c++ - map keys of abstract classes that are logically similar -

suppose user uses library kbase vbase classes. user adds pairs of keys , values of classes available him (he implemented them using library) - v , k. in library, when added, want keys logically similar (have same string) "merged" single key.

he're code sample i've tried :

//---------------------------library.h------------------------//  #include <map> #include <vector> #include <iostream>  using namespace std;     map<kbase*,vector<vbase*>> mapa;  void printmap(); void addtomap(kbase* k1, vbase* v2);  class kbase { public:     virtual ~kbase(){} };  class vbase { public:     virtual ~vbase(){} };  //---------------------------library.cpp------------------------//  void printmap() {     // expect 3 keys implemented.     (auto : mapa)         cout << i.first << endl; }  void addtomap(kbase* k1, vbase* v2) {     // create k1. if doesnt exist - vector created.     // otherwise, object obtained.     mapa[k1];     mapa[k1].push_back(v2); }  //------------------------user----------------------------//  #include "library.h"  // user's implementation. class k : public kbase { public:     k(string inputstring)     {         kstring = inputstring;     }     ~k(){}      string kstring; };  class v : public vbase { public:      v(string inputstring)     {         vstring = inputstring;     }     ~v(){}      string vstring; };  int main() {     // user adds keys , values     addtomap(new k("key1"), new v("value1"));     addtomap(new k("key1"), new v("value2"));     addtomap(new k("key1"), new v("value3"));      addtomap(new k("key2"), new v("value4"));     addtomap(new k("key2"), new v("value5"));     addtomap(new k("key2"), new v("value6"));      addtomap(new k("key3"), new v("value7"));     addtomap(new k("key3"), new v("value8"));     addtomap(new k("key3"), new v("value9"));      printmap();      return 0; } 

obviously, cant add base class key directly. , library not aware of classes implemented user (that inherit classes of library). again, how can library know if objects given logically similar.

should use different data structure purpose?

c++ map uses equivalence concept finding entry, defined via “less” comparison. 2 keys k1 , k2 considered equivalent if k1 “is not less than” k2 , k2 “is not less than” k1. need define functor comparing pointers kbase class.

class kbaselesscompare { public:     bool operator() (const kbase * k1, const kbase * k2)     {         // following, shall define comparison         return k1->name < k2->name;     } } 

then change map definition following:

map<const kbase*, vector<vbase*>, kbaselesscomapre> mapa;