Stored vs Computed Property Swift: A Developer’s Guide

In Swift, properties are essential components of classes, structures, and enumerations. They help define how data is stored and manipulated within objects. In Swift, you’ll commonly encounter two types of properties: stored properties and computed properties.

Let's break down stored vs computed property swift, their differences, and roles in Swift programming.

What Are Properties in Swift?

A property in Swift is a variable or constant that belongs to an instance of a class, structure, or enumeration. Properties store and retrieve values associated with an instance. Swift offers two main types of properties: stored properties and computed properties.

• Stored properties store values and are similar to instance variables in other languages. They can be constants or variables.

• Computed properties, on the other hand, do not store values directly. Instead, they provide a getter (and optionally a setter) that performs computations to return a value.

Properties can be used in various Swift constructs such as classes, structures, and enumerations, which allows you to manage data efficiently and perform computations as needed.

Difference Between Stored and Computed Properties

Stored properties are properties that store constant or variable values as part of an instance. They can either be constant stored properties (using let) or variable stored properties (using var). A stored property retains the value you assign until it’s modified or the object is deallocated.

In contrast, computed properties do not store a value directly. Instead, they compute a value every time they are accessed, typically using a getter. Computed properties may also have a setter, allowing them to update other properties indirectly.

Comparing Stored and Computed Properties

• Stored properties store values. They can be modified and directly accessed.

• Computed properties compute values on the fly without storing them.

• You can add property observers to stored properties to monitor changes, whereas computed properties allow you to update values based on complex calculations.

Understanding Stored Properties

In Swift, stored properties play a fundamental role in managing and storing data within classes and structures. They directly hold a value and are central to object state management.

What are Stored Properties?

A stored property is a property that stores constant or variable values as part of an instance. You define stored properties within classes, structures, or enumerations using the var keyword for variable properties or the let keyword for constant properties.

Example of a Variable Stored Property

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struct Car {
    var model: String // Variable stored property
    var year: Int
}

let car = Car(model: "Tesla Model S", year: 2023)
print(car.model) // Output: Tesla Model S

In this example, model and year are variable stored properties of the Car structure. Their initial values are set when an instance of Car is created.

Example of a Constant Stored Property

In the Laptop structure, brand is a constant stored property, meaning its value cannot be changed after initialization. This ensures that the property holds a fixed value throughout the lifetime of the instance.

Mutable vs Immutable Stored Properties

• Variable stored properties can be modified after initialization.

• Constant stored properties must be assigned an initial value at creation and cannot be modified afterward.

You can also define lazy stored properties, which are only initialized when they are first accessed.

Example of a Lazy Stored Property

Here, data is a lazy stored property that delays its initialization until it's accessed for the first time, optimizing resource usage. This is useful when the stored property’s value is expensive to compute or load.

Memory Management for Stored Properties

Swift employs Automatic Reference Counting (ARC) to manage the memory of stored properties efficiently. Each time an instance of a class is created, ARC keeps track of the number of strong references to the instance. When there are no more references, ARC automatically deallocates the instance and frees the memory.

• Stored properties that are constants (let) do not require memory management beyond their initial allocation because their values do not change.

• For variable stored properties (var), Swift manages the memory based on the value type (such as whether it's a reference type like a class, or a value type like a struct).

Example of Memory Management with Classes

In the above example, Person is a class with variable stored properties. When you create multiple references (reference1, reference2) to the same instance, any changes to the stored properties of one reference affect all other references. This is because classes are reference types, meaning that their stored properties are shared between all references pointing to the same instance.

For stored properties in value types like structures, each copy of the struct will have its own separate stored properties. This ensures that modifying the properties in one instance does not affect others.

Understanding Computed Properties

In Swift, computed properties allow you to calculate a value dynamically instead of storing it directly in memory. This makes them extremely powerful for managing derived data, making the code cleaner and more efficient.

What are Computed Properties?

A computed property doesn’t store a value directly. Instead, it provides a getter (and optionally a setter) to perform computations that return a value. Computed properties can be defined in classes, structures, and enumerations.

Syntax of a Computed Property

A computed property typically includes:

• A getter, which calculates and returns a value.

• Optionally, a setter, which updates other properties based on the new value assigned to the computed property.

Example of a Computed Property with a Setter

In this example, perimeter is a read-write computed property that calculates the perimeter based on the side. When the perimeter is updated, the setter recalculates and updates the value of side.

