title: What does it mean to set the declaration of a function equal to 0? How can you assign an integer to a function?
body: <p>I was browsing through the sources of a (prefer not to name) GUI Toolkit which wrapped up the Windows API when I found the following function definition in the window class:</p>
  
  <pre><code> virtual LRESULT CALLBACK wndProc (HWND, UINT, WPARAM, LPARAM) = 0;
  </code></pre>
  
  <p>What is happening here? How can you assign a function to an integer? Or does it assign it to <code>NULL</code>? Do you need to do this if you want to use function pointers in the wndproc?</p>