Many C++ programmers still use <iostream.h> instead of the newer, standard compliant <iostream> library. What are the differences between the two? First, the .h notation of standard header files was deprecated more than five years ago. Using deprecated features in new code is never a good idea. In terms of functionality, <iostream> contains a set of templatized I/O classes which support both narrow and wide characters, as opposed to <iostream.h> which only supports char-oriented streams. Third, the C++ standard specification of iostream’s interface was changed in many subtle aspects. Consequently, the interfaces and implementation of <iostream> differ from those of <iostream.h>. Finally, <iostream> components are declared in namespace std whereas <iostream.h> components are global.
Because of these substantial differences, you cannot mix the two libraries in one program. As a rule, use <iostream> unless you’re dealing with legacy code that is only compatible with <iostream.h>.

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