__ Weak and__ The difference between blocks

One 、 __weak Is a weak reference , Use weak The modifier reference count will not increase . The referenced object will be added to a weak_table in , The address of the object will be treated as key,weak The address of the pointer （ The value of the address is the address of the object pointer ） Array as value.

1. Initialize a weak Object or adding a weak reference to an object initializes a weak The pointer points to the address of the object and creates the corresponding weak reference table .
2. When the object is released, it will be transferred from weak_table Get all weak Pointer to an array of addresses , Then traverse the array and empty the array . Then completely empty this element from the table and clean up the records of the object .

Two 、 Use __block The decoration object is block Will not be strongly quoted , And the object is in block Can be changed and reassigned .

__block You can modify objects and basic data types , stay MRC and ARC You can use .__weak Only in ARC And can only decorate objects , stay block Cannot be reassigned .