mutating：只用在值类型中

struct LGStack {
    var items = [Int]()
    func push(_ item: Int) {
        items.append(item)
    }
}

对于结构体而言，我创建了一个items的数组，并且给定LGStack一个push方法，向items数组内append元素，这个时候，会提示错误，很显然不允许我修改items！

截屏2021-01-11 下午6.57.52.png

打开项目文件夹目录

cd /Users/zhou/Desktop/SwiftTwoPractice/SwiftTwoPractice

把 mian.swift编译成main.sil并打开（推荐使用vs code）

swiftc -emit-sil main.swift | xcrun swift-demangle >> ./main.sil && open main.sil
No application knows how to open /Users/zhou/Desktop/SwiftTwoPractice/SwiftTwoPractice/main.sil.

通过main.si可以看到为什么：

// LGStack.push(_:)
sil hidden @main.LGStack.push(Swift.Int) -> () : $@convention(method) (Int, @guaranteed LGStack) -> () {
// %0 "item"                                      // users: %11, %2
// %1 "self"                                      // user: %3
bb0(%0 : $Int, %1 : $LGStack):
  debug_value %0 : $Int, let, name "item", argno 1 // id: %2
  debug_value %1 : $LGStack, let, name "self", argno 2 // id: %3
  %4 = integer_literal $Builtin.Word, 1           // user: %6
  // function_ref _allocateUninitializedArray<A>(_:)
  %5 = function_ref @Swift._allocateUninitializedArray<A>(Builtin.Word) -> ([A], Builtin.RawPointer) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // user: %6
  %6 = apply %5<Any>(%4) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // users: %8, %7
  %7 = tuple_extract %6 : $(Array<Any>, Builtin.RawPointer), 0 // users: %20, %17
  %8 = tuple_extract %6 : $(Array<Any>, Builtin.RawPointer), 1 // user: %9
  %9 = pointer_to_address %8 : $Builtin.RawPointer to [strict] $*Any // user: %10
  %10 = init_existential_addr %9 : $*Any, $Int    // user: %11
  store %0 to %10 : $*Int                         // id: %11
  // function_ref default argument 1 of print(_:separator:terminator:)
  %12 = function_ref @default argument 1 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: %13
  %13 = apply %12() : $@convention(thin) () -> @owned String // users: %19, %17
  // function_ref default argument 2 of print(_:separator:terminator:)
  %14 = function_ref @default argument 2 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: %15
  %15 = apply %14() : $@convention(thin) () -> @owned String // users: %18, %17
  // function_ref print(_:separator:terminator:)
  %16 = function_ref @Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> () // user: %17
  %17 = apply %16(%7, %13, %15) : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> ()
  release_value %15 : $String                     // id: %18
  release_value %13 : $String                     // id: %19
  release_value %7 : $Array<Any>                  // id: %20
  %21 = tuple ()                                  // user: %22
  return %21 : $()                                // id: %22
} // end sil function 'main.LGStack.push(Swift.Int) -> ()'

debug_value %1 : $LGStack, let, name "self", argno 2 // id: %3
首先push方法提供默认的参数self，这也是我可以在push方法内访问到items的原因，那么self 就是 LGStack， 是用let进行修饰的，也就是说，是不可变的，这也就是报错的原因

也就是说我在修改items的时候，就是在修改LGStack本身，而且LGStack还是一个let，所以不允许！

通过mutating修饰之后

struct LGStack {
    var items = [Int]()
    mutating func push(_ item: Int) {
        print(item)
        items.append(item)
    }
}

var s = LGStack()
s.push(1)
print(s.items)

lldb打印结果为

1
(lldb) 

