Virtual Inheritance: what happens if the keyword is at some point forgotten?

Question

What happens if, in a large chain of inheritances, the virtual keyword is at some point forgotten?

For example:

struct I {};

struct A : virtual I {};

struct B : A, virtual I {};

struct C : B, /* virtual */ I {};   // ooops, distraction error

Is it like in the methods case, that once a method is virtual it stays virtual forever, or is the struct C reintroducing the diamond problem?

Is there a way to make the compiler check for this type of errors, in a similar way the new override keyword is checking for the correct overriding of the virtual methods?

How would the compiler know that this is an error? It's perfectly valid and reasonable code. Also, why would you be using virtual inheritance so much? — Nicol Bolas, Mar 08 '16 at 15:22
From the compiler view, it is not an error, and such code is reasonable (ish)? In all my years of coding never in my life I had to deal with diamond inheritance outside of the interview room. I think, if you have designed your system with such inheritance, just go back and redesign. — SergeyA, Mar 08 '16 at 15:26
@NicolBolas Why would you use non-virtual public inheritance so much? — curiousguy, Mar 11 '16 at 21:09

score 4 · Accepted Answer · answered Mar 08 '16 at 15:28

What happens here is as follows:

A gets I as part of its memory
B gets exactly A as part of its memory
C gets exactly B plus an extra I as part of its memory

So it's not a diamond, but more like a broken fork:

I
|
A
|
B   I
 \ /
  C

Also, it's not strictly an error—at least not a compilation error—but a feature of the language.

As for avoiding it, you should probably restrict your virtual inheritance work to when you're really focused anyways, and/or avoid it as much as you can.

score 0 · Answer 2 · answered Mar 11 '16 at 21:07

What happens if, in a large chain of inheritances, the virtual keyword is at some point forgotten?

struct A : virtual I {};

struct B : A, virtual I {};

Why would you even "reintroduce" I? It is already a virtual base class. You are just being redundant.

There is no reason to do that. The only possible reason would be to relax access control (or gain access to a private virtual base class), and the base class is already public here.

So the question is just meaningless. You can't forget a second virtual in a chain of inheritance when you never repeat the base class name.

Virtual Inheritance: what happens if the keyword is at some point forgotten?

2 Answers2