Writing a C++ Wrapper around Objective-C

Question

I want to call and work with Objective-C classes from within a C++ project on OS X. It is time to start moving towards all Objective-C, but we need to do this over some time.

How does one go about accomplishing this? Can anyone shed some light and provide an example?

Answer 1

Objective-C++ is a superset of C++, just as Objective-C is a superset of C. It is supported by both the gcc and clang compilers on OS X and allows you to instantiate and call Objective-C objects & methods from within C++. As long as you hide the Objective-C header imports and types within the implementation of a C++ module, it won't infect any of your "pure" C++ code.

.mm is the default extension for Objective-C++. Xcode will automatically do the right thing.

So, for example, the following C++ class returns the seconds since Jan 1., 1970:

//MyClass.h

class MyClass
{
  public:
    double secondsSince1970();
};

//MyClass.mm

#include "MyClass.h"
#import <Foundation/Foundation.h>

double MyClass::secondsSince1970()
{
  return [[NSDate date] timeIntervalSince1970];
}

//Client.cpp

...
MyClass c;
double seconds = c.secondsSince1970();

You will quickly find that Objective-C++ is even slower to compile than C++, but as you can see above, it's relatively easy to isolate its usage to a small number of bridge classes.