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SkoolBoyAtWork
SkoolBoyAtWork

Reputation: 97

Assign one interface pointer to another interface pointer in C++

Is it possible to assign the Interface pointer to another interface pointer where both interface pointers point to same class. below is my code for Test.h and Test.cpp

class Test
    {
        ITestPtr testPtr;
        ITestSecondPtr secondPtr;
    };

 Test:: Test()
    {
    testPtr.CreateInstance(__uuidof(MyNamespace::MyClass));
    secondPtr=testPtr;
    }

Here my ITestPtr and ITestSecondPtr points to same C# class MyClass. So to avoid double initilaization of C# class, I assigned testPtr to secondPtr. It is build successfully. But the Runtime throws the Access Violation Excpeption. Please let me know if any solution.

Upvotes: 0

Views: 440

Answers (1)

harper
harper

Reputation: 13690

No. It's not possible to assign a pointer to one COM interface to another. When you QueryInterface for IUnknown you will always get the same pointer to the implementing object. But when you query for a specific interface you will get different pointer.

The reason is that the pointer points to the VMT pointer. When you have different inherances fro IUnknown you need different VMTs. That's why QueryInterface must select the correct VMT at a different address.

The reason for the convetion to return always the same IUnkown pointer is that this allows to check the identity of an object despite you get different pointer for different intefaces.

So your requested solution: Don't (implicitly) cast COM interface pointer. Use QueryInterface() to get the right pointer.

More Details are here.

Upvotes: 1

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