一、Swift指针
1.Swift指针简介
swift中的指针分为两类
-
typed pointer
指定数据类型
指针,即UnsafePointer<T>
,其中T表示泛型 -
raw pointer
未指定数据类型
的指针(原生指针) ,即UnsafeRawPointer
swift与OC指针对比如下:
Swift | OC | 说明 |
---|---|---|
UnsafePointer<T> | const T * | 指针及所指向的内容都不可变 |
UnsafeMutablePointer<T> | T * | 指针及其所指向的内存内容均可变 |
UnsafeRawPointer | const void * | 指针指向不可变未知类型 |
UnsafeMutableRawPointer | void * | 指针指向可变未知类型 |
2.使用
2.1 raw pointer
注: 对于raw pointer,其内存管理是手动管理的,指针在使用完毕后需要手动释放
// 定义一个未知类型的的指针p,分配32字节大小的空间,8字节对齐
let p = UnsafeMutableRawPointer.allocate(byteCount: 32, alignment: 8)
// 存储数据
for i in 0..<4 {
p.storeBytes(of: i + 1, as: Int.self)
}
// 读取数据
for i in 0..<4 {
// 从首地址开始偏移读取
let value = p.load(fromByteOffset: i * 8, as: Int.self)
print("index: \(i) value: \(value)")
}
// 释放内存
p.deallocate()
//index: 0 value: 4
//index: 1 value: -5764607523034234880
//index: 2 value: 4461297666
//index: 3 value: 668503133614176
从运行结果中可以看到,这并不是我们想要的结果,这也是我们平常在使用的时候需要特别注意的。因为我们在读取的时候是从指针首地址进行不断的偏移读取的,但是存储的时候却都是存储在了首地址,所以存储的时候也要进行偏移。修改后的代码如下:
// 定义一个未知类型的的指针p,分配32字节大小的空间,8字节对齐
let p = UnsafeMutableRawPointer.allocate(byteCount: 32, alignment: 8)
// 存储数据
for i in 0..<4 {
// p.storeBytes(of: i + 1, as: Int.self)
// 修改后(每次存储的位置都有增加,也就是偏移)
p.advanced(by: i * 8).storeBytes(of: i + 1, as: Int.self)
}
// 读取数据
for i in 0..<4 {
// 从首地址开始偏移读取
let value = p.load(fromByteOffset: i * 8, as: Int.self)
print("index: \(i) value: \(value)")
}
// 释放内存
p.deallocate()
//index: 0 value: 1
//index: 1 value: 2
//index: 2 value: 3
//index: 3 value: 4
allocate 源码
/// Allocates uninitialized memory with the specified size and alignment.
///
/// You are in charge of managing the allocated memory. Be sure to deallocate
/// any memory that you manually allocate.
///
/// The allocated memory is not bound to any specific type and must be bound
/// before performing any typed operations. If you are using the memory for
/// a specific type, allocate memory using the
/// `UnsafeMutablePointer.allocate(capacity:)` static method instead.
///
/// - Parameters:
/// - byteCount: The number of bytes to allocate. `byteCount` must not be negative.
/// - alignment: The alignment of the new region of allocated memory, in
/// bytes.
/// - Returns: A pointer to a newly allocated region of memory. The memory is
/// allocated, but not initialized.
@inlinable
public static func allocate(
byteCount: Int, alignment: Int
) -> UnsafeMutableRawPointer {
// For any alignment <= _minAllocationAlignment, force alignment = 0.
// This forces the runtime's "aligned" allocation path so that
// deallocation does not require the original alignment.
//
// The runtime guarantees:
//
// align == 0 || align > _minAllocationAlignment:
// Runtime uses "aligned allocation".
//
// 0 < align <= _minAllocationAlignment:
// Runtime may use either malloc or "aligned allocation".
var alignment = alignment
if alignment <= _minAllocationAlignment() {
alignment = 0
}
return UnsafeMutableRawPointer(Builtin.allocRaw(
byteCount._builtinWordValue, alignment._builtinWordValue))
}
- 以指定的大小和对齐方式分配未初始化的内存
- 首先对对齐方式进行校验
- 然后调用
Builtin.allocRaw
方法进行分配内存 -
Builtin
是Swift
的标准模块,可以理解为调用(匹配)LLVM
中的方法
2.2 typed pointer
定义
/// Invokes the given closure with a pointer to the given argument.
///
/// The `withUnsafePointer(to:_:)` function is useful for calling Objective-C
/// APIs that take in parameters by const pointer.
///
/// The pointer argument to `body` is valid only during the execution of
/// `withUnsafePointer(to:_:)`. Do not store or return the pointer for later
/// use.
///
/// - Parameters:
/// - value: An instance to temporarily use via pointer.
/// - body: A closure that takes a pointer to `value` as its sole argument. If
/// the closure has a return value, that value is also used as the return
/// value of the `withUnsafePointer(to:_:)` function. The pointer argument
/// is valid only for the duration of the function's execution.
