结构体
-
在 Swift 标准库中,绝大多数的公开类型都是结构体,而枚举和类只占很小一部分
-
比如
、
、
、
、
、
等常见类型都是结构体
struct Date {
var year: Int
var month: Int
var day: Int
}
var date = Date(year: 2019, month: 6, day: 23)
-
所有的结构体都有一个编译器自动生成的初始化器(
,初始化方法、构造器、构造方法)
-
在第6行调用的,可以传入所有成员值,用以初始化所有成员(存储属性,Stored Property)
结构体的初始化器
- 编译器会根据情况,可能会为结构体生成多个初始化器,宗旨是:保证所有成员都有初始值
struct Point {
var x: Int
var y: Int
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(y: 10)
var p3 = Point(x: 10)
var p4 = Point()
报错如下图:需要保证都有值
![](https://img.haomeiwen.com/i1401554/91b2873b538977a6.png)
struct Point {
var x: Int = 0
var y: Int
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(y: 10)
var p3 = Point(x: 10)
var p4 = Point()
struct Point {
var x: Int
var y: Int = 0
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(y: 10)
var p3 = Point(x: 10)
var p4 = Point()
struct Point {
var x: Int = 0
var y: Int = 0
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(y: 10)
var p3 = Point(x: 10)
var p4 = Point()
// Tips:生成4种初始化器
思考:下面代码能编译通过吗?
struct Point {
var x: Int?
var y: Int?
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(y: 10)
var p3 = Point(x: 10)
var p4 = Point()
可选项都有个默认值nil
因此可以编译通过
自定义初始化器
- 一旦在定义结构体时自定义了初始化器,编译器就不会再帮它自动生成其他初始化器
struct Point {
var x: Int = 0
var y: Int
init(x: Int, y: Int) {
self.x = x
self.y = y
}
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(y: 10)
var p3 = Point(x: 10)
var p4 = Point()
窥探初始化器的本质
- 以下2段代码完全等效
struct Point {
var x: Int = 0
var y: Int = 0
}
var p = Point()
struct Point {
var x: Int
var y: Int
init() {
x = 0
y = 0
}
}
var p = Point()
推导过程:
-
自己写一个初始化器,断点看汇编。汇编断点 Xcode -> Debug Workflow -> Always show Disassembly
image.png
发现一模一样,所以推论正确。
![](https://img.haomeiwen.com/i1401554/0b9bd58994252ffc.png)
TestSwift`testStruct():
0x100002a10 <+0>: pushq %rbp
0x100002a11 <+1>: movq %rsp, %rbp
0x100002a14 <+4>: subq $0x10, %rsp
0x100002a18 <+8>: xorps %xmm0, %xmm0
0x100002a1b <+11>: movaps %xmm0, -0x10(%rbp)
-> 0x100002a1f <+15>: callq 0x100002a40 ; Point.init() -> Point in TestSwift.testStruct() -> () at main.swift:6
0x100002a24 <+20>: movq %rax, -0x10(%rbp)
0x100002a28 <+24>: movq %rdx, -0x8(%rbp)
0x100002a2c <+28>: addq $0x10, %rsp
0x100002a30 <+32>: popq %rbp
0x100002a31 <+33>: retq
结构体内存结构
struct Point {
var x: Int = 0
var y: Int = 0
var origin: Bool = false // 占1个字节
}
print(MemoryLayout<Point>.size) // 17
print(MemoryLayout<Point>.stride) // 24
print(MemoryLayout<Point>.alignment) // 8
![](https://img.haomeiwen.com/i1401554/f1abe7ed147d25d2.png)
类
- 类的定义和结构体类似,但编译器并没有为类自动生成可以传入成员值的初始化器
class Point { struct Point {
var x: Int = 0 var x: Int = 0
var y: Int = 0 var y: Int = 0
} }
let p1 = Point() 生成一个无参初始化器 let p1 = Point()
let p2 = Point(x: 10, y: 20) let p2 = Point(x: 10, y: 20)
let p3 = Point(x: 10) let p3 = Point(x: 10)
let p2 = Point(y: 20) let p2 = Point(y: 20)
class Point { Class 'Point' has no initializers
