Test = default将默认初始化其成员. 但是对于int或float类型，默认初始化不同于值初始化

Test = default will default initialize its members. but for type as int or float, default initialization is different than value-initialization

如此

Test t; // t.s and t.m have unitialized value

而

Test t{}; // t.s == 0 and t.m == 0.0f;