Re: multiple inheritance; consistency of 'virtual'
On 2011-03-10 02:58:14 -0500, m0shbear said:
How come with
class A{};
class B: public A{};
class C: public virtual A, public virtual B{};
, B needs to virtual inherit A so that ambiguity in C wrt members of A
can be resolved? Is this due to subtleties of vtable generation when
MI is used?
This really isn't related to virtual.
struct A { };
struct B : A { };
struct C : A, B { };
Here, too, references to C::A are ambiguous because there are two A's
in each C. You can say C::B::A to access the A base of B, but there's
no way to acess the direct A base of C.
Add virtual when the design requires virtual inheritance, that is, when
you have multiple base classes that should share data. The side effect
that adding virtaul can eliminate ambiguity is irrelevant: if your
design requires two A subobjects, then you need two A subobjects;
making them virtual bases changes the design, and should not be done
just because you ended up with an ambiguity.
The way to fix the ambiguity is to add another level of indirection:
struct Bprime : A { };
struct C : Bprime, B { };
--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)