Rule A3-1-3 (advisory, architecture / design / implementation,automated)
Implementation files, that are defined locally in the project, should have a file name extension of ".cpp".
Rationale
This is consistent with developer expectations to provide C++ implementation files with the standard file name extension. Note that compilers support various file name extensions for C++ implementation files.
See also
JSF December 2005 [8]: 4.9.2 AV Rule 54 Implementation files will always have a file name extension of ".cpp".