I found that: as long as we put the member function's definition in the class body, there won't be any "multi-definition" problem when the header file is included several times.
Then my question is:
why would I love to put a member function definition in a separated cpp file?
3: I've had some trouble placing member function definitions in the class body in a header file when there were multiple cpp files that required the definitions. Linker errors occurred.