How it works...

Defining a struct is very similar to defining an object class, and that is intentional. Much of the functionality available to a class is also available to a struct.

Within the PersonName struct, we have properties for the three components of the name and the fullName method we saw earlier to combine the three name components into a full name string.

Next, we have a method to change the family name property, which is why we defined the familyName property as a var variable instead of a let constant. This method assigns a new value to a property of the struct; it is mutating, or changing, the struct, and therefore needs to be marked with the mutating keyword. This keyword is enforced by the compiler to remind us that when we mutate a struct, a new copy of the original struct is created with the new value. This is known as value-type semantics.

To see this in action, consider the following code:

let alissasBirthName = PersonName(givenName: "Alissa", middleName: "May", familyName: "Jones") 
print(alissasName.fullName()) // Alissa May Jones 
var alissasCurrentName = alissasBirthName 
print(alissasName.fullName()) // Alissa May Jones 

So far, so good. We have created a PersonName struct and assigned it to a constant called alissasBirthName and a variable called alissasCurrentName.

When we change or "mutate" the alissasCurrentName variable, only this variable is changed; alissasBirthName is a copy, and so it doesn't have the amended family name, even though they were assigned from the same source:

alissasCurrentName.change(familyName: "Moon") 
print(alissasBirthName.fullName()) // Alissa May Jones 
print(alissasCurrentName.fullName()) // Alissa May Moon