派生类无法访问基类的受保护成员

Question

请考虑以下示例

class base
{
protected :
    int x = 5;
    int(base::*g);
};
class derived :public base
{
    void declare_value();
    derived();
};
void derived:: declare_value()
{
    g = &base::x;
}
derived::derived()
    :base()
{}

根据知识,只有基类的朋友和派生类可以访问基类的受保护成员,但在上面的示例中,我得到以下错误,"Error C2248 'base::x': cannot access protected member declared in class "但是当我添加以下行时

friend class derived;

声明它是朋友,我可以访问基类的成员,我在声明派生类时做了一些基本的错误吗？

Answer 1

派生类protected只能通过派生类的上下文访问基类的成员.换句话说,派生类不能通过protected基类访问成员.

当形成指向受保护成员的指针时,它必须在其声明中使用派生类:

struct Base {
 protected:
    int i;
};

struct Derived : Base {
    void f()
    {
//      int Base::* ptr = &Base::i;    // error: must name using Derived
        int Base::* ptr = &Derived::i; // okay
    }
};

你可以改变

g = &base::x;

至

g = &derived::x;