/// It is undefined behavior to try to mutate through the pointer argument
/// by converting it to `UnsafeMutablePointer` or any other mutable pointer
/// type. If you need to mutate the argument through the pointer, use
/// `withUnsafeMutablePointer(to:_:)` instead.
/// - Returns: The return value, if any, of the `body` closure.
@inlinable public func withUnsafePointer<T, Result>(to value: T, _ body: (UnsafePointer<T>) throws -> Result) rethrows -> Result
该函数一共两个参数:
- 第一个就是要获取其指针的变量
- 第二个是一个闭包,然后通过rethrows关键字重新抛出Result(也就是闭包表达式的返回值),闭包的参数和返回值都是泛型,关于这种写法可以缩写,详见后面的代码。
var a = 10
/**
通过Swift提供的简写的API,这里是尾随闭包的写法
返回值的类型是 UnsafePointer<Int>
*/
let p = withUnsafePointer(to: &a) { $0 }
print(p)
withUnsafePointer(to: &a) {
print($0)
}
// Declaration let p1:UnsafePointer<Int>
let p1 = withUnsafePointer(to: &a) { ptr in
return ptr
}
print(p1)
//0x00007ff7ba577d18
//0x00007ff7ba577d18
//0x00007ff7ba577d18
以上三种用法是我们最常用的三种方法,都能够打印出变量的指针。那么是否可以通过指针修改变量的值呢?下面我们就来研究一下:
通过指针获取变量值
要想改变值,首先就要能够访问到变量的值:
let p = withUnsafePointer(to: &a) { $0 }
print(p.pointee)
withUnsafePointer(to: &a) {
print($0.pointee)
}
let p1 = withUnsafePointer(to: &a) { ptr in
return ptr
}
print(p1.pointee)
let p2 = withUnsafePointer(to: &a) { ptr in
return ptr.pointee
}
print(p2)
//10
//10
//10
//10
通过指针修改变量值
如果使用的是withUnsafePointer是不能直接在闭包中修改指针的,但是我们可以通过间接的方式,通过返回值修改,给原来变量赋值的方式修改(其实这种方式很low)
a = withUnsafePointer(to: &a){ ptr in
return ptr.pointee + 2
}
print(a)
我们可以使用withUnsafeMutablePointer
,直接修改变量的值。
withUnsafeMutablePointer(to: &a){ ptr in
ptr.pointee += 2
}
还有另一种方式,就是通过创建指针的方式,这也是一种创建Type Pointer的方式:
// 创建一个指针,指针内存存储的是Int类型数据,开辟一个8*1字节大小的区域
let ptr = UnsafeMutablePointer<Int>.allocate(capacity: 1)
//初始化指针
ptr.initialize(to: a)
// 修改
ptr.pointee += 2
print(a)
print(ptr.pointee)
// 反初始化,与下面的代码成对调用,管理内存
ptr.deinitialize(count: 1)
// 释放内存
ptr.deallocate()
//10
//12
从这里我们可以看到,指针的值在修改后是变了的,但是原变量的值并没有改变。所以不能用于直接修改原变量。
3.实例
demo1
本案例是初始化一个指针,能够访问两个结构体实例对象。
首先定义一个结构体
struct Teacher {
var age = 18
var height = 1.65
}
下面我们通过三种方式访问指针中的结构体对象
- 通过下标访问
- 通过内存平移访问
- 通过successor()函数访问
// 分配两个Teacher大小空间的指针
let ptr = UnsafeMutablePointer<Teacher>.allocate(capacity: 2)
// 初始化第一个Teacher
ptr.initialize(to: Teacher())
// 初始化第二个Teacher
ptr.successor().initialize(to: Teacher(age: 20, height: 1.85))
// 错误的初始化方式,因为这是确定类型的指针,只需移动一步即移动整个类型大小的内存
//ptr.advanced(by: MemoryLayout<Teacher>.stride).initialize(to: Teacher(age: 20, height: 1.85))
// 通过下标访问
print(ptr[0])
print(ptr[1])
// 内存偏移
print(ptr.pointee)
print((ptr+1).pointee)
// successor
print(ptr.pointee)
print(ptr.successor().pointee)
// 反初始化,释放内存
ptr.deinitialize(count: 2)
ptr.deallocate()
//Teacher(age: 18, height: 1.65)
//Teacher(age: 20, height: 1.85)
//Teacher(age: 18, height: 1.65)
//Teacher(age: 20, height: 1.85)
//Teacher(age: 18, height: 1.65)
//Teacher(age: 20, height: 1.85)
demo2
下面我们就通过内存的方式,将实例对象绑定到我们自定义的HHObject
上。
struct HHObject{
var kind: UnsafeRawPointer
}
class Teachers {
var age: Int = 18
}
指针绑定:
/**
