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Tip of the Day
Language: COM/DCOM/COM+
Expertise: Beginner
Jan 19, 2000

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Use of void** in QueryInterface (COM)

Question:
When using Queryinterface in COM, why is the interface pointer cast to void** and why is it passed as a pointer to a pointer (double indirection)?

Answer:
This is because the function that accepts the void** pointer changes its value.

In general, when you call a function that changes the value of an int, you pass an int * to that function. However, when the argument to be changed is itself a pointer, you need to pass that pointer's address. In other words, you pass a pointer to a pointer. This exactly what void** is.

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