请比较 Swift 中的结构体和类的首要差异？

在Swift中，结构体和类是两种常用的数据类型，它们有一些相似之处，但也有一些首要差异。下面是它们的首要差异：

值类型 vs 引证类型：
- 结构体是值类型，而类是引证类型[1]。
- 当结构体被赋值给一个新的变量或常量，或许作为函数参数传递时，它们的值会被仿制。这意味着每个副本都有自己的独立内存空间，对一个副本的修正不会影响其他副本[1]。
- 类是引证类型，当类的实例被赋值给一个新的变量或常量，或许作为函数参数传递时，实际上是将引证传递给了新的变量或常量。这意味着多个变量或常量能够引证同一个实例，对一个引证的修正会影响到其他引证[1]。
内存办理：
- 结构体的实例是在栈上分配内存的，它们的生命周期与其地点的作用域相同。当作用域结束时，内存会主动开释[2]。
- 类的实例是在堆上分配内存的，它们的生命周期不受作用域的束缚。需求手动办理内存，当没有任何引证指向一个实例时，内存会被主动开释，这个进程由ARC（主动引证计数）来办理[2]。
承继：
- 类支撑承继，能够经过承继一个基类来获得其特点和办法，并且能够重写基类的办法[2]。
- 结构体不支撑承继，不能承继其他结构体或类[2]。
默许初始化器：
- 结构领会主动生成一个默许的成员初始化器，用于初始化一切的存储特点[2]。
- 类不会主动生成默许的成员初始化器，需求手动界说初始化器来初始化一切的存储特点[2]。
扩展：
- 结构体和类都能够运用扩展来增加新的功能，包含特点、办法和初始化器等[2]。

综上所述，结构体和类在Swift中有一些重要的差异，包含值类型 vs 引证类型、内存办理、承继和默许初始化器等。根据具体的需求和场景，挑选适宜的数据类型能够进步代码的可读性和功能。

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Swift 的 Copy-on-Write 是怎样完成的

Swift的Copy-on-Write是一种资源办理技术，用于在可修正的资源上完成高效的“仿制”操作。在Swift中，有引证类型（类）和值类型（结构体、元组、枚举）。值类型具有仿制语义，这意味着当将值类型赋值给变量或作为参数传递给函数时（除非是inout参数），其底层数据将被仿制。你将拥有两个具有相同内容的值，但分配在两个不同的内存地址上。Swift规范库为某些值类型（如Array）完成了一组机制，其中只要在产生变异时才会仿制值，而且仅当它有多个引证时才会仿制，由于假如该值是仅有引证的，就不需求仿制，能够直接在引证上进行变异。因而，只是将Array赋值给变量或将其传递给函数并不必定意味着它会被仿制，这真正进步了功能[2]。

Copy-on-Write并不是值类型的默许行为，它是在Swift规范库中为某些类型（如Array和Collections）完成的。这意味着并非规范库中的每个值类型都具有这种行为。除非你自己完成，否则你创立的值类型也不具备Copy-on-Write行为[2]。

要完成自界说值类型的Copy-on-Write行为，你能够自己完成。下面是一个示例代码，展现了如何运用引证类型来完成泛型值类型T的Copy-on-Write。基本上，它是一个办理引证类型的包装器，假如该值不是仅有引证的，则回来一个新实例。否则，它只是变异引证类型的值[2]。

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WHY Swift not ObjC?

There are several reasons why Swift is often preferred over Objective-C for iOS and macOS development. Here are some key advantages of Swift:

Safety and Performance: Swift was designed with a focus on safety and performance. It includes modern features like optionals, type inference, and automatic memory management, which help reduce common programming errors and improve code stability [1]. Additionally, Swift’s compiler is optimized for performance, resulting in faster execution compared to Objective-C [2].
Simplicity and Readability: Swift has a more concise and expressive syntax compared to Objective-C. It eliminates many of the cumbersome and error-prone aspects of Objective-C, such as manual memory management and the need for header files. Swift’s syntax is designed to be more readable and easier to understand, making it more approachable for beginners and more productive for experienced developers [1].
Interoperability: Swift is fully interoperable with Objective-C, allowing developers to use existing Objective-C code and frameworks in their Swift projects. This makes it easier to adopt Swift gradually and leverage the extensive ecosystem of Objective-C libraries and tools [1].
Active Development and Community Support: Swift is an open-source language with an active and growing community. It is continuously evolving, with regular updates and improvements from Apple and the community. This ensures that Swift remains modern, efficient, and well-supported for future development [2].
Playgrounds and Interactive Development: Swift’s Playgrounds feature allows developers to experiment, prototype, and test code in an interactive and visual environment. This makes learning and exploring Swift concepts more engaging and efficient [2].
Platform Compatibility: While Objective-C is primarily used for Apple platforms, Swift is designed to be cross-platform. It can be used not only for iOS and macOS development but also for server-side programming and Linux applications [1].

