Re: multiple inheritance

  To: bob smith
From: "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-s5y-this>

  To: bob smith
From: Joshua Cranmer <Pidgeot18@verizon.invalid>

On 8/1/2012 10:28 PM, bob smith wrote:


Because multiple inheritance is really, really, really complicated and
confusing for most users.

The short answer is the diamond problem:

class A { int varA; };
class B : A { int varB; };
class C : A { int varC; };
class D : B, C { int varD; };

There are two main points of contention in this kind of hierarchy: 1. How many
copies of varA should D have? Intuitively, one is probably what most people
would expect, but the implementations of B and C would have to cooperate in
realizing that their superclass may be shared with D. It also incurs a penalty
in runtime costs 2. How does initialization/override order get resolved? Is it
"BFS"-y (like D, B, C, A) or "DFS"-y (D, B, A, C)? There are even more
convoluted orders in practice (C3 appears to be the most common nowadays), but
this is the sort of stuff that tends to cause nasty sorts of little edge cases
in practice.

It is rare in practice that you need true multiple inheritance, in the sense of
inheritance of implementation; multiple inheritance of interface is common, and
this is as far as Java goes.

--
Beware of bugs in the above code; I have only proved it correct, not tried it.
-- Donald E. Knuth

-+- BBBS/Li6 v4.10 Dada-1
 + Origin: Prism bbs (1:261/38)
-+- Synchronet 3.16a-Win32 NewsLink 1.98
Time Warp of the Future BBS - telnet://time.synchro.net:24

--- BBBS/Li6 v4.10 Dada-1
 * Origin: Prism bbs (1:261/38)
--- Synchronet 3.16a-Win32 NewsLink 1.98
Time Warp of the Future BBS - telnet://time.synchro.net:24