Re: abstract class with virtual base class and non default constructor

From:

Victor Bazarov <v.bazarov@comcast.invalid>

Newsgroups:

comp.lang.c++

Date:

Sat, 15 Mar 2014 15:41:52 -0400

Message-ID:

<lg2ae8$ji1$1@dont-email.me>

On 3/15/2014 1:26 PM, Marcel M??ller wrote:

Virtual base classes are always constructed by the most derived class.
So why need an abstract class a call to the constructor of it's virtual
base?

#include <stdio.h>

class V
{protected:
   V(int i) { printf("V::V(%i)\n", i); }
};

class A : public virtual V
{protected:
   A() : V(42) {} // <-- required to keep the compiler happy
   virtual void PureFunc() = 0;
};

class D : public A
{public:
   D(int i) : V(i) {}
  protected:
   void PureFunc() {}
};

int main()
{ D d(7);
}

From my understanding the constructor call V(42) could never be
executed, isn't it? A cannot be instantiated and any class inheriting
from A must initialize V::V(int) explicitly.

I think making such a distinction between an abstract class and
non-abstract class would be too much for an average compiler. After
all, it might not be so obvious. For instance, the abstractness can be
inherited (and not direct like in your 'A'), or it the inability to
instantiate could be due to some other feature (and not a pure virtual
function). And with conditions like that the standard would be too
complex...

Or it might be just an omission, a defect, that you might want to report
to the committee...

V
--
I do not respond to top-posted replies, please don't ask