包含在main和class体中返回多个定义错误

I am trying to compile a main function which includes a class header. The main function as well as the class cxx file use a helper function called "SayHello". However, if I include the helper file "Hello.h" in the main and the class cxx file I get a multiple definition error. I put minimal examples below. Can you help me?I am trying to compile a main function which in