IUnknown::QueryInterface - OLE Programmer's Reference

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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
);

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."):

The set of interfaces accessible on an object through IUnknown::QueryInterface must be static, not dynamic. This means that if a call to QueryInterface for a pointer to a specified interface succeeds the first time, it must succeed again, and if it fails the first time, it must fail on all subsequent queries.
It must be symmetric if a client holds a pointer to an interface on an object, and queries for that interface, the call must succeed.
It must be reflexive if a client holding a pointer to one interface queries successfully for another, a query through the obtained pointer for the first interface must succeed.

It must be transitive

if a client holding a pointer to one interface queries successfully for a second, and through that pointer queries successfully for a third interface, a query for the first interface through the pointer for the third interface must succeed.

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