How to Initialize a Map of Unique pointer Objects sorted by a Object Variable

Question

Hello I am new to the c++ and have a problem with a Unique Pointer of a Object as a Key of a Map.

1. What does the template need to look like on std::map<std::unique_ptr<Person>,string,?> phonebookMap2;
   so the Person gets Sorted/Inserted initial by first name?
2. Or how do i sort the map, i tired it with sort(phonebookMap2.begin(),phonebookMap2.end(),sortfunction2); but then ther is this issue:

no match for 'operator-' (operand types are 'std::_Rb_tree_iterator<std::pair<const std::unique_ptr, std::__cxx11::basic_string > >' and 'std::_Rb_tree_iterator<std::pair<const std::unique_ptr, std::__cxx11::basic_string > >')

i have class looking like this:

#ifndef PERSON_H
#define PERSON_H
#include ..
class Person
{
  private:
  string m_firstName;
  string m_lastName;
  string m_address;

  public:
  Person();
  Person(const string& firstName, const string& lastName,
         const string& address);

  string getFirstName() const;
  string getLastName() const;
  string getAddress() const;


};

bool operator<(const Person& left, const Person& right){
    return left.getFirstName() < right.getFirstName();
};
#endif // PERSON_H
Main:
#include...
bool sortfunction2(const std::unique_ptr<Person> &x,
const std::unique_ptr<Person> &y) { return x->getFirstName() < y->getFirstName(); }
int main()
{

//Template missing 
std::map<std::unique_ptr<Person>,string,?> phonebookMap2;

phonebookMap2.insert(make_pair(std::make_unique<Person>("Max", "Mustermann", "Bahnstr. 17"),"06151 123456"));
phonebookMap2.insert(make_pair(std::make_unique<Person>("Hubert", "Kah", "Minnesängergasse 23"),"06151 654321"));

//Not working 
sort(phonebookMap2.begin(),phonebookMap2.end(),sortfunction2);<br />

Answer 1

You cannot std::sort a std::map. Elements in a std::map are sorted and you cannot change order, that would break invariants of the map.

You can provide the comparison as functor. Then you only need to specify the functors type as argument of std::map:

struct PersonCompare {
    bool operator()(const  std::unique_ptr<Person>& left, const  std::unique_ptr<Person>& right) const {
        return left->getFirstName() < right->getFirstName();
    }
};


int main() {

//Template missing 
std::map<std::unique_ptr<Person>,string,PersonCompare> phonebookMap2;

phonebookMap2.insert(make_pair(std::make_unique<Person>("Max", "Mustermann", "Bahnstr. 17"),"06151 123456"));
phonebookMap2.insert(make_pair(std::make_unique<Person>("Hubert", "Kah", "Minnesängergasse 23"),"06151 654321"));

}

Complete Example