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From: Jim McKelvey <mckelvey@bean.jpl.nasa.gov>
Subject: Re: string/cow/algorithm
Date: 1998/08/12
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Nathan Myers wrote:
> 
> John Potter<jpotter@falcon.lhup.edu> wrote:
> >I received some email responses to the original.  Not seeing any
> >posts, I will pass on the information to you and others.  Here is
> >another nasty problem:
> >
> > string s("abc");
> > string::iterator i(s.begin());
> > string c(s);
> > *i = 'z';
> > // What is the value of c[0]?
> >
> >A solution is for strings to have one of three states (shared, unique,
> >unsharable).  You have the first two now.  Any function which returns
> >a non-const reference to internals must take care of making the copy
> >and marking it unsharable.  Any copy function which gets an unsharable
> >to copy must do a deep copy.  In the above, s.begin() would do that
> >for s and the copy constructor would be forced to do a deep copy in
> >creating c.
> 


Maybe I'm missing something, but an object should never return a pointer
or reference to its internals.

If s is a string, an iterator on s would bump the reference counter on
the value of s. This means that if s changes, the iterator still refers
to the previous value of s.

The iterator should not be able to change s. It just represents a
position within s.

So the code becomes something like:

string s("abc");
string::iterator i(s);
string c(s);
s(i) = 'z';
// What is the value of c[0]?

c[0] is 'a'.
The only thing changed is s, which is "zbc".

The call to s(i) can verify that i is indeed an iterator on s. If
desired, it can then update the iterator to refer to the new value of s.
In general though, an iterator should be considered a snapshot.


It seems to me that the problem is really that the string class is
poorly designed.


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