# 05.Copy-on-write

## 什么是 COW

Copy on write

* COW是一种常见的计算机技术，有助于在复制结构时提高效率
  * 例如一个数组有100个元素，赋值到另一个数组，赋值所有元素，即最终两个数组内容相同
* 当使用COW的话
  * 当将两个变量指向同一数组时，他们指向相同的底层数据。两个变量指向相同的数据可能看起来矛盾，当修改第二个变量的时候，Swift才会去复制一个副本，第一个不会改变。
* 通过延迟复制操作，直到实际使用到的时候 才去复制，以此确保没有浪费的工作。

> COW是特别添加到Swift数组和字典的功能，自定义的数据类型不会自动实现。


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