Setting up my ATL COM callback functions

Question

A follow-up to this question, I have the following CerberusNative.idl file (this is an ATL project written in Visual C++ which exposes a COM object):

[
    object,
    uuid(AECE8D0C-F902-4311-A374-ED3A0EBB6B49),
    nonextensible,
    pointer_default(unique)
]
interface ICallbacks : IUnknown
{
    [id(1)] HRESULT UserExit([in] int errorCode, [in] BSTR errorMessage);
    [id(2)] HRESULT UserAttemptingReconnection();
    [id(3)] HRESULT UserReconnected();
};

[
    object,
    uuid(B98A7D3F-651A-49BE-9744-2B1D8C896E9E),
    dual,
    nonextensible,
    pointer_default(unique)
]
interface ICerberusSession : IDispatch {
    ...
    [id(5)] HRESULT SetCallbacks([in] ICallbacks* callbacks);
};

I am attempting to create an interface for setting up callback methods which route from the COM object back to an implementation of said methods from the caller.

I am trying to run the following code:

HRESULT prolonguedDisconnection(int code, BSTR *message) {
    std::wcout << code << ": " << message << std::endl;
}
HRESULT reconnecting() {
    std::wcout << "Reconnecting." << std::endl;
}
HRESULT reconnected() {
    std::wcout << "Reconnected." << std::endl;
}
...
CoInitialize(NULL);
CerberusNativeLib::ICallbacksPtr callbacks;
callbacks.CreateInstance(__uuidof(CerberusNativeLib::ICallbacks));
callbacks->UserExit = prolonguedDisconnection;
callbacks->UserAttemptingReconnection = reconnecting;
callbacks->UserReconnected = reconnected;
CerberusNativeLib::ICerberusSessionPtr session;
session.CreateInstance(__uuidof(CerberusNativeLib::CerberusSession));
session->SetCallbacks(callbacks);

However, I am not sure how to properly set up the callback methods. Any ideas on how to do this? I get this compiler error on lines such as callbacks->UserExit = prolonguedDisconnection;:

Error C2659 '=': function as left operand

Answer 1

I needed simply to implement the interface in a separate class:

class Callbacks : public CerberusNativeLib::ICallbacks {

#define MIDL_DEFINE_GUID(type,name,l,w1,w2,b1,b2,b3,b4,b5,b6,b7,b8) \
        const type name = {l,w1,w2,{b1,b2,b3,b4,b5,b6,b7,b8}}

MIDL_DEFINE_GUID(IID, IID_ICallbacks, 0xAECE8D0C, 0xF902, 0x4311, 0xA3, 0x74, 0xED, 0x3A, 0x0E, 0xBB, 0x6B, 0x49);

public:

    HRESULT(*user_exit) (int, BSTR) = NULL;
    HRESULT(*user_attempting_reconnection) () = NULL;
    HRESULT(*user_reconnected) () = NULL;

    Callbacks::Callbacks(HRESULT(*disconnected)(int, BSTR), HRESULT(*reconnecting)(), HRESULT(*reconnected)()) : m_cRef(0) {
        user_exit = disconnected;
        user_attempting_reconnection = reconnecting;
        user_reconnected = reconnected;
    }

    HRESULT __stdcall UserExit(int ErrorCode, BSTR ErrorMessage) {
        return user_exit(ErrorCode, ErrorMessage);
    }
    HRESULT __stdcall UserAttemptingReconnection() {
        return user_attempting_reconnection();
    }
    HRESULT __stdcall UserReconnected() {
        return user_reconnected();
    }
    HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, _COM_Outptr_ void __RPC_FAR *__RPC_FAR *ppvObject) {
        if (!ppvObject)
            return E_INVALIDARG;
        *ppvObject = NULL;
        if (riid == IID_IUnknown || riid == IID_ICallbacks)
        {
            *ppvObject = (LPVOID)this;
            AddRef();
            return NOERROR;
        }
        return E_NOINTERFACE;
    }
    ULONG STDMETHODCALLTYPE AddRef() {
        InterlockedIncrement(&m_cRef);
        return m_cRef;
    }
    ULONG STDMETHODCALLTYPE Release() {
        ULONG ulRefCount = InterlockedDecrement(&m_cRef);
        if (0 == m_cRef)
            delete this;
        return ulRefCount;
    }

private:
    ULONG m_cRef;
};

Then, to use it:

HRESULT result = CoInitialize(NULL);
if (result != S_OK)
{
    std::wcout << "Failed to initialize the COM library on the current thread or to identify the concurrency model as single-thread apartment (STA)." << std::endl;
    std::wcout << result << ": " << _com_error(result).ErrorMessage() << std::endl;
    std::wcout << "Press the enter key to exit." << std::endl;
    std::cin.get();
    return 0;
}
Callbacks *callbacks = new Callbacks(&prolonguedDisconnection, &reconnecting, &reconnected);
CerberusNativeLib::ICerberusSessionPtr session;
result = session.CreateInstance(__uuidof(CerberusNativeLib::CerberusSession));
if (result != S_OK)
{
    std::wcout << "Failed to create a new Cerberus session." << std::endl;
    std::wcout << result << ": " << _com_error(result).ErrorMessage() << std::endl;
    std::wcout << "Press the enter key to exit." << std::endl;
    std::cin.get();
    return 0;
}
result = session->SetCallbacks(callbacks);
if (result != S_OK)
{
    std::wcout << "Failed to set Cerberus session callbacks." << std::endl;
    std::wcout << result << ": " << _com_error(result).ErrorMessage() << std::endl;
    std::wcout << "Press the enter key to exit." << std::endl;
    std::cin.get();
    return 0;
}