title: Undefined symbols for architecture i386: "_OBJC_CLASS_$_MFMailComposeViewController"
body: <p>I train in the creation of programs for the iPhone using MFMailComposeViewController. However, an error occurs when compiling:</p>
  
  <pre><code>Undefined symbols for architecture i386:
     "_OBJC_CLASS_ $ _MFMailComposeViewController", Referenced from:
         objc-class-ref in ViewController.o
  ld: symbol (s) not found for architecture i386
  clang: error: linker command failed with exit code 1 (use-v to see invocation)
  </code></pre>
  
  <p>Please help, what is it? And how to fix it?</p>
  
  <p>Thanks in advance!</p>