Question: I've got a c++ app written using Boost/WXWidgets, targeting Windows and Mac OSX. However, I've got one issue that I can't solve using these libraries. My solution requires me to wrap an Objective C class so that I can call it from one of my c++ modules. My research so far tells me that I need to use Objective C++ written into source files with a .mm extension, allowing XCode to treat the file as a mix of Objective C and c++. I've found lots of articles detailing how to wrap c++ so that it can be called from ObjectiveC, but nothing that gives any detail on the reverse. Any links to articles or, better still, a worked example, would be greatly appreciated.

Answer: 
If you want a reusable pure C++ wrapper around an Objective C class, the [Pimpl idiom](http://www.gotw.ca/gotw/024.htm) works pretty well. The Pimpl idiom will make it so that there is no Objective C / Cocoa stuff visible in the header file that will be included by pure C++ code.

```
// FooWrapper.hpp

// Absolutely no Cocoa includes or types here!

class FooWrapper
{
public:
    int bar();

private:
    struct Impl; // Forward declaration
    Impl* impl;
};

// FooWrapper.mm

@import "FooWraper.hpp"
@import "Foundation/NSFoo.h"

struct FooWrapper::Impl
{
    NSFoo* nsFoo;
};

FooWrapper::FooWrapper() : impl(new Impl)
{
    impl->nsFoo = [[NSFoo alloc] init];
}

FooWrapper::~FooWrapper()
{
    [impl->nsFoo release];
    delete impl;
}

int FooWrapper::bar()
{
    return [impl->nsFoo getInteger];
}

```
objc provides c interfaces for the basic interfaces and types (`#include <objc/headers_you_need.h>`. therefore, you can use these interfaces in pure c/c++ TUs. then include libs like Foundation or AppKit in the mm and use objc types and messaging in the implementation.

the following is a very basic interface which is not typesafe, but i encourage you to make it typesafe for the objc type you are wrapping. this should be enough to get you started in the right direction.

```
// .hpp
namespace MON {
// could be an auto pointer or a dumb pointer; depending on your needs
    class t_MONSubclassWrapper {
    public:
// usual stuff here...

// example wrapper usage:
        void release();
    private:
        id d_objcInstance;
    };
} /* << MON */

// .mm
...
#include <Foundation/Foundation.h>

MON::t_MONSubclassWrapper::t_MONSubclassWrapper() :
  d_objcInstance([[MONSubclass alloc] init]) {
}
...
void MON::t_MONSubclassWrapper::release() {
    [d_objcInstance release];
}

```