Vacancy method in mathematical function is a method to calculate vacancy function. An empty function is a complete function. It has a function body, but there are no statements in the function body. In the process of program development, it is sometimes necessary to define a virtual function in the descendants of the class, but it is not necessary to maintain every virtual function in the virtual function chain in the derivative hierarchy and it needs to be implemented in detail. If the virtual function of a certain layer does not need to be re-implemented, but the description of the virtual function must be provided, then only an empty virtual function needs to be defined at the corresponding layer to maintain the existence of the virtual function path in the class derivation hierarchy.