Immutability

Immutability
Yung-Luen Lan

Friday, August 10, 12

我是誰？

• 藍永倫 @yllan
• 前 CocoaHeads Taipei Organizer.
• 寫過 (Open Source) Cocoa App.


我以為 OSS 是這樣


實際上得到的：
“堂堂⼀一個⽐比⼀一搬⼈人聰明的電腦程式設計師, 還是 Mac程式⼯工程師, 竟然講話像
是在⼆二⼿手煙密佈的骯髒網咖裡⾯面打電動玩具的⼩小混混講的話.

真的令⼈人不敢領教, 真的是讓⼈人很遺憾!! 堂堂⼀一個 Mac OS 軟體設計⼯工程師, 竟
然還說：我如果⼼心地很壞, 妳問我, 我會⽼老實回答嗎?

幹XXXXX 他⾺馬的, 我真的很⽣生氣, 很⽣生氣~~~ 竟然⼀一個程式設計師的⽔水準是
這樣的.”

“幹林勞奧”

“我⾮非常 90%肯定他的軟體不安全!!! 請⼤大家⾃自⼰己⼩小⼼心!!!
不是每⼀一個電腦⼯工程師都是正派的!!”


在主題
開始前

Slides 裡的
code
都是錯的


immutable |iˈmyo͞otəәbəәl|
adjective
unchanging over time or unable
to be changed: an immutable fact.


無法
修改

安全
第一

char *p = "hello";
p[0] = 'c';
// GOTCHA!


- (void) setUsers: (NSArray *)users
{
if (users != _users) {
[_users release];
_users = [users retain];
}
}

- (NSArray *) users
{
return _users;
}


CocoaHeads *ksBranch = [CocoaHeads new];

NSMutableArray *people = [NSMutableArray
arrayWithObjects: @"mikimoto", …, nil];

[ksBranch setUsers: people];

[people addObject: @"yllan"]; // BAD!


MUTABLE


Immutable in Cocoa

• NSArray
• NSDictionary
• NSSet
• NSString
•…


Mutable in Cocoa

• NSMutableArray
• NSMutableDictionary
• NSMutableSet
• NSMutableString
•…


init for Immutable

• 在 init 時就給定初始值。
• 到被 dealloc 前都無法修改。
• 像物件版的 const!


+ (id) classnameWithData: (id)data

- (id) initWithData: (id)data


沒有
用？

Derivation


- (id) arrayByAddingObject:

Array1

Array2


// init

// derive
- (classname *) classnameByDoSomething…


也要有Mutable


// init
// derive

@interface MutableClassname : Classname

// modify
- (void) doSomething…


CocoaHeads *ksBranch = [CocoaHeads new];

NSMutableArray *people = [NSMutableArray
arrayWithObjects: @"mikimoto", …, nil];

[ksBranch setUsers: people];

[people addObject: @"yllan"]; // BAD!
NSMutableArray 也是
NSArray！


NSArray *a = [NSArray new];
NSMutableArray *mA = [NSMutableArray new];

NSArray *b = [a copy];
NSMutableArray *mB = [mA copy];

[a arrayByAddingObject: @"Hello"];
[mA addingObject: @"Hello"];

[b arrayByAddingObject: @"Hello"];
[mB addingObject: @"Hello"];


NSArray *a = [NSArray new];
NSMutableArray *mA = [NSMutableArray new];

NSArray *b = [a copy];
NSMutableArray *mB = [mA copy];

[a arrayByAddingObject: @"Hello"];
[mA addingObject: @"Hello"];

[b arrayByAddingObject: @"Hello"];
[mB addingObject: @"Hello"]; // DAMN!


copy: immutable version

mutableCopy: mutable version


@interface Classname <NSCopying,
NSMutableCopying>
// init
// derive

// copy
- (id) copyWithZone: (NSZone *)zone
- (id) mutableCopyWithZone: (NSZone *)zone


不用真的
copy!


immutable 的話
只要 retain 就好！


Heavy RotationDerivation?


Persistent Data
Structure
come to the rescue!


m

43

m n

44

m n

45

m n

46

m n

47

優點

• 部分更新，省時間 O(lg n)
• 共享結構，省空間
n + k O(lg n) with k derivation
• ⾃自然擁有歷史紀錄


缺點

• Pointer 造成 overhead
• 對 CPU cache 較不友善


Mutable ↔ Immutable

Mutable Immutable
Convert
Update Derive

Immutable backed by Mutable
@interface MyArray {
NSMutableArray *_array; O(n) O(1) O(n)
}

Mutable backed by Immutable
O(n) O(n)
@interface MyMutableArray {
O(1)
}
NSArray *_array;
O(lg n) O(lg n)

50

[] is fast.


⽐比你想得複雜
• http://ridiculousﬁsh.com/blog/posts/
array.html

52

問題？

Immutability

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