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Date: Wed, 28 Nov 2007 12:04:45 -0800 (PST)
From: "Travis Vitek (JIRA)" <jira@apache.org>
To: stdcxx-dev@incubator.apache.org
Subject: [jira] Issue Comment Edited: (STDCXX-645) stream iterators into
 different streams compare equal
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    [ https://issues.apache.org/jira/browse/STDCXX-645?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12543180 ] 

vitek edited comment on STDCXX-645 at 11/28/07 12:04 PM:
----------------------------------------------------------------

24.5.1 p1 says that an istream_iterator becomes an end-of-stream iterator when the end of stream is reached. So checks are required so that two iterators that become end-of-stream iterators will compare equal.

#include <assert.h> 
#include <iterator> 
#include <sstream> 

int main () 
{ 
    std::istringstream a ("1"); 
    std::istream_iterator<int> i (a); 
    ++i;

    std::istringstream b ("2"); 
    std::istream_iterator<int> j (b); 
    ++j;

    assert (i == j); 

    return 0;
} 

This method needs to return true if both iterators are end-of-stream iterators, or they have the same non-null stream pointer. Something like this is more appropriate given the implementation of the other methods

template <class _TypeT, class _CharT, class _Traits, class _Distance>
bool
operator== (const istream_iterator<_TypeT, _CharT, _Traits, _Distance>& __x, 
             const istream_iterator<_TypeT, _CharT, _Traits, _Distance>& __y) 
{ 
    const bool __x_eos = !__x._C_strm || !*__x._C_strm;
    const bool __y_eos = !__y._C_strm || !*__y._C_strm;

    return (__x._C_strm == __y._C_strm) ? true : __x_eos == __y_eos;
}

The other option would be to change our internal definition of end-of-stream to be istream_iterators with a NULL stream pointer. Then operator++ could be changed to invalidate the stream pointer when the stream reaches eos. I think this would work.

istream_iterator& operator++ () {
    if (_C_strm && !!*_C_strm)
        *_C_strm >> _C_val;
    else
        _C_strm = 0;
    return *this;
}


      was (Author: vitek):
    24.5.1 p1 says that an istream_iterator becomes an end-of-stream iterator when the end of stream is reached. So checks are required so that two iterators that become end-of-stream iterators will compare equal.

#include <assert.h> 
#include <iterator> 
#include <sstream> 

int main () 
{ 
    std::istringstream a ("1"); 
    std::istream_iterator<int> i (a); 
    ++i;

    std::istringstream b ("2"); 
    std::istream_iterator<int> j (b); 
    ++j;

    assert (i == j); 

    return 0;
} 

This method needs to return true if both iterators are end-of-stream iterators, or they have the same non-null stream pointer. Something like this is more appropriate given the implementation of the other methods

template <class _TypeT, class _CharT, class _Traits, class _Distance>
bool
operator== (const istream_iterator<_TypeT, _CharT, _Traits, _Distance>& __x, 
             const istream_iterator<_TypeT, _CharT, _Traits, _Distance>& __y) 
{ 
    const bool __x_eos = !__x._C_strm || !*__x._C_strm;
    const bool __y_eos = !__x._C_strm || !*__x._C_strm;

    return (__x._C_strm == __y._C_strm) ? true : __x_eos == __y_eos;
}

The other option would be to change our internal definition of end-of-stream to be istream_iterators with a NULL stream pointer. Then operator++ could be changed to invalidate the stream pointer when the stream reaches eos. I think this would work.

istream_iterator& operator++ () {
    if (_C_strm && !!*_C_strm)
        *_C_strm >> _C_val;
    else
        _C_strm = 0;
    return *this;
}

  
> stream iterators into different streams compare equal
> -----------------------------------------------------
>
>                 Key: STDCXX-645
>                 URL: https://issues.apache.org/jira/browse/STDCXX-645
>             Project: C++ Standard Library
>          Issue Type: Bug
>          Components: 24. Iterators
>    Affects Versions: 4.1.3, 4.2.0
>            Reporter: Mark Brown
>
> As Travis says in his reply to my post here:
> http://www.nabble.com/stream-iterators-into-different-streams-compare-equal--tf4721505.html#a13498487:
>     Given 24.5.1.1 p1 and p2, it is pretty clear to me that the two iterators are both non-end-of-stream type, and they are both created on different streams. The streams are different, so the iterators should not compare equal. I guess one could claim that 24.5.1.2 p6 conflicts with 24.5 p3 because 'end-of-stream' isn't clearly defined, but in this particular case that doesn't matter.
> This program aborts with stdcxx but not with gcc:
> #include <assert.h>
> #include <iterator>
> #include <sstream>
> int main ()
> {
>     std::istringstream a ("1");
>     std::istream_iterator<int> i (a);
>     std::istringstream b ("2");
>     std::istream_iterator<int> j (b);
>     assert (!(i == j));
> } 

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