fstream confusion

This is a discussion on fstream confusion within the C++ Programming forums, part of the General Programming Boards category; My objective is to write to a file and read it back. All the reading is done after all the ...

  1. #1
    unleashed alphaoide's Avatar
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    fstream confusion

    My objective is to write to a file and read it back. All the reading is done after all the writing.
    Code:
    // first version
    ofstream outFile("file.txt", ios::binary | ios::out | ios::in);
    ifstream inFile("file.txt", ios::binary | ios::in);
    With codes above, file.txt is not created when I run it.

    Code:
    // second version
    ofstream outFile("studentVer1.txt", ios::binary | ios::out);
    ifstream inFile("studentVer1.txt", ios::binary | ios::in);
    With these ones, file.txt is created but reading is unsuccessful.

    When I do first version and create a blank file.txt, first run will write but fail to read, second run and after will write and read.

    So, whatsupp??

  2. #2
    and the hat of wrongness Salem's Avatar
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    You can't have two separate handles to a file which read and write to that file.

    If you want read and write access, then you have a single handle, and you use ios:ut | ios::in to open it, and seek() to move to where you want to read and write
    And don't forget flush() after writing
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  3. #3
    unleashed alphaoide's Avatar
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    I did this
    Code:
    fstream file("file.txt", ios::binary | ios::out | ios::in);
    and after each writing I followed by
    Code:
    file.flush();
    and right before I started reading
    Code:
    file.seekg(0);
    But...still no file.txt is created, and if I create blank file.txt, it will write and read fine. Any suspicion?
    Anybody has the same case before? I'm afraid that some other codes in my program that cause it, but it's a lot of codes to post. So, any suggestion what to check?

  4. #4
    carry on JaWiB's Avatar
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    I don't know about ios::binary, but using ios:ut|ios::in will open a file only if it already exists...
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  5. #5
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    To create a file if it doesn't exist, not overwrite the data in the file if it does exist, and start at the beginning of the file then you can try this:

    ofstream fout(filename, ios::in | ios:ut | ios::trunc);

    I'm not sure it does all that, but I think it does, and it won't hurt to try on a sample file. Alternatively look for existense first, create the file second if need be.
    Code:
    ofstrean fout;
    ifstream fin(filename);
    if(!fin.is_open())
    {
      fout.open(filename);
    }
    else
    {
      fin.close();
      fout.open(filename, ios::in | ios::out);
    }
    Using an fstream may be a little easier.

  6. #6
    unleashed alphaoide's Avatar
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    Adding ios::trunc WORKS. Thanx a bunch!
    EDIT: but I thought ios::trunc is the default action for ios:ut?
    So, if you use ios:ut | ios::in at the same time, ios::trunc is not a default action anymore?
    Last edited by alphaoide; 10-06-2003 at 09:50 AM.

  7. #7
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    Yup, weird, but that's the way I understand it.

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