pass member function from C++ CLI to native C callback

Question

I've got problems passing a member function of a C++ CLI class to a native C callback from a library.

To be precise its the Teamspeak 3 SDK.

You can pass a non member function using the following code without problem:

struct ClientUIFunctions funcs;

/* Initialize all callbacks with NULL */
memset(&funcs, 0, sizeof(struct ClientUIFunctions));
funcs.onConnectStatusChangeEvent        = onConnectStatusChangeEvent;

But I need to pass a pointer to a member function, for example:

    funcs.onConnectStatusChangeEvent        = &MyClass::onConnectStatusChangeEvent;

Any other idea how to use the event within a non static member function is welcome to.

Thanks in advance!

Answer 1

This can only be done via a static class function because C doesn't know anything about the vtable or what object the function is part of. See below for a C++ and Managed C++ example

This could however be a work around, build a wrapper class which handles all the callbacks you need.

#include <string.h>

struct ClientUIFunctions
{
     void (*onConnectStatusChangeEvent)(void);
};

class CCallback
{
public:
     CCallback()
      {
           struct ClientUIFunctions funcs;

           // register callbacks
           my_instance = this;

           /* Initialize all callbacks with NULL */
           memset(&funcs, 0, sizeof(struct ClientUIFunctions));
           funcs.onConnectStatusChangeEvent        = sOnConnectStatusChangeEvent;

      }

     ~CCallback()
      {
           // unregister callbacks
           my_instance = NULL;
      }

     static void sOnConnectStatusChangeEvent(void)
      {
           if (my_instance)
            my_instance->OnConnectStatusChangeEvent();
      }

private:
     static CCallback *my_instance;

     void OnConnectStatusChangeEvent(void)
      {
           // real callback handler in the object
      }
};

CCallback *CCallback::my_instance = NULL;

int main(int argc, char **argv)
{
    CCallback *obj = new CCallback();

    while (1)
    {
         // do other stuff
    }

    return 0;
}

Another possibility would be if the callback supports and void *args like void (*onConnectStatusChangeEvent)(void *args); which you can set from the plugin. You could set the object in this args space so in de sOnConnectStatusChangeEvent you would have something like this:

static void sOnConnectStatusChangeEvent(void *args)
    {
           if (args)
              args->OnConnectStatusChangeEvent();
    }

For managed C++ it should be something like this, however I can't get it to compile because it doesn't like the template brackets..

wrapper.h:

using namespace std;
using namespace System;
using namespace System::Runtime::InteropServices;
using namespace System::Text;

namespace Test
{
    struct ClientUIFunctions
    {
        void (*onConnectStatusChangeEvent)(void);
    };

    public delegate void ConnectStatusChangeEvent(void);

    public ref class ManagedObject
    {
    public:
        // constructors
        ManagedObject();

        // destructor
        ~ManagedObject();

        //finalizer
        !ManagedObject();

        event ConnectStatusChangeEvent^ OnConnectStatusChangeEvent {
            void add(ConnectStatusChangeEvent^ callback) {
                m_connectStatusChanged = static_cast<ConnectStatusChangeEvent^> (Delegate::Combine(m_connectStatusChanged, callback));
            }

            void remove(ConnectStatusChangeEvent^ callback) {
                m_connectStatusChanged = static_cast<ConnectStatusChangeEvent^> (Delegate::Remove(m_connectStatusChanged, callback));
            }

            void raise(void) {
                if (m_connectStatusChanged != nullptr) {
                    m_connectStatusChanged->Invoke();
                }
            }
        }

    private:
        ConnectStatusChangeEvent^ m_connectStatusChanged;   
    };

    class CCallback
    {
    public:
        static void Initialize(ManagedObject^ obj);
        static void DeInitialize(void);

    private:
        static void sOnConnectStatusChangeEvent(void);

        static gcroot<ManagedObject^> m_objManagedObject;
    };
}

wrapper.cpp:

#include <string.h>
#include "wrapper.h"

using namespace System;
using namespace Test;

void CCallback::Initialize(ManagedObject^ obj)
{
    struct ClientUIFunctions funcs;

    // register callbacks
    m_objManagedObject = obj;

    /* Initialize all callbacks with NULL */
    memset(&funcs, 0, sizeof(struct ClientUIFunctions));
    funcs.onConnectStatusChangeEvent        = sOnConnectStatusChangeEvent;

}

void CCallback::DeInitialize(void)
{
    // unregister callbacks
    m_objManagedObject = nullptr;
}

void CCallback::sOnConnectStatusChangeEvent(void)
{
    if (m_objManagedObject != nullptr)
        m_objManagedObject->OnConnectStatusChangeEvent();
}


// constructors
ManagedObject::ManagedObject()
{
    // you can't place the constructor in the header but just for the idea..
    // create wrapper
    CCallback::Initialize(this);          
}

// destructor
ManagedObject::~ManagedObject()
{
    this->!ManagedObject();
}

//finalizer
ManagedObject::!ManagedObject()
{
    CCallback::DeInitialize();        
}

gcroot<ManagedObject^> CCallback::m_objManagedObject = nullptr;

int main(array<System::String ^> ^args)
{
     ManagedObject^ bla = gcnew ManagedObject();

     while (1)
     {
      // do stuff
     }

     return 0;
}