Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: Inheritance & type conversion
Message-ID: <761@taumet.com>
Date: 7 Jun 91 15:58:38 GMT
References: <2348@taurus.cs.nps.navy.mil>
Organization: Taumetric Corporation, San Diego
Lines: 30

skip@taygeta.oc.nps.navy.mil (Skip Carter) writes:

>There are two classes, Base and Derived.

>Derived is derived from the Base class and has a method to do a
>	type conversion from Derived TO Base.

>Base has friends functions, report1 and report2.  Report1 takes a
>	reference to a Base, report2 takes the value of a Base,
>	as the parameter.

[
  sometimes the user conversion from Derived to Base is called for
  only one function, sometimes only for the other, depending on compiler.
]

The problem is that the language specification defines how a derived
class is converted to a base class and under certain circumstances
must result in a slice of the same object.  That is, under some
circumstances the conversion must NOT call a user conversion function.

Some cases are (at the moment) undefined as to whether a user conversion
will be called.  This leaves room for different results from different
implementations.

Therefore, it is bad practice to declare a user conversion from a
derived to a base class, since it sometimes will not be called.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com