41. What is the Singleton design pattern?

Singleton is a class that only allows creating a single instance of itselt.

單例設計模式是一個類，它只允許創建自己的單個實例。

構造函數防止他在單例類以外的地方被調用。

使用情景：need a single object resposible for some job that is shared by the whole application.

What is a globl state? public?

Why is singleton considered an anti pattern?

What is the difference between the singleton design pattern and the application singleton?應用程序單例模式：不同位置使用的單個對象，不強制執行其單例。