1.按在組合類的定義中出現的順序呼叫內嵌物件的建構函式（與初始化列表中出現的順序無關）
2.呼叫本類建構函式的函式體
組合類建構函式定義的一般形式一般為：
類名::類名（形參表）:內嵌物件1(形參表）,內嵌物件2(形參表）,……
{類的初始化}

#include <iostream>
#include <cmath>

using namespace std;
class point {
public:
    point(int xx=0,int yy=0)
    {
        x=xx;
        y=yy;
        cout
 
<<"create point"<<endl;
    }
    point(point &p);
    int getx(){return x;}
    int gety(){return y;}
private:
    int x,y;
};
point::point(point &p){
    x=p.x;
    y=p.y;
    cout<<"copy point "<<endl;

}
class line
{

public:
    line(point xp1,point xp2);
    line(line &l);
    double
 
 getlen(){return len;}
private:
    point p1,p2;
    double len;
};
line::line(point xp1,point xp2):p1(xp1),p2(xp2){
    cout<<"create line"<<endl;
    double x=double(p1.getx()-p2.getx());
    double y=double(p1.gety()-p2.gety());
    len=sqrt(x*x+y*y);
}
line::line(line &l):p1(l.p1),p2(l.p2){
    cout
 
<<"copy line"<<endl;
    len=l.len;
}

int main(int argc, char *argv[])
{
    point myp1(1,1),myp2(4,5);
    line lin(myp1,myp2);
    line lin2(lin);
    cout<<"length of lin:"<<lin.getlen()<<endl;
    cout<<"length of lin2:"<<lin2.getlen()<<endl;
    return 0;
}

結果如下:
create point
create point
copy point
copy point
copy point
copy point
create line
copy point
copy point
copy line
length of lin:5
length of lin2:5

可以看到在創造line的物件前,先呼叫了point類的複製建構函式建立了兩個point類的物件.之後才開始呼叫自己的建構函式.