C++类和对象补充

class Date
{
public:
 Date(int year, int month, int day)
 {
 _year = year;
 _month = month;
 _day = day;
 //可以进行多次赋值，但一般不这么做
 _year = 1;
 }
private:
 int _year;
 int _month;
 int _day;
};

class Date
{
public:
	Date(int year = 0, int month = 1, int day = 1)
		:_year(year)
		,_month(month)
	{
		_day = day;
	}
private:
	int _year;
	int _month;
	int _day;
};

class B
{
public:
	B(int i)
		:_i(i)
	{
	}
private:
	int _i;
};
class A
{
public:
	A(int a, int& b, int bb)
		:_a(a)
		,_b(b)
		,_bb(bb)
	{
	}
private:
	const int _a;//const成员变量
	int& _b;//引用成员变量
	B _bb;//自定义成员变量
};

class B
{
public:
	B()
	{
		cout << "B()" << endl;
	}
private:
	int _i;
};
class A
{
public:
	A(int a, int& b)
		:_a(a)
		,_b(b)
	{
	}
private:
	const int _a;//const成员变量
	int& _b;//引用成员变量
	B _bb;//自定义成员变量
};
int main()
{
	int n = 0;
	A a1(0, n);
	return 0;
}

class A
{
public:
	A(int a)
		:_a1(a)
		, _a2(_a1)
	{}
	void Print() {
		cout << _a1 << " " << _a2 << endl;
	}
private:
	int _a2;
	int _a1;
};
int main() 
{
	A aa(1);
	aa.Print();
	return 0;
}

class Date
{
public:
	Date(int year)
		:_year(year)
	{}
	explicit Date(int year)
		:_year(year)
	{}
private:
	int _year;
	int _month;
	int _day;
};
int main()
{
	Date d1(2020);
	// 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2019构造一个无名对象，最后用无名对象给d1对象进行赋值
	Date d2 = 2021;//explict禁止隐式类型转换，因此该句代码运行错误
}

class Sum
{
public:
    //调用构造函数
    Sum()
    {
        _sum += _i;
        ++_i;
    }
    //static修饰的成员函数，没有隐含的this指针，只能访问静态成员变量
    static int GetSum()
    {
        return _sum;
    }
private:
	//static修饰的成员变量为所有定义出来的类对象共有
    static int _i;
    static int _sum;
};
//静态成员变量的定义
int Sum::_i = 1;
int Sum::_sum = 0;
class Solution {
public:
    int Sum_Solution(int n) {
        Sum* p = new Sum[n];
        return Sum::GetSum();
    }
};

class Date
{
	//用关键字friend在类中声明函数为Date的友元函数
	friend ostream& operator<<(ostream& out, const Date& d);
public:
	Date(int year, int month, int day)
		:_year(year)
		,_month(month)
		,_day(day)
	{}
private:
	int _year;
	int _month;
	int _day;
};
ostream& operator<<(ostream& out, const Date& d)
{
	out << d._year << "/" << d._month << "/" << d._day;
	return out;
}
int main()
{
	Date d1(2021, 10, 20);
	cout << d1 << endl;
}

class A
{
	//声明B为A的友元类，则在B中可以访问A中的成员
	friend class B;
public:
	A(int a)
		:_a(a)
	{
	}
private:
	int _a;
	static int _i;
};
class B
{
public:
	B(int b)
		:_b(b)
	{}
	static int Count()
	{
		A::_i++;
		return A::_i;
	}
private:
	int _b;
};
int A::_i = 0;
int main()
{
	A a1(1);
	B b1(1);
	cout << b1.Count() << endl;
	cout << b1.Count() << endl;
	return 0;
}

class A
{
public:
	class B//内部类，是A的友元类
	{
	public:
	//B可以直接访问A的成员
		void Print(const A& a)
		{
			cout << a._a << endl;
			cout << _i << endl;
		}
	};
	A(int a)
		:_a(a)
	{}
private:
	int _a;
	static int _i;
};
int main()
{
	A::B b1;//注意B的调用方式
	A a1(1);
	b1.Print(a1);
	//但A的对象不能去访问B中的成员
	a1.b1;//error
}