Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!milano!cadillac!puma!vaughan
From: vaughan@puma.cad.mcc.com (Paul Vaughan)
Newsgroups: comp.lang.c++
Subject: const
Message-ID: <10863@cadillac.CAD.MCC.COM>
Date: 4 Sep 90 23:02:49 GMT
Sender: news@cadillac.CAD.MCC.COM
Lines: 58


Awhile back I posted a message asking if const was really worth the
trouble.  Everyone who replied either on the net or privately thought
that making a program const correct was indeed worth the effort.
These were all pretty subjective opinions--it's kind of hard to
measure how much difference it makes.

So, here's a const question.

Consider the following function


const char* substring(const char* string, const char* sub) 
// substring searches for the first occurence of sub in string and
//returns a pointer to it or 0 if none is found.
{ ....}

If I'm working on a const char* (and, therefore, don't plan on changing
any part of it) this works just fine.  However, if I'm working on a
char* and was planning to change part of it and was using substring to
find that part, this will cause a problem because substring returns a
const.  For example,

char* foo = "barfoobar";
*substring(foo,"foo") = `\0`;

will give an error due to the modification of a non-const return value.


I can't declare substring as 

char* substring(const char* string, const char* sub) 

because it is returning a pointer to part of the string that is passed
in.  I would declare it

char* substring(char* string, const char* sub)

but then I can't use it to deal with const char*'s, and, of course, I can't
overload it based on the return type!


So what do I do?  My solution so far has been to declare it 

const char* substring(const char* string, const char* sub)

and when I want to work on a char*, I do a cast, like this

char* foo;
char* bar = (char*) substring(foo, "foo");

Given the semantics of the function and that I'm passing in a char*, I
can verify the safety of the cast without much trouble.  Is there a
better way?


 Paul Vaughan, MCC CAD Program | ARPA: vaughan@mcc.com | Phone: [512] 338-3639
 Box 200195, Austin, TX 78720  | UUCP: ...!cs.utexas.edu!milano!cadillac!vaughan