那么我在sil面看一下，mutating到底做了什么：

// LGStack.push(_:)
sil hidden @main.LGStack.push(Swift.Int) -> () : $@convention(method) (Int, @inout LGStack) -> () {
// %0 "item"                                      // users: %22, %11, %2
// %1 "self"                                      // users: %23, %3
bb0(%0 : $Int, %1 : $*LGStack):
  debug_value %0 : $Int, let, name "item", argno 1 // id: %2
  debug_value_addr %1 : $*LGStack, var, name "self", argno 2 // id: %3
  %4 = integer_literal $Builtin.Word, 1           // user: %6
  // function_ref _allocateUninitializedArray<A>(_:)
  %5 = function_ref @Swift._allocateUninitializedArray<A>(Builtin.Word) -> ([A], Builtin.RawPointer) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // user: %6
  %6 = apply %5<Any>(%4) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // users: %8, %7
  %7 = tuple_extract %6 : $(Array<Any>, Builtin.RawPointer), 0 // users: %20, %17
  %8 = tuple_extract %6 : $(Array<Any>, Builtin.RawPointer), 1 // user: %9
  %9 = pointer_to_address %8 : $Builtin.RawPointer to [strict] $*Any // user: %10
  %10 = init_existential_addr %9 : $*Any, $Int    // user: %11
  store %0 to %10 : $*Int                         // id: %11
  // function_ref default argument 1 of print(_:separator:terminator:)
  %12 = function_ref @default argument 1 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: %13
  %13 = apply %12() : $@convention(thin) () -> @owned String // users: %19, %17
  // function_ref default argument 2 of print(_:separator:terminator:)
  %14 = function_ref @default argument 2 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: %15
  %15 = apply %14() : $@convention(thin) () -> @owned String // users: %18, %17
  // function_ref print(_:separator:terminator:)
  %16 = function_ref @Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> () // user: %17
  %17 = apply %16(%7, %13, %15) : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> ()
  release_value %15 : $String                     // id: %18
  release_value %13 : $String                     // id: %19
  release_value %7 : $Array<Any>                  // id: %20
  %21 = alloc_stack $Int                          // users: %22, %28, %26
  store %0 to %21 : $*Int                         // id: %22
  %23 = begin_access [modify] [static] %1 : $*LGStack // users: %27, %24
  %24 = struct_element_addr %23 : $*LGStack, #LGStack.items // user: %26
  // function_ref Array.append(_:)
  %25 = function_ref @Swift.Array.append(__owned A) -> () : $@convention(method) <τ_0_0> (@in τ_0_0, @inout Array<τ_0_0>) -> () // user: %26
  %26 = apply %25<Int>(%21, %24) : $@convention(method) <τ_0_0> (@in τ_0_0, @inout Array<τ_0_0>) -> ()
  end_access %23 : $*LGStack                      // id: %27
  dealloc_stack %21 : $*Int                       // id: %28
  %29 = tuple ()                                  // user: %30
  return %29 : $()                                // id: %30
} // end sil function 'main.LGStack.push(Swift.Int) -> ()'

sil hidden @main.LGStack.push(Swift.Int) -> () : $@convention(method) (Int, @inout LGStack) -> ()
特别关注这里，给LGStack 添加了一个修饰符@inout（输入输出参数）

debug_value_addr %1 : $*LGStack, var, name "self", argno 2 // id: %3
这里的self的修饰符变成了var，变成了一个可变的self，并且self访问的是一个地址debug_value_addr，而不是原始value的值了debug_value

本质上mutating这个关键字就做了一件事情，默认给结构体LGStack添加了一个intou关键字，这个inout关键字传递的过程中传递的就是所谓的引用

在函数的声明中，默认的参数都是不可变的

func swap(_ a: Int,_ b: Int) {
    let temp = a
    a = b
    b = temp
}

这个代码会出现报错：

截屏2021-01-11 下午7.25.07.png

说明默认参数是不可变的，想要可变，需要添加inout关键字

func swap(_ a: inout Int,_ b: inout Int) {
    let temp = a
    a = b
    b = temp
}

var age = 10
var age2 = 30

//传递的是地址
swap(&age, &age2)

print(age)
print(age2)

这里调用swap函数传递的是地址（&），也就是说inout关键字传递的是地址

注意：

• mutating：修饰的是方法
• inout：修改的是参数类型