c ++将基类分配给Derived

Question

为什么这不会编译？我错过了什么？

class Base
{
    public:
        Base() : data(0) { }
    protected:
        int data;
};

class Derived : public Base
{
    public:
        Derived() : Base() { }
        Derived& operator=(const Base& _base)
        {
            data = _base.data; // protected member Base::data is not accessible through a "Base" pointer or object.
        }
};

为什么会这样，因为数据成员具有受保护的可见性（以便派生类可以访问它）。