* 时钟类构造函数 */

We can also declare "Create" outside Base. But is more relevant to limit it's scope to Base

We can also declare "Create" outside Base. But is more relevant to limit it's scope to Base

第一种： 使用初始化列表。

non-explicit-one-argument ctor 可以把别的东西转换为Fraction这种

explicit-one-argument ctor 可以把别的东西转换为Fraction这种

直接调用构造方法传递，基类构造方法有参数，派生类必须通过构造方法，在初始化列表中传递参数