Redefining static const values in derived classes C++

Question

If I create a static const in the base class of my hierarchy, can I redefine its value in a derived class?

edit:

#include <iostream>

class Base
{
public:
  static const int i = 1;
};

class Derived : public Base
{
public:
  static const int i = 2;
};

int main()
{
  std::cout << "Base::i == " << Base::i << std::endl;
  std::cout << "Derived::i == " << Derived::i << std::endl;  

  Base * ptr;

  ptr= new Derived;

  std::cout<< "ptr=" << ptr->i << std::endl;

  return 0;
}

... ptr refers to Base::i , which is undesirable.

Answer 1

Access via ptr to static members is via its declared type Base * and not its runtime type (sometimes Base * , sometimes Derived * ). You can see this with the following trivial extension of your program:

#include <iostream>

class Base
{
public:
    static const int i = 1;
};

class Derived : public Base
{
public:
    static const int i = 2;
};

int main()
{
    std::cout << "Base::i == " << Base::i << std::endl;
    std::cout << "Derived::i == " << Derived::i << std::endl;  

    Base *b_ptr = new Derived;
    std::cout<< "b_ptr=" << b_ptr->i << std::endl;

    Derived *d_ptr = new Derived;
    std::cout<< "d_ptr=" << d_ptr->i << std::endl;

    return 0;
}

Output:

Base::i == 1
Derived::i == 2
b_ptr=1
d_ptr=2

Answer 2

No. It's const , so you cannot modify its value.

But you can declare a new static const of the same name for the derived class, and define its value there.

#include <iostream>

class Base
{
public:
  static const int i = 1;
};

class Derived : public Base
{
public:
  static const int i = 2;
};

int main()
{
  std::cout << "Base::i == " << Base::i << std::endl;
  std::cout << "Derived::i == " << Derived::i << std::endl;  
  return 0;
}

Answer 3

you must initiate the const memember variable in the Constructor member initialization list.

you can not modify the const variable.