var x: Int
var y: Int
}
let p1 = Point() 'Point' cannot be constructed because it has no accessible initializers
类的初始化器
-
如果类的所有成员都在定义的时候指定了初始值,编译器会为类生成无参的初始化器
-
成员的初始化是在这个初始化器中完成的
下面2段代码是完全等效的:
class Point {
var x: Int = 10
var y: Int = 20
}
let p1 = Point()
class Point {
var x: Int
var y: Int
init() {
x = 10
y = 20
}
}
let p1 = Point()
结构体与类的本质区别
- 结构体是值类型(枚举也是值类型),类是引用类型(指针类型)
class Size { struct Point { func test() {
var width = 1 var x = 3 var size = Size()
var height = 2 var y = 4 var point = Point()
} } }
下图都是针对64bit环境 (size对象的内存地址 -> 指针变量地址 占8字节, size对象:如👉🏻图 占32个字节)
![](https://img.haomeiwen.com/i1401554/ef9db3145fc8949e.png)
推导过程:
0、了解基本调试命令
汇编模式下
-
fn + control + F7 : 指令单步执行,当遇到函数调用时会跳入函数内部。
-
fn + control + F6: 指令单独执行,当遇到函数调用时不会跳入函数内部。
多****线程****之间的切换:
-
control + shift + F7: 切换到当前线程,并执行单步指令。
-
control + shift + F6: 切换到当前线程,并跳转到函数调用的者的下一条指令。
1、怎么证明在堆空间,占空间?
调用了alloc malloc的在对空间。
2、先看一下结构体
![](https://img.haomeiwen.com/i1401554/50407ddcd235c264.png)
![](https://img.haomeiwen.com/i1401554/ab63d3f201abab48.png)
![](https://img.haomeiwen.com/i1401554/ca939fbf7d874d45.png)
发现根本没有调用任何alloc,malloc相关,在栈空间
3、看下类是否在堆空间
![](https://img.haomeiwen.com/i1401554/3607be82e48fae2d.png)
![](https://img.haomeiwen.com/i1401554/e061adbf7ae71806.png)
![](https://img.haomeiwen.com/i1401554/bfef3ba89d3af680.png)
![](https://img.haomeiwen.com/i1401554/509724a6a5d103dc.png)
![](https://img.haomeiwen.com/i1401554/0e00d619aa276483.png)
![](https://img.haomeiwen.com/i1401554/ca5b48adebd90d86.png)
此处和马哥说的感觉函数差点名字,我猜测应该是系统升级了?确实是有malloc分配空间。
func testClassAndStruct () {
class Size {
var width = 1
var height = 2
}
struct Point {
var x = 3
var y = 4
}
print("MemoryLayout<Size>.stride", MemoryLayout<Size>.stride)
print("MemoryLayout<Point>.stride", MemoryLayout<Point>.stride)
print("---------")
var size = Size()
print("size变量的地址", Mems.ptr(ofVal: &size))
print("size变量的内存", Mems.memStr(ofVal: &size))
print("size所指向的内存地址", Mems.ptr(ofRef: size))
print("size所指向的内存内容", Mems.memStr(ofRef: size))
print("---------")
var point = Point()
print("point变量的地址", Mems.ptr(ofVal: &point))
print("point变量的内存", Mems.ptr(ofVal: &point))
}
testClassAndStruct()
![](https://img.haomeiwen.com/i1401554/1b98e3b4a73e20e4.png)
无论Size对象存多少东西,指针变量占用都是8字节。 (64位的情况下)
对象的对空间申请过程
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在Swift中,创建类的实例对象,要向堆空间申请内存,大概流程如下
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Class.__allocating_init()
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libswiftCore.dylib:****swift_allocObject
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libswiftCore.dylib:****swift_slowAlloc
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libsystem_malloc.dylib:****malloc
-
-
通过class_getInstanceSize可以得知:类的对象至少需要占用多少内存
class Point {
var x = 11
var test = true
var y = 22
}
