c++ - Override initial value of a member variable of base class

Question

I am very confused with the inheritance right now. I planned to simply override the initial value of a variable. In the following code i just inherit the base class and try to get it's name, which is saved as a string withing the class. I expected that the derived class can override this value, however it does not do it.

My expected out put was

Derived
Derived

However i get

Base
Base

What is the correct way to implement the following?

#include <iostream>
#include <string>

struct Base {
    virtual ~Base() = default;
    virtual void id(){
        std::cout << id_ << std::endl;
    }
    std::string id_ = "Base";
};   



struct Derived : public Base {
    virtual ~Derived() = default;
    std::string id_ = "Derived";
};   


int main(){
    Base* b = new Derived();
    Derived* d = new Derived();
    b->id();
    d->id();
    delete d;
    delete b;                                                                                                          
    return 0;
}

Answer 1

What is the correct way to implement the following?

Without meaning to sound difficult, it really depends on exactly what you want to achieve.

I'm going to make a guess that we're simply be asking an object what type it is, regardless of which interface we're calling on:

struct Base {
  virtual ~Base() = default;

  virtual const std::string id() const {
    return "Base";
  }
};

struct Derived : Base {
  virtual const std::string id() const override {
    return "Derived";
  }
};

Here's another way:

struct Base {
  virtual Base(std::string ident = "Base") 
  : _id(std::move(ident))
  {}

  virtual ~Base() = default;

  std::string& id() const {
    return _id;
  }

private:
  std::string _id;

};

struct Derived : Base {
  Derived() : Base("Derived") {}
};

And another, using value is interface, but note that this will disable the assignment operator

struct Base {
  virtual Base(std::string ident = "Base") 
  : id(std::move(ident))
  {}

  virtual ~Base() = default;

  const std::string id;

};

struct Derived : Base {
  Derived() : Base("Derived") {}
};

This list is by no means exhaustive.