CAShapeLayer双指针

Question

I'm trying to declare a CAShapeLayer **dot_p; but Xcode is giving me the following error:

Pointer to non-const type 'CAShapeLayer *' with no explicit ownership

What is going on?

Answer 1

As you may know, Objective-C uses an automatic reference counting system (ARC) for memory management. In order for that to work, it needs to understand the implicit ownership rules for any pointer so it knows if setting the pointer needs to increment the retain count for the object.

ARC has sensible defaults, so if you say:

CAShapeLayer *foo;

It assumes that you want a strong reference. So the above is equivalent to writing:

__strong CAShapeLayer *foo;

But if you have a pointer to a pointer, it isn't able to guess what the ownership rules should be. If you want ARC to increment the reference count of your CAShapeLayer when the dot_p pointer is set, you could write:

CAShapeLayer *__strong *dot_p;

If you don't want it to increment the retain count you could write:

CAShapeLayer *__weak *dot_p;

Or

CAShapeLayer *__unsafe_unretained *dot_p;

If you aren't sure whether you want it to be strong or not, then I'd suggest thinking very hard about the ownership rules in your app, otherwise you're very likely to introduce a retain cycle (memory leak) or dangling pointer (crash).