var p = Point()
class_getInstanceSize(type(of: p)) // 40
class_getInstanceSize(Point.self) // 40
- 在Mac、iOS中的malloc函数分配的内存大小总是16的倍数 (这个是系统内部的优化,不用太纠结)
func testClassAndStruct () {
let ptr = malloc(1)
print(malloc_size(ptr))
let ptr1 = malloc(17)
print(malloc_size(ptr1))
}
testClassAndStruct()
![](https://img.haomeiwen.com/i1401554/524a36d60fe3e6e2.png)
值类型
-
值类型赋值给var、let或者给函数传参,是直接将所有内容拷贝一份
-
类似于对文件进行copy、paste操作,产生了全新的文件副本。属于深拷贝(deep copy)
struct Point {
var x: Int
var y: Int
}
func test() {
var p1 = Point(x: 10, y: 20)
var p2 = p1
}
![](https://img.haomeiwen.com/i1401554/ca8534a1535c70ca.png)
用下面代码利用汇编来验证结论是否正确:
func testValueType() {
struct Point {
var x: Int
var y: Int
}
var p1 = Point(x: 10, y: 20)
var p2 = p1
p2.x = 11
p2.y = 22
print("123")
}
testValueType()
推导过程:
![](https://img.haomeiwen.com/i1401554/09babeb4eab38ccd.png)
1、分析上述代码
0xa -> 10 0x14-> 20 然后执行init方法
movl $0xa, %edi
movl $0x14, %esi
callq 0x100002a00 ; Point.init(x: Swift.Int, y: Swift.Int) -> Point in TestSwift.testValueType() -> () at main.swift:45
2、执行 si 进入callq 大概意思是把10 20 赋值 rax rdx
TestSwift`init(x:y:) in Point #1 in testValueType():
-> 0x100002a00 <+0>: pushq %rbp
0x100002a01 <+1>: movq %rsp, %rbp
0x100002a04 <+4>: movq %rsi, %rdx
0x100002a07 <+7>: movq %rdi, %rax
0x100002a0a <+10>: popq %rbp
0x100002a0b <+11>: retq
3、结合代码看汇报分析
movq %rax, -0x10(%rbp) // rbp - 0x10 == 0x1000(假设地址是这个低地址) p1的内存地址
movq %rdx, -0x8(%rbp) // rbp - 0x8 == 0x1008 (连续的地址值)
movq %rax, -0x20(%rbp) // rbp - 0x20 == p2的内存地址
movq %rdx, -0x18(%rbp) // rbp - 0x18 ==
movq $0xb, -0x20(%rbp) // 11 22 给了p2的内存
movq $0x16, -0x18(%rbp)
rax == 10
rdx == 20
值类型的赋值操作
var s1 = "Jack"
var s2 = s1
s2.append("_Rose")
print(s1) // Jack
print(s2) // Jack_Rose
var a1 = [1, 2, 3]
var a2 = a1
a2.append(4)
a1[0] = 2
print(a1) // [2, 2, 3]
print(a2) // [1, 2, 3, 4]
var d1 = ["max" : 10, "min" : 2]
var d2 = d1
d1["other"] = 7
d2["max"] = 12
print(d1) // ["other": 7, "max": 10, "min": 2]
print(d2) // ["max": 12, "min": 2]
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在Swift标准库中,为了提升性能,String、Array、Dictionary、Set采取了Copy On Write的技术
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比如仅当有“写”操作时,才会真正执行拷贝操作
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对于标准库值类型的赋值操作,Swift 能确保最佳性能,所有没必要为了保证最佳性能来避免赋值
-
建议:不需要修改的,尽量定义成 let
值类型的赋值操作
struct Point {
var x: Int
var y: Int
}
var p1 = Point(x: 10, y: 20)
p1 = Point(x: 11, y: 22)
![](https://img.haomeiwen.com/i1401554/ae31cb18b38ba7ee.png)
引用类型
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引用赋值给
、
或者给函数传参,是将内存地址拷贝一份
-
类似于制作一个文件的替身(快捷方式、链接),指向的是同一个文件。属于浅拷贝(shallow copy)
class Size {
var width: Int
var height: Int
init(width: Int, height: Int) {
self.width = width
self.height = height
}
}
func test() {
var s1 = Size(width: 10, height: 20)
var s2 = s1
}
s2.width = 11
s2.height = 22
// 请问s1.width和s1.height是多少?
