派生类构造函数与析构函数的构造规则

1.派生类的构造函数可以不显示式的写出基类的构造函数。例如：

Third()
{e=0;}

此时，系统自动调用基类的无参构造函数（没有，则为默认的构造函数）。也可显示式的指出调用基类的哪一个构造函数。例如：

Third(int x,int y,int z):Second(x,y)
{
e=z;
}

2.要调用的基类构造函数的参数可在派生类的构造函数中明确给出。例如：

Third(int x,int y,int z):Second(x,y)
{
e=z;
}

也可不给出：

Second():First(1,1)
{
c=0;
d=0;
}

还可以对派生类构造函数的参数进行一些交换，再传递给要调用的基类的构造函数，例如：

Second(int x,int y):First(x+1,y+1)
{
c=x;
d=y;
}

一个完整的例子如下：

#include<iostream>
using namespace std;
class First{
public:
	First()
	{
		a=0;
		b=0;
	}
	First(int x,int y)
	{a=x;
	 b=y;
	}
	~First()
	{
		cout<<"destructing First"<<endl;
	}
	void show()
	{
     cout<<"\n a="<<a<<"\n b="<<b<<endl;
	}
private:
	int a;
	int b;
};
class Second:public First{
public:
	Second():First(1,1)
	{
		c=0;
		d=0;
	}
	Second(int x,int y):First(x+1,y+1)
	{
		c=x;
		d=y;
	}
	Second(int x,int y,int m,int n):First(m,n)
	{
		c=x;
		d=y;
	}
	~Second()
	{
		cout<<"destructing Second"<<endl;
	}
	void show()
	{
		First::show();
		cout<<"c="<<c<<"d="<<d<<endl;
	}
private:
	int c,d;

};
class Third:public Second{
public:
	Third()
	{e=0;}
	Third(int x,int y,int z):Second(x,y)
	{
         e=z;
	}
	Third(int x,int y,int z,int m,int n):Second(x,y,m,n)
	{
		e=z;
	}
	~Third()
	{
		cout<<"destructing Third"<<endl;
	}
	void show()
	{
		Second::show();
		cout<<"e="<<e<<endl;
	}
private:
	int e;
};
int main()
{
	First f;
	f.show();
	cout<<"======================"<<endl;
	Second d1;
	d1.show();
	cout<<"======================"<<endl;
	Second d2(10,20,30,40);
    d2.show();
	cout<<"====================="<<endl;
	Second d3(12,13);
	d3.show();
	cout<<"======================="<<endl;
	Third t;
	t.show();
	cout<<"======================"<<endl;
	Third t1(14,15,16);
	t1.show();
	cout<<"======================="<<endl;
	Third t2(50,60,70,80,90);
	t2.show();
	return 0;

}