Structures are value types. When we pass instances of a structure within our application, we pass a copy of the structure and not the original structure. Classes are reference types, therefore when we pass an instance of a class, within our application, a reference to the original instance is passed. It is very important to understand this difference. We will give a very high-level view here, and will provide additional details in the Memory management section at the end of this chapter.

When we pass structures within our application, we are passing copies of the structures and not the original structures. Since the function gets its own copy of the structure, it can change it as needed without affecting the original instance of the structure.

When we pass an instance of a class within our application, we are passing a reference to the original instance of the class. Since we're passing the instance of the class to the function, the function is getting a reference to the original instance; therefore, any changes made within the function will remain once the function exits.

To illustrate the difference between value and reference types, let's look at a real-world object: a book. If we have a friend who wants to read Mastering Swift, we could either buy them their own copy or share ours.

If we bought our friend their own copy of the book, then any notes they made within the book would remain in their copy of the book and would not be reflected in our copy. This is how passing by value works with structures and variables. Any changes that are made to the structure or variable within the function are not reflected in the original instance of the structure or variable.

If we share our copy of the book, then any notes they made within the book would stay in the book when they return it to us. This is how passing by reference works. Any changes that are made to the instance of the class remains when the function exits.

To read more about value versus reference types, see the Memory management section at the end of this chapter.