title: What header defines NULL in C++?
body: <p>According to <a href="https://rads.stackoverflow.com/amzn/click/com/0321714113" rel="noreferrer" rel="nofollow noreferrer">C++ primer</a>, <code>&lt;cstdlib&gt;</code> header defines <code>NULL</code>. <a href="http://www.cplusplus.com/reference/clibrary/cstddef/NULL/" rel="noreferrer">cpluspplus</a> says it is defined in <code>&lt;cstddef&gt;</code>.</p>
  
  <p>Ultimately, if the right header is not included, I thought <code>NULL</code> can't be referenced. </p>
  
  <p>From what i can see, however it can be referenced and produce programs and that compile and run without warnings or errors, after including only <code>&lt;iostream&gt;</code></p>
  
  <p>Please help me understand this.</p>