Re: Replacing blank lines in my .txt data file

From:
"kanze" <kanze@gabi-soft.fr>
Newsgroups:
comp.lang.c++.moderated
Date:
24 Aug 2006 09:34:59 -0400
Message-ID:
<1156407927.493751.25300@75g2000cwc.googlegroups.com>
Carl Barron wrote:

In article <1156351815.444658.218570@p79g2000cwp.googlegroups.com>,
kanze <kanze@gabi-soft.fr> wrote:

Ulrich Eckhardt wrote:

    [...]

                      The idiomatic way is this:

while( getline(in,line))
  ... // handle line

if(in.eof())
  ... // reached EOF
else
  ... // failed before reaching EOF


Just a detail, but the only way getline can normally fail is if
the last line is not terminated by a '\n'. And of course, this
sets EOF as well. If you want to reliably handle an incorrectly
terminated last line:


if the maximum filesize of a file on an OS is greater than
string::max_size(), then it is possible to extract string::max_size()
chars before finding a '\n' and this will set the failbit while there is
no real problem with the stream and its not at 'eof'.


The thought did occur to me. It's true that it's a possibility
according to the standard. But in practice, on most systems
today, you'll get a bad_alloc long before hitting max_size.
Whereas there are a lot of bad editors and other programs out
there which will generate text files which don't end with a
'\n'.

attempting to store the char in the string should set
bad_alloc but should be caught in getline or the special case
would not occur, and not need specif


I'm not sure about that. The specification of std::getline(),
both in C++98 and the latest draft, certainly doesn't say so.
On the other hand, it doesn't say that an exception from one of
the streambuf functions should be caught and handled either,
although I'm certain that this is the intent.

Presumably, the general specifications in ?27.6.1.1 should hold,
particularly (from paragraph 4): "If one of these called
functions [rdbuf()->sbumpc(), rdbuf()->sgetc()] throws an
exception, then unless explicitly noted otherwise, the input
function sets badbit in error state." But paragraphs 1 and 2
seem to say that this applies only to member functions of
basic_istream. The description of the effects of each of the >>
operators in ?27.6.1.2.3 starts by saying "Behaves like a
formatted input member[...]", but this text is missing in the
description of getline. And of course, this text very
explicitly limits the mapping to exceptions from the streambuf
functions. (There, too, I'm sure that the intent was to map
exceptions from rdbuf()->sgetn() or rdbuf()->snextc() as well.
And I'm not sure whether the intent was to map exceptions from
rdbuf()->sync() or one of the rdbuf()->pubseek's.)

this can be simulated with a stream whose streambuffer class
always returns '1' as the char extracted.


True, but that's not the usual case. And on most systems,
you'll probably get a bad_alloc earlier, at least with
basic_istream< char >.

In these days of 'super large files' and 'networked files with
main frame capacity sources' seems like a case to reckon with.


Not too much with basic_istream< char >, I don't think. With
basic_istream< wchar_t > on a 32 bit system, on the other
hand...

do
{
   std::getline(in,line);
   if(in.eof())
   {
      // end line missing in stream


Or you've finished reading a perfectly good file.

   }
   if(!n)
   {
      if(s.size()==s.max_size())
      {
      // string is really truncated at max_size() chars
      // delimiter not reached


The correct test for this condition should be:

    if ( in.fail() && ! in.eof() )

But I'm not sure that the standard is correct on this one,
either. If you store max_size characters, and the next
character is a '\n', I don't see why it should fail.

      }
   }
}while(in);

looks like a more safe approach...


Except that it doesn't work. There is no way to distinguish
after the fact whether you've tried to read beyond end of file,
or whether you've actually read a line which wasn't terminated.

Try it. Read from an istringstream initialized with "abc\n",
and then from one initialized with "abc\nxyz". Using just
getline and testing after, there is no way to distinguish
between the two. If you want to be sure, you need the peek()
before.

--
James Kanze GABI Software
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