使用指向派生类中基类的指针访问受基类保护的成员

考虑以下代码：

#include <iostream>

using std::endl;
using std::cout;

template<typename T>
class B{
protected:
    T value;
    B* ptr;
public:
    B(T t):value(t), ptr(0){}
};

template<typename T>
class D: public B<T>{
public:
    void f();
    D(T t):B<T>(t){}
};

template<typename T>
void D<T>::f(){
    cout << this->value << endl;     //OK!
    this->ptr = this;
    cout << this->ptr->value << endl;      //error! cannot access protected member!!
    B<T>* a = this;
    cout << a->value <<endl;       //error! cannot access protected member!!
}


int main(){
    D<double> a(1.2);
    a.f();
    return 0;
}

似乎可以使用 this 指针，而不是其他指针。

编译器是否将它们视为不同的实例化？

It seems that the member of the base class can be directly accessed using this pointer, but not the other pointers.
Does the compiler consider them as different instantiation?