类继承时 operator+ 的问题

[BillLeecn]

有一个基类和一个子类。我想要使两个 Sub 对象相加是返回一个 Sub 对象

class Base{
       friend Base operator+(Base&, Base&);
};
class Sub: public{
        friend Sub operator+(Sub& a, Sub& b){
                return a + b;
        }
};

这样的代码将会导致编译错误
error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
请问怎样改代码才能实现这样的用法（基类 Base 不能改）

[liutengfeigo]

return a + b;
你因该去调用构造函数啊比如 return Sub(a.xxx+b.xxx);

[MK]

楼主的思路是什么？
什么都没有，继承的时候少了基类名啊

[fallening]

尽量少放在类里边

class Base;
class Derive;

const Base
operator + ( const Base&, const Base& );

const Derive
operator + ( const Derive&, const Derive& );

[we_sky2008]

friend Sub operator+(Sub& a, Sub& b)
{
    return a + b;
}

这个会导致无穷递归的

[jizhongqing]

class Base
{
public:
Base(int x = 0, int y = 0) : m_x(x), m_y(y) {}

friend Base operator+(const Base& a, const Base& b);

// 这里不写 get，set了，
// int GetX()
// {
// return m_x;
// }
// int GetY()
// {
// return m_y;
// }

//private:
int m_x;
int m_y;
};

Base operator+(const Base& a, const Base& b)
{
Base c;
c.m_x = a.m_x + b.m_x;
c.m_y = a.m_y + b.m_y;

return c;
}

class Sub: public Base
{
public:
Sub(int x = 0, int y = 0, int z = 0) : Base(x, y), m_z(z) {}
friend Sub operator+(const Sub& a, const Sub& b);

//private:
int m_z;
};

Sub operator+(const Sub& a, const Sub& b)
{
// 直接使用父类的成员不太好，实际中父类应该提供访问函数。。。
Sub c;
c.m_x = a.m_x + b.m_x;
c.m_y = a.m_y + b.m_y;
c.m_z = a.m_z + b.m_z;

return c;
}


int _tmain(int argc, _TCHAR* argv[])
{
Base a(1, 2), b(3, 4), c;
c = a + b;

Sub d(1,2), e(3, 4, 5), f;
f = d + e;

return 0;
}

[cattycat]

你并没有实现operator+啊。
可以
Sub temp;
temp.x=a.x+b.x;
return temp;
类似这样的。

[selooloo]

Base对象和Sub对象相加时会产生二义性

[tananade]

呵呵，你根本没有定义成功，正如上面所说的，这会递归
属于递归定义

[wyz007134]

你们没发现编译器的报错很搞笑么？
error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
它好像很无奈...