C语言的指针操作
在c语言中申明一个变量并通过指针修改该变量的值
int a = 1;
int *p = &a;
*p = 2;
printf("a value is %d\\\\n",a);
a value is 2
c语言操作结构体指针操作
- 申明一个叫User的结构体
typedef struct User{
int ID;
int age;
} User;
- 申明一个结构体User变量user 设置ID值为1 age值为18
User user;
user.ID = 1;
user.age = 3;
- 通过指针对变量user的值进行修改
//申明一个结构体指针变量userpointer指向user的地址
User *userpointer = &user;
//修改ID的值为2
int *idPointer = (int *)userpointer;
*idPointer = 2;
//获得属性age所在指针
int *agePointer = ++ idPointer;
//修改user的age值为4
*agePointer = 4;
printf("user ID = %d, age = %d", user.ID, user.age);
user ID = 2, age = 4
Swift
Swift并不推荐对指针进行直接操作, 但仍提供了几种可以直接操作内存的指针类型，以下是c与Swift的语法对应表 ，使用Type 做类型占用
对于返回值、变量和参数，使用一下对应表
| C 语法 | Swift 语法 |
|---|---|
| const Type * | UnsafePointer<Type> |
| Type * | UnsafeMutablePointer<Type> |
对于类，使用一下语法对应
| C 语法 | Swift 语法 |
|---|---|
| Type * const * | UnsafePointer<Type> |
| Type * __strong * | UnsafeMutablePointer<Type> |
| Type ** | AutoreleasingUnsafeMutablePointer<Type> |
在Swift中无类型的指针，原始内存可以用UnsafeRawPointer 和UnsafeMutableRawPointer来表示
如果像不完整结构体的这样的c指针的值的类型无法用Swift来表示，则用OpaquePointer来表示
更多指针相关知识详见文档
接下来，进行Swift版结构体指针操作
申明User结构体
struct User {
var ID: Int
var age: Int
}
申明变量，并获取变量地址, 创建一个user变量 ID初始值问为1 age初始值为3
通过指针设置ID值为2 设置age值为3
var user = User(ID: 1, age: 3)
let userPointer = withUnsafePointer(to: &user, {$0})//UnsafePointer<User>
//打印user指针的值
print(userPointer.pointee)
//User(ID: 1, age: 3)
//获取user ID的指针
let userIDPointer = unsafeBitCast(userPointer, to: UnsafeMutablePointer<Int>.self)
//设置ID的值为2
userIDPointer.pointee = 2
print(userPointer.pointee)
//User(ID: 2, age: 3)
//获取user age的指针
let agePointer = userIDPointer.advanced(by: 1)
agePointer.pointee = 4
print(userPointer.pointee)
//User(ID: 2, age: 4)
虽然c和Swift的语法差距有点大，但是原理是相同的
接下来我们看一个更复杂一点的例子
public struct Person {
var age: Int
var firstName: String
var lastName: String
var phoneNumber: PhoneNumber
}
public struct PhoneNumber {
var number: String
var type: String
}
创建person变量 并通过指针操作 获取对应的属性值
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")
到这里，我们已经学会了 通过指针获取值，通过指针设置对应的值
接下来 就可以运用这些知识发挥自己的创造力了
参考资料
结构体的内存空间分配原理
Swift 中的指针使用