![](https://img.haomeiwen.com/i1401554/da27a9b0e84496b9.png)
推导过程:此过程比较困难,先看下汇编的寄存器模块 00-汇编语言
1、了解下常用的指令
![](https://img.haomeiwen.com/i1401554/70c5c101de75420a.png)
2、推导分析
-
rax一般是函数返回的对象地址值
-
0x10(%rbp) 这里根据前面的判断是一个局部变量,结合代码推导出是 s1
-
0x60(%rbp) 这里根据前面的判断是一个局部变量,结合代码推导出是 s2
![](https://img.haomeiwen.com/i1401554/571d5218c910905a.png)
3、查看一下内存存放是否和猜想一致
-
断点处拿到rax 地址
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Xcode -> Debug -> Debug WorkFLow -> View Memory
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前8个字节放类信息地址
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引用计数
![](https://img.haomeiwen.com/i1401554/3f0b50a1f0a09d12.png)
4、查看一下11 22 哪里修改的 寻找立即数
![](https://img.haomeiwen.com/i1401554/869dd6243756c98f.png)
引用类型的赋值操作
class Size {
var width: Int
var height: Int
init(width: Int, height: Int) {
self.width = width
self.height = height
}
}
var s1 = Size(width: 10, height: 20)
s1 = Size(width: 11, height: 22) 指向了新的对象
![](https://img.haomeiwen.com/i1401554/8ba877208541fdd0.png)
值类型、引用类型的 let
struct Point {
var x: Int
var y: Int
}
class Size {
var width: Int
var height: Int
init(width: Int, height: Int) {
self.width = width
self.height = height
}
}
let p = Point(x: 10, y: 20)
p = Point(x: 11, y: 22)\
p.x = 33
p.y = 44
let s = Size(width: 10, height: 20)
s = Size(widht: 11, height: 22)
s.width = 33
s.height = 44
let str = "Jack"
str.append("_Rose")
let arr = [1, 2, 3]
arr[0] = 11
arr.append(4)
测试:
func testInstanceSize() {
class Point {
// 16
var x = 11 // 8
var test = true // 1
var y = 22 // 8
} // 33 40 48
let p = Point() // malloc
print(class_getInstanceSize(type(of: p)))
print(class_getInstanceSize(Point.self)) // [Point class] [p class]
print(Mems.size(ofRef: p))
}
嵌套类型
struct Poker {
enum Suit : Character {
case spades = "♠", hearts = "♥", diamonds = "♦", clubs = "♣"
}
enum Rank : Int {
case two = 2, three, four, five, six, seven, eight, nine, ten
case jack, queen, king, ace
}
}
print(Poker.Suit.hearts.rawValue)
var suit = Poker.Suit.spades
suit = .diamonds
var rank = Poker.Rank.five
rank = .king
枚举、结构体、类都可以定义方法
- 一般把定义在枚举、结构体、类内部的函数,叫做方法
class Size {
var width = 10
var height = 10
func show() {
print("width=\(width), height=\(height)")
}
}
let s = Size()
s.show() // width=10, height=10
struct Point {
var x = 10
var y = 10
func show() {
print("x=\(x), y=\(y)")
}
}
let p = Point()
p.show() // x=10, y=10
enum PokerFace : Character {
case spades = "♠", hearts = "♥", diamonds = "♦", clubs = "♣"
func show() {
print("face is \(rawValue)")
}
}
let pf = PokerFace.hearts
pf.show() // face is ♥
-
方法占用对象的内存么?
- 不占用
-
方法的本质就是函数
-
方法、函数都存放在代码段
思考:
以下结构体、类对象的内存结构是怎样的?
struct Point {
var x: Int
var b1: Bool
var b2: Bool
var y: Int
}
var p = Point(x: 10, b1: true, b2: true, y: 20)
class Size {
var width: Int
var b1: Bool
var b2: Bool
var height: Int
init(width: Int, b1: Bool, b2: Bool, height: Int) {
self.width = width
self.b1 = b1
self.b2 = b2
self.height = height
}
}
var s = Size(width: 10, b1: true, b2: true, height: 20)
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