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POC++ Iterate over logfile content by timestamp
 

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  1. POC++ Iterate over logfile content by timestamp
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    <p>I have a log file with log lines. Each line is composed of a timestamp and a message.</p> <pre><code>timestamp1 blablabla timestamp2 foo timestamp3 bar etc... </code></pre> <p>My class <code>LogFile</code> has a map as member to match each timestamp to an ifstream position.<br> Now I would like to create a custom iterator over these timestamps.</p> <p>example : </p> <pre class="lang-c prettyprint-override"><code>LogFile myFile("file.log"); for (LogFile::iterator it = myFile.begin(); it != myFile.end(); it++) std::cout &lt;&lt; it-&gt;message &lt;&lt; std::endl; </code></pre> <p>output :</p> <pre><code>blablabla foo bar </code></pre> <p>I also want the iterator to be able to get decremented.</p> <p>Now I don't really know how to implement it in an efficient way.</p> <p>The most simple would be to open-seek-read-close the file with each iterator increment. But is that efficient ? I read that open/seek/close was quite expensive.</p> <p>Maybe a better solution is to have a <code>LogFile::open()</code> method to open the file, keeping it open to do all the incrementation we want, and finally close the file with a <code>LogFile::close()</code> method.</p> <p>Do you have some tips over this ? I'm sure this not the first time someone has to deal this kind of problem.</p> <p>EDIT A few more details :</p> <p>My class LogFile has a member of type <code>std::map&lt;Time, std::streampos&gt;</code> in order to store the links between timestamps and stream position.<br> I need to increment <em>and</em> decrement the iterator. Hence I think a map would be more appropriate, since I will use <code>the std::map::find(Time)</code> a lot (complexity o(log(n)), instead of <code>std::vector::find(Time)</code> with a o(n) complexity).</p> <p>The log file is very big (~20Mo) and my application has to run on a limited resources embeded system. So I can't store the whole file in the ram, I have to bufferize and take only the part I need on a given time.</p> <p>So yes, I think I'm gonna deal with the "open() and close() once" method.</p> <p>In the case of increment, I dont have to <code>std::ifstream::seekg()</code> a lot. But in the case of decrement, I dont see another way to seekg for each timestamp. Is that really the best way?</p>
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    1. COhave a look at http://www.boost.org/doc/libs/1_54_0/libs/iterator/doc/index.html#iterator-facade-and-adaptor. These classes allow for quick iterator adaption.
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      1. POC++ Iterate over logfile content by timestamp
      1. USWerner Erasmus
 

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