title: Differences among including <xstring>, <cstring>, <string> and <wstring> in C++
body: <p>I have seen the following <code>#include</code> directives:</p>
  
  <pre><code>#include &lt;xstring&gt;
  
  #include &lt;cstring&gt;
  
  #include &lt;string&gt;
  
  #include &lt;wstring&gt;
  </code></pre>
  
  <p>What are the differences among these include directives? Did I miss any others that should be considered part of this group?</p>