Read-Only vs Read-Write Computed Properties

• Read-only computed properties only provide a getter, which computes and returns a value. They cannot be assigned a new value directly.

• Read-write computed properties provide both a getter and a setter, allowing both the computation and the modification of values based on the computed property.

Read-Only Computed Property Example

In this example, area is a read-only computed property because it only provides a getter that calculates the area based on the radius but does not have a setter to modify the property.

Read-Write Computed Property Example

In this example, distanceFromOrigin is a read-write computed property. The getter calculates the current distance from the origin using the Pythagorean theorem. The setter modifies the x and y values to adjust the point’s distance from the origin when a new value is assigned.

Key Differences: Stored vs Computed Properties

When working with properties in Swift, understanding the core differences between stored properties and computed properties is crucial for efficient coding and performance optimization. Let's break down how these two types of properties differ in terms of value storage and memory management.

Value Storage vs Value Computation

A stored property directly holds a value in memory. Whether you declare a variable stored property or a constant stored property, the property reserves memory to store its value. You can think of stored properties as being responsible for storing an initial value (or default value) that persists throughout the life cycle of the object or struct instance.

• Variable stored properties can be modified after initialization.

• Constant stored properties hold a value that cannot be changed once it's set.

Example of a Stored Property

In this example, name is a variable stored property that can be modified after the object is initialized, while birthYear is a constant stored property that cannot be changed once assigned.

How Computed Properties Calculate Values

Unlike stored properties, computed properties do not store values directly. Instead, they calculate and return a value each time they are accessed. A computed property uses a getter to return a calculated value and, optionally, a setter to modify related other properties.

Example of a Computed Property

In this example, the area property is a computed property that calculates its value every time it’s accessed. It doesn't store the result; it simply computes the value based on the current radius.

Initialization and Memory Impact

Stored properties must be assigned an initial value either during initialization or via a default value. Swift ensures that stored properties are always initialized before an object is used. Since they occupy memory, the amount of memory allocated depends on the size and type of the property.

• Constant stored properties are assigned a value at initialization and retain that value throughout the object's lifetime.

• Variable stored properties can change their value during the object's lifecycle, but both types occupy memory once initialized.

Example of Memory Impact in Stored Properties

Here, width is a stored property with a default value, which means that its memory is allocated as soon as the Rectangle instance is created.

Computed Properties: No Memory Overhead

Computed properties differ from stored properties in terms of memory usage. Since they don't store any data, there’s no memory overhead for storing their values. Instead, they execute code each time they are accessed, which might result in performance costs if the calculation is complex or involves external resources.

Example of Computed Properties and Memory Efficiency

In this example, area does not consume any additional memory because it’s calculated each time based on the width and height. Unlike a stored property, it doesn’t retain a value in memory.

When to Use Stored Properties

When working with properties in Swift, knowing when to use stored properties versus other types, like computed properties, is essential for building efficient and performant applications. Stored properties are valuable in many scenarios where you need to hold and manipulate data directly in memory.

Best Use Cases for Stored Properties

Stored properties are ideal in situations where you need to store constant or variable data that is accessed repeatedly. These are a few common use cases:

1. Simple Data Storage

When you need to store values directly and repeatedly access them without any additional computation, stored properties are perfect. For example, storing a user's name, age, or preferences in a class or struct makes accessing and modifying this information efficient.

2. Constant Data That Doesn’t Change

For constant data that won’t change throughout the lifetime of an instance, constant stored properties (let) provide a way to store initial values that are accessed multiple times.

In this case, using constant stored properties ensures the values remain unchanged, ideal for representing fixed data like the brand and year of a car.

3. Data That Is Directly Related to the Object’s State

If the value directly reflects or represents an object's internal state, such as dimensions, position, or user settings, stored properties are ideal for maintaining the consistency of the object state.

4. Objects Requiring a Default Value

Stored properties can also provide a default value upon initialization, useful when you want to ensure an object always has some base data.

In these cases, stored properties provide the most straightforward solution for managing values that are critical to an object’s core identity or state.

Performance Considerations for Stored Properties

While stored properties offer simplicity and ease of use, they also come with performance trade-offs that you should consider, especially in memory-constrained or performance-sensitive applications.

1. Memory Consumption

Since stored properties allocate memory to store their values, objects with multiple stored properties can lead to increased memory usage. Each stored property holds its value directly in memory, and the more stored properties an object has, the larger its memory footprint.

