title: What does it mean for a CTLine to have "string access"?
body: <p>I'm trying to solve a <a href="https://stackoverflow.com/questions/19417776/how-do-i-locate-the-cgrect-for-a-substring-of-text-in-a-uilabel">hairy problem with UILabel</a>, and I've gotten most of it figured out, except for one thing: I'm having a challenge understanding what it means for a <code>CTLine</code> to have "string access".</p>
  
  <p>The method that I'd like to use is <code>CTLineGetOffsetForStringIndex</code>. <a href="https://developer.apple.com/library/Mac/documentation/Carbon/Reference/CTLineRef/Reference/reference.html#//apple_ref/c/func/CTLineGetOffsetForStringIndex" rel="noreferrer">Here's a link to the documentation for the method.</a></p>
  
  <p>Here's the part of the documentation that I don't understand (emphasis is mine):</p>
  
  <blockquote>
    <p>The primary offset along the baseline for charIndex, <strong>or 0.0 if the
    line does not support string access.</strong></p>
  </blockquote>
  
  <p>When I'm running this method, I'm getting <code>0.0</code> back, so I guess that means the line doesn't support string access - but what does that mean, exactly?</p>