是否有必要在以下C ++代码中使用“new”运算符？

Question

The code is:

class base{
    base(){}
    virtual base* copy()const=0;
    virtual ~base(){}
};
class derived:public base{
    derived(){}
    base* copy()const;
    ~derived(){}
};
base* derived::copy()const{
   return new derived(*this);
}

Is it necessary to use the new operator in the function copy() or why the code use the new operator?

Should I directly return this pointer, like this:

const base* derived::copy()const{
   return this;// note: this pointer is const.
}

Answer 1

To put it extremely simply, no.

The this keyword in C++ is a small bit of syntactic sugar meaning "pointer to the current instance of this object".

A copy method, by English-language definition, returns a new object, identical to the first in every way but occupying a different location in memory . Returning this from a copy method would, quite naturally, break with this paradigm, because it would be returning a pointer to the object being "copied" .

Answer 2

your function

base* derived::copy()const{
   return new derived(*this);
}

seems to be correct - you have to use the "new" Operator here. Otherwise, you would create a local instance (of class derived) and return a pointer to that local instance. After execution of your method, the local instance will get invalid (because it is out of scope).