Differences between assign, retain, and copy

I. assign attributes

When the data type is int, float, or other native types, you can use assign. Otherwise, memory leakage may occur. For example, if you use malloc to allocate a piece of memory and assign its address to pointer a, If you want pointer B to share the block of memory, then assign a value to assgin B. Assgin is used, and a and B point to the same memory. But now the problem arises. Can a be released directly when a no longer needs this memory? Certainly not, because a does not know whether B is still using this memory. If a is released, B will cause the program to crash when using this memory.

Ii. retain attributes

The retain attribute is proposed to solve the above problem. It uses reference counting. In the above example, we set a reference count for the memory, when the memory is allocated and assigned to a, the reference count is 1. when a is assigned to B, the reference count is increased to 2. if a no longer uses this memory, it only needs to reduce the reference count by 1, indicating that it no longer owns this memory. When B no longer uses this memory, it also reduces the reference count by 1. When the reference count is changed to 0, it means that the memory is no longer referenced by any pointer, and the system can directly release it. In this case, the system automatically calls the dealloc function, and the memory is recycled.

Iii. copy attributes

Copy is used when you do not want a and B to share a piece of memory. A and B each have their own memory. For more information, see the next article.