This is particularly important when working with objects that are instantiated frequently or when memory is a constraint. For instance, if you have a class with several stored properties, each instance of that class will consume additional memory.

In this case, the data property consumes a significant amount of memory, making this class heavier to instantiate multiple times.

2. Fast Access Times

The main benefit of stored properties is their fast access time. Since the values are already stored in memory, accessing them is nearly instantaneous. This makes stored properties ideal when you need to access or modify values frequently and cannot afford the performance overhead of computing values dynamically.

Here, accessing the pages property is quick since the value is directly stored, making stored properties ideal for performance-sensitive applications where fast data retrieval is crucial.

3. Avoiding Redundant Computations

Another performance advantage of stored properties is that they avoid redundant calculations. In contrast to computed properties, which recalculate their value every time they are accessed, stored properties simply hold the value in memory. This can result in significant performance improvements when dealing with data that doesn’t need to change frequently.

4. Lazy Stored Properties for Memory Efficiency

Swift offers lazy stored properties that help manage memory more efficiently by delaying the initialization of a stored property until it’s first accessed. This can significantly reduce the initial memory overhead, especially if the property’s value is resource-intensive to calculate.

In this example, data is only initialized when it's accessed, improving memory usage and performance, especially if the value is computationally expensive to generate.

Key Takeaways for Performance

• Stored properties provide faster access times because the values are already stored in memory, making them ideal for frequently accessed data.

• They consume more memory as each stored property reserves space in memory for the entire lifecycle of the object.

• Lazy stored properties help optimize memory usage by deferring initialization until the property is accessed.

• For objects with complex or large amounts of data, careful consideration should be given to how much memory is allocated for stored properties.

When to Use Computed Properties

Computed properties provide a powerful mechanism for dynamically calculating values in Swift, which makes them ideal in scenarios where the value is derived from other properties or external data. Unlike stored properties, computed properties do not store data but rather calculate it each time they are accessed.

Best Use Cases for Computed Properties

1. Dynamic Data Calculations

Computed properties are excellent when you need to calculate values on the fly based on other stored properties. For example, if the value of a property is dependent on other data, you can use a computed property to derive that value without storing additional information.

In this case, the area property is a computed property that dynamically calculates the rectangle's area based on its width and height. This is useful when the value depends on properties that may change over time.

2. Avoiding Redundant Stored Data

If a value can be derived from existing stored properties and doesn’t need to be saved separately, computed properties help avoid redundancy. This is particularly useful when you have related data that can be computed rather than duplicating it in memory.

Here, circumference is derived from radius, eliminating the need to store it separately and ensuring that any changes to radius are reflected in the circumference.

3. Deriving Values Indirectly

Computed properties are ideal when the value needs to be calculated based on more complex logic or external conditions, such as aggregating values from multiple stored properties.

In this case, total is computed based on the basic and bonus amounts. Using a computed property helps keep the data consistent without requiring multiple updates.

4. Providing Custom Getters and Setters

When you need to control how a property is read or written, computed properties offer an elegant solution with custom getters and setters. This is useful for enforcing certain constraints or adding validation logic when modifying the property.

This example shows how the perimeter property can adjust the value of side when set. It provides a flexible and reusable way to handle property logic without the need for manual intervention.

Performance Considerations for Computed Properties

While computed properties are highly flexible and useful, they come with performance considerations that you should be aware of:

1. Overhead from Repeated Calculations

Since computed properties calculate their value each time they are accessed, repeated accesses can result in performance overhead, especially if the computation is complex. In situations where the value is frequently read but rarely changes, it may be more efficient to store the result in a stored property.

In this case, every time totalSum is accessed, the entire array is iterated over to calculate the sum. For large datasets, this can result in significant performance hits, especially if the value is accessed frequently. A stored property or lazy stored property might be more appropriate in this scenario if the data doesn't change often.

2. Reduced Memory Usage

One of the key advantages of computed properties is their ability to save memory by not storing redundant data. Since computed properties calculate values on demand, they don’t occupy additional memory like stored properties. This can be crucial in memory-constrained environments, such as mobile applications where minimizing memory footprint is important.

The fahrenheit property in this example doesn't store any data, making the struct more memory-efficient while still providing access to the computed temperature in Fahrenheit.

3. Lazy Computation for Expensive Operations

If the computation involved in a computed property is expensive and doesn't need to be performed every time the property is accessed, you can optimize performance using lazy stored properties. This allows the value to be calculated once, cached, and reused, minimizing repeated expensive computations.