In summary, Swift offers a safer, more modern, and more efficient alternative to Objective-C. Its simplicity, readability, and interoperability make it an attractive choice for iOS and macOS development.

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if a struct is an immutable value type, why we can mutate the size property?

In Swift, structs are generally considered to be immutable types because they are copied on mutation. However, it is possible to mutate properties of a struct instance, including the size property, even if the struct is declared as a constant (let).

Here’s why we can mutate the size property in an immutable struct:

Copy-on-Write (COW) Optimization: When a struct is assigned to a new variable or passed as a function argument, a copy of the struct is made. However, the actual copying of the struct’s data is deferred until a mutation is attempted. This optimization is known as Copy-on-Write (COW) [1].
In-Place Mutation: In many cases, the Swift compiler optimizes the mutation of a struct’s property by performing the mutation in-place, without creating a new copy of the entire struct. This means that the original struct instance is modified directly, even if it is declared as a constant (let). However, logically, you should still consider the struct as immutable because the mutation is not observable from outside the struct [1].
let vs var: The ability to mutate properties of a struct instance is not related to whether the struct is declared as a constant (let) or a variable (var). Both let and var allow you to mutate the properties of a struct instance. The difference between let and var is that let declares the struct instance itself as a constant, meaning you cannot reassign a new value to the struct instance, while var allows reassignment of the struct instance [1].

In summary, while structs in Swift are generally considered to be immutable types, the ability to mutate properties of a struct instance, including the size property, is possible due to the Copy-on-Write optimization and in-place mutation. However, it is important to remember that the mutation is not observable from outside the struct, and the struct should still be treated as immutable.

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What are the basic categories of types in Swift?

In Swift, there are two basic categories of types: named types and compound types.

Named Types:
- Named types are types that can be given a specific name when they are defined.
- Examples of named types in Swift include:
  - Classes: Classes are reference types that can have properties, methods, and inheritance.
  - Structures: Structures are value types that can have properties and methods. They are typically used for simpler data structures.
  - Enumerations: Enumerations define a group of related values and can have associated values and methods.
  - Protocols: Protocols define a set of methods, properties, and other requirements that a type must conform to.
Compound Types:
- Compound types are types that are composed of multiple values.
- Examples of compound types in Swift include:
  - Tuples: Tuples allow you to group multiple values of different types into a single compound value.
  - Functions: Functions are a type that can take parameters and return a value.
  - Closures: Closures are self-contained blocks of functionality that can be passed around and used in your code.

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What is a Tuple in Swift?

A tuple in Swift is a lightweight data structure that allows you to group multiple values together into a single compound value. It is a convenient way to store and pass around related values without defining a custom struct or class.

Here are some key features of tuples in Swift:

Multiple Values: Tuples can contain two or more values of any type, including different types. For example, a tuple can hold a combination of integers, strings, booleans, or even other tuples.
Type Safety: Each value within a tuple can have its own type, and the types of the values are enforced by the Swift compiler. This ensures type safety and helps catch any type mismatches at compile-time.
Accessing Values: You can access the individual values within a tuple using dot syntax followed by the index or name of the value. For example, if you have a tuple (name: String, age: Int), you can access the name value using tupleName.name and the age value using tupleName.age.
Function Return Values: Tuples are commonly used as return types for functions when you need to return multiple values. This allows you to conveniently return multiple values without defining a custom struct or class.
Pattern Matching: Tuples can be used in pattern matching to extract values and perform conditional logic based on the values. This is particularly useful when working with functions that return tuples or when using switch statements.

Here’s an example of creating and using a tuple in Swift:

let person = (name: "John", age: 30, isEmployed: true)
print(person.name)        // Output: John
print(person.age)         // Output: 30
print(person.isEmployed)  // Output: true

In the example above, we create a tuple person with three values: name, age, and isEmployed. We can access each value using dot syntax.

Tuples are a flexible and lightweight way to group related values together in Swift, providing a convenient alternative to defining custom data structures.

