IRootStorage

The IRootStorage interface contains a single method that switches a storage object to a different underlying file and saves the storage object to that file. The save operation occurs even with low memory conditions and uncommitted changes to the storage object. A subsequent call to IStorage::Commit is guaranteed to not consume any additional memory.

When to Implement
Storage objects that are based on a file should implement IRootStorage in addition to the IStorage interface. For storage objects that are not file-based, this interface is not necessary.

OLE provides an implementation of a storage object, including the IRootStorage interface, as part of its compound file implementation.

When to Use
The primary use for the IRootStorage interface is to save a storage object to a file during low memory conditions. Typically, the container application calls the IRootStorage interface to switch to a new file.

You can call IStorage::QueryInterface with IID_IRootStorage to obtain a pointer to the IRootStorage interface.

Methods in Vtable Order

IUnknown Methods	Description
QueryInterface	Returns pointers to supported interfaces.
AddRef	Increments the reference count.
Release	Decrements the reference count.

IRootStorage Method	Description
SwitchToFile	Copy the file underlying this root storage object, then associate this storage with the copied file.