40d3a699
From: daniel.kruegler@googlemail.com
On 2012-01-21 00:07, Daryle Walker wrote:
> Let's say I'm reading a list of numbers:
>
> template< typename T, typename Ch, class Tr>
> std::list
> read_my_udt( std::basic_istream &is )
> {
> Ch const lp = is.widen('('), ca = is.widen(','), rp = is.widen(')');
> Ch c;
> std::list result;
>
> if ( !(is>> c) )
> return std::list();
> if ( !Tr::eq(c, lp) )
> {
> T sole_element;
>
> is.unget();
> is>> sole_element;
> result.push_back( sole_element );
> }
> else
> {
> //...
> }
> return result;
> }
>
> I didn't get into reading a comma-separated, parentheses-bounded list
because the single-element case is sufficient. If there's a format error,
from a read setting "eof" or "failbit", I can just return an empty list and
use the stream's state to pass
on
> the error. But what if there's some sort of exception thrown from list
operations? Should "badbit" be set, or should exceptions not having anything
to do with stream operations get passed up without affecting the stream's
state? After all, the stream
> is still technically usable, although it has all the elements that weren't
read yet at the time of the throw.
IMO you already answered your question: I also see no reason, why the
stream should be considered in a "bad state" just because of a failing
operation. There is no contract that requires you to modify the stream
state in any way here. But if you use this template to implement some IO
extractor and if you want to mimic the behaviour of the standard library
in this regard, you should indeed catch the exception within this
extractor, set the badbit state, and rethrow the exception unless the
stream's exception() flag bit-and-combined with badbit is not zero. If
you look at the discussion of LWG issue 309,
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#309
you can get an idea how this can be done.
HTH & Greetings from Bremen,
Daniel Krügler
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