Question

I am trying to figure out the syntax to register a callback with this 3rd party software. I think it is probably a basic question, I just am not too familiar with c++. They have a method for registering a callback function so their code can call a function in my code when an event happens. They provided a working example that registers the callback from the main file, but I want to know how to do it when working inside a class

Their method signature:

smHTRegisterHeadPoseCallback(smEngineHandle engine_handle, void *user_data, smHTHeadPoseCallback callback_fun);

Working example from the main file:

void STDCALL receiveHeadPose(void *,smEngineHeadPoseData head_pose, smCameraVideoFrame video_frame)
{
 ...
}

void main()
{
 ...
 smHTRegisterHeadPoseCallback(engine_handle,0,receiveHeadPose)
 ...
}

But I want to use this from my class

MyClass.h

class FaceEngine
{
public:
 void STDCALL receiveFaceData(void *, smEngineFaceData face_data, smCameraVideoFrame video_frame);
    ...

MyClass.cpp

void FaceEngine::Start(void)
{
 rc = smHTRegisterFaceDataCallback(hFaceAPIEngine,0,&FaceEngine::receiveFaceData);
        ...

Results in this compiler error:

Error 1 error C2664: 'smHTRegisterFaceDataCallback' : cannot convert parameter 3 from 'void (__stdcall FaceEngine::* )(void *,smEngineFaceData,smCameraVideoFrame)' to 'smHTFaceDataCallback' d:\stuff\programming\visual studio 2008\projects\tut02_vertices\faceengine.cpp 43 Beard

If my question isn’t clear please let me know how I can clarify.

Answer 1

You need to use a trampoline function as member functions can’t be called without an instance:

void STDCALL trampoline(void* data, smEngineHeadPoseData hp, smCameraVideoFrame vf) {
    static_cast<FaceEngine*>(data)->receiveFaceData(hp, vf);
}

// ... register:
smHTRegisterFaceDataCallback(hFaceAPIEngine, static_cast<void*>(this), &trampoline);

Note that this uses the user-data parameter to pass the this pointer, so the trampoline function knows what instance to call the member function on.