朋友ostream＆运营商<<无法访问私人会员

Question

Roughly, this is what my code looks like:

template<typename K, typename V>
class A{
 private:
   size_t num_;
 public:
   A(initializer_list< something<K,V> > smthng);
   friend ostream& operator<<(ostream &out, const A &as){
       size_t number = num_;
  };  
};

template<typename K, typename V>
A<K,V>::A(initializer_list< something<K,V> > smthng){

    size_t sz = 5;
    num_ = sz;
}

For some reason my code will always give "error: invalid use of non-static data member" as an error when I attempt to compile. Obviously the code above isn't what I have exactly, but this is the only error I'm getting. I thought the benefit of using the friend function wasa thaat you can access private members, but I can't seem to do so.

Answer 1

A friend function is an external function wrt the class you are defining it in. You can access private members, but you have no this pointer available.

Threfore, you should change

size_t number = num_; 

with

size_t number = as.num_;

inside the operator<<() implementation.

Answer 2

As it currently stands, you are using the num_ variable as if it were a static member variable, that is, as if it does not belong to a particular object of a class but belongs to the class as a whole. What you need to do is to indicate the object whose private member variable num_ you need to access via you << operator. You need to do:

class A{

private:
    int num_;

public:
    A();

friend std::ostream& operator<<(std::ostream &out, const A &as)
{
   int number = as.num_;
   out << number << std::endl;
}  
};

A::A()
{

   int sz = 5;
   num_ = sz;
}

int main()
{
   A object;
   std::cout << object << std::endl;

   return 0;
}  

This works fine for me. Try to compile this as it is and you will see it displaying 5 on the console without errors. The segmentation fault is probably due to another error on your code.

Answer 3

I have same issue with the operator, it is not accessing the private members of the class.

class X {
    char* xName;
    double dd;
public:
    X(char*, double);
    X(const X&);
    ~X();
    X& operator=(const X&);
    friend ostream& operator<<(ostream&, X&);
};

//in cpp

ostream& operator<<(ostream& o, X& obj) {

    /*when i try the following i have error msg saying that private member xName is
   inaccessible*/
  o>> obj.xName >> endl;

}