String 与 NSString 的联系与差异：

String 和 NSString 是在不同的编程语言中表明字符串的类型。String 是 Swift 语言中的字符串类型，而 NSString 是 Objective-C 语言中的字符串类型。它们之间有一些差异和联系。

差异：

可变性：String 是不可变的，即一旦创立就不能修正其内容。而 NSString 是可变的，能够经过 NSMutableString 类来修正其内容。
语法：String 运用双引号（”）来表明字符串，而 NSString 运用@”来表明字符串。
类型揣度：String 能够运用类型揣度，不需求显式声明变量的类型。而 NSString 需求显式声明变量的类型为 NSString。
办法和特点：String 和 NSString 有一些相同的办法和特点，但也有一些不同的办法和特点。例如，String 有一个 count 特点来获取字符串的字符数，而 NSString 则有一个 length 办法来获取字符串的字符数。

联系：

桥接：String 和 NSString 之间能够进行桥接，即能够彼此转化。能够运用 as 关键字将 String 转化为 NSString，也能够运用 as? 或 as! 进行可选类型转化。同样，能够运用 as 关键字将 NSString 转化为 String。
同享基础完成：String 和 NSString 在底层同享相同的字符串表明办法，即 Unicode 编码。因而，它们能够彼此转化而不会导致数据丢失。

综上所述，String 和 NSString 是不同编程语言中表明字符串的类型，它们有一些差异和不同的语法，但也能够彼此转化并同享相同的字符串表明办法。

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defer 使⽤场景：

iOS中的defer句子能够在即将脱离当时代码块时履行一系列句子，用于履行一些必要的整理作业。下面是一些常见的iOS中运用defer的场景：

文件操作：在翻开文件后，能够运用defer句子来保证文件在脱离当时代码块时被封闭，以防止忘掉封闭文件或产生过错而导致文件未能正常封闭[1]。

func readFile() {
    let file = open("file.txt")
    defer {
        close(file)
    }
    // 读取文件内容
}

内存办理：在手动分配内存的情况下，能够运用defer句子来保证在脱离当时代码块时开释内存，以防止忘掉开释内存或产生过错而导致内存走漏[1]。

func allocateMemory() {
    let pointer = UnsafeMutablePointer<Int>.allocate(capacity: 1)
    defer {
        pointer.deallocate()
    }
    // 运用指针进行操作
}

加锁/解锁：在运用锁进行临界区操作时，能够运用defer句子来保证在脱离当时代码块时解锁，以防止忘掉解锁或产生过错而导致死锁[1]。

func performCriticalTask() {
    objc_sync_enter(lock)
    defer {
        objc_sync_exit(lock)
    }
    // 履行临界区操作
}

调用completion block：在某些情况下，一个函数可能有多个分支，可能会忘掉调用completion block，运用defer句子能够保证在脱离当时代码块时调用completion block，以防止潜在的bug[1]。

func performTask(completion: () -> Void) {
    defer {
        completion()
    }
    // 履行任务
}

调用super办法：在重写父类办法时，能够运用defer句子来保证在脱离当时代码块之前调用super办法，以便在super办法之前进行一些准备作业[1]。

override func viewDidLoad() {
    defer {
        super.viewDidLoad()
    }
    // 准备作业
}

这些是一些常见的iOS中运用defer的场景，它们能够帮助咱们在脱离当时代码块时履行必要的整理作业，进步代码的可读性和可维护性。

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iOS struct 在堆区仍是栈区？

在iOS中，struct（结构体）一般是分配在栈区（stack）而不是堆区（heap）[1]。栈区是一种后进先出（LIFO）的数据结构，用于存储局部变量和函数调用的上下文。下面是关于iOS中struct在栈区的一些特点：

栈区分配：
- 当一个函数被调用时，函数内的局部变量和参数会被分配在栈区。
- 结构体实例作为局部变量时，会被分配在栈区。
- 栈区的分配和开释是主动进行的，不需求手动办理内存。
栈区的特点：
- 栈区的拜访速度非常快，由于它是线性的数据结构，能够直接拜访最新分配的内存。
- 栈区的大小是有限的，一般比堆区小。
- 栈区的内存分配和开释是主动的，不需求手动办理内存。

需求注意的是，假如结构体包含引证类型的特点（如类实例），那么这些引证类型的特点实际上是存储在堆区的，而结构体本身仍然是分配在栈区的[1]。

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特点中，值特点，和核算特点有什么差异？

值特点（Stored Properties）和核算特点（Computed Properties）是iOS开发中特点的两种不同类型，它们有以下差异：

存储办法：值特点是直接存储值的特点，它们在内存中有自己的存储空间。而核算特点不直接存储值，它们经过核算或其他逻辑来获取值。
存储方位：值特点一般存储在实例的内存中或许类的静态存储区中。而核算特点没有自己的存储方位，它们的值是经过核算得到的。
拜访办法：值特点的值能够直接读取和设置，类似于一般的变量。而核算特点的值是经过核算得到的，不能直接设置，只能经过核算逻辑来获取。
依赖联系：核算特点能够依赖于其他特点的值进行核算。当依赖的特点产生变化时，核算特点会从头核算并回来新的值。而值特点的值是固定的，不会因其他特点的变化而改变。

