VC++ ATL/COM - Step 3.3 Adding a COM Interface - Dual / Custom Interface

3.3 Dual / Custom Interface

Dual: Vtable has the following two important features:

1. custom interface functions

2. late binding IDispatch Interface

Custom: Vtable has only custom interface functions so that it can be faster and can only be accessed via c++ and can not be used with VB, Java or any other scripting language.

Using IDispatch and IUnknown interface

The following lines does the magic for queryinterface

BEGIN_COM_MAP(CMathControl)

COM_INTERFACE_ENTRY(IMathControl)

COM_INTERFACE_ENTRY(IDispatch)

END_COM_MAP()

If you have chosen Custom interface instead of Dual interface, the above lines COM MAP code will look like the following:

BEGIN_COM_MAP(CMathControl)

COM_INTERFACE_ENTRY(IMathControl)

END_COM_MAP()

The big difference here is there is no definition for IDispatch.

[

object,

uuid(B129C4F7-81EA-47DA-AA22-4120C487C3FB),

dual,

nonextensible,

helpstring("IMathControl Interface"),

pointer_default(unique)

]

interface IMathControl : IDispatch{

};

[

uuid(CEF82C0A-246B-4869-A44E-0707CCD480F7),

version(1.0),

helpstring("SFTComServer 1.0 Type Library")

]

library SFTComServerLib

{

importlib("stdole2.tlb");

[

uuid(005C1FA1-FC7E-4FBB-8F12-15D410416F59),

helpstring("MathControl Class")

]

coclass MathControl

{

[default] interface IMathControl;

};

I you have chosen custom interface, you will not find the dual keyword in idl definition and it will be derived from IUnknown.

[