title: How do I check if a CGPoint has been initialised?
body: <p>I would like to initially set a CGPoint property to a particular point (middle of screen). Other methods may subsequently wish to change this property. My thoughts were to initialise it if empty in the getter, but I get the message invalid argument type 'struct CGPoint' to unary expression. I also tried using if property == nil or 0 but no joy.</p>
  
  <p>Any thoughts?</p>
  
  <pre><code>-(CGPoint)graphOrigin
  {
      // initialise to centre of screen if has not been set
      if(!_graphOrigin) // this expression is causing the problem
      {
          CGPoint origin = CGPointMake(self.bounds.origin.x + self.bounds.size.width / 2, self.bounds.origin.y + self.bounds.size.height / 2);
  
          _graphOrigin = origin;
      }
  
  return _graphOrigin;
  </code></pre>
  
  <p>}</p>