从返回基类指针的派生类对象调用方法

Question

I have a misunderstanding with some c++ OOP concepts, for example If I have a base class that returns a pointer to an object of the same class, say:

class base{
public:
    int baseVal;
    base* foo(){
        return new base;
    }
}

and a derived class that inherits method foo() :

class derived : public base{
public:
    int derivedVal;
    //some other members
}

My problem is "What will happen when I call the method foo() from a derived object? are there any way -instead of overriding- to make it return an object of derived not base ?" for example:

int main(){
    derived obj1;
    derived * obj2P = obj1.foo();
    cout << obj2P->derivedVal << endl;
    return 0;
}

How may I handle something like that?

Answer 1

You can achieve what you want with a templated base class.

template<typename derived_t>
class base {
public:
  derived_t* foo() {
    return new derived_t;
  }
};

class derived : public base<derived> {
public:
  int derivedVal;
  //some other members
};

See also the CRTP pattern.

Although, it is generally bad practice to return an owning raw pointer and you should prefer to return smart pointer like unique_ptr :

template<typename derived_t>
class base {
public:
  std::unique_ptr<derived_t> foo() {
    return std::make_unique<derived_t>();
  }
};