title: C++ Include Guards for Standard Headers
body: <p>I am wondering if/ what include guards on files like <code>windows.h</code>, <code>math.h</code>, <code>iostream</code>, <code>stdio</code>... etc.</p>
  
  <p>Since I have those headers included multiple times in different files. Do those files already have guards built in or is there a definition defined?</p>
  
  <p>I am just wondering what the standards are for that kind of thing.</p>