c++ - Overloading of << operator using iterator as a parameter -

i`d print enum values text , use overloading. suppose have following code:

#include <iostream> #include <map> #include <string> #include <vector> #include <unordered_set>  enum enm{     one,     2 };  class complex{ public: friend std::ostream& operator<<(std::ostream& out, std::unordered_multiset<int>::const_iterator i){     switch (*i){         case one:{             return out<<"one";         }         case two:{             return out << "two";         }     } } void func(std::unordered_multiset<int> _v); };  void complex:: func(std::unordered_multiset<int> _v){     _v.insert(one);     _v.insert(two);     (std::unordered_multiset<int>::const_iterator i(_v.begin()), end(_v.end()); != end; ++i){         std::cout <<"num: " << *i <<std::endl; //need here "one", "two" instead of 0, 1     } }  int main(){     complex c;     std::unordered_multiset<int> ms;     c.func(ms);     return 0;    } 

the problem variant doesn`t work. so, 0, 1 instead of one, two. have no ideas how properly. thank help!

i'm assuming changed i *i in order program compile. in order print iterator must i, fails compiler error.

the problem insertion operator defined friend within class on first declaration, lookup find operator can depend on namespaces , classes associated argument types, lookup referred adl or koenig lookup.

since std::ostream , unordered_multiset::const_iterator not asscoiated complex (see adl#details), lookup fails find insertion operator.

the solution declare function outside of class normal unqualified lookup operator takes place:

std::ostream& operator<<(std::ostream&, std::unordered_multiset<int>::const_iterator); class complex { .. }; 

i recommend define operator outside of class, doesn't seem need access private/protected members of complex (part of purpose of befriending entities).