Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!softrue!kearns
From: kearns@softrue.UUCP (Steven Kearns)
Newsgroups: comp.lang.c++
Subject: typesafe downward casting
Message-ID: <17.UUL1.3#8618@softrue.UUCP>
Date: 29 Mar 91 15:45:40 GMT
Organization: Software Truth
Lines: 21

There has been much discussion lately about adding stuff to C++
so that people can assign a Base* to a Derived* without a cast.  
People feel "guilty" when they use a cast, as if they are doing something
unsafe.

If you look at things from a "program verification" viewpoint, the issue
becomes much clearer.  A cast of (Base*) to Derived* is just a function
call that assumes that the Base* really points to a Derived*;  if you
can prove to yourself (and God) that this is so, then using the cast
is perfectly safe, and you should not feel guilty.  

If we added stuff to C++ to take care of this "problem", then we would
also have to add new types so that people do not call sqrt(x) when
x is a negative number, or so they do not call a/b when b is 0.  

-steve

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* Steven Kearns            ....uunet!softrue!kearns    *
* Software Truth           softrue!kearns@uunet.uu.net *
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