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From: philip@pescadero.stanford.edu (Philip Machanick)
Subject: Re: C++ Static Members and Inheritance
Message-ID: <1991May5.224348.12578@neon.Stanford.EDU>
Sender: news@neon.Stanford.EDU (USENET News System)
Organization: Computer Science Department, Stanford University
References: <5069@servax0.essex.ac.uk> <7510@harrier.ukc.ac.uk>
Date: Sun, 5 May 1991 22:43:48 GMT
Lines: 38

In article <7510@harrier.ukc.ac.uk> mr3@ukc.ac.uk (M.Rizzo) writes:
>In article <5069@servax0.essex.ac.uk> whisd@essex.ac.uk (Whiteside S D B)
writes:
>
>>I had a base class in which I declared a static data member.
>>
>>I then derived TWO separate classes which both inherited from the base class.
>>
>>Then these classes were instanstiated as objects I found that for BOTH the
static data member (a pointer) had the SAME value.
[...]
>The only solution I found was to re-declare the static data members
>in every subclass I needed them.  However this should be done with
>care. If a base class member function refers to a static data member,
>then if this function is inherited by a derived class re-declaring the
>same static member, the base class static member will still be the one
>referenced by the inherited function.
>
>What we need here is virtual static data members !  Unfortunately there
>are no such thingies :-(
>
>To solve this problem you can define two virtual non-static functions
>to read and write the member - though again this must be done for
>every derived class where the static member is declared. Macros are very
>handy here !
>
>Does anybody know of another (possibly better) approach ?
[...]
Make the statics private (rather than protected). Then you are forced to
redeclare them for derived classes. This still doesn't help you if you call a
function higher up the hierarchy - the only solution I can propose here is that
you pass the statics as parameters at all times. This, combined with the use of
private, ensures you get the right one.

Again - a better approach would be appreciated.

Philip Machanick