title: what happens if you call an event declared virtual in the header file that is not defined
body: <p>I am new to C++ and trying to understand a line from <a href="https://svn.code.sf.net/p/extremetuxracer/code/trunk/src/states.cpp" rel="nofollow">this</a> code: </p>
  
  <pre><code>current-&gt;Loop(g_game.time_step);
  </code></pre>
  
  <p>-- where current is a pointer to an object of type State. </p>
  
  <p>In the <a href="https://svn.code.sf.net/p/extremetuxracer/code/trunk/src/states.h" rel="nofollow">header</a> file for the state the loop function is declared virtual:</p>
  
  <pre><code>virtual void Loop(double time_step) {}
  </code></pre>
  
  <p>But in the <a href="https://svn.code.sf.net/p/extremetuxracer/code/trunk/src/states.cpp" rel="nofollow">States.Cpp</a> file the header function is not defined. </p>
  
  <p>What happens if you call a function that is declared but not defined? The code comes from a working open-source game. This line executes without error. </p>