关于下标操作符重载，书C++ Primer例子错误，求解答

class Foo 

{ 

public: 

    Foo()

    {

        for (int i = 0; i < 10; i++)

        {

            data.push_back(i);

        }

    }

    int &operator[] (const size_t); 

    const int &operator[] (const size_t) const; 

    // other interface members 

private: 

    std::vector<int> data; 

    // other member data and private utility functions 

}; 



// 下标操作符本身可能看起来像这样： 

int& Foo::operator[] (const size_t index) 

{ 

    return data[index];  // no range checking on index 

} 

const int& Foo::operator[] (const size_t index) const 

{ 

    return data[index];  // no range checking on index 

} 



int _tmain(int argc, _TCHAR* argv[])

{

    Foo a;

    a[0] = 145;   // 作左值

    int n = a[0]; // 作右值

    return 0;

}