Why would ofstream not work sometimes?

This is a discussion on Why would ofstream not work sometimes? within the C++ Programming forums, part of the General Programming Boards category; Check out the following code: Code: obook.write(reinterpret_cast< const char * >(&newBook), sizeof(Book)); ...where obook is an ofstream variable that opens ...

  1. #1
    Registered User johnnyd's Avatar
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    Unhappy Why would ofstream not work sometimes?

    Check out the following code:

    Code:
    obook.write(reinterpret_cast< const char * >(&newBook), sizeof(Book));
    ...where obook is an ofstream variable that opens a file using ios::ate and newBook is an instance of a class I made.

    The problem is that when I write to the file, around 70% of the time, it will work. There is however, a good 30% of the time where it just writes garbage. Even when I'm entering the exact same data twice (exactly the same except the key field). Why would this happen?

    Has anyone ever had this problem? I'd appreciate your insights. Thanks.
    Excuse me, while I water my money tree.

  2. #2
    Registered User Codeplug's Avatar
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    Are you ensuring the state of obook is still good()?
    Is obook opened in binary?

    gg

  3. #3
    Registered User johnnyd's Avatar
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    No it's not being opened in binary. It's in text mode. Here's how I play with it:

    Code:
    obook.open("books.dat", ios::ate);
    if (obook){
       newBook = Book(isbn, author, title, publisher, pPlace, status, year, copy);
       obook.write(reinterpret_cast< char * >(&newBook), sizeof(Book));
       obook.close();
       newBook.show();
       cout<<"\n\nBook data written successfully.";
       getch();
       }
    else{
       cout<<"\n\nCannot open transaction file."<<endl;
       getch();
       return;
       }
    That's pretty harmless isn't it? I mean it works most of the time without a problem! I just don't understand why simple code can go rouge sometimes.

    As for making sure if it is still good(), to be perfectly honest with you, I was never taught that. What's that?

    Thanks again Codeplug.
    Excuse me, while I water my money tree.

  4. #4
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    As Codeplug says, you should open the file in binary mode:

    obook.open("books.dat", ios::ate | ios::binary);

  5. #5
    Registered User johnnyd's Avatar
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    I've actually tried that. It doesn't work. And exactly as how you typed it too!
    Excuse me, while I water my money tree.

  6. #6
    Registered User Codeplug's Avatar
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    The reason it is only working sometimes is because you are not opening the file in binary mode. When you use text mode, any output can be modified so that newlines are represented correctly for the native OS.
    When you read the help for ostream::write(), it says:
    If the underlying file was opened in text mode, additional carriage return characters may be inserted.
    So, if any of the bytes in your class contained 0x0A (line feed), a 0x0D (carriage return) is inserted before it to make it look like a true "newline" in DOS/Windows.

    If you aren't reading and writting text, you must use binary.

    gg

  7. #7
    Registered User Codeplug's Avatar
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    If you have an existing data file that was created with text-mode code, consider it corrupted and start fresh.

    If you are still having problems, then post your program and steps to repro the problem - we'll figure it out.

    gg

  8. #8
    Registered User johnnyd's Avatar
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    When you say text, do you mean that all of the variables in the class should be of the char* type? or do you mean text as in a literal string like "The quick brown fox was nuked by the lazy dog"?
    Excuse me, while I water my money tree.

  9. #9
    Registered User johnnyd's Avatar
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    Gotcha.

    Starting afresh... (I wish they told us these things in school and in books)
    Excuse me, while I water my money tree.

  10. #10
    Registered User Codeplug's Avatar
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    When you open an [o,i]fstream without ios::binary, then you opened the file in text-mode.

    (I edited my last post)

    gg

  11. #11
    Registered User johnnyd's Avatar
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    Thumbs up

    I just tested it and it works. All the time.

    I've come to realise why binary mode wasn't working for me. It was because I used fstream to create the file handler, and not ofstream. ofstream works beautifully. But I find that rather curious though. If you have any Idea why that would happen, please comment. A classmate is reading these posts as well, so it would benefit both of us.

    Thanks yet again Codeplug. You are for me now what Prelude was for me last year. You guys rock.
    Excuse me, while I water my money tree.

  12. #12
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    With fstream you might try this:
    obook.open("books.dat", ios::ate | ios::out | ios::binary);

    And then for reading:
    obook.open("books.dat", ios::in | ios::binary);

  13. #13
    Registered User johnnyd's Avatar
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    That's interesting Swoopy, I've never thought of that one. I'll give it a shot.
    Excuse me, while I water my money tree.

  14. #14
    Registered User Codeplug's Avatar
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    You mean "why text mode" wasn't working for you?

    There isn't any difference between ofstream and fstream except ofstream includes ios::out in it's open mode by default (and fstream includes ifstream interfaces as well). In other words, these two declarations will behave exactly the same:
    Code:
    fstream out("data.dat",ios::out);
    ofstream out("data.dat");
    That goes for any other mode flags you use, as long as the fstream version has ios::out.

    gg

  15. #15
    Registered User johnnyd's Avatar
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    Got it. It's a shame though that C's implementation of files was so much simpler and more reliable. I get the feeling that somehow they were trying to reinvent the wheel with the text mode and it didn't work out so well - as far as standardisation is concerned anyways.
    Excuse me, while I water my money tree.

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