IStorage : IUnknown

This interface provides methods for creating and managing the root storage, child storage, and stream objects. These methods can create, open, enumerate, move, copy, rename, or delete the elements in the storage object.

At a Glance

The IStorage interface supports the creation and management of structured storage objects. Structured storage allows hierarchical storage of information within a single file, and is often referred to as “a file system within a file”. Elements of a structured storage object are storages and streams. Storages are analogous to directories, and streams are analogous to files. Within a structured storage there will be a primary storage object that may contain substorages, possibly nested, and streams. Storages provide the structure of the object, and streams contain the data, which is manipulated through the IStream interface.

The IStorage interface provides methods for creating and managing the root storage object, child storage objects, and stream objects. These methods can create, open, enumerate, move, copy, rename, or delete the elements in the storage object.

An application must release its IStorage pointers when it is done with the storage object to deallocate memory used. There are also methods for changing the date and time of an element.

There are a number of different modes in which a storage object and its elements can be opened, determined by setting values from the STGM enumeration. One aspect of this is how changes are committed. You can set direct mode, in which in which changes to an object are immediately written to it, or transacted mode, in which changes are written to a buffer until explicitly committed. The IStorage interface provides methods for committing changes and reverting to the last-committed version. Other storage modes set, for example, a stream can be opened in read only mode or read/write mode. For more information, refer to the STGM enumeration.

Other methods provide a means to gain access to information about a storage object and its elements through the STATSTG structure.

When to Implement

Generally, you would not implement this interface unless you were defining a new storage scheme for your system. COM provides a compound file implementation of the IStorage interface that supports transacted access. COM provides a set of helper APIs to facilitate using the compound file implementation of storage objects. Refer to IStorage - Compound File Implementation.

When to Use

Call the methods of IStorage to manage substorages or streams within the current storage. This management includes creating, opening, or destroying sub-storages or streams, as well as managing aspects such as time stamps and names. You can also commit changes or revert to previous version for storages opened in transacted mode. The methods of IStorage do not include means to read and write data—this is reserved for IStream, which manages the actual data. While the IStorage and IStream interfaces are used to manipulate the storage object and its elements, the IPersistStorage interface contains methods that are called to serialize the storage object and its elements to a disk file.