Accessing to base class of derived class by derived class C++

Question

The function whoAmI() is supposed to return:

I am a Man
I am a Omnivore

But it just returns "I am a Man" twice:

class Animal
{
public:
  string className;
};

class Omnivore:public Animal
{
 public:
    Omnivore()
    {
        className = "Omnivore";
    }
};

class Man:public Omnivore
{
public:
    Man() {
        className = "Man";
    }
     void whoAmI()
     {
         cout << "I am a " << Omnivore::className << endl;
         cout << "I am a " << Omnivore::Animal::className << endl;
     }
};

There is only one `className` for a given `this`, regardless of how you refer to it. — François Andrieux, Feb 01 '18 at 16:54
Because `Omnivore::className` and `Omnivore::Animal::className` are two names for one string variable - one variable cannot have two values at the same time — PiotrNycz, Feb 01 '18 at 16:54
Make `className` a (virtual or static) function, then it will work. — Slava, Feb 01 '18 at 16:55
Check out the [Definitive C++ Book Guide/List](https://stackoverflow.com/q/388242/2069064). — Barry, Feb 01 '18 at 16:56

songyuanyao · Accepted Answer · 2018-02-01T17:07:12.433

There's only one Animal::className, which is initialized to empty std::string by Animal's constructor, then assigned to "Omnivore" by Omnivore's constructor, then assigned to Man by Man's constructor. So you got the same results, because they refer to same data member.

You can make them have their own data member with the same name; but note that it's not a good idea, the names in derived class will hide those in base class. e.g.

class Animal
{
public:
  string className;
};

class Omnivore:public Animal
{
 public:
    string className;
    Omnivore()
    {
        className = "Omnivore";
    }
};

class Man:public Omnivore
{
public:
    string className;
    Man() {
        className = "Man";
    }
     void whoAmI()
     {
         cout << "I am a " << Omnivore::className << endl;         // "Omnivore"
         cout << "I am a " << Omnivore::Animal::className << endl; // empty
         cout << "I am a " << className << endl;                   // "Man"
     }
};

score 1 · Answer 2 · answered Feb 01 '18 at 16:59

It is usually done this way:

class Animal
{
public:
  static string className() { return "Man"; };
};

class Omnivore:public Animal
{
public:
  static string className() { return "Omnivore"; };
  void whoAmI()
  {
      cout << "I am a " << Omnivore::className() << endl;
      cout << "I am a " << Animal::className() << endl;
  }
};

Accessing to base class of derived class by derived class C++

2 Answers2