Extension Source Type
.h Header files. Header files contains class, type, function, and constant declarations.
.m Source files. This is the typcial extension used source files and can contain both Objective-C
and C code.
.mm Source files. A source file with this extnesion can contain C++ code in additon to Objective-C
and C code. This extension should be used only if you actually refer to C++ classes or features
from your Objective-C code.