Swift 里的指针

Posted 2021-11-24 huahuahu

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基础知识

指针的内存状态

typed?	initiated?
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之前分配的内存可能被释放，使得指针指向了未被分配的内存。
有两种方式可以使得指针指向的内存处于Uninitialized状态：

1. 刚刚被分配内存
2. 内存被deinitialized

var bytes: [UInt8] = [39, 77, 111, 111, 102, 33, 39, 0]
let uint8Pointer = UnsafeMutablePointer<UInt8>.allocate(capacity: 8)

此时，uint8Pointer处于Uninitialized状态。

已经被初始化的内存，可以通过pointee属性或者下标方式访问。

let ptr: UnsafePointer<Int> = ...
// ptr.pointee == 23
// ptr[0] == 23

Typed Pointers

	指向内存能否改动	可否进行边界检查
UnsafePointer	?	?
UnsafeMutablePointer	?	?
UnsafeBufferPointer	?	?
UnsafeMutableBufferPointer	?	?

Raw Pointers

即指向的内存没有特定类型。

Use raw pointers and buffers to access memory for loading and storing as raw bytes.

• automated memory management ?
• type safety ?
• alignment guarantees ?

指针类型转换

前提条件

Memory that has been bound to a type can be rebound to a different type only after it has been deinitialized or if the bound type is a trivial type

• 内存已经被 deinitialized
• 内存指向的类型是 trivial 的

被 deinitialized 的地址，有三种出路：

1. 以相同类型被 reinitialized
2. bound to a new type
3. deallocated

typed pointer 状态转换

untrivial pointee

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trivial pointee

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相比之下，pointee 类型是 trivial 的，可以进行更多的操作。