EOF Statement in C++

This is a discussion on EOF Statement in C++ within the C++ Programming forums, part of the General Programming Boards category; It is commonly advised that using the EOF flag for file input is erroneous in C++. The follwoing article explains ...

  1. #1
    Registered User Vespasian's Avatar
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    EOF Statement in C++

    It is commonly advised that using the EOF flag for file input is erroneous in C++. The follwoing article explains why:

    EOF And Reading Text Files (C++) - C++ Tutorials | Dream.In.Code

    I especially don't get the paragraph:

    Uh oh! First the program tells you that its read 6 names when there are only 5 names in the file, and then it goes on to display the last name twice.

    What has happened is that the while loop has been repeated 6 times, because it is repeating until namefile has its internal "EOF" flag set. Remember that ifstream is not a file, its a stream, which means that it cannot know whether or not there is any more data in a file until after a failed attempt to read past the end of the file. After it read "Harry" from the file, the stream had not failed, therefore the EOF flag was still unset.

    There's an even worse (more subtle) problem with this, based on how the >> operator works; it will always stop reading at the first 'whitespace' character it encounters - a space, a newline, a carriage return or a tab.
    In lamens terms, why, using EOF, does the last entry or character get repeated twice?

  2. #2
    CSharpener vart's Avatar
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    because eof flag is set AFTER the failed read attemp

    So 5th successful read will not set the flag even when this record is last one in the file
    Next 6th attempt will fail, set the flag to true and leave teh buffer with last read data (from 5th read)

    After loop prints that 5th result second time it will exit.

    So instead of checking the eof flag - the result of the read operation should be checked
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    Registered User Vespasian's Avatar
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    Thanks.

    What other solution is proposed apart from the one mentioned in the link.

    I.e. they propose:

    while (fileinput >> stream)

    Is this the best and only option?

  4. #4
    CSharpener vart's Avatar
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    you can implement any other approach that checks the state of the stream before using the read operation result
    Code:
       for(;;)
        {
           fileinput >> result;
           if(!fileinput )
           {
              // read failed
              if( fileinput.eof())
              {
                 //end of file - just exit the loop
                 break;
              }
              if( fileinput.bad())
              {
                  //Read/writing error on i/o operation
                  //log error and exit the loop
    
                  break;
              }
              if( fileinput.fail())
              {
                  //Logical error on i/o operation
                  //need to skip mailformed data and continue reading
    
                  ...
    
                  continue;
              }
              break;
           }
    
           //if we got here - reading was successful - use result
        }
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Vespasian View Post
    Thanks.

    What other solution is proposed apart from the one mentioned in the link.

    I.e. they propose:

    while (fileinput >> stream)

    Is this the best and only option?
    It's certainly not the best. This checks if the read failed, which could be for many reasons.
    It's usually a good enough way to do it.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    Registered User whiteflags's Avatar
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    Just in case it's relevant, I want to mention that you should compare the return value of a function that might return end-of-file, such as istream::peek(), to char_traits<return_type>::eof().

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