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### 696. `istream::operator>>(int&)` broken

Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: C++11 Submitter: Martin Sebor Opened: 2007-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.formatted.arithmetic].

View all issues with C++11 status.

Discussion:

From message c++std-lib-17897:

The code shown in 31.7.5.3.2 [istream.formatted.arithmetic] as the "as if" implementation of the two arithmetic extractors that don't have a corresponding `num_get` interface (i.e., the `short` and `int` overloads) is subtly buggy in how it deals with `EOF`, overflow, and other similar conditions (in addition to containing a few typos).

One problem is that if `num_get::get()` reaches the EOF after reading in an otherwise valid value that exceeds the limits of the narrower type (but not `LONG_MIN` or `LONG_MAX`), it will set `err` to `eofbit`. Because of the if condition testing for `(err == 0)`, the extractor won't set `failbit` (and presumably, return a bogus value to the caller).

Another problem with the code is that it never actually sets the argument to the extracted value. It can't happen after the call to `setstate()` since the function may throw, so we need to show when and how it's done (we can't just punt as say: "it happens afterwards"). However, it turns out that showing how it's done isn't quite so easy since the argument is normally left unchanged by the facet on error except when the error is due to a misplaced thousands separator, which causes `failbit` to be set but doesn't prevent the facet from storing the value.

[ Batavia (2009-05): ]

> We believe this part of the Standard has been recently adjusted and that this issue was addressed during that rewrite.

> Move to NAD.

[ 2009-05-28 Howard adds: ]

> I've moved this issue from Tentatively NAD to Open.

> The current wording of N2857 in 30.4.3.2.3 [facet.num.get.virtuals] p3, stage 3 appears to indicate that in parsing arithmetic types, the value is always set, but sometimes in addition to setting `failbit`.

> - If there is a range error, the value is set to min or max, else
> - if there is a conversion error, the value is set to 0, else
> - if there is a grouping error, the value is set to whatever it would be if grouping were ignored, else
> - the value is set to its error-free result.

> However there is a contradictory sentence in 30.4.3.2.3 [facet.num.get.virtuals] p1.

> 31.7.5.3.2 [istream.formatted.arithmetic] should mimic the behavior of 30.4.3.2.3 [facet.num.get.virtuals] (whatever we decide that behavior is) for `int` and `short`, and currently does not. I believe that the correct code fragment should look like:

> >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> >     iostate err = ios_base::goodbit;
> >     long lval;
> >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> >     if (lval < numeric_limits<int>::min())
> >     {
> >       err |= ios_base::failbit;
> >       val = numeric_limits<int>::min();
> >     }
> >     else if (lval > numeric_limits<int>::max())
> >     {
> >       err |= ios_base::failbit;
> >       val = numeric_limits<int>::max();
> >     }
> >     else
> >       val = static_cast<int>(lval);
> >     setstate(err);

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change 30.4.3.2.3 [facet.num.get.virtuals], p1:

> -1- Effects: Reads characters from `in`, interpreting them according to `str.flags()`, `use_facet<ctype<charT> >(loc)`, and `use_facet< numpunct<charT> >(loc)`, where `loc` is `str.getloc()`. If an error occurs, `val` is unchanged; otherwise it is set to the resulting value.

Change 31.7.5.3.2 [istream.formatted.arithmetic], p2 and p3:

>     operator>>(short& val);

> > -2- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

> > >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> > >     iostate err = iostate_base::goodbit;
> > >     long lval;
> > >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> > >     if (err != 0)
> > >       ;
> > >     else if (lval < numeric_limits<short>::min()
> > >       || numeric_limits<short>::max() < lval)
> > >          err = ios_base::failbit;
> > >     if (lval < numeric_limits<short>::min())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<short>::min();
> > >     }
> > >     else if (lval > numeric_limits<short>::max())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<short>::max();
> > >     }
> > >     else
> > >       val = static_cast<short>(lval);
> > >     setstate(err);

>     operator>>(int& val);

> > -3- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

> > >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> > >     iostate err = iostate_base::goodbit;
> > >     long lval;
> > >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> > >     if (err != 0)
> > >       ;
> > >     else if (lval < numeric_limits<int>::min()
> > >       || numeric_limits<int>::max() < lval)
> > >          err = ios_base::failbit;
> > >     if (lval < numeric_limits<int>::min())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<int>::min();
> > >     }
> > >     else if (lval > numeric_limits<int>::max())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<int>::max();
> > >     }
> > >     else
> > >       val = static_cast<int>(lval);
> > >     setstate(err);