在该类中，我应该如何写以获得“ 2”作为输出？

Question

#include <iostream>
using namespace std;

class ParentClass
{
public:
    int id;

    ParentClass(int id)
    {
      this->id = id;
    }

    void print()
    {
      cout << id << endl;
    }
  };

class ChildClass: public ParentClass
{
public:
    int id;

    ChildClass(int id): ParentClass(1)
    {
      this->id = id;
    }
};

int main()
{
    ChildClass c(2);
    c.print();

    return 0;
}

I run this C++ file and I get '1' as output, I want to know how should I write to get '2' as output ? Or say that how to access a redefined variable from a derived class.

Answer 1

You cannot access ChildClass::id from ParentClass::print . More generally: You cannot access members of a derived class, from a member function of the parent.

You can access ChildClass::id within the member functions of ChildClass .

ChildClass::id isn't a "redefined" variable. It is a separate, unrelated variable that happens to have the same identifier, but in different scope. There is no concept of "redefining" a member variable in a derived class, in C++.

---

I recommend considering, whether it makes sense for ChildClass to have two different ids.

Answer 2

Your ChildClass is doing some wrong or discutable stuff:
1. it's "shadowing" the int id member of ParentClass
2. it's calling the ParentClass constructor passing 1 (instead of "id")
3. it's using assignements in constructors instead of initializer lists
4. ParentClass::id visibility may be decreased from public to protected or private.

Try the following modified code:

#include <iostream>

class ParentClass
{
public:
    int id;

    ParentClass(int id) : id(id) {}

    void print()
    {
      std::cout << id << std::endl;
    }
  };

class ChildClass: public ParentClass
{
public:
    ChildClass(int id): ParentClass(id) {}
};

int main()
{
    ChildClass c(2);
    c.print();

    return 0;
}