C++必修：类与对象(三)

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1. 隐式类型转换

在学习C语言时我们就明白，当我们进行赋值时，如果赋值两边的类型不同时就可能发生隐式类型转换。

1.1. 内置类型

在发生隐式类型转换时，如果都是内置类型就会先开辟一个临时变量，再将右操作数强制类型转换为左操作数的类型，最后用这个临时变量对左操作数进行赋值。注意：这个临时变量具有常性，不可修改。

int main(){double j = 1.1;int i = j;//隐式类型转换int& a = j;//errorconst int& b = j;//okreturn 0;}

因为临时变量具有常性，所以无法被修改。如果赋值给普通引用就会造成权限的放大，所以只能用常引用。

1.2. 自定义类型

如果将一个内置赋值给自定义类型，那么编译器也会先创造一个自定义类型的**临时变量，**然后用这个内置类型调用构造函数对临时变量初始化，最后用这个临时变量对左操作数进行拷贝构造。。注意：这个临时变量也具有常性，不可修改。

class Date{public:Date(int year = 1, int month = 1, int day = 1){_year = year;_month = month;_day = day;}Date(const Date& d){_year = d._year;_month = d._month;_day = d._day;}void Print(){cout Date d1 = 2023;//发生隐式类型转换Date d2 = { 2023,2 };d1.Print();d2.Print();return 0;}public:explicit Date(int year = 1, int month = 1, int day = 1){_year = year;_month = month;_day = day;}Date(const Date& d){_year = d._year;_month = d._month;_day = d._day;}void Print(){cout Date d1 = 2023;//发生隐式类型转换Date d2 = { 2023,2 };d1.Print();d2.Print();return 0;}public:    static int Print()//静态成员函数{cout public:static void Print()//静态成员函数{cout A a;a.Print();A::Print();return 0;}cout return x + y;}cout return x + y;Print();}friend void Print(const Date& d);//友元函数public:Date(int year = 1, int month = 1, int day = 1){_year = year;_month = month;_day = day;}Date(const Date& d){_year = d._year;_month = d._month;_day = d._day;}private:int _year;int _month;int _day;};void Print(const Date&d){cout public:Date(int year = 1, int month = 1, int day = 1){_year = year;_month = month;_day = day;}Date(const Date& d){_year = d._year;_month = d._month;_day = d._day;}int GetYear()const{return _year;}int GetMonth()const{return _month;}int GetDay()const{return _day;}private:int _year;int _month;int _day;};void Print(const Date& d){cout friend class Date; // 声明日期类为时间类的友元类，则在日期类中就直接访问Time类public:Time(int hour = 0, int minute = 0, int second = 0): _hour(hour), _minute(minute), _second(second){}private:int _hour;int _minute;int _second;};class Date{public:Date(int year = 1900, int month = 1, int day = 1): _year(year), _month(month), _day(day){}void SetTimeOfDate(int hour, int minute, int second){// 直接访问时间类私有的成员变量_t._hour = hour;_t._minute = minute;_t._second = second;}private:int _year;int _month;int _day;Time _t;};public:class B//B是A的友元{private:int _m;};private:int _a;int _b;};private:static int k;int h;public:class B // B天生就是A的友元{public:void foo(){cout A::B b;b.foo();return 0;}private:int k;int h;public:class B // B天生就是A的友元{public:int _a;};};int main(){A a;cout public:Date(int year = 1, int month = 1, int day = 1){cout cout cout cout Date();//匿名对象Date d;return 0;}const Date& dc = Date();//匿名对象也具有常性Date d;return 0;}public:Date(int year = 1, int month = 1, int day = 1){cout cout cout cout int months[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };int day = months[month];if (month == 2 && (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)){day++;}return day;}assert(month  0 && month