How to glue COM server to methods in C++

Question

I have a lot of experience in C++ but am new to COM, MFC and MIDL. I'm trying to create a 32 bit COM server from an IDL file supplied by a third party. The IDL section looks like this:

[uuid(3CA6AC95-6F14-4D36-93BC-A9AAC9230FEA)]
dispinterface IMyEvent
{
    properties:
    methods:
        [id(1), helpstring("do the connect")]
            SCODE Connect([in] IDispatch *GetBackToMe);
        [id(2), helpstring("do the disconnect")]
            SCODE Disconnect();
};
[uuid(5695B1AB-0D7C-4C8B-AE07-6BF8F8B10393)]
coclass MyEvent
{
    [default] dispinterface IMyEvent;
};

MIDL generated the following:

MIDL_DEFINE_GUID(IID, DIID_IMyEvent, 0x3CA6AC95, 0x6F14, 0x4D36, 0x93, 0xBC, 0xA9, 0xAA, 0xC9, 0x23, 0x0F, 0xEA);
MIDL_DEFINE_GUID(CLSID, CLSID_MyEvent, 0x5695B1AB, 0x0D7C, 0x4C8B, 0xAE, 0x07, 0x6B, 0xF8, 0xF8, 0xB1, 0x03, 0x93);
typedef interface IMyEvent IMyEvent;
typedef class MyEvent MyEvent;
EXTERN_C const IID DIID_IMyEvent;
MIDL_INTERFACE("3CA6AC95-6F14-4D36-93BC-A9AAC9230FEA")
    IMyEvent : public IDispatch
{
};
EXTERN_C const CLSID CLSID_MyEvent;

I have written the following so far:

class MyEvent
{
public:
    SCODE Connect(IDispatch *GetBackToMe);
    SCODE Disconnect();
};
SCODE MyEvent::Connect(IDispatch *GetBackToMe)
{
    // TODO
    return 0;
}
SCODE MyEvent::Disconnect()
{
    // TODO
    return 0;
}

What glue do I need to make this work?

Don't use MFC to implement COM interfaces, you really should use ATL. And consider COM to be an adapter layer on top of your C++ business logic. E.g. translate exceptions to HRESULT, translate COM types to C++ types etc. Have a look at Active Template Library (ATL) Tutorial. You start by creating an IDL file then building a tlb from that which you wrap it in a resource dll so you can register the typelibrary. Afer that you can implement with ATL and using #import. — Pepijn Kramer
– Pepijn Kramer, Commented Mar 4 at 15:54
MyEvent must implement IMyEvent, among other things. PS: you should use what's called connection points to do callbacks devblogs.microsoft.com/oldnewthing/20130611-00/?p=4113 — Simon Mourier
– Simon Mourier, Commented Mar 4 at 15:58
Further resources : ATL::IDispatchImp, and Using IDispEventImpl ... be prepared for a bit of a learning curve to do it right. — Pepijn Kramer
– Pepijn Kramer, Commented Mar 4 at 15:59
Thanks @PepijnKramer, I had been wondering for a long time whether I should be using MFC or ATL and you have answered that well. It seems that ATL is preferred for simple use cases, which mine is. — Stephen
– Stephen, Commented Mar 6 at 13:23

Remy Lebeau · Accepted Answer · 2025-03-06 15:30:47Z

2

Your MyEvent class needs to implement the IMyEvent interface. That means deriving from the interface type, and defining implementations of its methods. Also since IMyEvent is a dispinterface, that means you also need to implement all of the methods that are inherited from IUnknown and IDispatch, too:

class MyEvent : public IMyEvent
{
private
    ULONG m_refCnt = 0;

public:
    MyEvent() = default;

    // IUnknown
    HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void **ppvObject) override;
    ULONG STDMETHODCALLTYPE AddRef() override;
    ULONG STDMETHODCALLTYPE Release() override;