By using a lazy stored property, you can perform the computation only when it’s actually needed, making it more efficient than a computed property that recalculates the result every time.

4. Increased Read Time for Complex Computations

The more complex the logic in the computed property, the longer the read time becomes. For properties that require intensive calculations, this can slow down the performance of your app if the property is accessed frequently.

If performance is critical and the result doesn’t change often, it might be worth storing the result rather than recalculating it each time. A hybrid approach, such as caching the value after the first calculation, might offer a balance between memory usage and computation time.

The Bottom Line: Choosing Between Stored and Computed Properties

When developing in Swift, knowing when to use stored properties vs. computed properties is crucial for writing efficient, maintainable, and optimized code. Each type of property serves distinct purposes, and understanding their differences helps in choosing the right one based on your use case.

When to Use Stored Properties

• Use stored properties when you need to hold and persist data directly in memory. These are ideal for simple and constant values like user settings, configuration data, or fixed attributes of an object that don’t require recalculation.

• Best for constant and frequently accessed values: Stored properties are fast to access since they are directly stored in memory, making them suitable when performance is a priority.

• Memory Considerations: Stored properties consume memory for every instance, which may be a concern when dealing with large data sets or high volumes of instances. If you need to store large datasets or expensive resources, consider using lazy stored properties to delay initialization until necessary.

When to Use Computed Properties

• Use computed properties when a value can be dynamically calculated rather than stored. They are ideal for derived values that are dependent on other properties, or when you need to enforce specific rules or calculations each time the property is accessed.

• Avoid Redundancy: Computed properties are excellent for avoiding redundant data storage. Instead of duplicating data across multiple properties, use a computed property to calculate values on the fly. This approach is more memory efficient and ensures consistency when values change.

• Performance Considerations: While computed properties reduce memory usage, they can introduce overhead when accessed frequently, especially if the computation is complex. If performance becomes an issue, consider caching results or converting the computed property into a stored property with a single calculation.

Balancing Memory and Performance

• For expensive properties to calculate but don’t change often, consider using lazy stored properties to delay initialization until needed.

• For values that depend on other properties and must be recalculated, use computed properties but be mindful of performance if the property is accessed often.

In essence, stored properties are best when you need to store values directly, while computed properties shine when you need to calculate values dynamically. By balancing the trade-offs of memory usage and computational overhead, you can choose the property type that best fits your application’s needs, ensuring efficient and readable Swift code.

struct Laptop {
    let brand: String // Constant stored property
    let year: Int
}

let macbook = Laptop(brand: "Apple", year: 2022)
print(macbook.brand) // Output: Apple

struct DataLoader {
    lazy var data: String = loadData() // Lazy stored property

    func loadData() -> String {
        return "Data loaded from the server."
    }
}

var loader = DataLoader()
print(loader.data) // Output: Data loaded from the server.

class Person {
    var name: String // Variable stored property
    var age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

var john = Person(name: "John", age: 30)
var reference1 = john
var reference2 = john

reference1.name = "John Doe" // Modifying the stored property
print(reference2.name) // Output: John Doe

var propertyName: PropertyType {
    get {
        // return the computed value
    }
    set(newValue) {
        // update other properties based on newValue
    }
}

struct Square {
    var side: Double

    // Computed property to get and set the perimeter
    var perimeter: Double {
        get {
            return 4 * side
        }
        set {
            side = newValue / 4
        }
    }
}

var square = Square(side: 4.0)
print(square.perimeter) // Output: 16.0
square.perimeter = 20
print(square.side) // Output: 5.0

struct Circle {
    var radius: Double

    // Read-only computed property to calculate the area
    var area: Double {
        return .pi * radius * radius
    }
}

let circle = Circle(radius: 5.0)
print(circle.area) // Output: 78.54

struct Point {
    var x: Double
    var y: Double

    // Computed property to get and set the distance from origin
    var distanceFromOrigin: Double {
        get {
            return sqrt(x * x + y * y)
        }
        set(newDistance) {
            let ratio = newDistance / distanceFromOrigin
            x *= ratio
            y *= ratio
        }
    }
}

var point = Point(x: 3.0, y: 4.0)
print(point.distanceFromOrigin) // Output: 5.0
point.distanceFromOrigin = 10.0
print(point.x) // Output: 6.0
print(point.y) // Output: 8.0

