title: C++ Help to define and use a vector of vector of a struct
body: <p>My intention is to build such a data structure in C++:</p>
  
  <pre><code>struct callbackDataUnit {
      std::string columnName;
      std::string columnData;
  };
  
  std::vector&lt;callbackDataUnit&gt; callbackRow;
  
  std::vector&lt;callbackRow&gt; callbackSet; &lt;--- Invalid... It needs a type here
  </code></pre>
  
  <p>The compiler first complains about the lack os <code>static</code> on <code>callbackRow</code>. Even if I use <code>static</code> there, it still does not compile as the structure is naturally invalid</p>
  
  <p>I would like to take this opportunity to understand a little more about C++ (I´m a beginner on that area), so here goes my questions:</p>
  
  <p>a) Why do we need the <code>static</code> qualifier here ?</p>
  
  <p>b) How can I solve this matrix of the first variables ? I could create 3 classes here (<code>CallbackDataUnit</code>, <code>CallbackRow</code> and <code>CallbackSet</code>) but I feel I would be missing real C++ power here. Would it make sense to make <code>callbackRow</code> a single element <code>struct</code>, so that it can be added to <code>callbackSet</code> ?</p>
  
  <p>Thanks for helping</p>