There is no constructor defined to initialize it to another value .
我们没有声明构造函数用其它值来初始化它。
Scala also requires you to initialize a var when it is declared .
scala同时也要求你在声明一个var时将其初始化。
No constructor logic is required to initialize these fields , in contrast to most other object-oriented languages .
初始化这些字段不需要逻辑上的构造函数，这和很多面向对象语言不同。
If a concrete field in a trait does not have a suitable default value , there is no " fail-safe " way to initialize the value .
如果trait的一个具体字段没有合适的默认值，那么就没有一个“万无一失”的方式来初始化这个值了。
This will create a new nsmanagedobjectcontext instance , and initialize it with the default nspersistentstorecoordinator . This is all that 's necessary to create a usable scratch context .
