How a friend class can access a private member of a nested class?

Question

Consider the following example:

class SIP{
    public:
        friend std::ostream& operator<<(std::ostream& os, const SIP& c);
    private:
        class BusStop;
        std::vector<BusStop*> mbusStops;
};

class SIP::BusStop{
    private:
        struct BusInfo;
        std::vector<BusInfo*> mbusStopTerminal;
};

struct SIP::BusStop::BusInfo{
    std::string from;
    std::string to;
};

std::ostream& operator<<(std::ostream &os, const SIP &c) {
    for (std::vector<SIP::BusStop*>::const_iterator it = c.mbusStops.begin(); 
         it != c.mbusStops.end(); it++){
        for (std::vector<SIP::BusStop::BusInfo*>::const_iterator it2 = mbusStopTerminal.begin(); 
             it2 != mbusStopTerminal.end(); it2++){
        }
    }
    return os;
}

It won't compile, because the BusInfo struct is private. Friend classes can't access private members of nested classes by default. What should I do in that situation? Is there any workaround?

Answer 1

You could add a stop-printing function to SIP:

class SIP{
    public:
        friend std::ostream& operator<<(std::ostream& os, const SIP& c);
    private:
        void printStops(std::ostream& os);
        class BusStop;
        std::vector<BusStop*> mbusStops;
};

std::ostream& operator<<(std::ostream &os, const SIP &c) {
    c.printStops(os);
    return os;
}

or you could just add operators all the way down:

class SIP{
    public:
        friend std::ostream& operator<<(std::ostream& os, const SIP& c);
    private:
        class BusStop;
        std::vector<BusStop*> mbusStops;
};

class SIP::BusStop{
    private:
        friend std::ostream& operator<<(std::ostream& os, const BusStop& c);

        struct BusInfo;
        std::vector<BusInfo*> mbusStopTerminal;
};

struct SIP::BusStop::BusInfo{
    std::string from;
    std::string to;
};

std::ostream& operator<<(std::ostream &os, const SIP::BusStop::BusInfo &i)
{
   // Whatever
}

std::ostream& operator<<(std::ostream &os, const SIP::BusStop &c)
{
    for (std::vector<SIP::BusStop::BusInfo*>::const_iterator it = mbusStopTerminal.begin(); 
         it != mbusStopTerminal.end(); it++){
        os << **it;
    }   
}

std::ostream& operator<<(std::ostream &os, const SIP &c) {
    for (std::vector<SIP::BusStop*>::const_iterator it = c.mbusStops.begin(); 
         it != c.mbusStops.end(); it++){
        os << **it;
    }
    return os;
}

or any combination of approaches that suits your code.