title: Using strtok with a std::string
body: <p>I have a string that I would like to tokenize.
  But the C <code>strtok()</code> function requires my string to be a <code>char*</code>.
  How can I do this simply?</p>
  
  <p>I tried:</p>
  
  <pre><code>token = strtok(str.c_str(), " "); 
  </code></pre>
  
  <p>which fails because it turns it into a <code>const char*</code>, not a <code>char*</code></p>