C++ classes as instance variables of an Objective-C class

Question

I need to mix Objective-C and C++. I would like to hide all the C++ stuff inside one class and keep all the others plain Objective-C. The problem is that I want to have some C++ classes as instance variables. This means they have to be mentioned in the header file, which gets included by other classes and C++ starts spreading to the whole application. The best solution I was able to come with so far looks like this:

#ifdef __cplusplus
#import "cppheader.h"
#endif

@interface Foo : NSObject
{
    id regularObjectiveCProperty;
    #ifdef __cplusplus
    CPPClass cppStuff;
    #endif
}

@end

This works. The implementation file has an mm extension, so that it gets compiled as Objective-C mixed with C++, the #ifdef unlocks the C++ stuff and there we go. When some other, purely Objective-C class imports the header, the C++ stuff is hidden and the class does not see anything special. This looks like a hack, is there a better solution?

Answer 1

This sounds like a classic use for an interface/@protocol. Define an objective-c protocol for the API and then provide an implementation of that protocol using your Objective-C++ class. This way clients need only know about the protocol and not the header of the implementation. So given the original implementation

@interface Foo : NSObject
{
    id regularObjectiveCProperty;
    CPPClass cppStuff;

}

@end

I would define a protocol

//Extending the NSObject protocol gives the NSObject
// protocol methods. If not all implementations are
// descended from NSObject, skip this.
@protocol IFoo <NSObject>

// Foo methods here
@end

and modify the original Foo declaration to

@interface Foo : NSObject <IFoo>
{
    id regularObjectiveCProperty;
    CPPClass cppStuff;
}

@end

Client code can then work with type id<IFoo> and does not need to be compiled as Objective-C++. Obviously you can pass an instance of Foo to these clients.