title: Read large txt file in c++
body: <p>I'd like to read a file of about 5MB in memory ...
  the file has this format (it is a text file)</p>
  
  <pre><code>ID 3:  0 itemId.1 0 itemId.2 0 itemId.5 1 itemId.7 ........................ 20 itemId.500
  ID 50:  0 itemId.31 0 itemId.2 0 itemId.4 2 itemId.70 ........................ 20 itemId.2120
  .....
  </code></pre>
  
  <p>how can I do this efficiently in c++?</p>