C語言的指針操作

在c語言中申明一個變量并通過指針修改該變量的值

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(jié)構(gòu)體指針操作

• 申明一個叫User的結(jié)構(gòu)體

typedef struct User{
    int ID;
    int age;
} User;

• 申明一個結(jié)構(gòu)體User變量user 設(shè)置ID值為1 age值為18

User user;
user.ID = 1;
user.age = 3;

• 通過指針對變量user的值進(jìn)行修改

//申明一個結(jié)構(gòu)體指針變量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所在指針
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并不推薦對指針進(jìn)行直接操作, 但仍提供了幾種可以直接操作內(nèi)存的指針類型闪幽，以下是c與Swift的語法對應(yīng)表 鲫凶，使用Type 做類型占用

對于返回值保檐、變量和參數(shù)笨觅，使用一下對應(yīng)表

對于類顶吮，使用一下語法對應(yīng)

在Swift中無類型的指針颁股，原始內(nèi)存可以用UnsafeRawPointer 和UnsafeMutableRawPointer來表示
如果像不完整結(jié)構(gòu)體的這樣的c指針的值的類型無法用Swift來表示惜索，則用OpaquePointer來表示

更多指針相關(guān)知識詳見文檔

接下來购笆，進(jìn)行Swift版結(jié)構(gòu)體指針操作

申明User結(jié)構(gòu)體

struct User {
    var ID: Int
    var age: Int
}

申明變量，并獲取變量地址, 創(chuàng)建一個user變量 ID初始值問為1 age初始值為3
通過指針設(shè)置ID值為2 設(shè)置age值為3

var user = User(ID: 1, age: 3)
let userPointer = withUnsafePointer(to: &user, {$0})//UnsafePointer<User>
//打印user指針的值
print(userPointer.pointee)
//User(ID: 1, age: 3)

//獲取user ID的指針
let userIDPointer = unsafeBitCast(userPointer, to: UnsafeMutablePointer<Int>.self)
//設(shè)置ID的值為2
userIDPointer.pointee = 2
print(userPointer.pointee)
//User(ID: 2, age: 3)

//獲取user age的指針
let agePointer = userIDPointer.advanced(by: 1)
agePointer.pointee = 4
print(userPointer.pointee)
//User(ID: 2, age: 4)

雖然c和Swift的語法差距有點(diǎn)大耘柱，但是原理是相同的

接下來我們看一個更復(fù)雜一點(diǎn)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 
}

創(chuàng)建person變量 并通過指針操作 獲取對應(yīng)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")

到這里如捅，我們已經(jīng)學(xué)會了 通過指針獲取值，通過指針設(shè)置對應(yīng)的值
接下來 就可以運(yùn)用這些知識發(fā)揮自己的創(chuàng)造力了

參考資料
結(jié)構(gòu)體的內(nèi)存空間分配原理
Swift 中的指針使用