title: convert std::string to nsstring issue
body: <p>Hi 
  I use the following to convert <code>std::string</code> to NSString but it return (null)  while trying to display the value of the nsstring </p>
  
  <p>I need it to return instead of (null) empty at conversion time any suggestion </p>
  
  <pre><code>StudyDate=[NSString stringWithCString:studyDate length:strlen(studyDate)]; 
  </code></pre>
  
  <p>any suggestion to avoid null values </p>
  
  <p>best regards </p>