title: What does const& do?
body: <p>I do not understand what is going on. I am just learning C++ and I see something like this a lot:</p>
  
  <pre><code>double some_function(const Struct_Name&amp; s) {
      ...
  }
  </code></pre>
  
  <p>Why the <code>const</code> if we are passing by reference?</p>