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IUnknown::QueryInterface
Returns a pointer to a specified interface on a component to which a client
currently holds an interface pointer. This function must call IUnknown::AddRef on the pointer it returns.
HRESULT QueryInterface(
REFIID iid,
| //Specifies the requested interface's IID
| void ** ppvObject
| //Receives an indirect pointer to the object
| );
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Parameters
iid
[in] Specifies the IID of the interface being requested.
ppvObject
[out] Receives a pointer to an interface pointer to the object on return. If
the interface specified in riid is not supported by the object, ppvObj is set to NULL.
Return Value
S_OK if the interface is supported, S_FALSE if not.
Comments
The QueryInterface method gives a client access to other interfaces on an
object.
For any one object, a specific query for the IUnknown interface on any of the object's interfaces must always return the same
pointer value. This allows a client to determine whether two pointers point to the
same component by calling QueryInterface on both and comparing the results. It is specifically not the case that
queries for interfaces (even the same interface through the same pointer) must
return the same pointer value.
There are four requirements for implementations of QueryInterface (In these cases, "must succeed" means "must succeed barring catastrophic
failure."):
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if a client holds a pointer to an interface on an object, and queries for that
interface, the call must succeed.
