Undefined Reference, Symbols not found for architecture x86_64

Question

I made this simple classes and I don't know why I'm keep getting error; here's my code:

    using namespace std;

    class Player{
    public:
        Player(string name);
        virtual ~Player();
        virtual void fight() = 0;
    private:
        string _name;
    };       


    #include "Player.h"
    #include <iostream>
    using namespace std;

    class Elf : public Player{
    public:
         Elf(string elf_name);
        ~Elf();
        void fight(){
            cout << "I kill you!" << endl;
        }
    private:
        string _elf_name;
    };

    class Dwarf : public Player{
    public:
        Dwarf(string dwarf_name);
        ~Dwarf();
        void fight(){
            cout<< "Where's The Arkengem!?" << endl;
        }
    private:
        string _dwarf_name;
    };

Here's the main:

    #include <iostream>
    #include <vector>
    #include "Player.h"
    #include "Elf.h"
    #include "Dwarf.h"
    using namespace std;

    int main(){
        vector<Player*> playerGroup;
        Elf* aElf = new Elf("Legolas");
        Dwarf* aDwarf = new Dwarf("Gimli");

        playerGroup.push_back(aElf);
        playerGroup.push_back(aDwarf);
        vector<Player*>::const_iterator itr;
        for(itr = playerGroup.begin(); itr != playerGroup.end(); itr++){
            cout << "  " << *itr;
        }

         return 0;
    }

That's the error message:

Undefined symbols for architecture x86_64:

"Elf::Elf(std::__1::basic_string, std::__1::allocator >)", referenced from: _main in Principale.o

"Dwarf::Dwarf(std::__1::basic_string, std::__1::allocator >)", referenced from: _main in Principale.o ld: symbol(s) not found for architecture x86_64

Thank you in advance.