【C++】踏上C++学习之旅（十）：深入“类和对象“世界，掌握编程黄金法则（五）（最终篇，内含初始化列表、静态成员、友元以及内部类等等）

class Date
{
public:
	//这种是在函数体内赋值的玩法
	Date(int year = 1, int month = 1, int day = 1)
	{
		_year = year;
		_month = month;
		_day = day;
	}

private:
	int _year;
	int _month;
	int _day;
};

int main()
{
	//这个就是成员变量的整体初始化的地方
	Date d1(2024,11,22);

	return 0;
}

class Date
{
public:
	//这种是在函数体内赋值的玩法
	Date(int year = 1, int month = 1, int day = 1)
		:_year(year)
		,_month(month)
		,_day(day)
	{}

private:
	int _year;
	int _month;
	int _day;
};

class A
{
public:
	A(int a, char ref)
	{
		//如果不使用初始化列表初始化的话，会出现编译错误
		_a = a;
		_ref = ref;
	}
private:
	const int _a;
	char& _ref;
};

class B
{
public:
	B(int a) {}
};

class A
{
public:
	//初始化列表的语法：以冒号开头，逗号进行分割,括号的内容是成员变量该被赋值的值
	A(int a, char ref)
		:_a(a)
		,_ref(ref)
		,_b(1)
	{}
private:
	const int _a;
	char& _ref;
	B _b; //这个对象_b的构造函数是要显示传递参数的，为此只能走初始化列表
};

public:
	
	Date(int year = 1, int month = 1, int day = 1)
		:_year(2024)
		,_month(11)
		,_day(22)
	{}

//相当于是在初始化列表中，做了这么一件事
private:
	int _year = 2024; //给成员变量一个缺省值
	int _month = 11;
	int _day = 22;
};

class B
{
public:
	B(int a) {}
};

class A
{
public:
	A(int a, char ref)
		:_a(a)
		,_ref(ref)
		,_b(1)
	{}
private:
	const int _a; //const成员变量
	char& _ref;   //引用成员变量
	B _b;		  //自定义类型且没有默认的构造函数
};

class Time
{
public:
	Time(int hour = 0)
		:_hour(hour)
	{
		cout << "Time()" << endl;
	}
private:
	int _hour;
};

class Date
{
public:
	Date(int day)
	{}
private:
	int _day;
	Time _t;
};

int main()
{
	Date d(1);

	return 0;
}

class A
{
public:
	A(int n)
		:_a(n)
		,_b(_a)
	{
		cout << "_a:" << _a << endl;
		cout << "_b:" << _b << endl;
	}
private:
	int _b;
	int _a;
};

int main()
{
	A a(100);

	return 0;
}

class A
{
public:
	A(int a)
		:_a(a)
	{
		cout << "A(int a)" << endl;
	}
private:
	int _a;
};

int main()
{
	A aa = 10;
	return 0;
}

class A
{
public:
	A(int a, int b)
		:_a(a)
	{
		cout << "A(int a)" << endl;
	}
private:
	int _a;
	int _b;
};

int main()
{
	A aa = 10;
	return 0;
}

class Date
{
public:
	// 1. 单参构造函数，没有使用explicit修饰，具有类型转换作用
	// explicit修饰构造函数，禁止类型转换---explicit去掉之后，代码可以通过编译
	explicit Date(int year)
		:_year(year)
	{}
	/*
	// 2. 虽然有多个参数，但是创建对象时后两个参数可以不传递，没有使用explicit修饰，具有类型转
   换作用
	// explicit修饰构造函数，禁止类型转换
	explicit Date(int year, int month = 1, int day = 1)
	: _year(year)
	, _month(month)
	, _day(day)
	{}
	*/
	Date& operator=(const Date& d)
	{
		if (this != &d)
		{
			_year = d._year;
			_month = d._month;
			_day = d._day;
		}
		return *this;
	}
private:
	int _year;
	int _month;
	int _day;
};

void Test()
{
	Date d1(2022);
	// 用一个整形变量给日期类型对象赋值
	// 实际编译器背后会用2023构造一个无名对象，最后用无名对象给d1对象进行赋值
	d1 = 2023;
	// 将1屏蔽掉，2放开时则编译失败，因为explicit修饰构造函数，禁止了单参构造函数类型转换的作用
}

//static成员的初始化
class A
{
private:
	static int _a;
};
//在类外面进行初始化
//方式：就是在变量名前指明类域
int A::_a = 10;

class A
{
public:
	static int GetStatic()
	{
		return _a;
	}
private:
	static int _a;
};
//在类外面进行初始化
int A::_a = 10;

int main()
{
	A aa;
	cout << aa.GetStatic() << endl;
}

class A
{
public:
	int StaticNum()
	{
		return _a;
	}

private:
	static int _a;
};
//在类外面进行初始化
int A::_a = 10;

int main()
{
	A aa;
	cout << aa.StaticNum() << endl;
}

class A
{
public:
	A() { ++_scount; }
	A(const A& t) { ++_scount; }
	~A() { --_scount; }
	static int GetACount() { return _scount; }
private:
	static int _scount;
};

int A::_scount = 0;

void TestA()
{
	cout << A::GetACount() << endl;
	A a1, a2;
	A a3(a1);
	cout << A::GetACount() << endl;
}

class Date
{
public:
	Date(int year, int month, int day)
		: _year(year)
		, _month(month)
		, _day(day)
	{}
	// d1 << cout; -> d1.operator<<(&d1, cout); 不符合常规调用
	// 因为成员函数第一个参数一定是隐藏的this，所以d1必须放在<<的左侧
	ostream& operator<<(ostream& _cout)
	{
		_cout << _year << "-" << _month << "-" << _day << endl;
		return _cout;
	}
private:
	int _year;
	int _month;
	int _day;
};

class Date
{
	friend ostream& operator<<(ostream& _cout, const Date& d);
	friend istream& operator>>(istream& _cin, Date& d);
public:
	Date(int year = 1900, int month = 1, int day = 1)
		: _year(year)
		, _month(month)
		, _day(day)
	{}
private:
	int _year;
	int _month;
	int _day;
};

ostream& operator<<(ostream& _cout, const Date& d)
{
	_cout << d._year << "-" << d._month << "-" << d._day;
	return _cout;
}

istream& operator>>(istream& _cin, Date& d)
{
	_cin >> d._year;
	_cin >> d._month;
	_cin >> d._day;
	return _cin;
}

int main()
{
	Date d;
	cin >> d;
	cout << d << endl;
	return 0;
}

class Time
{
	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类中的私有成员变量
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;
};

class A
{
private:
	static int k;
	int h;
public:
	class B // B天生就是A的友元
	{
	public:
		void foo(const A& a)
		{
			cout << k << endl;//OK
			cout << a.h << endl;//OK
		}
	};
};
int A::k = 1;

int main()
{
	A::B b;
	b.foo(A());

	return 0;
};

class A
{
public:
	A(int a = 1)
		:_a(a)
	{
		cout << _a << endl;
		cout << "A(int a = 1)" << endl;
	}
	~A()
	{
		cout << _a << endl;
		cout << "~A()" << endl;
	}

private:
	int _a;
};

int main()
{
	//有名对象
	A aa1(10);
	//匿名对象
	A(100);

	return 0;
}