Writing at the beginning of a file, keeping file contents

Question

I have a text file where I want to write. I want to keep the file content always. I want to write following a "FIFO" (last write always on the top line of the file).

I try using fout.open("filename"); with ate mode to keep file content, after that use seekg(0) trying to take back writing cursor to the begining of the file. Didn't work.

The unique way I found to do that I think it's so time-expensive, copy all the file content to a temporary file. Write want I want to write and after that write the content of the temp file at the end of the target file.

There must be an easy way do this operation?

Create a temporary file. Write your new content to it. Then copy the old content to the new temporary file. Finally, delete the old file and rename the temporary file to the original name. — uselpa, Nov 26 '12 at 20:58
@uselpa is that not exactly what OP says they are doing now? — Blorgbeard, Nov 26 '12 at 20:58
Can't you physically append to the end, but logically see it as appending on top (basically you append to the end but "read it backwards") — leo, Nov 26 '12 at 20:58
@Blorgbeard - my understanding is that he writes the file twice, I suggest only writing once. — uselpa, Nov 26 '12 at 21:00
Was discussed here as possible feature of filesystems http://stackoverflow.com/questions/5560191/prepending-data-to-a-file — pbhd, Nov 26 '12 at 21:14
If your new text is not supposed to replace previous text, and is not at the end, there is _no_ other solution than that proposed by @uselpa. Operating systems don't support insertion of text in a file; you can only overwrite or append to the end. (Physically, it's hard to see how insertion could be implemented.) — James Kanze, Nov 26 '12 at 21:44
@Jorge: You are not qualified to decide that, as evidenced by the fact that you asked the question. — Lightness Races in Orbit, Nov 27 '12 at 03:13

score 5 · Accepted Answer · answered Nov 26 '12 at 21:02

Jorge, no matter what you will have to rewrite the entire file in memory. You cannot simply keep the file where it is and prepend memory, especially since it's a simple text file (maybe if there was some form of metadata you could...)

Anyways, your best chance is to flush the old contents into a temporary location, write what you need and append the old contents.

score 1 · Answer 2 · answered Nov 26 '12 at 21:38

I'm not sure what you're asking for. If you want to add a line to the beginning of the file, the only way is to open a new, temporary file, write the line, copy the old file into after the new line, then delete the old file and rename the temporary.

If the original line has a fixed length, and you want to replace it, then all you have to do is open the file with both ios_base::in and ios_base::out.

score 0 · Answer 3 · answered Nov 26 '12 at 21:54

First, you should realize that files are historically streams, i.e. they can only be read and written in one direction. This comes from the times when files were stored on tapes, which could move in one direction (at that time).

However, if you only want to prepend, then you can just store your file backwards. Sounds silly? Maybe, but this would work with just a little overhead.

Apart from that, with current OS's you will need to make a copy to prepend. While files are not streams anymore, and can be accessed randomly on a harddisk, they are still made to grow in one direction. Of course you could make a filesystem, where files grow in both directions, but I have not heard of one.

score 0 · Answer 4 · answered Aug 21 '13 at 08:34

With <fstream> you may use the filebuf class.

    filebuf myfile;
    myfile.open ("test.txt", ios::in | ios::out);
    if (!myfile.is_open()) cout << "cannot open" << endl;
    myfile.sputn("AAAA", 4);
    myfile.close();

    filebuf myfile2;
    myfile2.open ("test.txt", ios::in | ios::out);
    if (!myfile2.is_open()) cout << "cannot open 2" << endl;
    myfile2.sputn("BB", 2);
    myfile2.close();

score -4 · Answer 5 · answered Nov 26 '12 at 20:58

-4

write to a string in order you want, then flush to the file

answered Nov 26 '12 at 20:58

iddqd

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Writing at the beginning of a file, keeping file contents

5 Answers5