C / C++ / MFC

George_George wrote:

1. What is the purpose of doing so "in practice"? Force user to use IDispatch other than efficient vtable?

George_George wrote:

2. Could we use vtable even if dual property is not added to interface? For example, using QueryInterface to get some interface other than IUnknown and IDispatch, then invoke its member function just the same as invoking through vtable?

CPallini wrote:

In practice you use IDispatch on VTABLE capable client only if you need late binding.

CPallini wrote:

You can always use VTABLE on all of the COM Interfaces. what dual really means is that the same functionalities that you can access via IDispatch mechanism are also available as standard COM methods (i.e. are part of the interface itself).

George_George wrote:

You mean QueryInterface for IDispatch interface? What mean later binding?

Dim objSer as Object
set objSer = CreateObject("MSCOMMLib.MSComm")

George_George wrote:

So you mean even if we do not explicitly add dual to the property, we can still use dual (by vtable and by IDispatch.invoke()) functions if underlying component support it?

If yes, what is the function of dual keyword? It does not enforce anything strictly, right?

CPallini wrote:

For instance (maybe syntax is not correct, I havent VB6 at hand)

Dim objSer as Objectset objSer = CreateObject("MSCOMMLib.MSComm")

is late binding, i.e. MSComm object is ignored at compile time and you can only access objSer functionality through IDispatch. Late binding is often useful.

George_George wrote:

If I remember correctly (I am not VB expert), in VB to access some object in COM, we have to program in the way above, does it mean VB always use later binding?

2.

Do you mean there are any other ways to access VB in COM, that is not later binding?

George_George wrote:

3.

Later binding means runtime binding, right? In runtime, using invoke method on the IDispatch interface of the component?

George_George wrote:

1.

For the later binding case (suppose client will use invoke, other than vtabke), I am wondering during compile/build phase, how will the client (C++, VB, VBScript, etc.) utilize the TLB file? Does the contained interface function information be read and checked?

(I think the answer is no)

George_George wrote:

So, all the interface/function/parameter checking work in done during runtime?

Dim objSer as Objectset
objSer = CreateObject("MSCOMMLib.MSComm")

Private Sub Command1_Click()
    Dim obj As Object
    Set obj = CreateObject("George.George")
End Sub

George_George wrote:

once we inherit from IDispatch interface, then it is always a dual interface, right?

George_George wrote:

What type of implementation in C++ for a component is not a dual interface? I can not imagine a sample

Rajkumar R wrote:

I think ur confusion is once derived from IDispatch in C++, u have access through vtbl (atleast of IDispatch) and also through Invoke(), dual implicit and why u need dual keyword. Is it?

George_George wrote:

but actually in implementation, I implemented the dual approach.

George_George wrote:

Could the client of the component (e.g. in case of C++ client) utilize vtable other than through IDispatch?

rp_suman wrote:

How to check the instance of the class is valid