c++ - friend keyword for operator overloading -

i tried write simple example <<-operator-overloading. before, i've never used keyword "friend". not work without it. mistake did or why need friend keyword here?

class rational{  public:  rational(double num, double count)     : n(num),c(count){}   rational& operator+=(const rational& b){     this->n+= b.n;     this->c+= b.c;     return *this;  }   friend std::ostream& operator<<(std::ostream& os, const rational& obj)  {  std::cout << obj.n << "," << obj.c << std::endl;  return os;  }  private:  double n;  double c; }; 

thanks

you want stream objects internals not accessible through class' public interface, operator can't @ them. have 2 choices: either put public member class streaming

class t {   public:     void stream_to(std::ostream& os) const {os << obj.data_;}   private:     int data_; }; 

and call operator:

inline std::ostream& operator<<(std::ostream& os, const t& obj) {    obj.stream_to(os);    return os; } 

or make operator friend

class t {   public:     friend std::ostream& operator<<(std::ostream&, const t&);   private:     int data_; }; 

so can access class' private parts:

inline std::ostream& operator<<(std::ostream& os, const t& obj) {    os << obj.data_;    return os; }