ATL Multiple Dual Interfaces

Exposing a Single IDispatch Interface

ATL provides the method to expose multiple dual interferences on a single object. This can be achieved by deriving the multiple specializations of IDispatchImpl. This class contains the static members of type CComTypeInfoHolder. This static member manages the information type of dual interface. ATL also allows multiple queries for the IDispatch interface. In such cases, you also need to use the COM_INTERFACE_ENTRY2 macro. It specifies the base class of the IDispatch. 

Example:

COM_INTERFACE_ENTRY2(IDispatch, IMyDualInterface)

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Here only one IDispatch interface is exposed. In such cases, you can only access the objects of your class.

Combining Multiple Dual Interfaces in a Single Implementation of IDispatch

ATL doesn't have the feature to combine Multiple Dual Interfaces in a Single Implementation of IDispatch. You can manually implement such features. The following are the different ways to combine multiple dual interfaces into a single implementation:

• Creating a templated class that contains all the separate IDispatch interfaces.
   
• Creating a new object to carry out the QueryInterface function. QueryInterface() is used to obtain an interface pointer of an another class from the exiting interface pointer.
   
• Using the nested objects with typeinfo. The typeinfo is a header file which defines several types associated with the type identification operator, which yields information about both static and dynamic types.
   

The above approaches are not practically feasible. This might result in code complexity and namespace collisions.

Nonextensible Attribute

The nonextensible attribute gives information to client languages (like Visual Basic) that can be used to enable full code verification during compile time. This attribute is helpful when the dual interface is not enabled during runtime. This attribute prevents bugs in the client code during runtime.

Syntax:

[nonextensible]

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Let's understand with an example.

Code:

#include "header.h"
[module(name="ATLFIRELib")];
[export] typedef long CODESTUDIO;


[dual, nonextensible, ms_union, oleautomation,
uuid("96842514-9E4C-2804-AB98-030098666741")]
__interface CodingNinjas
{
   CODESTUDIO procedure (int x);
};

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Explanation:

 In the above code snippet, we had created an interface with the name CodingNinjas. It is a dual and nonextensible attribute. It is helpful when the dual interface is not enabled during the runtime of the code.

Frequently Asked Questions

What is an Object Model?

The Object Model visualizes the software elements as objects. 

Is it good to combine Multiple Dual Interfaces in a Single Implementation of IDispatch?

No, combining Multiple Dual Interfaces into a Single Implementation of IDispatch would increase the complexity of the code.

What is a Virtual Table?

A virtual table is a lookup table of functions used to solve function calls in dynamic binding.

Conclusion

In this article, we have discussed the details of Multiple Dual Interfaces and nonextensible Attributes in ATL.

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