Swift Refs: Strong, Weak & Unowned Explained
Swift is a powerful, modern language for iOS and macOS development that makes writing code much easier and more efficient. One of the most important features of Swift is its use of references. In this article, we will explore the three types of references available in Swift: strong, weak, and unowned. We will look at how they are used, their pros and cons, and when each should be used.
What are References?
References are a way to keep track of an object in memory. When an object is created in Swift, it is stored in memory and a reference to that object is created. This reference can then be used to access the object from anywhere in the code.
Strong References
The most common type of reference is the strong reference. Strong references are the default type of reference used in Swift and it is responsible for keeping an object alive. Any time an object has one or more strong references to it, it will stay in memory until all of the strong references are removed.
let objectA = ObjectA()
let objectB = ObjectB(objectA) // objectA now has a strong reference
In the example above, objectA now has a strong reference from objectB, so it will stay in memory until objectB is deallocated.
Pros and Cons
Strong references have the advantage of being easy to use and understand. They are also the most reliable type of reference, since they guarantee that the object will remain in memory as long as there is at least one strong reference to it.
On the other hand, strong references can cause memory leaks if they are not managed properly. If two objects have strong references to each other, they will both stay in memory even if they are no longer needed. This is known as a “retain cycle” and it can cause memory usage to grow over time.
Weak References
Weak references are the opposite of strong references. They are used to keep track of an object without keeping it alive. If an object has no other strong references to it, it will be deallocated and the weak reference will be set to nil.
let objectA = ObjectA()
weak var objectB = ObjectB(objectA) // objectA now has a weak reference
In the example above, objectA now has a weak reference from objectB. If objectA does not have any other strong references to it, it will be deallocated and objectB’s weak reference will be set to nil.
Pros and Cons
Weak references are useful for avoiding retain cycles. Since they do not keep an object alive, they can help avoid memory leaks. On the other hand, they can be tricky to use since they may become nil at any time. It is important to check the value of a weak reference before using it to make sure it is not nil.
Unowned References
Unowned references are similar to weak references, but they do not become nil when the object they are referencing is deallocated. Instead, they will contain an invalid reference.
let objectA = ObjectA()
unowned var objectB = ObjectB(objectA) // objectA now has an unowned reference
In the example above, objectA now has an unowned reference from objectB. If objectA does not have any other strong references to it, it will be deallocated and objectB’s unowned reference will remain unchanged.
Pros and Cons
Unowned references are useful for avoiding retain cycles, just like weak references. However, they are less safe than weak references since they do not become nil when the object they are referencing is deallocated. It is important to check the value of an unowned reference before using it to make sure it is not invalid.
Conclusion
References are an important part of Swift and are used to keep track of objects in memory. The three types of references available in Swift are strong, weak, and unowned. Each type has its own pros and cons, and it is important to choose the right one for the job. Strong references are the most reliable, but can lead to memory leaks if not managed properly. Weak and unowned references are useful for avoiding retain cycles, but can be less safe than strong references.