Swift結構體指標操作

在c語言中申明一個變數並通過指標修改該變數的值

int a = 1;

int *p = &a;
*p = 2;

printf("a value is %d\\\\n",a);
a value is 2

c語言操作結構體指標操作

申明一個叫User的結構體

typedef struct User{
    int ID;
    int age;
} User;
申明一個結構體User變數user 設定ID值為1 age值為18

User user;
user.ID = 1;
user.age = 3;
通過指標對變數user的值進行修改

//申明一個結構體指標變數userpointer指向user的地址
User *userpointer = &user;

//修改ID的值為2
int  *idPointer = (int *)userpointer;
*idPointer = 2;

//獲得屬性age所在指標
int *agePointer = ++ idPointer;
//修改user的age值為4
*agePointer = 4;
printf("user ID = %d, age = %d", user.ID, user.age);
user ID = 2, age = 4

Swift

Swift並不推薦對指標進行直接操作,  但仍提供了幾種可以直接操作記憶體的指標類型，以下是c與Swift的文法對應表 ，使用 Type 做類型佔用

對於傳回值、變數和參數，使用一下對應表

C 文法 Swift 文法
const Type * UnsafePointer
Type * UnsafeMutablePointer
對於類，使用一下文法對應

C 文法 Swift 文法
Type const UnsafePointer
Type __strong UnsafeMutablePointer
Type ** AutoreleasingUnsafeMutablePointer
在Swift中無類型的指標，原始記憶體可以用UnsafeRawPointer 和UnsafeMutableRawPointer來表示

如果像不完整結構體的這樣的c指標的值的類型無法用Swift來表示，則用OpaquePointer來表示

更多指標相關知識詳見 文檔

接下來，進行Swift版結構體指標操作

申明User結構體

struct User {
    var ID: Int
    var age: Int
}
申明變數，並擷取變數地址, 建立一個user變數 ID初始值問為1 age初始值為3

通過指標設定ID值為2 設定age值為3

var user = User(ID: 1, age: 3)
let userPointer = withUnsafePointer(to: &user, {$0})//UnsafePointer

 
//列印user指標的值
print(userPointer.pointee)
//User(ID: 1, age: 3)

//擷取user ID的指標
let userIDPointer = unsafeBitCast(userPointer, to: UnsafeMutablePointer
 
  .self)
//設定ID的值為2
userIDPointer.pointee = 2
print(userPointer.pointee)
//User(ID: 2, age: 3)

//擷取user age的指標
let agePointer = userIDPointer.advanced(by: 1)
agePointer.pointee = 4
print(userPointer.pointee)
//User(ID: 2, age: 4)
 
雖然c和Swift的文法差距有點大，但是原理是相同的

接下來我們看一個更複雜一點的例子

public struct Person {
      var age: Int
      var firstName: String
      var lastName: String
      var phoneNumber: PhoneNumber
}
public struct PhoneNumber {
      var number: String
      var type: String
}
建立person變數 並通過指標操作 擷取對應的屬性值

let phone = PhoneNumber(number: "186xxxxxxxx", type: "work")
var person = Person(age: 24, firstName: "Bing", lastName: "lin", phoneNumber: phone)
let rawPointer = withUnsafePointer(to: &person, { UnsafeRawPointer($0)})

let age = rawPointer.load(fromByteOffset: 0, as: Int.self)
let firstName = rawPointer.load(fromByteOffset: 8, as: String.self)
let lastName  = rawPointer.load(fromByteOffset: 32, as: String.self)
let phoneInfo = rawPointer.load(fromByteOffset: 56, as: PhoneNumber.self)

print("age: \\\\(age) firstName: \\\\(firstName) lastName: \\\\(lastName) , phoneNumber: \\\\(phoneInfo)")
//age: 24 firstName: Bing lastName: lin , phoneNumber: PhoneNumber(number: "186xxxxxxxx", type: "work")
到這裡，我們已經學會了 通過指標擷取值，通過指標設定對應的值

接下來 就可以運用這些知識發揮自己的創造力了