c++ - Insertion into map not sorted properly -

i confused why records not sorted in map.

 class point{  public:    string s1,s2;  public:        point(string string1, string string2){             s1=string1;s2=string2;     }   };   bool operator<(const point &p1, const point &p2)  {     cout<<"p1.s1.compare(p2.s1)="<<p1.s1.compare(p2.s1)<<" p1.s1="<<p1.s1<<"  p2.s1="<<p2.s1<<endl;     return p1.s1.compare(p2.s1);  }  int main(int argc, char *argv[])  {     std::map<point,int>m1;     point p1("hello","hi");     m1.insert(std::make_pair(p1,1));     point p2("abc","kbd");     m1.insert(std::make_pair(p2,2));     point p3("hell","hi");     m1.insert(std::make_pair(p3,3));      std::map<point,int>::iterator it=m1.begin();     while(it!=m1.end())     {         cout<<"m1.first="<<it->first.s1<<"  m1.first.s2="<<it->first.s2<<"  m1.second="<<it->second<<endl;         it++;     }     return 0;  } 

the output is

m1.first=hell  m1.first.s2=hi  m1.second=3 m1.first=abc  m1.first.s2=kbd  m1.second=2 m1.first=hello  m1.first.s2=hi  m1.second=1 

but output expected was

m1.first=abc  m1.first.s2=kbd  m1.second=2 m1.first=hell  m1.first.s2=hi  m1.second=3 m1.first=hello  m1.first.s2=hi  m1.second=1 

could please clarify, way insertion works in rb tree, or other problem exists.

the operator< function needs return true if lhs supposed less rhs. std::string::compare() not return such value. returns negative value, zero, or positive value.

what need use:

return (p1.s1.compare(p2.s1) < 0 ); 

or use defined operator< strings.

return (p1.s1 < p2.s1);