Header Filenames

Files such as iostream are called include files (because they are included in other files) or header files (because they are included at the beginning of a file). C++ compilers come with many header files, each supporting a particular family of facilities. The C tradition has been to use the h extension with header files as a simple way to identify the type of file by its name. For example, the C math.h header file supports various C math functions. Initially, C++ did the same. For instance, the header file supporting input and output was named iostream.h. But C++ usage has changed. Now the h extension is reserved for the old C header files (which C++ programs can still use), whereas C++ header files have no extension. There are also C header files that have been converted to C++ header files. These files have been renamed by dropping the h extension (making it a C++-style name) and prefixing the filename with a c (indicating that it comes from C). For example, the C++ version of math.h is the cmath header file. Sometimes the C and C++ versions of C header files are identical, whereas in other cases the new version might have a few changes. For purely C++ header files such as iostream, dropping the h is more than a cosmetic change, for the h-free header files also incorporate namespaces, the next topic in this chapter. Table 2.1 summarizes the naming conventions for header files.

Table 2.1. Header File Naming Conventions

In view of the C tradition of using different filename extensions to indicate different file types, it appears reasonable to have some special extension, such as .hpp or .hxx, to indicate C++ header files. The ANSI/ISO committee felt so, too. The problem was agreeing on which extension to use, so eventually they agreed on nothing.