c++ - Trying to sort a vector of objects by any class member -

i trying figure out how sort vector of city objects. each object has cityname member , citycode member. i'd accessor functions automatically sort vector display on screen in proper sorted order. think i'm close, can please tell me i'm doing wrong? much.

//specification file city class  #ifndef city_h #define city_h #include <string>  using namespace std;  class city { protected:     string cityname;     string citycode;  public:     //constructor     city();     city(string name, string code);      //setter     void setcityname(string name);     void setcitycode(string code);      //getter     string getcityname();     string getcitycode();       bool sortbycityname(const city & c1, const city & c2)     {         return c1.cityname < c2.cityname;     }      bool sortbycitycode(const city & c1, const city & c2)     {         return c1.citycode < c2.citycode;     } }; #endif 

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//implementation file city class  #include "city.h"  //constructor city::city() {     cityname = "";     citycode = ""; }  city::city(string name, string code) {     cityname = name;     citycode = code; }   //setter void city::setcityname(string name) {     cityname = name; }  void city::setcitycode(string code) {     citycode = code; }   //getter string city::getcityname() {     return cityname; }  string city::getcitycode() {     return citycode; } 

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//specification file citylist class  #ifndef citylist_h #define citylist_h #include <iostream> #include <string> #include <vector> #include <algorithm> #include "city.h"  using namespace std;  class citylist { private:     vector<city> cities;  public:     //constructor     citylist();      //setter     void addcity(string name, string code);      //getter     void getcitiesbyname();     void getcitiesbycode();       friend bool city::sortbycityname(const city & c1, const city & c2);     friend bool city::sortbycitycode(const city & c1, const city & c2);  }; #endif 

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//implementation file citylist class  #include "citylist.h"  //constructor citylist::citylist() {     cities.push_back(city("atlanta          ", "ggg"));     cities.push_back(city("orlando          ", "fff"));     cities.push_back(city("dallas/fort worth", "eee"));     cities.push_back(city("new york city    ", "ddd"));     cities.push_back(city("hawaii           ", "ccc"));     cities.push_back(city("chicago          ", "bbb"));     cities.push_back(city("los angeles      ", "aaa")); }   //setter void citylist::addcity(string name, string code) {     cities.push_back(city(name, code)); }   //getter void citylist::getcitiesbyname() {     sort (cities.begin(), cities.end(), sortbycityname);     (city &c : cities)         cout << "\t\t\t    " << c.getcityname() << "\t" << c.getcitycode() << endl; }  void citylist::getcitiesbycode() {     sort (cities.begin(), cities.end(), sortbycitycode);     (city &c : cities)         cout << "\t\t\t    " << c.getcitycode() << "\t\t" << c.getcityname() << endl; } 

your sort calls using sortbycityname , sortbycitycode, member functions not free-standing functions. sort call doesn't call member functions on object instance, member functions don't match signature , can't find them. can fix in 2 different ways: can either take them out of class altogether, or can make them static members , use class specifier e.g.

sort (cities.begin(), cities.end(), city::sortbycityname); 

p.s. 1 trick use in these situations have single functor class can sort either criteria depending on parameter.

struct comparecities {     comparecities(bool byname) : _byname(byname) {}     bool operator()(const city & c1, const city & c2)     {         if (_byname)             return c1.cityname < c2.cityname;         else             return c1.citycode < c2.citycode;     }     bool _byname; };