Cocoaphony

Mac and iPhone, on the brain

Wrapping C++ - Take 2, Part 1

Last year, I presented an approach to wrapping C++. Since then, I’ve been introduced to other approaches, particularly from gf who helped me better understand opaque objects. Since I do a lot of cross-language work, I’ve had some opportunity to play with and expand this, and so I’d like to update my C++ wrapping approach.

First, to remind everyone of the problem: you have a C++ object that you want to consume in Objective-C. That’s easy in ObjC++, but if you make an ivar that references a C++ class, then the header file can only be included by ObjC++ classes. This quickly spreads .mm files throughout your project, creating all kinds of headaches. ObjC is a beautiful thing, and C++ is fine, but ObjC++ is a crazy land that should be carefully segregated from civilized code. So how do we do it?

We create a thin wrapper object to provide an ObjC face on a C++ object. The challenge is how to hide C++ classes from the header file. The answer is to put them in a struct that you forward declare so you don’t have to expose its contents. Structs are almost identical with C++ classes, but their forward declaration syntax is C-compatible (unlike class). Let’s look at how this is done.

For this example, we will consider a C++ class called RN::Wrap. It holds a simple string with set and get accessors.

class Wrap {
public:
    Wrap(string str) : m_string(str) {};
    string getString() { return m_string; };
    void setString(string str) { m_string = str; };
private:
    string m_string;
};

We wrap this into Objective-C++ using an opaque structure, RNWrapOpaque:

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

Since we only include a raw pointer to RNWrapOpaque, we don’t have to declare anything else about it in the header file. If we tried to store the actual struct here (rather than a pointer), then that would defeat the purpose. This is the only raw pointer we will need.

Since structs are almost identical to classes in C++, we can use class features like constructors and destructors in the implementation (.mm file):

struct RNWrapOpaque {
public:
    RNWrapOpaque(string aStr) : wrap(aStr) {};
    RN::Wrap wrap;
};

Now initializing and using the opaque object is easy in ObjC++ code:

- (id)initWithString:(NSString *)aString {
    self = [super init];
    if (self != nil) {
        self.cpp = new RNWrapOpaque([aString UTF8String]);
    }
    return self;
}

- (void)dealloc {
    delete _cpp;
    _cpp = NULL;    
    [super dealloc];
}

- (void)setString:(NSString *)aString {
    self.cpp->wrap.setString([aString UTF8String]);
}

- (NSString *)string {
    return [NSString stringWithUTF8String:
        self.cpp->wrap.getString().c_str()];
}

A nice side effect of this usages is that C++ objects are easy to detect through the self.cpp prefix. Memory management is easy since there is only one raw C++ pointer.

Using the class requires no special work. It’s pure ObjC:

RNWrap *wrap = [[[RNWrap alloc] initWithString:@"my string"] autorelease];
NSLog(@"wrap = %@", [wrap string]);

In part two, we’ll discuss how to extend this approach to more complex problems such as smart pointers, listeners/delegates, and bindings.