C++

<iostream.h> or <iostream>

• <iostream> is the standard compliant library. <iostream.h> is deprecated since many many years.
• <iostream> contains a set of templatized I/O classes which support both narrow and wide characters (by contrast, <iostream.h> classes are confined to char exclusively).
• Third, the C++ standard specification of iostream's interface was changed in many subtle aspects. Consequently, the interfaces and implementation of <iostream> differ from <iostream.h>.
• Finally, <iostream> components are declared in namespace std whereas <iostream.h> components are declared in the global scope.

Note that both libraries cannot be mixed in one program.