Re: [boost] (infinite) sequences using recurrence relations

From: John E. Potter (jpotter_at_[hidden])
Date: 2001-08-10 14:22:43

On Fri, 10 Aug 2001, Douglas Gregor wrote:

> Then how does the condition: "a == b and (a, b) in the domain of == implies 
> *a equivalent to *b" from Table 72 hold up for istream_iterators or 
> istreambuf_iterators?
> 
> istream_iterator i(cin), j;
> ++i; // make sure we've actually read a value from the stream
> j = i;
> ++j;
> if (i == j) // true, because they reference the same stream
>   assert(*i == *j); // not necessarily so
> 
> This will compile happily, run, and fail at the assertion, and yet my fire 
> extinguisher remains untouched...

For input iterators in general, i and j are not in the domain of
operator== because incrementing j invalidated i and it produced
undefined behavior.  For istream_iterator in particular, there is
a defect in the standard if the specification for operator==
defines the domain.  That's what happens when an implementation is
used as the basis for standardization.

I would be quite happy with the domain being at least one end iterator
and at most one non end iterator.

bool operator== (istream_iterator<T> lhs, istream_iterator<T> rhs) {
   if (! lhs.end() && ! rhs.end())
      throw domain_error();
   return lhs.end() && rhs.end();
   }

John