下面是一个示例，展现了值特点和核算特点的差异：

struct Circle {
    var radius: Double // 值特点
    var area: Double { // 核算特点
        return Double.pi * radius * radius
    }
}
var circle = Circle(radius: 5.0)
print(circle.area) // 输出: 78.53981633974483
circle.radius = 7.0
print(circle.area) // 输出: 153.93804002589985

在上面的示例中，radius 是值特点，它直接存储了圆的半径。area 是核算特点，它经过核算 radius 的平方乘以来获取圆的面积。当修正 radius 的值时，area 会从头核算并回来新的面积值。

总结来说，值特点是直接存储值的特点，而核算特点是经过核算或其他逻辑来获取值的特点。挑选运用哪种特点类型取决于特点的特性和需求。

声明⼀个只要⼀个参数没有回来值闭包的别号

在iOS开发中，声明一个只要一个参数没有回来值的闭包的别号能够运用typealias关键字。以下是一个示例：

typealias SimpleClosure = (Int) -> Void
func performClosure(closure: SimpleClosure) {
    closure(10)
}
let myClosure: SimpleClosure = { number in
    print("Number: (number)")
}
performClosure(closure: myClosure) // 输出: Number: 10

在上面的示例中，咱们运用typealias关键字创立了一个名为SimpleClosure的闭包类型别号，它承受一个Int类型的参数并且没有回来值。然后，咱们声明了一个名为myClosure的闭包变量，它契合SimpleClosure的类型界说。最终，咱们经过调用performClosure函数并传递myClosure闭包来履行闭包。

经过运用闭包类型别号，咱们能够更方便地声明和运用具有相同类型的闭包变量或参数。

什么是泛型，swift哪些地方运用了泛型？

泛型（Generics）是一种在编程中运用的技术，它答应咱们编写通用且可重用的代码，防止重复。经过运用泛型，咱们能够创立能够适用于不同数据类型的函数、类和结构体。

在Swift中，泛型被广泛应用于以下几个方面：

泛型函数（Generic Functions）：咱们能够创立能够适用于任何数据类型的函数。经过在函数界说中运用类型参数（type parameter），咱们能够在函数内部运用这些参数来处理不同类型的数据。例如，咱们能够创立一个泛型函数来打印恣意类型的数据 [2]。

func displayData<T>(data: T) {
  print("Data Passed:", data)
}
// 运用泛型函数打印不同类型的数据
displayData(data: "Swift") // 输出: Data Passed: Swift
displayData(data: 5) // 输出: Data Passed: 5

泛型类（Generic Classes）：类也能够是泛型的，这意味着咱们能够创立一个能够适用于不同类型数据的类。经过在类界说中运用类型参数，咱们能够在类内部运用这些参数来界说特点、办法等。例如，咱们能够创立一个泛型类来存储恣意类型的数据 [2]。

class Information<T> {
  var data: T
  init(data: T) {
    self.data = data
  }
  func getData() -> T {
    return self.data
  }
}
// 运用泛型类存储不同类型的数据
var intObj = Information<Int>(data: 6)
print(intObj.getData()) // 输出: 6
var strObj = Information<String>(data: "Swift")
print(strObj.getData()) // 输出: Swift

泛型束缚（Generic Constraints）：有时候，咱们希望泛型只能承受特定类型的数据，而不是恣意类型。在这种情况下，咱们能够运用泛型束缚来束缚泛型的类型范围。例如，咱们能够创立一个泛型函数，只承受契合特定协议的数据类型 [2]。

func addition<T: Numeric>(num1: T, num2: T) {
  print("Sum:", num1 + num2)
}
// 运用泛型函数进行加法运算
addition(num1: 5, num2: 10) // 输出: Sum: 15
addition(num1: 5.5, num2: 10.8) // 输出: Sum: 16.3

在上述示例中，咱们运用了Numeric协议作为泛型束缚，这意味着泛型函数只能承受契合Numeric协议的数据类型（如Int、Double等）。

泛型在Swift中的应用使得咱们能够编写更加通用和灵敏的代码，进步了代码的重用性和可维护性。

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请解释 Swift 中的泛型是什么？并描述一下它的实质？

