Unexpected size for class that inherits twice from same class

Question

Here the data member of the base class is private. Please explain this code. why the output of this program is 80.

  #include<iostream> 
  using namespace std; 

  class base { 
      int arr[10]; 
   }; 
      
  class b1: public base { }; 
      
  class b2: public base { }; 
      
  class derived: public b1, public b2 {}; 
      
  int main(void) 
  { 
    cout << sizeof(derived); 
    return 0; 
  }

This is the diamond problem, see [How does virtual inheritance solve the "diamond" (multiple inheritance) ambiguity?](https://stackoverflow.com/questions/2659116/how-does-virtual-inheritance-solve-the-diamond-multiple-inheritance-ambiguit) — Botje, Aug 31 '20 at 11:29
Are you expecting a private member to occupy less space than other members? — molbdnilo, Aug 31 '20 at 11:35
@Botje There is no diamond here. `derived` just has two subobjects of `base` type — user7860670, Aug 31 '20 at 11:35
what output did you expect? Why do you think `80` would be a problem? — 463035818_is_not_an_ai, Aug 31 '20 at 11:39
@user7860670 The inheritance diagram looks like a diamond and OP has the exact problem listed in [the C++ faq](https://isocpp.org/wiki/faq/multiple-inheritance#mi-diamond) — Botje, Aug 31 '20 at 11:40
Please formulate your question better; now people are guessing and make fun of it. 'Botje' already explained why the size is 80 and not 40 if that's what you had expected. — gast128, Aug 31 '20 at 11:50

score 0 · Answer 1 · answered Aug 31 '20 at 11:41

You misunderstand what private modifier does. In your code snippet it simply prevents non-members and non-friends of A from accessing it. It's just an access-control modifier. Instances of child class will have data members of its parent, even though child class won't have (direct) access to it.

NOTE : if private, child class cannot peek in parent even though it contains it.

Unexpected size for class that inherits twice from same class

1 Answers1