Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!att!ulysses!andante!alice!bs
From: bs@alice.UUCP (Bjarne Stroustrup)
Newsgroups: comp.lang.c++
Subject: Re: Efficiency of Multiple Inheritance
Summary: alternative implementations
Message-ID: <9465@alice.UUCP>
Date: 12 Jun 89 13:08:05 GMT
References: <1484@bacchus.dec.com>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 18



MI can indeed  be implemented that way. The technique is often referred
to as using `thunks' because a similarity to the technique for inplementing
call by name in Algol60. It has the obvious benefit of saving a memory
reference and an addition in each virtual function call and a word in
every virtual table entry. You don't in general require an extra stack
frame (though certain well known computer manufactures does make life
difficult), a jump suffices on most architectures.

I use the `add a delta' technique in my implementation for portability.
I mention the principle of `not requiring a user to pay for what he/she
doesn't use' in my paper `Multiple Inheritance for C++' (Proc EUUG conference
May 1987, Helsinki and the release notes for release 2.0). I had to
chose between the memory reference + addition cost and adding several
days to the porting time (from a base of a day or so). I chose portability.
Other implementor will have different constrints and goals and may chose
differently.