the constructor initializer is specified only on the constructor definition, not its declaration.
构造函数初始化只在构造函数的定义中而不是声明中指定。
a constructor initializer list specifies initial values for one or more data members of the class.
构造函数的初始化列表为类的一个或多个数据成员指定初值。
the constructor initializer is a list of member names, each of which is followed by that member's initial value in parentheses.
构造函数的初始化式是一系列成员名,每个成员后面是括在圆括号中的初始值。