Re: Dealing with a Diamond of Death

James Kanze <>
Thu, 30 Oct 2008 03:01:58 -0700 (PDT)
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)
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