Upcast vs downcast7/5/2023 ![]() Next, you initialize it with an instance of Vehicle, which has eight wheels. In the code block above, you declare a reference named truck, which is of type Truck. Here’s an example of upcasting: let truck:Truck = Vehicle(wheels:8) Upcasting their objects to the Vehicle class uses this generalization, letting us run operations using loops and other routine paradigms. We already know that these are descendants of the same class, Vehicle, and they are bound to have some similarities. Upcasting lets you store any kind of Vehicle in a reference of type Vehicle: Car, Truck, etc. Think of it as moving up in the class hierarchy. An object of a subclass is type cast into an object of its superclass. We mentioned earlier that Car or Truck could be type cast to Vehicle. ![]() Let’s consider the example using Car, Truck, and Vehicle. Upcasting is used to generalize a series of subclasses of a class by using a variable of the class itself. You would not be able to cast its objects to any of the types given above. Now, let’s say there was another class called Ship, which had no relation to Truck, Car, or Vehicle. ![]() You can cast an object of type Car to Vehicle, and vice-versa, or Truck to Vehicle, and vice-versa. Let’s say a class called Vehicle has two subclasses, Car and Truck. For example, you obviously can’t change the type of an object holding a string from string to integer because it wouldn’t make sense in real life to treat a sentence as a number.įor type casting to occur, the concerned object’s original type and the new type must be either subclasses or superclasses of one another. ![]() Instead, it changes the type used to describe the object. Type casting does not change the object concerned. While type casting may not make much sense at first, it helps to simplify many routines in modern applications. Type casting is a programming paradigm used in most object-oriented languages that allows developers to treat an object of one type like that of another. From now on, we’ll refer to primitive data types and classes by types. Most of a modern application’s logic relies on custom objects that are made possible via classes. Not all class variables hold primitive data like integers and strings. Type casting provides a way to gain some flexibility back. The constraints of using types force a language to lose a big chunk of its flexibility. For example, in languages like PHP and C, you can alter data types to a certain extent to gain more flexibility in your code. Weakly-typed languages are not as strict in this regard. Every variable in Swift has a type associated with it, and once a type has been assigned to a variable, it cannot store data of any other type. Swift is a strongly-typed programming language. Even if a type mismatch does not explicitly break the flow, it may go unnoticed during the compilation process, leading to unexpected results when running a program. However, if a type mismatch causes a break in the flow, you’ll see a compilation error. ![]() If data stored in a variable is eligible for the operations carried out later in the flow, processes will run accordingly. Classifying data types prevents type mismatch, which is a common error at compile time. As each class differs from the others, so do the data types, allowing developers to distinguish variables according to what kind of data they hold. What are types in Swift?Ī type is, in essence, the primitive equivalent of a class that is used to denote the kind of data stored in a variable. In this guide, we’ll take a look at type casting in Swift, a popular concept in modern object-oriented programming languages. Like almost every other programming language, Swift has its own rules and syntax. Type casting in SwiftĬreated by Apple in 2014, Swift is a popular open-source language for building iOS applications that has garnered a strong community of developers and a wealth of third-party content. Kumar Harsh Follow Technical writer and software developer based in India. ![]()
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