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Compare contents of two istreams using STL (C++)

Posted on 2006-11-15
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Last Modified: 2008-01-09
Hi all

Can anybody give me a quick code snippet to compare the contents of two istreams?

In particular, I would like to be able to use this code to compare a stringstream with an ifstream, but hopefully it will generalise to pairs of istreams in general?

Code should work with MinGW and Linux (GCC). The application is for testing some routines that makes ostream output; I want to check the expected output against the real output. I'm just not sure what the preferred and efficent approach here would be, in line with STL principles.

Thanks for any ideas!

Cheers
JP
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Question by:jdpipe
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6 Comments
 
LVL 12

Expert Comment

by:rajeev_devin
ID: 17953894
You can do like this

bool isEqual(istream & in1, istream & in2)
{
      char ch1, ch2;
      while (in1.get(ch1))
      {
            if (!in2.get(ch2) || (ch1 != ch2))
                  return (false);
      }
      if (!in2.get(ch2))
            return (true);
      else
            return (false);

      return (false);
}
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LVL 12

Expert Comment

by:rajeev_devin
ID: 17953899
That function can be used in this way.

ifstream fs("test.txt"); // test.txt file sould also contain "This is a test string." to be equal. Else it will return false.
stringstream ss;
ss << "This is a test string.";

bool equal = isEqual(fs, ss);
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LVL 7

Author Comment

by:jdpipe
ID: 17953949
Hi Rajeev,

I'm concerned that the approach you suggested might be sensitive to line endings on different platforms. Would that be true?

What about a line-by-line approach?

Cheers
JP
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LVL 12

Expert Comment

by:rajeev_devin
ID: 17953959
>> I'm concerned that the approach you suggested might be sensitive to line endings on different platforms. Would that
>> be true?
That won't be a problem.
Since in C++ streams line ending is considered as '\n'. Whatever be the platform.
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LVL 12

Expert Comment

by:rajeev_devin
ID: 17953978
You can try with line comparison. like this.

char ch1[2048]; // you need a char array.
in1.getline(ch1, 2048);

But with line comparison you need to do more error checking as you are not sure about the line size.
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Accepted Solution

by:
rajeev_devin earned 800 total points
ID: 17954284
For line-by-line comparison you can use this.

bool isEqual(istream & in1, istream & in2)
{
      string str1, str2;

      do
      {
            getline(in1, str1);
            getline(in2, str2);

            if (str1 != str2)
                  return (false);

            if (in1.eof() && !in2.eof())
                  return (false);

            if (!in1.eof() && in2.eof())
                  return (false);

      }while(!in1.eof() && !in2.eof());

      return (true);
}
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