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?

molbdnilo · Accepted Answer · 2015-01-08 13:31:38Z

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.

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