使用withUnsafeMutablePointer获取到的指针是UnsafeMutablePointer<T>类型
UnsafeMutablePointer<T>没有bindMemory方法
所以此处引入Unmanaged
*/
//let ptr = withUnsafeMutablePointer(to: &t) { $0 }
/**
Unmanaged 指定内存管理,类似于OC与CF交互时的所有权转换__bridge
Unmanaged 有两个函数:
- passUnretained:不增加引用计数,也就是不获得所有权
- passRetained: 增加引用计数,也就是可以获得所有权
以上两个函数,可以通过toOpaque函数返回一个
UnsafeMutableRawPointer 指针
*/
let ptr4 = Unmanaged.passUnretained(Teachers()).toOpaque()
//let ptr = Unmanaged.passRetained(t).toOpaque()
/**
bindMemory :将指针绑定到指定类型数据上
如果没有绑定则绑定
已绑定则重定向到指定类型上
*/
let h = ptr4.bindMemory(to: HHObject.self, capacity: 1)
print(h.pointee)
//HHObject(kind: 0x00007efc5e4d0e60)
demo3
在实际开发中,我们往往会调用其他人的api完成一些代码,在指针操作过程中,往往会因为类型不匹配造成传参问题,下面我们就来看一个例子:
首先我们定义一个打印指针的函数:
func printPointer(p: UnsafePointer<Int>) {
print(p)
print("end")
}
示例代码:
var tul = (10,20)
withUnsafeMutablePointer(to: &tul) { tulPtr in
printPointer(p: UnsafeRawPointer(tulPtr).assumingMemoryBound(to: Int.self))
}
//0x00007ff7b564cc60
//end
assumingMemoryBound
是假定内存绑定,目的是告诉编译器已经绑定过Int
类型了,不需要在检查内存绑定。
那么我们将元组换成结构体呢?我们此时想要获取结构体中的属性。
struct Test {
var a: Int = 10
var b: Int = 20
}
var t = Test()
withUnsafeMutablePointer(to: &t) { ptr in
printPointer(p: UnsafeRawPointer(ptr).assumingMemoryBound(to: Int.self))
}
//0x00007ff7b77b8c50
//end
那么如果我想想获取结构体中的属性呢?
withUnsafeMutablePointer(to: &t) { ptr in
// let strongRefPtr = withUnsafePointer(to: &ptr.pointee.b) { $0 }
// printPointer(p: strongRefPtr)
// let strongRefPtr = UnsafeRawPointer(ptr).advanced(by: MemoryLayout<Int>.stride)
let strongRefPtr = UnsafeRawPointer(ptr) - MemoryLayout<Test>.offset(of: \Test.b)!
printPointer(p: strongRefPtr.assumingMemoryBound(to: Int.self))
}
以上提供了三种方式实现访问结构体中的属性。
demo 4
使用withMemoryRebound
临时更改内存绑定类型,withMemoryRebound
的主要应用场景还是处理一些类型不匹配的场景,将内存绑定类型临时修改成想要的类型,在闭包里生效,不会修改原指针的内存绑定类型。
var age: UInt64 = 18
let ptr = withUnsafePointer(to: &age) { $0 }
// 临时更改内存绑定类型
ptr.withMemoryRebound(to: Int.self, capacity: 1) { (ptr) in
printPointer(p: ptr)
}
func printPointer(p: UnsafePointer<Int>) {
print(p)
print("end")
}
4.总结
至此我们对Swift中指针的分析基本就完事了,现在总结如下:
- Swift中的指针分为两种:
-
raw pointer
:未指定类型(原生)指针,即:UnsafeRawPointer
和unsafeMutableRawPointer
-
type pointer
:指定类型的指针,即:UnsafePointer<T>
和UnsafeMutablePointer<T>
- 指针的绑定主要有三种方式:
-
withMemoryRebound
:临时更改内存绑定类型 -
bingMemory(to: capacity:)
:更改内存绑定的类型,如果之前没有绑定,那么就是首次绑定,如果绑定过了,会被重新绑定为该类型 -
assumingMemoryBound
:假定内存绑定,这里就是告诉编译器,我就是这种类型,不要检查了(控制权由我们决定)
- 内存指针的操作都是危险的,使用时要慎重
二、内存管理
跟OC一样,Swift也是采取基于引用计数的ARC内存管理方案(针对堆空间),Swift的ARC中有3种引用。
1.弱引用
protocol Livable : AnyObject {}
class Person {}
weak var p0: Person?
weak var p1: AnyObject?
weak var p2: Livable?
unowned var p10: Person?
unowned var p11: AnyObject?
unowned var p12: Livable?
说明:
- weak、unowned只能用在类实例上面
- 因为weak可以设置为nil,所以必须用可选项
- 会发生改变,所以需要用var
2.循环引用
循环引用就是两个对象相互持有,无法释放会导致内存泄漏,和OC一样,所以重点看一下闭包的循环引用,其实也就和block一样
循环引用的解决:weak、unowned 都能解决循环引用的问题,unowned 要比 weak 少一些性能消耗,在生命周期中可能会变为 nil 的使用 weak,初始化赋值后再也不会变为 nil 的使用unowned
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