一道题

定义基类Base，有两个共有成员函数fn1()、fn2()，私有派生出Derived类，如果想在Derived类的对象中使用基类函数fn1()，应怎么办？
我的代码:

#include
using namespace std;

class Base
{
public:
    Base(){}
    ~Base(){}
    void fn1();
    void fn2();
};

void Base::fn1()
{
    cout << "Base fn1 is used." << endl;
}

void Base::fn2()
{
     cout << "Base fn2 is used." << endl;
}

class Derived :public Base
{
public:
    Derived(){}
    ~Derived(){}
};

int main()
{
    Derived n;
    n.fn1();
    n.fn2();
    return 0;
}

教程的代码：

#include 
using namespace std;

class Base
{
public:
    int fn1() const { cout << "Base fn1 is used." << endl; return 1; }
    int fn2() const { cout << "Base fn2 is used." << endl; return 2; }
};

class Derived : private Base
{
public:
    int fn1() { cout << "Derived fn1 is used." << endl; return **Base::fn1();** };
    int fn2() { cout << "Derived fn2 is used." << endl; return **Base::fn2();** };
};

int main()
{
    Derived derive;
    derive.fn1();
    derive.fn2();
    return 0;
}

不是很明白加粗地方的想法。