填空题

下面程序的输出结果是 【15】 。
#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;

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