    // IDispatch
    HRESULT STDMETHODCALLTYPE GetTypeInfoCount(UINT *pctinfo) override;
    HRESULT STDMETHODCALLTYPE GetTypeInfo(UINT iTInfo, LCID lcid, ITypeInfo **ppTInfo) override;        
    HRESULT STDMETHODCALLTYPE GetIDsOfNames(REFIID riid, LPOLESTR *rgszNames, UINT cNames, LCID lcid, DISPID *rgDispId) override;
    HRESULT STDMETHODCALLTYPE Invoke(DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags, DISPPARAMS *pDispParams, VARIANT *pVarResult, EXCEPINFO *pExcepInfo, UINT *puArgErr) override;

    // IMyEvent
    SCODE Connect(IDispatch *GetBackToMe) override;
    SCODE Disconnect() override;
};

HRESULT STDMETHODCALLTYPE MyEvent::QueryInterface(REFIID riid, void **ppvObject)
{
    if (!ppvObject) return E_POINTER;
    *ppvObject = nullptr;
    if (riid == DIID_IMyEvent)
        *ppvObject = (IMyEvent*)this;
    else if (riid == IID_IDispatch)
        *ppvObject = (IDispatch*)(IMyEvent*)this;
    else if (riid == IID_IUnknown)
        *ppvObject = (IUnknown*)(IMyEvent*)this;
    else
        return E_NOINTERFACE;
    AddRef();
    return S_OK;
}

ULONG STDMETHODCALLTYPE MyEvent::AddRef()
{
    return ++m_refCnt;
}

ULONG STDMETHODCALLTYPE MyEvent::Release()
{
    if (--m_refCnt == 0) {
        delete this;
        return 0;
    }
    return m_refCnt;
}

HRESULT STDMETHODCALLTYPE MyEvent::GetTypeInfoCount(UINT *pctinfo)
{
    // TODO
    return E_NOTIMPL;
}

HRESULT STDMETHODCALLTYPE MyEvent::GetTypeInfo(UINT iTInfo, LCID lcid, ITypeInfo **ppTInfo)
{
    // TODO
    return E_NOTIMPL;
}   

HRESULT STDMETHODCALLTYPE MyEvent::GetIDsOfNames(REFIID riid, LPOLESTR *rgszNames, UINT cNames, LCID lcid, DISPID *rgDispId)
{
    // TODO
    return E_NOTIMPL;
}

HRESULT STDMETHODCALLTYPE MyEvent::Invoke(DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags, DISPPARAMS *pDispParams, VARIANT *pVarResult, EXCEPINFO *pExcepInfo, UINT *puArgErr)
{
    // TODO
    return E_NOTIMPL;
}

SCODE MyEvent::Connect(IDispatch *GetBackToMe)
{
    // TODO
    return 0;
}

SCODE MyEvent::Disconnect()
{
    // TODO
    return 0;
}

You also need to implement and register a class that implements the IClassFactory or IClassFactory2 interface, so that COM can create instances of your MyEvent class when requested.

See MSDN for more details:

Component Object Model (COM)

COM Clients and Servers

COM Server Responsibilities

edited Mar 6 at 15:30

answered Mar 4 at 19:53

Remy Lebeau

609k36 gold badges516 silver badges875 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Pepijn Kramer Mar 6 at 13:33

Hey Remy, nice work. But with ATL you can just derive from CRTP/mixin class ATL::IDispatchImpl : learn.microsoft.com/en-us/cpp/atl/reference/… which I think is a lot less verbose :)

Remy Lebeau Mar 6 at 15:32

@PepijnKramer yes, using a framework like ATL to wrap much of the work is a good idea. But having a good understanding of what's actually happening under the hood is important, too.

Pepijn Kramer Mar 6 at 16:00

I kind of agree, but after working with COM from since 1998... It is in the end a complex tech. SO if OP only needs to do this once. Going into detail how COM hooks into C++ might not be necessary, and the somewhat more declaritve style of ATL might be less frustrating. And normally I would even advise agains using the whole IDispatch machinery but seems OP got that as a given.

Pepijn Kramer Mar 6 at 16:01

The naming in the idl file is a bit unfortunate too , since this is not a (connectionpoint) event but just a regular callback interface. But sometimes it is what it is :)

Collectives™ on Stack Overflow

How to glue COM server to methods in C++

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related