# Value / Reference Type
## 三種記憶體區間
#### 1. static memory：靜態變數(static)，永遠存在的變數(ex. Swift中用特殊機制Lazy初始化)
#### 2. stack memory：變數在方法結束會自動回收
#### 3. heap memory：變數由使用者自己決定何時回收(ex. Swift中用ARC處理)
* 參考連結：
* [C 語言程式記憶體配置概念](https://blog.gtwang.org/programming/memory-layout-of-c-program/)
* [stack vs heap：執行時期儲存兩大要角](https://antrash.pixnet.net/blog/post/70456505)
* [stack 與 heap 的底層概念](https://nwpie.blogspot.com/2017/05/5-stack-heap.html)
* [MEMORY IN C – THE STACK, THE HEAP, AND STATIC](https://craftofcoding.wordpress.com/2015/12/07/memory-in-c-the-stack-the-heap-and-static/)
## Swift 裡的 Value Type 與 Reference Type
* Value Type：Int, String, Array, Dict, Enum, Struct
* 記錄在 stack memory
* 因為變數在方法結束會自動回收
* 所以賦值或是傳值都是"新的一份資料"，即為 Pass By Value
* Reference Type：Class, Closure
* 記錄位址在stack memory，紀錄資料在heap memory
* 因為變數在方法結束不會回收，是利用ARC回收
* 所以賦值或是傳值都是"同一份資料"，即為 Pass By Reference
* Value Type 和 Reference Type 都只是基於操作記憶體的方式不同，而產生的簡稱
* Value Type 和 Reference Type 萬年爭議哪個好？操作安全性？記憶體消耗？
* 參考連結：[Value Type vs. Reference Type](https://medium.com/@wuufone/%E5%AD%B8%E6%9C%83-swift-%E7%9A%84%E9%97%9C%E9%8D%B5-value-type-vs-reference-type-50d3034596a8)
```swift
var a = "0050"
var b = a
a = "2330"
print(b) // "0050"
// String 賦值是複製一份 String
func resetData(input: Int){
input = "" // Error 無法編譯，input 預設是 let
}
```
```swift
struct Stock{
var name: String = "0050"
}
var a = Stock()
var b = a
a.name = "2330"
print(b.name) // "0050"
// struct 賦值是複製一份 struct
func resetData(input: Stock){
input.name = "" // Error 無法編譯，input 預設是 let
}
```
```swift
class Stock{
var name: String = "0050"
}
var a = Stock()
var b = a
a.name = "2330"
print(b.name) // "2330"
// class 賦值是用同一份 class
func resetData(input: Stock){
input.name = "Null"
}
resetData(input: a)
print(a.name)
// class 傳值是用同一份 class，可以在方法內修改
```
## 課後練習題(可以在playground上跑)
```swift
struct Car{
var number = 0
init(number: Int) {
self.number = number
}
}
var car0 = Car(number: 0)
var car1 = Car(number: 1)
var cars = [car0, car1]
var newCars = cars
cars.sort { $0.number > $1.number }
dump(cars) // [{number 1}, {number 0}]
dump(newCars) // [?, ?]
car0.number = 2
dump(cars) // [?, ?]
dump(newCars) // [?, ?]
cars[0].number = 3
dump(cars) // [?, ?]
dump(newCars) // [?, ?]
```
```swift
class Car{
var number = 0
init(number: Int) {
self.number = number
}
}
var car0 = Car(number: 0)
var car1 = Car(number: 1)
var cars = [car0, car1]
var newCars = cars
cars.sort { $0.number > $1.number }
dump(cars) // [{number 1}, {number 0}]
dump(newCars) // [?, ?]
car0.number = 2
dump(cars) // [?, ?]
dump(newCars) // [?, ?]
cars[0].number = 3
dump(cars) // [?, ?]
dump(newCars) // [?, ?]
```
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