Re: Interfaces in C++

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 10 Oct 2008 07:20:56 -0700 (PDT)
Message-ID:
<452097c2-60b7-42d1-9dd5-c9ccfa450a84@k37g2000hsf.googlegroups.com>
On Oct 10, 11:09 am, anon <a...@no.invalid> wrote:

Pete Becker wrote:

On 2008-10-07 02:07:33 -0400, anon <a...@no.invalid> said:

Maik wrote:

On 6 Okt., 21:12, "A.Gallus" <u...@rz.uni-karlsruhe.de> wrote:

Bingo, that did the job, thx!

class A
{
   virtual void myfunc() = 0;
};

class B : public A
{


You should consider to prefer
  class B : public virtual A
to avoid being bitten by:

http://www.parashift.com/c++-faq-lite/multiple-inheritance.html#faq-25=

..8

which will most likely happen if one uses the interface pattern.


I think that is worse performance wise then just this:
class B : public A
At least thats what I read here:
http://www.agner.org/optimize/#optimize


And int is faster than double, so I won't use
floating-point. Unfortunately, that often gives the wrong
answer.


What I suggested is "do not use it, unless you have to"


We're talking here about a very general case of an interface
which extends another interface. The correct "default" is to
use virtual inheritance, since it is always "correct"; with
non-virtual inheritance, you artificially restrict the ways the
client code can use the interface.

Seriously: virtual and non-virtual inheritance have rather
different meanings, and performance concerns (whether real
or imaginary) never overrule correct semantics.


http://en.wikipedia.org/wiki/Virtual_inheritanceexplains my
understanding of the virtual inheritance.

You haven't said what are differences in virtual and
non-virtual inheritance meanings.


He probably supposed that that was known.

Whether I am going to use a virtual inheritance depends on the
design, and I would not suggest to people to use it because it
might prevent a bug.


Certainly, it depends on the design. If I'm inheriting a
concrete implementation class directly from a single interface,
and the concrete implementation class is not designed to serve
further as a base class, then I don't bother with virtual
inheritance. If I'm extending an interface (in the sense that
Java uses the word "extends"), then I do, systematically. If
I'm providing a concrete implementation class which uses the
template method pattern to allow for further client
customization, it depends, but virtual inheritance has the
advantage of allowing more flexibility to the client.

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