writing to the beginning of a file

This is a discussion on writing to the beginning of a file within the C++ Programming forums, part of the General Programming Boards category; Hi all, I have a file on c:\ and wish to write to the beginning of it, but not overwrite ...

  1. #1
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    writing to the beginning of a file

    Hi all,

    I have a file on c:\ and wish to write to the beginning of it, but not overwrite any of the content that is already there. Basically the contents of the file looks like this

    This is line 2
    This is line 3
    This is line 4
    This is line 5

    I wish to open this file and write "This is line 1" at the beginning of it so it looks (suprisingly) like this

    This is line 1
    This is line 2
    This is line 3
    This is line 4
    This is line 5

    This is my code so far

    Code:
    	ofstream out("c:/myfile.txt", ios::app);
    	out.seekp(0, ios::beg);
    	out<<"This is line 1\r\n";
            out.close();
    But this still writes to the end of the file.

    Any ideas?

    Thanks

  2. #2
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    what you could do is open a new file for writing, write line 1 then copy the data from the other file in. Delete old file (using system function) and rename the new file. I know there is a function to del and rename but I don't know their names.
    Amish

  3. #3
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    thanks for the reply :-)

    Surely there is a more elegant way to do this than to create different files.

  4. #4
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    Nope.

  5. #5
    Registered User hk_mp5kpdw's Avatar
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    You can read the original contents into an STL container of some sort. You can then insert the new line at the beginning of the container. Close and reopen the original file for writing and write out the contents of the container back to the original file. This means you don't need to create a new file, delete of the old file, and rename the new file back to the old name. Still, it's not a "pretty" process, there is no clean file insertion function.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  6. #6
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    Nope.
    Yep. Read the contents of the file into a string. Write the new line to the file, then write the contents of the string back to the file.

  7. #7
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    or write the line you want inserted first to a string , concatenate that file to the string, then output it to the file.

  8. #8
    and the hat of wrongness Salem's Avatar
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    > Close and reopen the original file for writing and write out the contents of the container back to the original file.
    It does on the other hand mean that you've either got the old file or the new file.
    Bugs / crashes / power fails will ensure that you end up with nothing if you simply re-use the existing file.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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