title: What's the meaning of "this" keyword here?
body: <p>A very short C# function.</p>
  
  <pre><code>    public static int SizeInBytes(this byte[] a)
      {
          return sizeof (int) + a.Length*sizeof (byte);
      }
  </code></pre>
  
  <p>What does "this" keyword mean in this function? What's the equivalent of this keyword in C++? Besides, what is this function trying to calculate exactly? </p>