title: Use of undeclared identifier
body: <p>I'm trying to use <a href="https://github.com/hollance/AudioBufferPlayer/" rel="nofollow">this project</a> which is a synthesizer for Objective-C for an iPhone application I'm building. However, I'm having trouble with the <code>MHAudioBufferPlayer</code> class. </p>
  
  <p>In the <a href="https://github.com/hollance/AudioBufferPlayer/blob/master/MHAudioBufferPlayer/MHAudioBufferPlayer.m" rel="nofollow"><code>MHAudioBufferPlayer.m</code> class</a>, I'm getting a bunch of <code>Use of undeclared identifier</code> errors for <code>_gain</code>, <code>_playing</code>, and <code>_audioFormat</code>. This makes sense, as those identifiers are never declared with an underscore in front of them. However, they are declared in the <code>MHAudioBufferPlayer.h</code> class <b>without</b> the underscores. </p>
  
  <p>I'm sort of confused by this as I'm new to Objective-C. Does an underscore denote a special action to be taken? Is it supposed to be translated into <code>self.gain</code>, <code>self.playing</code>, etc.? How can I fix this? Or is this code just buggy?</p>
  
  <pre><code>- (id)initWithSampleRate:(Float64)sampleRate channels:(UInt32)channels bitsPerChannel:(UInt32)bitsPerChannel packetsPerBuffer:(UInt32)packetsPerBuffer
  {
      if ((self = [super init]))
      {
          _playing = NO;
          _playQueue = NULL;
          _gain = 1.0;
  
          _audioFormat.mFormatID         = kAudioFormatLinearPCM;
          _audioFormat.mSampleRate       = sampleRate;
          _audioFormat.mChannelsPerFrame = channels;
          _audioFormat.mBitsPerChannel   = bitsPerChannel;
          _audioFormat.mFramesPerPacket  = 1;  // uncompressed audio
          _audioFormat.mBytesPerFrame    = _audioFormat.mChannelsPerFrame * _audioFormat.mBitsPerChannel/8;
          _audioFormat.mBytesPerPacket   = _audioFormat.mBytesPerFrame * _audioFormat.mFramesPerPacket;
          _audioFormat.mFormatFlags      = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
  
          _packetsPerBuffer = packetsPerBuffer;
          _bytesPerBuffer = _packetsPerBuffer * _audioFormat.mBytesPerPacket;
  
          [self setUpAudio];
      }
      return self;
  }
  </code></pre>