有一行代码看不懂，请大家来指点一下！！！

C/C++ code

#include <iostream.h>class B1{     public:           B1(int i){cout<<"constructing B1\n";}};class B2{     public:           B2(int j){cout<<"constructing B2\n"}};class B3{     public:           B3(){cout<<"constructing B3\n"}};class C: public B2,public B1,public B3//派新类{     public:           C(int a,int b,int c,int d):B1(a),memberB2(d),memberB1(c),B2(b){}/*[color=#FF0000]中一行代码是什么意思为什么这样写小弟不太明白请教一下大家,小弟新手刚开始自学C++请大家耐心给小弟讲解一下[/color]*/     private:           B1 memberB1;           B2 memberB2;           B3 memeberB3;} void main(){    C obj(1,2,3,4);}