Swift-06.指针(UnsafePointer)

Swift中也有专门的指针类型，这些都被定性为“Unsafe”(不安全的)。

常见的有以下4种类型：

• UnsafePointer<Pointee>类似于 const Pointee *
• UnsafeMutablePointer<Pointee> 类似于 Pointee *
• UnsafeRawPointer 类似于 const void *
• UnsafeMutableRawPointer 类似于 void *

    var age = 10
    func test1(_ ptr: UnsafeMutablePointer<Int>) {
        ptr.pointee += 10 
    }
    func test2(_ ptr: UnsafePointer<Int>) { 
        print(ptr.pointee)
    }
    test1(&age)
    test2(&age) // 20
    print(age) // 20

    var age = 10
    func test3(_ ptr: UnsafeMutableRawPointer) {
        ptr.storeBytes(of: 20, as: Int.self) 
    }
    func test4(_ ptr: UnsafeRawPointer) { 
        print(ptr.load(as: Int.self))
    }
    test3(&age)
    test4(&age) // 20
    print(age) // 20

• 获得指向某个变量的指针:

var age = 11
var ptr1 = withUnsafeMutablePointer(to: &age) { $0 } 
var ptr2 = withUnsafePointer(to: &age) { $0 } 
ptr1.pointee = 22
print(ptr2.pointee) // 22
print(age) // 22

var ptr3 = withUnsafeMutablePointer(to: &age) { UnsafeMutableRawPointer($0) } 
var ptr4 = withUnsafePointer(to: &age) { UnsafeRawPointer($0) } 
ptr3.storeBytes(of: 33, as: Int.self)
print(ptr4.load(as: Int.self)) // 33
print(age) // 33

• 获得指向堆空间实例的指针：

// UnsafeRawPointer(bitPattern:)函数用来生成指向某一地址的指针
var ptr = UnsafeRawPointer(bitPattern: 0x100001234)

class Person {}
// 类似于 *p = person
var person = Person()
// p = &person， p是person变量的地址值
var ptr = withUnsafePointer(to: &person) { UnsafeRawPointer($0) }
// ptr.load(as: UInt.self) 类似于 *p， 取出地址中存储的值，也就是Person实例在堆上的地址值
var heapPtr = UnsafeRawPointer(bitPattern: ptr.load(as: UInt.self)) 
print(heapPtr!)

• 创建指针：
  • 非泛型

   // 创建
   var ptr = malloc(16)
   // 存
   ptr?.storeBytes(of: 11, as: Int.self)
   ptr?.storeBytes(of: 22, toByteOffset: 8, as: Int.self)
   // 取
   print((ptr?.load(as: Int.self))!) // 11 
   // fromByteOffset: 地址偏移
   print((ptr?.load(fromByteOffset: 8, as: Int.self))!) // 22 
   // 销毁
   free(ptr)
  
   var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1) 
   ptr.storeBytes(of: 11, as: Int.self)
   // advanced: 也是地址偏移
   ptr.advanced(by: 8).storeBytes(of: 22, as: Int.self) 
   print(ptr.load(as: Int.self)) // 11
   print(ptr.advanced(by: 8).load(as: Int.self)) // 22 
   // 销毁
   ptr.deallocate()
  

  • 泛型
    基础数据类型：

  var ptr = UnsafeMutablePointer<Int>.allocate(capacity: 3) 
   ptr.initialize(to: 11)
   // successor()后继, 表示向后走<Int>长度个字节
   ptr.successor().initialize(to: 22) 
   ptr.successor().successor().initialize(to: 33)
   print(ptr.pointee) // 11 
   // 这里的 + 指的是指针的加，步长为<Int>长度个字节
   print((ptr + 1).pointee) // 22 
   print((ptr + 2).pointee) // 33
  
   print(ptr[0]) // 11
   print(ptr[1]) // 22
   print(ptr[2]) // 33
   ptr.deinitialize(count: 3) 
   ptr.deallocate()
  

  对象类型：

   class Person {
       var age: Int
       var name: String
       init(age: Int, name: String) {
           self.age = age
           self.name = name 
       }
       deinit { print(name, "deinit") } 
   }
   var ptr = UnsafeMutablePointer<Person>.allocate(capacity: 3) 
   ptr.initialize(to: Person(age: 10, name: "Jack"))
   (ptr + 1).initialize(to: Person(age: 11, name: "Rose")) 
   (ptr + 2).initialize(to: Person(age: 12, name: "Kate"))
   // Jack deinit
   // Rose deinit
   // Kate deinit 
   ptr.deinitialize(count: 3) 
   ptr.deallocate()
  

• 指针之间的转换

   var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1) 
   ptr.assumingMemoryBound(to: Int.self).pointee = 11
   (ptr + 8).assumingMemoryBound(to: Double.self).pointee = 22.0 
  
   // 只改变类型，而不改变其值
   //（值指的是二进制数据，也就是你告诉计算机这是什么类型，计算机就按什么类型解读这些二进制数据）
   print(unsafeBitCast(ptr, to: UnsafePointer<Int>.self).pointee) // 11
   print(unsafeBitCast(ptr + 8, to: UnsafePointer<Double>.self).pointee) // 22.0 
   ptr.deallocate()
  
   // unsafeBitCast是忽略数据类型的强制转换，不会因为数据类型的变化而改变原来的内存数据 
   // 类似于C++中的reinterpret_cast
  
   class Person {}
   var person = Person()
   var ptr = unsafeBitCast(person, to: UnsafeRawPointer.self) 
   print(ptr)
  

• 关于指针的强制类型转换可参考这篇