Returns a pointer to a specified interface on an object to which a client currently holds an interface pointer. This function must call IUnknown::AddRef on the pointer it returns.
HRESULT QueryInterface( REFIID iid, void ** ppvObject );
S_OK if the interface is supported, E_NOINTERFACE if not.
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."):
Implementations of QueryInterface must never check ACLs. The main reason for this rule is because COM requires that an object supporting a particular interface always return success when queried for that interface. Another reason is that checking ACLs on QueryInterface does not provide any real security because any client who has access to a particular interface can hand it directly to another client without any calls back to the server. Also, because COM caches interface pointers, it does not call QueryInterface on the server every time a client does a query.
For an explanation of the requirement values, see Requirements (COM).
Windows NT/2000/XP: Requires Windows NT 3.1 or later.
Windows 95/98: Requires Windows 95 or later.
Header: Declared in unknwn.h.