Using the C++ I/O stream libraries

The following information applies to the USL I/O Stream Class Library and to the Standard C++ I/O stream classes.

Unit-buffering incurs a significant performance penalty. Unit-buffering can be enabled by setting the ios::unitbuf flag. It is enabled for the cerr object by default.
The sync_with_stdio() function enables unit-buffering of standard streams, to ensure their synchronization with C standard streams. However, a runtime performance penalty is incurred to ensure this synchronization. For more information about sync_with_stdio(), see Using the Standard C++ Library I/O Stream Classes.
In most cases, calls to functions in the USL or ANSI C++ I/O stream libraries are mapped to calls to the I/O functions of the C standard library. For this reason, direct calls to the C I/O functions are recommended for applications that must have the best possible performance. This does not mean that these types of applications cannot or should not contain any iostream.h calls. However, you might want to ensure that iostream.h I/O calls do not appear on the critical path; it is safe to keep them for unused debugging code.
Note: If you access the same file with both C and C++ I/O stream classes, undefined results will occur.