c++ - 为什么我可以通过指向派生对象的基类指针访问派生私有(private)成员函数？

Question

#include<iostream>

using namespace std;
class base
{
public:
    virtual void add() {
        cout << "hi";
    }
};

class derived : public base
{
private:
    void add() {
        cout << "bye";
    }
};

int main()
{
    base *ptr;
    ptr = new derived;
    ptr->add();
    return 0;
}

输出是再见

我对如何实现没有问题。我了解您使用 vtables，并且 derived 的 vtable 包含新的 add() 函数的地址。但是 add() 是私有(private)的，当我尝试在类外访问它时，编译器不应该产生错误吗？不知怎的，这似乎不对。

Answer 1

add() 仅在 derived 中是私有(private)的，但您拥有的 静态类型 是 base* -因此 base 的访问限制适用。
通常，您甚至无法在编译时知道指向 base 的指针的动态类型是什么，例如根据用户输入进行更改。

这是根据 C++03 §11.6:

The access rules (clause 11) for a virtual function are determined by its declaration and are not affected by the rules for a function that later overrides it.
[...] Access is checked at the call point using the type of the expression used to denote the object for which the member function is called [...]. The access of the member function in the class in which it was defined [...] is in general not known.