This is more efficient for class types, since it just needs to be constructed directly; whereas with assignment, they must be first initialized using the default constructor, and then assigned a different value.
这对类类型来说更有效，因为它只需要直接构造；而赋值时，它们必须先使用默认构造函数进行第一次初始化，然后再赋予一个不同的值。