Reference types
Struts 是值类型 class 是引用类型
类不会提供自动自动初始化方法，需要自己实现。
类里面的存储类型必须在初始化方法结束之前完成赋值。
In Swift, a structure is an immutable value. A class, on the other hand, is a mutable reference.
在swift中 Struts 是一个不可变的值，Class是可变的引用。
struct 的赋值是copy class的赋值是指针指向的。具体见下方代码。
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
堆和栈区别 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.
系统使用栈来存储在执行的即时线程上的任何东西;它由CPU严格管理和优化。当函数创建一个变量时，堆栈存储该变量，然后在函数退出时将其销毁。由于栈是如此良好的组织，它是非常高效的，因此相当快。
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.
系统使用堆来存储其他对象引用的数据。堆通常是一个大的内存池，系统可以从内存池中请求和动态分配内存块。堆不会像堆栈那样自动破坏它的对象;需要额外的工作来做到这一点。相比与堆栈，创建和删除堆上的数据是一个较慢的过程。
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.
当你创建一个类的实例的时间，会向系统申请在堆上的一块内存用来存储类的实例，包括name 和 lastName。同时在栈上存储内存地址。
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.
== 是比较两个值是否一致，=== 是比较两个引用地址是否一致。
Structures
- 用于表示值
- 隐式的值copy
- 数据不可变
- 内存分配速度快（栈）
Classes
- 用于表示对象
- 隐式的对象共享
- 数据可变
- 内存分配较慢（堆）
Key points
- 和structures一样，类是具有方法和属性的命名类型（named type）
- 类通过指针共享对象
- 类的实例叫做对象（objects）
- 对象是可变的
