title: standard predefined empty c++ string?
body: <p>I am wondering do we have a predefined static const empty string in modern c++?</p>
  <p>There is a multitude of cases where we need a reference to an empty string (say as a default parameter). All classic examples use temp created by default constructor.</p>
  <p>It will be much cheaper to use reference to something from the library. And it will be really annoying (and waste of memory) if each project would define its own empty string.</p>
  <p>If it is not there already all what is needed is one line (c++17) addition to the library.</p>
  <pre><code>static const std::basic_string&lt;char&gt; null;
  </code></pre>