Interacting with C++ classes from Swift

Question

I have a significant library of classes written in C++. I'm trying to make use of them through some type of bridge within Swift rather than rewrite them as Swift code. The primary motivation is that the C++ code represents a core library that is used on multiple platforms. Effectively, I'm just creating a Swift based UI to allow the core functionality to work under OS X.

There are other questions asking, "How do I call a C++ function from Swift." This is not my question. To bridge to a C++ function, the following works fine:

Define a bridging header through "C"

#ifndef ImageReader_hpp
#define ImageReader_hpp

#ifdef __cplusplus
extern "C" {
#endif

    const char *hexdump(char *filename);
    const char *imageType(char *filename);

#ifdef __cplusplus
}
#endif

#endif /* ImageReader_hpp */

Swift code can now call functions directly

let type = String.fromCString(imageType(filename))
let dump = String.fromCString(hexdump(filename))

My question is more specific. How can I instantiate and manipulate a C++ Class from within Swift? I can't seem to find anything published on this.

Answer 1

I've worked out a perfectly manageable answer. How clean you'd like this to be is entirely based upon how much work you're willing to do.

First, take your C++ class and create C "wrapper" functions to interface with it. For example, if we have this C++ class:

class MBR {
    std::string filename;

public:
    MBR (std::string filename);
    const char *hexdump();
    const char *imageType();
    const char *bootCode();
    const char *partitions();
private:
    bool readFile(unsigned char *buffer, const unsigned int length);
};

We then implement these C++ functions:

#include "MBR.hpp"

using namespace std;
const void * initialize(char *filename)
{
    MBR *mbr = new MBR(filename);

    return (void *)mbr;
}

const char *hexdump(const void *object)
{
    MBR *mbr;
    static char retval[2048];

    mbr = (MBR *)object;
    strcpy(retval, mbr -> hexdump());
    return retval;
}

const char *imageType(const void *object)
{
    MBR *mbr;
    static char retval[256];

    mbr = (MBR *)object;
    strcpy(retval, mbr -> imageType());
    return retval;
}

The bridge header then contains:

#ifndef ImageReader_hpp
#define ImageReader_hpp

#ifdef __cplusplus
extern "C" {
#endif

    const void *initialize(char *filename);
    const char *hexdump(const void *object);
    const char *imageType(const void *object);

#ifdef __cplusplus
}
#endif

#endif /* ImageReader_hpp */

From Swift, we can now instantiate the object and interact with it like so:

let cppObject = UnsafeMutablePointer<Void>(initialize(filename))
let type = String.fromCString(imageType(cppObject))
let dump = String.fromCString(hexdump(cppObject))                
self.imageTypeLabel.stringValue = type!
self.dumpDisplay.stringValue = dump!

So, as you can see, the solution (which is actually rather simple) is to create wrappers that will instantiate an object and return a pointer to that object. This can then be passed back into the wrapper functions which can easily treat it as an object conforming to that class and call the member functions.

Making It Cleaner

While this is a fantastic start and proves that it is completely feasible to use existing C++ classes with a trivial bridge, it can be even cleaner.

Cleaning this up would simply mean that we remove the UnsafeMutablePointer<Void> from the middle of our Swift code and encapsulate it into a Swift class. Essentially, we use the same C/C++ wrapper functions but interface them with a Swift class. The Swift class maintains the object reference and essentially just passes all method and attribute reference calls through the bridge to the C++ object!

Having done this, all of the bridging code is completely encapsulated in the Swift class. Even though we are still using a C bridge, we are effectively using C++ objects transparently without having to resort to recoding them in Objective-C or Objective-C++.

Answer 2

Swift has no C++ interop currently. It's a long-term goal, but is very unlikely to happen in the near future.

Answer 3

In addition to your own solution, there is another way to do it. You can call or directly write C++ code in objective-c++.

So you can create an objective-C++ wrapper on top of your C++ code and create a suitable interface.

Then call objective-C++ code from your swift code. To be able to write objective-C++ code you may have to rename file extension from .m to .mm

Do not forget to release memory allocated by your C++ objects when suitable.

Answer 4

You can use Scapix Language Bridge to automatically bridge C++ to Swift (among other languages). Bridge code automatically generated on the fly directly from C++ header files. Here is an example:

C++:

#include <scapix/bridge/object.h>

class contact : public scapix::bridge::object<contact>
{
public:
    std::string name();
    void send_message(const std::string& msg, std::shared_ptr<contact> from);
    void add_tags(const std::vector<std::string>& tags);
    void add_friends(std::vector<std::shared_ptr<contact>> friends);
};

Swift:

class ViewController: UIViewController {
    func send(friend: Contact) {
        let c = Contact()

        contact.sendMessage("Hello", friend)
        contact.addTags(["a","b","c"])
        contact.addFriends([friend])
    }
}

Answer 5

As another answer mentioned, using ObjC++ to interact is much easier. Just name your files .mm instead of .m and xcode/clang, gives you access to c++ in that file.

Note that ObjC++ does not support C++ inheritance. I you want to subclass a c++ class in ObjC++, you can't. You will have to write the subclass in C++ and wrap it around an ObjC++ class.

Then use the bridging header you would normally use to call objc from swift.