如何构建一个名字成员在类外可以被修改而在内部是只读的对象

c++有太多的trick，下面是一个实例，代码如下：

class Father
{
public:
    Father()
        :count(0) {cout << "father create " <<  ++count << " times!" << endl;}
   
    ~Father() {cout << "father destory!" << endl;}

protected:
    int count;
};

class Son : public Father
{
public:
    Son(int i)
        : Father(), ival(i)
    {cout << "son create " << count << " times" << endl;
     cout << ival << endl;}

    ~Son()
    {
        cout << "son destory!" << endl;
    }
private:
    const int& ival;
};

i