[boost] Re: problem with new iterators in 1.31 Release candidate # 2

From: David Abrahams (dave_at_[hidden])
Date: 2004-01-31 21:47:53

Robert Ramey <ramey_at_[hidden]> writes:

> I have a problem with the latest version of the new iterators.
> This is illustrated in the following example.
>
> #include <strstream>
> #include <iterator>
> #include <boost/iterator/counting_iterator.hpp>
>
> main(){
> 	std::istrstream is("abcdefg");
> 	typedef boost::counting_iterator<std::istream_iterator<char> > ci;
> 	const ci end = std::istream_iterator<char>();  // note the const !!!

I don't see why the const should be relevant.

> 	ci begin = std::istream_iterator<char>(is);

Hmm, the implicit conversions from istream_iterator to the
counting_iterator above are troubling.  I think those constructors
should be explicit.

> 	unsigned int size;
>
> 	// the following should fail at compilation ?

Why?  OK, maybe it would be better to make it fail because the
iterator is not in fact a random-access iterator.

> 	// in fact it compiles are returns a value of -7 !!!
> 	size = begin - end;
>
> 	// the following should compile and return a value of 7
> 	// in fact, it compiles but goes into an infinite loop
> 	size = end - begin;
> }

I don't see why you think the first should compile but the second
should not.  If they're not random-access iterators, it seems to me
that there's no reason you should be able to subtract them in any
order.  If they were random-access iterators, it would be unreasonable
to expect the compiler to detect which one had an earlier position
and disallow one subtraction.

> Note that this problem did not appear with the first release
> candidate.
>
> Also, if one follows the links from the main documentation page, one
> is directed to the documentation of the old iterators.  This is
> extremely confusing.  This has been pointed out before.

That's a serious problem which I'll fix immediately.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com