struct Person {
    var name: String // Variable stored property
    let birthYear: Int // Constant stored property
}

var person = Person(name: "Alice", birthYear: 1990)
print(person.name) // Output: Alice
person.name = "Bob" // The name can be changed because it's a variable stored property
// person.birthYear = 1991 // Error: Cannot assign to 'birthYear' because it's a constant stored property

struct Circle {
    var radius: Double

    // Computed property to calculate the area
    var area: Double {
        return .pi * radius * radius
    }
}

let circle = Circle(radius: 5.0)
print(circle.area) // Output: 78.54

struct Rectangle {
    var width: Double = 5.0 // Default value
    var height: Double
}

let rect = Rectangle(height: 10.0)
print(rect.width) // Output: 5.0

struct Rectangle {
    var width: Double
    var height: Double

    // Computed property to calculate the area
    var area: Double {
        return width * height
    }
}

let rect = Rectangle(width: 5.0, height: 10.0)
print(rect.area) // Output: 50.0

struct User {
    var name: String // Stored property for user name
    var age: Int // Stored property for user age
}

var user = User(name: "Alice", age: 30)
print(user.name) // Output: Alice

struct Car {
    let brand: String // Constant stored property
    let year: Int
}

let car = Car(brand: "Tesla", year: 2023)
print(car.brand) // Output: Tesla

struct Rectangle {
    var width: Double // Stored property
    var height: Double // Stored property
}

var rect = Rectangle(width: 10.0, height: 20.0)
print(rect.width) // Output: 10.0

struct Counter {
    var count: Int = 0 // Stored property with a default value
}

var counter = Counter()
print(counter.count) // Output: 0

class LargeObject {
    var name: String
    var description: String
    var data: [Int] = Array(repeating: 0, count: 1000000) // Large stored property
}

struct Book {
    var title: String // Stored property
    var pages: Int // Stored property
}

let book = Book(title: "Swift Programming", pages: 400)
print(book.pages) // Fast access to stored value

struct User {
    var name: String // Stored property
    var profilePicUrl: String // Stored property
}

let user = User(name: "Alice", profilePicUrl: "https://example.com/alice.jpg")
print(user.profilePicUrl) // Access without any computation

struct DataLoader {
    lazy var data: String = loadData() // Lazy stored property

    func loadData() -> String {
        return "Data loaded from the server."
    }
}

var loader = DataLoader() // Data is not loaded yet
print(loader.data) // Data is loaded only when accessed

struct Rectangle {
    var width: Double
    var height: Double

    // Computed property to calculate area dynamically
    var area: Double {
        return width * height
    }
}

let rect = Rectangle(width: 10.0, height: 5.0)
print(rect.area) // Output: 50.0

struct Circle {
    var radius: Double

    // Computed property to calculate circumference
    var circumference: Double {
        return 2 * .pi * radius
    }
}

let circle = Circle(radius: 3.0)
print(circle.circumference) // Output: 18.84

struct Salary {
    var basic: Double
    var bonus: Double

    // Computed property to calculate total salary
    var total: Double {
        return basic + bonus
    }
}

let salary = Salary(basic: 3000, bonus: 500)
print(salary.total) // Output: 3500.0

struct Square {
    var side: Double

    // Computed property to get and set the perimeter
    var perimeter: Double {
        get {
            return 4 * side
        }
        set {
            side = newValue / 4
        }
    }
}

var square = Square(side: 5.0)
print(square.perimeter) // Output: 20.0
square.perimeter = 16.0
print(square.side) // Output: 4.0

struct Data {
    var numbers: [Int]

    // Computed property that calculates the sum every time it's accessed
    var totalSum: Int {
        return numbers.reduce(0, +)
    }
}

let dataset = Data(numbers: Array(1...1000))
print(dataset.totalSum) // Calculation happens every time

struct TempConverter {
    var celsius: Double

    // Computed property for Fahrenheit
    var fahrenheit: Double {
        return celsius * 9 / 5 + 32
    }
}

let temp = TempConverter(celsius: 25.0)
print(temp.fahrenheit) // Output: 77.0

struct ExpensiveComputation {
    lazy var result: Int = performExpensiveTask() // Lazy stored property

    func performExpensiveTask() -> Int {
        // Simulate a time-consuming task
        return (1...1_000_000).reduce(0, +)
    }
}

let computation = ExpensiveComputation()
print(computation.result) // Task is performed only once