Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!brunix!sdm
From: sdm@cs.brown.edu (Scott Meyers)
Newsgroups: comp.lang.c++
Subject: Re: constructors and initializers
Keywords: constructor, default, class, array, definition
Message-ID: <75068@brunix.UUCP>
Date: 9 May 91 15:33:14 GMT
References: <1991May2.165955.28385@iitmax.iit.edu> <701@taumet.com> <1991May8.155352.18313@alias.com>
Sender: news@brunix.UUCP
Reply-To: sdm@cs.brown.edu (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 45

In article <1991May8.155352.18313@alias.com> rae@alias.com (Reid Ellis) writes:
| A note about initializers in constructors.  Recently, the following
| class was mentioned under the subject line "HELP: default constructor
| for array of class":
| 
| 	class Point {
| 		int x;
| 	public:
| 		Point() { x = 200; }
| 		Point(int xc = 100) { x = xc; }
| 	};
| 
| Not to belabor the point [pun intentional] but the constructors should
| be written as:
| 
| 	class Point {
| 		int x;
| 	public:
| 		Point() : x(200) {}
| 		Point(int xc = 100) : x(xc) {}
| 	};
| 
| i.e. use initializers rather than assignment in a constructor.  Why?
| The following line of code won't work with the first version, but will
| work with the second version:
| 
| 	const Point kPoint(23);

But it will work.  It's perfectly acceptable to invoke constructors (and
destructors) on const objects -- see the ARM section 9.3.1, p. 178.

Although the ARM doesn't explicitly say it anywhere, it seems to be the
case that a const object doesn't become const until after construction is
complete (including the body of the constructor), and it ceases to be const
once destruction commences.  This seems to me to be the only practical
semantics, since you can't always do everything you want to through the
member initialization list.

For the more empirical in the audience, the above code also compiles
and runs under cfront 2.0, 2.1, and g++ 1.39.

Scott

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