Swift3.0 指针

2018-05-23

最近试着用Swift基于AudioQueue写一个音乐播放器，但是AudioQueue的API都是C函数，避免不了操作指针。经常卡在指针上，于是决定先静下心来学习一下Swift中的指针。baidu了很多，但是没有系统讲解Swift指针的文章，一边学一边记录，写下这些作为积累。这次学习分为四个部分:UnsafePointer, UnsafeMutablePointer, UnsafeRawPointer, UnsafeMutableRawPointer。有不对的地方欢迎指出！
在Swift中，指针由结构体 struct UnsafePointer<Pointee> 或 struct UnsafeMutablePointer<Pointee> 表示，但是不能通过”&”获取一个UnsafePointer 或 UnsafeMutablePointer的实例，只能作为inout参数使用。UnsafePointer相对于UnsafeMutablePointer，其所指向的地址的值是不可变的

一、UnsafeMutablePointer

被UnsafeMutablePointe引用的内存有三种状态：
\1. Not Allocated
\2. Allocated but not initialized
\3. Allocated and initialized
只有在状态3时，可以安全的使用pointee属性来set和get。

1.初始化

allocate() initialize() deinitialize deallocate() 这四个方法来管理这个指针，并确保指针的pointee不会出错

(1) allocate

1	let a_unsafe_mutable_pointer = UnsafeMutablePointer<Int>.allocate(capacity: 0)1

Tips:
\1. 需要指明要声明的指针类型

allocate是静态方法

(2) initialize

1
2
3

a_unsafe_mutable_pointer.initialize(to: 5)
//或
a_unsafe_mutable_pointer.pointee = 5123

这两种方法的结果是相同的

(3) deinitialize

1	a_unsafe_mutable_pointer.deinitialize(count: 1)1

该方法貌似没有真正deintialize，只是返回了一个指向相同地址的UnsafeMutableRawPointer,并且a_unsafe_mutable_pointer.pointee仍然是可以正常使用

(4) deallocate

1	a_unsafe_mutable_pointer.deallocate(capacity: 1)1

此时a_unsafe_mutable_pointer不可用了。

2.互相转化

(1) UnsafeMutablePointer -> Swift

1	let a : Int = a_unsafe_mutable_pointer.pointee1

(2) Swift -> UnsafeMutablePointer

1	let a : UnsafeMutablePointer = &a 1

以上代码是错误的，只能作为函数参数使用,如下所示

func printPointer(ptr:UnsafeMutablePointer<Int>) {
        print("UnsafeMutablePointer:\(ptr)")
        print("pointee:\(ptr.pointee)")
}
printPointer(ptr:&a)12345

3.暂时绑定

将该内存暂时绑定为一种类型，例子中将Int型绑定为Int8

a_unsafe_mutable_pointer.initialize(to: 0x0102030A)
a_unsafe_mutable_pointer.withMemoryRebound(to: Int8.self, capacity: 1, {
    ptr in
    print(String(ptr.pointee,radix:16)) //1
    print(String(ptr.advanced(by: 1).pointee,radix:16)) //2
    print(String(ptr.advanced(by: 2).pointee,radix:16)) //3
    print(String(ptr.advanced(by: 3).pointee,radix:16)) //a
})12345678

二、UnsafePointer

UnsafePointer中的pointee属性只能get不能set。
UnsafePointer中没有allocate方法。

1.初始化

1	let a_unsafe_pointer = UnsafePointer<Int>.init(a_unsafe_mutable_pointer)1

可以由UnsafeMutablePointer、OpaquePointer或其他UnsafePointer创建一个UnsafePointer指针。其他与UnsafeMutablePointer类似

2.互相转换

(1) UnsafeMutablePointer -> UnsafePointer

1	let a_unsafe_pointer = UnsafePointer<Int>.init(a_unsafe_mutable_pointer)1

(2) UnsafePointer -> UnsafeMutablePointer

1	let a_unsafe_mutable_pointer = UnsafePointer<Int>.init(mutating:a_unsafe_pointer)1

(3) UnsafePointer -> Swift

1	let a : Int = a_unsafe_pointer.pointee1

(4) Swift -> UnsafePointer

与UnsafeMutablePointer类似，只能作为函数参数引用

3.withUnsafePointer方法

withUnsafePointer方法可以直接使用指针,但是不能改变pointee的值

var a = 0
var b = a
a = withUnsafePointer(to: &a, {
        ptr in
        return ptr.pointee + 2 
        //此时，新开辟空间，令a指向该地址，其值为2
        //b仍指向a原来的地址，值为0
})

var _ = withUnsafePointer(to: &b, {
            ptr in
            let size = MemoryLayout<Int>.size //8
            //advance(by:Int)移动指针到a
            let movedPointer = ptr.advanced(by: -size) 
            print("b:\(ptr.pointee)") //b = 0
            print("a:\(movedPointer.pointee)") //a = 2
        })1234567891011121314151617

三、UnsafeMutableRawPointer

UnsafeMutableRawPointer按我的理解就是无类型的原始指针

1.分配内存

1
2
3

var size = MemoryLayout<Int>.size
//其中alignTo是开辟内存中的对齐，不是很了解，貌似会影响效率，太底层了，不太懂
var a_unsafe_mutable_raw_pointer = UnsafeMutableRawPointer.allocate(bytes: size, alignedTo: size)123

这时，这个a_unsafe_mutable_raw_pointer感觉没啥用，还是需要转换为UnsafeMutablePointer来对内存进行操作。

2.初始化

初始化内存为Int类型，并赋值为1

1	a_unsafe_mutable_raw_pointer.initializeMemory(as: Int.self, to: 1)1

3.绑定UnsafeMutablePointer

绑定了UnsafeMutablePointer,使用UnsafeMutablePointer才能对内存进行赋值,有两种方法绑定：
(1) bindMemory()
该方法绑定内存为指定类型并返回一个UnsafeMutablePointer<指定类型>的指针

1	var a_unsafe_mutable_pointer = a_unsafe_mutable_raw_pointer.bindMemory(to: Int.self, capacity: 1)1

(2) assumingMemoryBound()
该方法意思是直接转换这个原始指针为一个UnsafeMutablePointer<指定类型>的指针

1	var a_unsafe_mutable_pointer = a_unsafe_mutable_raw_pointer.assumingMemoryBound(to: Int.self)1

-
这样，就可以使用这个a_unsafe_mutable_pointer进行其他操作了。

4.deallocate

最后，需要释放内存，首先要deinitialize，再deallocate

1 2	a_unsafe_mutable_pointer.deinitialize() a_unsafe_mutable_raw_pointer.deallocate(bytes: size, alignedTo: size)12

5.转换

(1) Swift -> UnsafeMutableRawPointer

1 2	var a = 10 var ptr = UnsafeMutableRawPointer(&a)12

(2) UnsafeMutableRawPointer -> Swift
其过程应当为UnsafeMutableRawPointer转换为UnsafeMutablePointer，再由UnsafeMutablePointer转换为Swift指针

四、UnsafeRawPointer

UnsafeRawPointer只能由其他指针用init方法得到，与UnsafePointer类似，没有allocate静态方法。但是，与UnsafeMutableRawPointer类似的有两种绑定方法，绑定成UnsafePointer指针。