title: Vector of vectors, reserve
body: <p>Suppose I want to represent a two-dimensional matrix of <code>int</code> as a vector of vectors:</p>
  
  <pre><code>std::vector&lt;std::vector&lt;int&gt; &gt; myVec;
  </code></pre>
  
  <p>The inner dimension is constant, say 5, and the outer dimension is less than or equal to <code>N</code>. To minimize reallocations I would like to reserve space:</p>
  
  <pre><code>myVec.reserve(N);
  </code></pre>
  
  <p>What size is assumed for the inner vector? Is this purely implementation dependent? How does this effect the spatial locality of the data? Since the inner dimension is a constant is there a way to tell the compiler to use this constant size? How do these answers change if the inner vector's size changes?</p>