IClassFactory::CreateInstance method (unknwn.h)
Creates an uninitialized object.
[in] IUnknown *pUnkOuter,
[in] REFIID riid,
[out] void **ppvObject
If the object is being created as part of an aggregate, specify a pointer to the controlling IUnknown interface of the aggregate. Otherwise, this parameter must be NULL.
A reference to the identifier of the interface to be used to communicate with the newly created object. If pUnkOuter is NULL, this parameter is generally the IID of the initializing interface; if pUnkOuter is non-NULL, riid must be IID_IUnknown.
The address of pointer variable that receives the interface pointer requested in riid. Upon successful return, *ppvObject contains the requested interface pointer. If the object does not support the interface specified in riid, the implementation must set *ppvObject to NULL.
This method can return the standard return values E_INVALIDARG, E_OUTOFMEMORY, and E_UNEXPECTED, as well as the following values.
|The specified object was created.
|The pUnkOuter parameter was non-NULL and the object does not support aggregation.
|The object that ppvObject points to does not support the interface identified by riid.
A COM server's implementation of CreateInstance must return a reference to an object contained in an apartment that belongs to the server's DCOM resolver. It must not return a reference to an object that is contained in a remote apartment.
The IClassFactory interface is always on a class object. The CreateInstance method creates an uninitialized object of the class identified with the specified CLSID. When an object is created in this way, the CLSID must be registered in the system registry with the CoRegisterClassObject function.
The pUnkOuter parameter indicates whether the object is being created as part of an aggregate. Object definitions are not required to support aggregation - they must be specifically designed and implemented to support it.
The riid parameter specifies the IID (interface identifier) of the interface through which you will communicate with the new object. If pUnkOuter is non-NULL (indicating aggregation), the value of the riid parameter must be IID_IUnknown. If the object is not part of an aggregate, riid often specifies the interface though which the object will be initialized.
For OLE embeddings, the initialization interface is IPersistStorage, but in other situations, other interfaces are used. To initialize the object, there must be a subsequent call to an appropriate method in the initializing interface. Common initialization functions include IPersistStorage::InitNew (for new, blank embeddable components), IPersistStorage::Load (for reloaded embeddable components), IPersistStream::Load, (for objects stored in a stream object) or IPersistFile::Load (for objects stored in a file).
In general, if an application supports only one class of objects, and the class object is registered for single use, only one object can be created. The application must not create other objects, and a request to do so should return an error from IClassFactory::CreateInstance. The same is true for applications that support multiple classes, each with a class object registered for single use; a call to CreateInstance for one class followed by a call to CreateInstance for any of the classes that should return an error.
To avoid returning an error, applications that support multiple classes with single-use class objects can revoke the registered class object of the first class by calling CoRevokeClassObject when a request for instantiating a second is received. For example, suppose there are two classes, A and B. When CreateInstance is called for class A, revoke the class object for B. When B is created, revoke the class object for A. This solution complicates shutdown because one of the class objects might have already been revoked (and cannot be revoked twice).
|Minimum supported client
|Windows 2000 Professional [desktop apps | UWP apps]
|Minimum supported server
|Windows 2000 Server [desktop apps | UWP apps]