title: Changing source file type to Objective-C++ causes linker error when using GameKit
body: <p>So I am using Xcode 6.1 on my Objective-C app.
  My app uses the GameKit framework.</p>
  
  <p>I tried to call external C++ methods from my GameViewController (which is Objective-C source.)</p>
  
  <p>To make GameViewController.mm compile, I had to set the type to 'Objective-C++ Source' manually.</p>
  
  <p>However, even though the source now compiles, it will fail to link with:</p>
  
  <pre><code>    Undefined symbols for architecture x86_64:
   "_OBJC_CLASS_$_GLKViewController", referenced from:
        _OBJC_CLASS_$_GameViewController in GameViewController.o
    "_OBJC_METACLASS_$_GLKViewController", referenced from:
        _OBJC_METACLASS_$_GameViewController in GameViewController.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>I can only get rid of the linker error by setting the type back to 'Objective-C Source' and removing the calls to the external C++ functions.</p>
  
  <p>Why does the linker fail if my source is compiled as 'Objective-C++ Source' instead of 'Objective-C Source' type?</p>
  
  <p>Note that I properly link against GameKit in the Build-phases.</p>