Programming with Objective-C  -- 属性的缺省实现

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithObjects/WorkingwithObjects.html#//apple_ref/doc/uid/TP40011210-CH4-SW6

Properties Are Atomic by Default

By default, an Objective-C property is atomic:

Note: The opposite of readonly is readwrite. There’s no need to specify the readwrite attribute explicitly, because it is the default.

Most Properties Are Backed by Instance Variables

By default, a readwrite property will be backed by an instance variable, which will again be synthesized automatically by the compiler.

You Can Customize Synthesized Instance Variable Names

As mentioned earlier, the default behavior for a writeable property is to use an instance variable called _propertyName.

By default, both Objective-C properties and variables maintain strong references to their objects. This is fine for many situations, but it does cause a potential problem with strong reference cycles.

Use Strong and Weak Declarations to Manage Ownership

By default, object properties declared like this:

use strong references for their synthesized instance variables. To declare a weak reference, add an attribute to the property, like this:

Note: The opposite to weak is strong. There’s no need to specify the strong attribute explicitly, because it is the default.

Note: The opposite to __weak is __strong. Again, you don’t need to specify __strong explicitly, because it is the default.