Reference types
Struts 是值類型 class 是引用類型
類不會(huì)提供自動(dòng)自動(dòng)初始化方法，需要自己實(shí)現(xiàn)。
類里面的存儲(chǔ)類型必須在初始化方法結(jié)束之前完成賦值。
In Swift, a structure is an immutable value. A class, on the other hand, is a mutable reference.
在swift中 Struts 是一個(gè)不可變的值，Class是可變的引用。
struct 的賦值是copy class的賦值是指針指向的。具體見(jiàn)下方代碼。
class Person {
var name : String
init(_ name : String) {
self.name = name
}
}
var t1 = Person.init("Yunis")
var t2 = t1
print(t1.name)//Yunis
print(t2.name)//Yunis
t1.name = "Change"
print(t1.name)//Change
print(t2.name)//Change
struct Contact {
var name: String
}
var t3 = Contact.init(name: "Yunis")
var t4 = t3
print(t3.name)//Yunis
print(t4.name)//Yunis
t3.name = "Change"
print(t3.name)//Change
print(t4.name)//Yunis
堆和棧區(qū)別 The heap vs. the stack
The system uses the stack to store anything on the immediate thread of execution; it is tightly managed and optimized by the CPU. When a function creates a variable, the stack stores that variable and then destroys it when the function exits. Since the stack is so well organized, it’s very efficient, and thus quite fast.
系統(tǒng)使用棧來(lái)存儲(chǔ)在執(zhí)行的即時(shí)線程上的任何東西;它由CPU嚴(yán)格管理和優(yōu)化。當(dāng)函數(shù)創(chuàng)建一個(gè)變量時(shí)，堆棧存儲(chǔ)該變量，然后在函數(shù)退出時(shí)將其銷毀。由于棧是如此良好的組織，它是非常高效的，因此相當(dāng)快。
The system uses the heap to store data referenced by other objects. The heap is generally a large pool of memory from which the system can request and dynamically allocate blocks of memory. The heap doesn’t automatically destroy its objects like the stack does; additional work is required to do that. This makes creating and removing data on the heap a slower process, compared to on the stack.
系統(tǒng)使用堆來(lái)存儲(chǔ)其他對(duì)象引用的數(shù)據(jù)。堆通常是一個(gè)大的內(nèi)存池，系統(tǒng)可以從內(nèi)存池中請(qǐng)求和動(dòng)態(tài)分配內(nèi)存塊。堆不會(huì)像堆棧那樣自動(dòng)破壞它的對(duì)象;需要額外的工作來(lái)做到這一點(diǎn)。相比與堆棧，創(chuàng)建和刪除堆上的數(shù)據(jù)是一個(gè)較慢的過(guò)程。
class Person {
var name : String
var lastName : String
init(_ name : String,_ lastName : String) {
self.name = name
self.lastName = lastName
}
var FullName : String {
return"\(name) \(lastName)"
}
}
var variable = Person.init("Yunis","Last")
When you create an instance of a class, your code requests a block of memory on the heap to store the instance itself; that’s the first name and last name inside the instance on the right side of the diagram. It stores the address of that memory in your named variable on the stack; that’s the reference stored on the left side of the diagram.
當(dāng)你創(chuàng)建一個(gè)類的實(shí)例的時(shí)間，會(huì)向系統(tǒng)申請(qǐng)?jiān)诙焉系囊粔K內(nèi)存用來(lái)存儲(chǔ)類的實(shí)例，包括name 和 lastName。同時(shí)在棧上存儲(chǔ)內(nèi)存地址。
Object identity
var t1 = Person.init("Yunis","Last")
var t2 = t1
var t3 = Person.init("Yunis","Last")
t1 === t2//true
t1 === t3//true
t2 === t3//true
t2 = t3
t1 === t2//true
t1 === t3//true
t2 === t3//true
t1.FullName == t2.FullName//true
t1.FullName == t3.FullName//true
t2.FullName == t3.FullName//true
Jut as the == operator checks if two values are equal, the === identity operator
compares the memory address of two references.
== 是比較兩個(gè)值是否一致，=== 是比較兩個(gè)引用地址是否一致。
Structures
- 用于表示值
- 隱式的值copy
- 數(shù)據(jù)不可變
- 內(nèi)存分配速度快（棧）
Classes
- 用于表示對(duì)象
- 隱式的對(duì)象共享
- 數(shù)據(jù)可變
- 內(nèi)存分配較慢（堆）
Key points
- 和structures一樣，類是具有方法和屬性的命名類型（named type）
- 類通過(guò)指針共享對(duì)象
- 類的實(shí)例叫做對(duì)象（objects）
- 對(duì)象是可變的
