Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-ncis!helios.ee.lbl.gov!pasteur!ucbvax!decwrl!labrea!rutgers!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: static members
Message-ID: <8695@alice.UUCP>
Date: 7 Jan 89 05:01:32 GMT
References: <701@cadillac.CAD.MCC.COM>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 69

In article <701@cadillac.CAD.MCC.COM>, rpj@redcloud.cad.mcc.com (Rich Johns) writes:

> In Stroustroup on page 275, section 8.5.1 it states:
> 
>    "No initializer can be specified for a static member, 
>     and it cannot be of a class with a constructor."

> This could mean two things(at least):

> 1) if a class has a constructor, that class may NOT have 
>    static members.

> 2) You cannot have a static data member which needs a 
>    constructor.

(2) is correct.  The reason for this is that the definition
of a static data member appears as part of the class definition,
which is generally replicated in several modules.  If the
member requires a constructor, that constructor would effectively
be called in each module, thus initializing the member many times.

> After discussing this with someone, I am inclined to 
> think number 2 is correct. As such are the following
> assumptions correct:

>  a) a static data member is allowed, as long as:
>     1) the member does not require a constructor.
>     2) the member is private

It doesn't have to be private.

>  b) a static const data member  is allowed, as long as:
>     1) the member does not require a constructor.
>     2) the member is private

It doesn't have to be private.

>  c) a static const data member CAN have an initial value.

Yes indeed.

>  d) a static function member is not allowed.

It would make sense, wouldn't it?

	class Widget {
		static int n;
	public:
		Widget() { n++; /* other stuff */ }
		~Widget() { n--; /* other stuff */ }
		static int count() { return n; }
		/* other stuff */
	};

This is an example of a Widget class that, among other things,
counts how many Widgets there are in the universe.  Thus if I say

	Widget x;

	cout << x.count() << "\n";

it prints 1 (unless there are other Widgets I didn't mention).
Of course if there aren't any Widgets at all I can't use this
notation to count them, but I should be able to call Widget::count.

Anyway, it's an interesting idea, but not presently implemented.
-- 
				--Andrew Koenig
				  ark@europa.att.com