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For example, i open ifstream, and i want it to read not from current position to end, like always, but in reverse direction (from current position to start)?

hakeris1010
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    Might be an XY problem; if you want the data backwards then read it forwards and read the resulting data backwards. – OMGtechy Feb 14 '15 at 19:11
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    I'd like to know the reason for _that_ :D – Ray Feb 14 '15 at 19:11
  • Seek to the end, then seek backwards X bytes, where X is the number of bytes you want to read. Then seek X + Y bytes backwards, where Y is the number of bytes you want to read next, etc. – Some programmer dude Feb 14 '15 at 19:14
  • Usually, operating systems do not support it. You have to determine the size of the file, seek to the end - one page, read a page, process and repeat. (Here, you can determine the page size) – erenon Feb 14 '15 at 19:14
  • `std::ifstream file("input.txt"); std::vector contents { std::istreambuf_iterator{file}, std::istreambuf_iterator{} };` .. and then iterate `contents` using `rbegin()` and `rend()`. Well this should work great for smaller files. – Nawaz Feb 14 '15 at 19:19
  • @Mats Not a duplicate. This OP says nothing about line-by-line. Please be more careful with your dupe hammer. – Lightness Races in Orbit Feb 14 '15 at 19:35
  • Sorry, missed that it said line-by-line in that one too (I saw it in the title of another one similar to it). I can't believe this isn't a dupe tho'. – Mats Petersson Feb 14 '15 at 19:38

2 Answers2

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You can use random file operations in C++ for this.

For ifstream ss, you can write the command ss.seekg(0, ios::end) to move your pointer in the text file to the end of the text file. Now you can move x number of bytes from your current location using the ss.seekg(-x, ios::current) current, and then read the data from that location in the forward direction. This is however a messy solution as you would need to know how many bits to move backward, or have to calculate that repeatedly. Refer to http://www.learncpp.com/cpp-tutorial/137-random-file-io/ for more information on Random file operations in C++.

therainmaker
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if you want to read single characters, you can to something like this:

    is.seekg(0,is.end);
    while(is.tellg()>1)
    {
            is.seekg(-2,ios::cur);
            char ch;
            is.read(&ch,1);
            cout<<ch<<" ";
    }
mchouhan_google
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