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Language: C++
Expertise: All

Jan 2, 2001

Although the <iostream.h> library was deprecated for several years, many C++ users still use it in new code instead of using 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 5 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. 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. Because of these substantial differences, you cannot mix the two libraries in one program. As a rule, use <iostream> in new code and stick to <iostream.h> in legacy code that is incompatible with the new <iostream> library.