虚基类构造函数的参数必须由最新派生出的类负责初始化，即使不是直接继承，
示例程序的虚基类的构造函数只执行一次：
#include <iostream>
#include <string>
#include <cstring>
using namespace std;
class base
{
protected :
int x;
public :
base( int x1)
{
x = x1;
cout << " constructing base , x = " << x << endl;
}
};
class base1 : virtual public base
{
int y;
public :
base1( int x1, int y1) : base(x1)
{
y = y1;
cout << " constructing base1 , y = " << y << endl;
}
};
class base2 : virtual public base
{
int z ;
public :
base2( int x1, int z1) : base(x1)
{
z = z1;
cout << " constructing base2, z = " << z << endl;
}
};
class derived : public base1, public base2
{
int xyz;
public :
derived( int x1, int y1, int z1, int xyz1) : base(x1),base2(x1,z1),base1(x1,y1)
{
xyz = xyz1;
cout << " constructing derived, xyz = " << xyz << endl;
}
};
int main ()
{
derived ob(1,2,3,4);
return EXIT_SUCCESS;
}
在derived构造函数中，如果没有base(x1)将报错，因为derived的直接基类中有虚继承base的，且base的构造函数是有参数的，可以形象的理解为“ derived继承了base ” ，所以需要在初始化列表中对父类初始化。
程序运行结果 ：
constructing base , x = 1
constructing base1 , y = 2
constructing base2 , z = 3
constructing derived, xyz = 4