下面程序输出结果为 B() is called! D1() is called! D2() is called! D12() is called! 请将程序填写完整 #include <iostream> using namespace std; class B { public: B(){cout<<"B() is called!"<<endl;} }; class D1: (1) { public: D1(){cout<<"D1() is called!"<<endl;} }; class D2: (2) { public: D2(){cout<<"D2() is called!"<<endl;} }; class D12: (3) { public: D12(){cout<<"D12() is called!"<<endl;} }; int main() { D12 d12; return 0; }

下面是程序的完整代码：

#include <iostream> 
using namespace std; 

class B { 
public: 
    B(){cout<<"B() is called!"<<endl;} 
}; 

class D1: public B { 
public: 
    D1(){cout<<"D1() is called!"<<endl;} 
}; 

class D2: public B { 
public: 
    D2(){cout<<"D2() is called!"<<endl;} 
}; 

class D12: public D1, public D2 { 
public: 
    D12(){cout<<"D12() is called!"<<endl;} 
}; 

int main() { 
    D12 d12; 
    return 0; 
}

程序中，类B是基类，类D1和D2是公共继承自B的派生类，类D12是多重继承自D1和D2的派生类。在派生类D12的构造函数中，先调用D1和D2的构造函数，再调用自己的构造函数，因此输出结果为 B() is called! D1() is called! D2() is called! D12() is called!