OOP Bisic-desctructor can be virtual,while constructer can not? why?

A:
    virtual function uses virtual calling.  virtual calling is a  kind of operation mechanism which can run on the condition of part information being provided. it allows us to call a function that we only know the interface while not know accurate object type of the function. 
    However if we create an object, you must know what kind of the object is accrurately! Therefore, the constructor can not be virtual.