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Class objects are reference types that refer to the way they are stored and referenced internally. To see how these reference type semantics work, consider the following code:

class MovieReview { 
    let movieTitle: String 
    var starRating: Int // Rating out of 5 
    init(movieTitle: String, starRating: Int) { 
        self.movieTitle = movieTitle 
        self.starRating = starRating 
    } 
} 
 
// Write a review 
let shawshankReviewOnYourWebsite = MovieReview(movieTitle: "Shawshank Redemption", starRating: 3) 
 
// Post it to social media 
let reviewLinkOnTwitter = shawshankReviewOnYourWebsite 
let reviewLinkOnFacebook = shawshankReviewOnYourWebsite 
 
print(reviewLinkOnTwitter.starRating) // 3 
print(reviewLinkOnFacebook.starRating) // 3 
 
// Reconsider my review  
shawshankReviewOnYourWebsite.starRating = 5 
 
// The change visible from anywhere with a reference to the object  
print(reviewLinkOnTwitter.starRating) // 5 
print(reviewLinkOnFacebook.starRating) // 5

We created a review object and assigned that review to two separate constants. As an object is a reference type, it is a reference to the object that is stored in the constant, rather than a new copy of the object. Therefore, when we reconsider our review and rightly give The Shawshank Redemption five stars, we are changing the underlying object, and all references that access that underlying object will see that the starRating property has changed.