CODEWITHSUNDEEP
c++classinheritanceradixderived
Tirtha
Tirtha

Reputation: 33

does a derived class object contain private members of the base class? What does it look like in memory?

class base{
private:
  int a;
public:
  int b;
  void setData(){
    a = 10; b = 5;
  }
};
class derived: public base{
private:    
   int c;
public:
  // b is inherited
};

void main(){
  derived D1;

  D1.setData();
}

I learned that private members aren't inherited. So, the private variable a (in the base class) is not present in the derived class object. My question is when D1.setData() is called, how can it assign a value to a, if a doesn't exist? I'm pretty sure I have this concept wrong, so can some one explain how this works? Are the base class members also created when the derived class object is created?

Upvotes: 0

Views: 2027

Answers (2)

VoidStar
VoidStar

Reputation: 5421

Private member variables are inherited, but cannot be accessed externally. The memory pattern of inheritance is simple:

class base {
    int x;
};

class subclassCpp : public base {
    int y;
};

class subclassCStyle {
    base a;
    int y;
};

Now, subclassCpp and subclassCStyle have the exact same memory pattern, regardless of private/protected status, etc. This should illustrate how the memory is laid out.

For the other question, "Are the base class members also created when the derived class object is created?"

The answer is yes, a constructor is always invoked on the base class. If you don't add this yourself, the default constructor is automatically invoked. If the base class doesn't have a default constructor, it won't let you create a subclass constructor that doesn't properly invoke whatever constructor is necessary to initialize the base. So there is no legal way you can end up with a base class that hasn't been initialized when the subclass was created, as long as a constructor in the subclass was executed.

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726569

I learned that private members aren't inherited.

Of course they are inherited: otherwise, all member functions of the base class that need these private variables would be broken. Although private members are not accessible to the inheriting class, the base class retain full access to them.

Can some one explain how this works?

Layout of the inheriting class derived includes a place to store a, the private member of the base class. All methods of the base class can access base::a. At the same time, it remains inaccessible to the derived class: any attempt to access a from derived would cause a compile-time error.

Upvotes: 1

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