Swift，字符串和内存地址 [英] Swift, Strings and Memory Addresses

问题描述

对于Swift如何管理String(s)

There is something I am not understanding about how Swift manages memory address of String(s)

此处foo和boo是指向相同存储位置的2个指针.

Here foo and boo are 2 pointers to the same memory location.

class Foo { }

let foo = Foo()
let boo = foo

unsafeAddressOf(foo) // "UnsafePointer(0x7FCD13719BE0)"
unsafeAddressOf(boo) // "UnsafePointer(0x7FCD13719BE0)" 

好.

let word0 = "hello"
let word1 = word0

现在word0和word1是value types，但是这里涉及到copy on write机制.

Now word0 and word1 are value types but here the copy on write mechanism is involved.

[...]但是，Swift仅在绝对必要时才在幕后执行实际复制. Swift会管理所有值复制以确保最佳性能，因此您不应避免分配以尝试抢占该优化. 那么为什么它们有2个不同的内存地址?

So why do they have 2 different memory addresses?

unsafeAddressOf(word0) // "UnsafePointer(0x7FCD1342ACE0)"
unsafeAddressOf(word1) // "UnsafePointer(0x7FCD13414260)"

3.更多

还请注意，String是某种程度上符合的struct a>到AnyObject.

3. More

Also please note that String is a struct that somehow conforms to AnyObject.

通过Xcode 7 GM Playground和Swift 2.0进行了测试.

Tested with Xcode 7 GM Playground and Swift 2.0.

推荐答案

func unsafeAddressOf(object: AnyObject) -> UnsafePointer<Void>

带有AnyObject参数，即 class 的实例. 它将指针返回到用于所引用对象的存储 通过object.

takes an AnyObject parameter, i.e. an instance of a class. It returns the pointer to the storage used for the object referenced by object.

addressOf()不能与 struct 变量一起使用:

addressOf() cannot be used with struct variables:

struct Foo { }
var f = Foo()
let a = unsafeAddressOf(f)
// error: cannot invoke 'unsafeAddressOf' with an argument list of type '(Foo)'

String是struct，但是 ，当传递给需要对象的函数时，它会自动桥接到NSString.所以

String is a struct, however, it is automatically bridged to NSString when passed to a function expecting an object. So

let word0 = "hello"
let p1 = unsafeAddressOf(word0)

实际执行

let p1 = unsafeAddressOf(word0 as NSString)

您不是获得word0变量的地址，而是指向该变量的指针. 桥接的NSString对象的内存位置.

You get not the address of the word0 variable, but the pointer to the memory location of the bridged NSString object.

您似乎无法对这种桥接是否做出任何假设 返回相同的NSString对象(或更常见的是，相同的 基础对象)在同一Swift字符串上重复执行.在操场上，甚至

It seems that you cannot make any assumptions on whether this bridging returns the identical NSString object (or more generally, the same Foundation object) when done repeatedly on the same Swift string. In a Playground, even

let word0 = "hello"
let p1 = unsafeAddressOf(word0)
let p2 = unsafeAddressOf(word0)
let p3 = unsafeAddressOf(word0)

返回三个不同的地址(但在编译后返回相同的地址 项目).进行了相同的观察(对于数组和字典) 在数组与字典之间的不同桥接中.

returns three different addresses (but the same addresses in a compiled project). The same observation (for arrays and dictionaries) was made in A different bridging between Array and Dictionary.

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