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;}