virtual keyword in inheritance

Question

Possible Duplicate:
In C++, what is a virtual base class?

In this code when an object of DR is created, the string "Hello World" should be printed 4 times, instead it is printed just 3 times.Why is it so ? From what I guess it is because of the mid1 and mid2 being virtually inherited. Can somebody explain me what happens when we virtually inherit a class and more importantly when it is useful and why ?

#include <iostream>

struct BS
{
  BS()
  {
    std::cout << "Hello World" << std::endl;
  }
  unsigned int color;
};

struct mid1 : virtual public BS { };
struct mid2 : virtual public BS { };
struct mid3 : public BS { };
struct mid4 : public BS { };

struct DR : public mid1, public mid2, 
            public mid3, public mid4 { };

int main(int argc, char** argv) 
{ 
  DR d;
  return 0; 
}

[Take your pick.](https://www.google.com/search?q=virtual+inheritance) — chris, Jan 04 '13 at 19:50
Did you mean ""Hello World" should be printed 3 times, and so it is."? — Luchian Grigore, Jan 04 '13 at 19:51
No from whatever limited knowledge I have it calculated it as 4. But it is printing 3.I want to know the reason. — jairaj, Jan 04 '13 at 19:52
For the record, [this answer](http://stackoverflow.com/questions/21558/in-c-what-is-a-virtual-base-class#21607) explains it pretty well. — chris, Jan 04 '13 at 19:52
@K-ballo: I know as the compiler gives it so. I want to know the reason. — jairaj, Jan 04 '13 at 19:55
@jairaj, It should be a lot easier to properly count them after reading that answer. — chris, Jan 04 '13 at 19:58
I'm not certain, but I _think_ if you rearrange `DR`s parents, the answer _might_ change to two. — Mooing Duck, Jan 04 '13 at 20:01
I voted down. As showned by chris, you didn't do any Googling here. — undu, Jan 04 '13 at 21:50

score 7 · Accepted Answer · answered Jan 04 '13 at 20:31

Let's look at a simplified example:

class base {};

class mid1 : public base {};
class mid2 : public base {};

class derived1 : public mid1, public mid2;

class mid1a : virtual public base {};
class mid2a : virtual public base {};

class derived2 : public mid1a, public mid2a {};

If we draw object diagrams for these, we get something like this:

enter image description here

When the intermediate classes use virtual inheritance, the derived class contains only a single instance of the base class, instead of a separate instance from each intermediate class.

In your case, that leads to three instances of the base class instead of four.

virtual keyword in inheritance

1 Answers1