title: string initializing as NULL in C++
body: <pre><code>string a=NULL;
  </code></pre>
  
  <p>it gives error. Why and how can I initialize string as NULL?</p>
  
  <p>but when I write</p>
  
  <pre><code>string a="foo";
  </code></pre>
  
  <p>this it works fine.</p>