下面程序的输出结果是【 】。 include <iostream> using namespace std; class base { protected: int a; public: base(){cout<<'0':} }; class basel: virtual public base { public: base1(){ cout<<'1';} }; class base2 : virtual public base { public: base2(){cout<<'2';} }; class derived : public base1,public base2 { public: derived () {cout<<'3'; } } int main () { derived obj; cout<<end1; return 0; }