Re: Why C++ Is Not =?windows-1252?Q?=93Back=94?=
On 12/6/2012 7:33 AM, BGB wrote:
On 12/5/2012 10:00 PM, kurt krueckeberg wrote:
This is what the Google coding guidelines for C++ says about multiple
inheritance:
Multiple Inheritance
Only very rarely is multiple implementation inheritance actually
useful. We allow multiple inheritance only when at most one of the
base classes has an implementation; all other base classes must be
pure interface classes tagged with the Interface suffix.
What makes about as much sense as their guideline on (not) using
exceptions... At least the later versions kinda admit that their
guidelines follow being locked in messed up codebases.
yeah, 'tis an issue...
how often does using MI actually make sense?...
How you measure "often"? Or more interestingly why?
The base guideline is "do the right thing" or "use the/a right tool".
You make decision on the particular situation. Where MI will or will
not make sense. Entirely unrelated on MI's usefulness in other situations.
IME one completely sensible use of MI is where you mix different
hierarchies. Like an UI framework like MFC and the implementation of
your model and other stuff. A usual bad way is to just use classwizard
and punch in the code wherever it placed //TODO. While it is ways better
to write the logic separately, independent of an UI, and glue them
together -- possibly using trivial templates.
I suspect though that this is a reason for MI being generally absent
from many/most newer languages.
The reason is probably simpler: authors of those didn't want to mess
with object layout issues. It beats me why disallowing code from
interfaces, not only data... I'm yet to hear a on-FUD argument here.
like, it seemed like a good idea early
on, but tended not really to be generally worth the added funkiness and
implementation complexity, and the single-inheritance + interfaces model
is mostly-good-enough but is much simpler to implement.
If you put down a language that aims for just a small subset of possible
problems it is okay to restrict. Certainly if in that realm the dropped
tools are really not needed.
C++ aimed for the most general arena. And most people admit that when
you need MI you really need it, and the alternatives are pretty crappy.
Implementation-wise it is not such a big burden. More so on user side:
with MI in the mix you can't just keep casting stuff around carelessly.
But who said you should rather aim to support sloppy casting habits?
actually, it is sort of like inheritance hierarchies:
many people seem to imagine big/complex hierarchies more similar to that
of a taxonomy (with many levels and a "common ancestor" for pretty much
everything);
Those cosmic hierarchies in C++ was forced in early times due to lack of
templates (or template-based libraries). Than as that feature (plus
native RTTI) entered you could just cut the obsolete roots and tangles.
more often, I have personally rarely seen cases where more than 2 or 3
levels are needed (and very often don't have *any* parent class).
Yeah.