Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!decwrl!ie0013.dec.com!vilot
From: vilot@ie0013.dec.com (Michael J. Vilot)
Newsgroups: comp.lang.c++
Subject: Re: Errors in Constructors
Message-ID: <8902011503.AA10122@decwrl.dec.com>
Date: 1 Feb 89 18:05:00 GMT
Organization: Digital Equipment Corporation
Lines: 34


 
> how to handle an error situation in the constructor for the class.
> ... I would like to provide some way for the program
> creating the object to detect this and handle it appropriately. How
> can I go about doing this?

> mutchler@sun.UUCP
> sun!zule!mutchler

I use the approach of storing status information in the object (originally
outlined by Jon Shopiro in note <7188@alice.UUCP> 17 Aug 87).

If there's a possibility that a constructor can fail, then the notion of a
"null" object of that class -- that is, a well-formed object that simply has
no interesting data values -- is an essential part of the abstraction provided
by the class.

This gives clients of the class an abstract way to test if the constructor
succeeded (similar to testing for a NULL pointer after calling malloc() in 
old C).  Providing member functions for querying the status rounds out the 
idea -- and delegates the details of how to react to the situation to the
client.

An alternate approach is to invoke an error reporting function, although I 
tend to avoid this in constructors. See pp. 207-209, and the discussion of the
_new_handler on pp. 92/93 in "The C++ Programming Language" for an outline of
this alternative.

Hope this helps,

Mike Vilot			Self-employed.  Consulting at:
e-mail:	       			vilot%csdpie.dec@decwrl.dec.com
when that fails:    		(508)467-3631 [for now]