无法使用朋友类从基类访问私有成员

Question

This is my code:

class Base
{
    friend class SubClass;
    int n;
    virtual int getN()
    {
        return n;
    }
};

class SubClass: public Base
{
public:
    SubClass() {}
    SubClass(const SubClass& s) {}


};

int _tmain(int argc, _TCHAR* argv[])
{
    SubClass s;
    int x = s.getN();

    return 0;
}


error C2248: 'Base::getN' : cannot access private member declared in class 'Base'

What more do I have to do to access my private members from Base ?

Answer 1

Your friend declaration means that SubClass gets to access it in the scope of SubClass .

If you want users of a class to access something, at some point you need to a write public: function:

class SubClass : public Base
{
public:
    int getN()
    {
        return Base::getN();
    }
};

You can write a using declaration to bring a base class function into the current class:

class SubClass : public Base
{
public:
    // getN is considered declared at this point now (in public)
    using Base::getN(); 
};

Answer 2

为什么不将n声明为protected呢？

Answer 3

The code in main is not trying to access a SubClass method, it's trying to access a Base method - that's why it doesn't work.

Try adding an override in SubClass :

virtual int getN()
{
    return Base::getN();
}

Answer 4

getN() is never made public, so tmain() can't call it.

Make a sub class a friend of a base class is not the right way to do this anyway. It is a confusing subversion of the type system. If you want to access non-public members of a base class from a sub class but not have them public, make them protected.