Retrieve OpenGL Context from Qt 5.5 on OSX

In the latest Qt 5.5, the QOpenGLWidget is much better and has less bugs than the QGLWidget, but it doesn't supply the good API to retrieve the native OpenGL context. On Mac OSX the situation would be a bit more difficult, since the platform is in Obj-C not C\C++ as Windows and Linux.

So what you have to do to get the native Cocoa OpenGL context is to follow these steps.

Write a header file CCocoaGLContext.h to declare the function interface.

#pragma once

void * GetCocoaGLContext(void * x);

Write a Obj-C CCocoaGLContext.mm file, super easy

#import <QCocoaNativeContext>

#import "CCocoaGLContext.h"

void * GetCocoaGLContext(void * x)
{
    QCocoaNativeContext * p = (QCocoaNativeContext *)(x);
    if (p)
    {
        NSOpenGLContext * x = p->context();
        return [x CGLContextObj];
    }
    return 0;
}

In the application, that would be easy to get the native OpenGL handle.

QVariant NativeHandle(GLContext->nativeHandle());
#if defined(__APPLE__)
CGLContextObj GLContext = reinterpret_cast<CGLContextObj>(GetCocoaGLContext(NativeHandle.data()));
CGLShareGroupObj  GLShareGroup  = CGLGetShareGroup(GLContext);
#elif defined(_MSC_VER)
QWGLNativeContext WGLContext = NativeHandle.value<QWGLNativeContext>();
#else
QGLXNativeContext GLXContext = NativeHandle.value<QGLXNativeContext>();
#endif

It's super useful to initialize the OpenCL with GL Interop on 3 platforms.