Can you explain what the term 'Retain' means in the context of memory management?

In the context of memory management in iOS, 'Retain' refers to the reference counting mechanism. When an object is "retained," its reference count increases by one, indicating that someone owns this object and it should not be deallocated.

When the object is no longer needed, 'release' is called, decreasing the reference count. If the count reaches zero, the object is automatically deallocated.

Thus, 'retain' is an operation that ensures the object remains in memory as long as there are active references to it. This is a key memory management mechanism in Objective-C and Swift (before ARC, and also within ARC).

Example in Objective-C:

NSObject *obj = [[NSObject alloc] init]; // retain count = 1
[obj retain]; // retain count = 2
[obj release]; // retain count = 1
[obj release]; // retain count = 0, object is deallocated