Re: File reading problem

From:

Eric Lilja <mindcooler@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 15 Jul 2007 07:33:22 -0700

Message-ID:

<1184510002.280119.146010@w3g2000hsg.googlegroups.com>

On 15 Juli, 16:18, Eric Lilja <mindcoo...@gmail.com> wrote:

On 15 Juli, 15:12, Robert Bauck Hamar <roberth+n...@ifi.uio.no> wrote:

Eric Lilja wrote:

Hi! I have a program with a class that needs to be able to write
itself to a file in clear text format. The file has two integers and
vector of struct objects. The struct has a string that can consist of
one or more words and a few integers. I'm able to create the file
properly, as confimed by viewing it in a text editor, but something
goes wrong when I tried to read it. I've made a test program
illustrating the problem:

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

struct s_t
{
   s_t() {}
   s_t(const string& si, int xi, int yi) : s(si), x(xi), y(yi) {}
   string s;
   int x, y;

   friend ostream& operator<<(ostream& os, const s_t& rhs);
   friend istream& operator>>(istream& is, s_t& rhs);
};

ostream&
operator<<(ostream& os, const s_t& rhs)
{
   os << rhs.s << endl;
   os << rhs.x << ' ' << rhs.y;

   return os;
}

istream&
operator>>(istream& is, s_t& rhs)
{
   getline(is, rhs.s);

   is >> rhs.x >> rhs.y;

   return is;
}

void
readit()
{
   int x = 0, y = 0;
   vector<s_t> vec;

   ifstream in("foo.txt");

   in >> x >> y;

After this, the next character in the input stream should be a '\n'.

{
s_t temp;

This will now read to the first newline in the stream and put the result
into temp.s. Since the first character is a newline, temp.s will be empty.
Then in will try to put the contents of what's stored as a string into
temp.x. In this case, in finds 'f', and fails. Thus the reading of the two
ints will become a noop, and the test fails.

I suggest you look up the ignore() member function of istream. You need to
ignore the rest of the line.

I solved it by adding this to my overloaded operator>>:
if (is.peek() == '\n')
{
is.ignore();}

before the getline() call. Now that program works just fine. However,
the *real* program tries to read even though there are no more entries
in the file. Hmm.

And I solved that by checking the return value of the getline() call
and returning
the stream directly if it fails.

      while (in >> temp) vec.push_back(temp);
   }

   in.close();

   cout << x << ' ' << y << endl;
   cout << vec.size() << endl;
   for (unsigned int i = 0; i < vec.size(); i++)
      cout << vec.at(i) << endl;
}

int
main()
{
   int x = 4711, y = 1337;
   vector<s_t> vec;

   vec.push_back(s_t("foo bar", 33, 22));
   vec.push_back(s_t("bar baz", 11, 99));

   ofstream out("foo.txt");

   out << x << ' ' << y << endl;
   for (unsigned int i = 0; i < vec.size(); i++)
   {
      out << vec.at(i);

      if (i < vec.size() - 1)
         out << endl;
   }

   out.flush();
   out.close();

   readit();
}

When I run it, I get this output:
4711 1337
0

so it reads the first two numbers ok but reading the s_t objects
doesn't work because the vector is empty afterwards...

Any help appreciated.

It's a common beginner error. Try to hand trace the code. Write down the
contents of the file on paper with control characters, and see where the
next char to be read is.

--
rbh