g++编译报错：undefined reference to symbol

问题描述：
(Template implementation not found)
Common error messages are error LNK2001, error LNK1120, error LNK2019 for Microsoft Visual Studio and undefined reference to symbolName for GCC.
ref: https://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix

原因解析：
ref: https://stackoverflow.com/a/495056