Question

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.

Any thoughts?

-(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;

}

Answer 1

A CGPoint is a struct, so you can’t set it to nil or NULL (it’s not a pointer). In a sense, there’s really no “uninitialized” state. Perhaps you could use {0.0, 0.0} to designate an unset CGPoint, but that’s also a valid coordinate. Or you could use negative x and y values to flag an “uninitialized” point, since negative values can’t be valid drawing points, but that’s a bit of a hack, too.

Probably your best bet is to do one of two things:

1. Store the property as a pointer to a CGPoint. This value can be set to NULL when uninitialized. Of course, you have to worry about mallocing and freeing the value.

2. Store the CGPoint alongside a BOOL called pointInitialized or somesuch, initially set to NO, but set to YES once the point has been initialized. You can even wrap that up in a struct:

   struct {
       CGPoint point;
       BOOL initialized;
   } pointData;