Rule A3-1-2 (required, architecture / design / implementation,automated)
Header files, that are defined locally in the project, shall have a file name extension of one of: ".h", ".hpp" or ".hxx".
Rationale
This is consistent with developer expectations to provide header files with one of the standard file name extensions.
Example
//% $Id: A3-1-2.cpp 266557 2017-02-07 13:08:19Z piotr.tanski $
//#include <h3.h> // Compliant
//#include <h1.hpp> // Compliant
//#include <h2.hxx> // Compliant
//#include <h4.cpp> // Non-compliant
//#include <h5.c> // Non-compliant
//#include <h6.hdr> // Non-compliant
//#include <h7.inc> // Non-compliant
See also
JSF December 2005 [8]: 4.9.2 AV Rule 53 Header files will always have a file name extension of ".h".