "funky" Apple extension you refer to). Objective-C++ is a superset of C++, just as Objective-C is a superset of C. In Objective-C++, you can make objc-style message passing calls (like [some-objc-object callMethod];) from within a C++ function. Conversely, you can call C++ functions from within ObjC code like:

@interface MyClass {
    MyCPPClass *cppInstance;
}
@end

@implementation MyClass
- (id)init {
    if(self = [super init]) {
        cppInstance = new MyCPPClass();
    }
    return self;
}
- (void) dealloc {
    if(cppInstance != NULL) delete cppInstance;
    [super dealloc];
}
- (void)callCpp {
    cppInstance->SomeMethod();
}
@end

You can find out more about Objective-C++ in the Objective-C language guide. The view layer can then be pure Objective-C.

The second option is to use a cross-platform C++ toolkit. The Qt toolkit might fit the bill. Cross-platform toolkits are generally despised by Mac users because they do not get all the look and feel details exactly right and Mac users expect polish in the UI of Mac applications. Qt does a surprisingly good job, however, and depending on the audience and the use of your app, it may be good enough. In addition, you will lose out on some of the OS X-specific technologies such as Core Animation and some QuickTime functionality, though there are approximate replacements in the Qt API. As you point out, Carbon will not be ported to 64-bit. Since Qt is implemented on Carbon APIs, Trolltech/Nokia have had to port Qt to the Cocoa API to make it 64-bit compatible. My understanding is that the next relase of Qt (currently in release candiate) completes this transition and is 64-bit compatible on OS X. You may want to have a look at the source of Qt 4.5 if you're interested in integrating C++ and the Cocoa APIs.

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ⁱ For a while Apple made the Cocoa API available to Java, but the bridge required extensive hand-tuning and was unable to handle the more advanced technologies such as Key-Value Bindings described above. Currently dynamically typed, runtime-bound languages like Python, Ruby, etc. are the only real option for writing a Cocoa app without Objective-C (though of course these bridges use Objective-C under the hood).