Get Ivar value from CGPointin Objective-C

Question

Hi,

Thanks for looking at my question.

I have been trying to look around and I have been playing around with low level IVars to get information from the classes. I am having trouble trying to load the values that come from CGPoint.

CGPoint point;   
object_getInstanceVariable(obj, [columnName UTF8String], (void **)&point);
NSLog(@"%@", NSStringFromCGPoint(point)); 

The output of this should be {220,180}

The i am actually getting {220, 1.72469e-38}

Could anyone help me out and explain why this might be happening?

Answer 1

Pointers are mixed up here. point is an instance, so &point is a pointer (void *-like), not a pointer to a pointer (void **-like). You need something like this:

CGPoint *point;   
object_getInstanceVariable(obj, [columnName UTF8String], (void **)&point);
NSLog(@"%@", NSStringFromCGPoint(*point)); 

Answer 2

The real problem here is that a CGPoint is not an object -- it's a plain-old-C struct. :-)

Answer 3

object_getInstanceVariable() only copies a pointer-worth of data at the start of the ivar's offset. That's all the storage you give it, after all, with that final void ** argument. When you know you have more than a pointer-worth of data, as in this case, you can use ivar_getOffset() to find where the data starts then copy out as many bytes as you need:

Ivar iv = object_getInstanceVariable(obj, [columnName UTF8String], NULL);
ptrdiff_t offset = ivar_getOffset(iv);
CGPoint point = *(CGPoint *)((char *)obj + offset);

In this case, dereferencing a CGPoint * causes sizeof(CGPoint) bytes to be copied from the referenced address; in the general case, you could memcopy() or bcopy() data from the computed address into an appropriately sized buffer.

The next problem would be computing the correct size at runtime…