ios delegates the job of actually reading and writing bytes to the abstract class, streambuf, which is designed to provide buffered streams (compatible with C, in the GNU implementation). See Using the streambuf layer for information on the facilities available at the streambuf level.

ios::ios (streambuf* sb [, ostream* tie]) 
Constructor
The ios constructor by default initializes a new ios, and if you supply a streambuf sb to associate with it, sets the state, good, in the new ios object. It also sets the default properties of the new object. You can also supply an optional second argument, tie, to the constructor; if present, it is an initial value for ios::tie, to associate the new ios object with another stream.
ios::~ios () 
Destructor
The ios destructor is virtual, permitting application-specific behavior when a stream is closed—typically, the destructor frees any storage associated with the stream and releases any other associated objects.

• Checking the state of a stream
• Choices in formatting
• Changing stream properties using manipulators
• Extended data fields
• Synchronizing related streams
• Reaching the underlying streambuf