泛型（Generics）是 Swift 中的一种特性，它答应咱们编写通用且可重用的代码，防止重复。泛型类型或函数在当时作用域中创立了束缚，要求输入值契合这些要求。

泛型的实质是参数化类型，它答应咱们在编写代码时不指定具体的类型，而是运用占位符来表明类型。这样一来，咱们能够编写一次代码，适用于多种类型，进步了代码的灵敏性和可重用性。

运用泛型能够使现有代码更具可重用性。一个经典的比如是运用一个元素的仓库，也就是 Swift 中的数组。在没有运用泛型的情况下，咱们可能会创立一个只支撑整数的仓库（IntStack），当咱们需求一个字符串的仓库时，就需求创立一个新的仓库（StringStack），这样会导致代码重复和维护多个类型的问题。而运用泛型，咱们能够界说一个通用的仓库（Stack），其中的元素类型由泛型参数 Element 决议，然后防止了代码重复和多个类型的维护。

除了在类型级别上运用泛型，咱们还能够在办法中运用泛型。例如，咱们能够编写一个打印办法，该办法承受一个契合 CustomStringConvertible 协议的元素，并将其转化为字符串后打印出来。经过运用泛型，咱们能够在办法界说中运用类型参数 T，并对 T 增加束缚，要求它契合 CustomStringConvertible 协议。

泛型还能够作为回来类型运用。例如，咱们能够编写一个将恣意元素转化为数组的办法，该办法的回来类型是一个泛型数组。经过运用泛型回来类型，咱们能够在调用该办法时根据实际情况确认回来的数组类型。

在 Swift 中，咱们还能够运用相关类型和协议扩展来创立带有束缚的泛型代码。经过在协议扩展中运用相关类型，咱们能够对特定类型的数组增加特定的办法或特点。

总之，泛型答应咱们编写可重用的代码，防止重复，并进步代码的灵敏性和可维护性。但是，运用泛型需求必定的思维办法来将函数泛化，使其具有通用性。在编写代码时，应该一直从强类型的角度动身，只要在的确需求泛型代码提供的灵敏性时才运用泛型。

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What’s the difference between`opaque type`and`generic`?

In Swift, both opaque types and generics are used to provide type abstraction and flexibility. However, there are some key differences between the two.

Generics:

Generics allow the code that calls a function to pick the type for that function’s parameters and return value in a way that’s abstracted away from the function implementation [1].
With generics, the type information is preserved and can be accessed and manipulated by the caller of the function.
Generics are useful when you want to write code that can work with different types without sacrificing type safety.
Generic functions and types are defined using type parameters, which are placeholders for actual types that will be provided when the code is used.
The type parameters can be used throughout the function or type to define the behavior and constraints.

Opaque Types:

Opaque types, on the other hand, are used when you want to hide the underlying type and provide a consistent interface without exposing the implementation details [1].
Opaque types are the reverse of generics. Instead of allowing the caller to choose the type, the implementation of the function or type chooses the type and returns an opaque type to the caller.
The opaque type hides the actual type and only exposes the interface defined by a protocol or a concrete type.
Opaque types are useful when you want to provide an abstract interface without revealing the implementation details or when you want to optimize performance by using concrete types instead of protocols [2].
Opaque types are defined using the some keyword followed by a protocol or a concrete type. The actual type is determined by the implementation and is not visible to the caller.

Summary:

Generics allow the caller to choose the type, while opaque types hide the underlying type and provide a consistent interface.
Generics preserve type information and allow type manipulation, while opaque types hide the type information and only expose the interface.
Generics are useful for writing code that works with different types, while opaque types are useful for providing abstract interfaces and optimizing performance.

Learn more:

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作者信息

Swift/ObjC 语言基础

请比较 Swift 中的结构体和类的首要差异？

Swift 的 Copy-on-Write 是怎样完成的

WHY Swift not ObjC?

if a struct is an immutable value type, why we can mutate the size property?

What are the basic categories of types in Swift?

What is a Tuple in Swift?

String 与 NSString 的联系与差异：

defer 使⽤场景：

iOS struct 在堆区仍是栈区？

特点中，值特点，和核算特点有什么差异？

声明⼀个只要⼀个参数没有回来值闭包的别号

什么是泛型，swift哪些地方运用了泛型？

请解释 Swift 中的泛型是什么？并描述一下它的实质？

What’s the difference betweenopaque typeandgeneric?

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What’s the difference between`opaque type`and`generic`?