What is the reason for the "Undefined symbols for x86_64 architecture:" error in C ++?

I would like to know why these files (or maybe others in my project) do not want to link. As far as I know, it's okay to leave the constructor and destructor if you define them in the .h file, right? I did it with another .h in my project and it doesn't complain. What's happening?

Error message:

Undefined symbols for architecture x86_64:
  "BasicArena::BasicArena()", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

      

        
        
        
      

    

AreaInterface.h (for assignment purposes we are not allowed to edit this file)

class ArenaInterface
{
    public:
        ArenaInterface(){}
        virtual ~ArenaInterface(){}
        // lots of pure virtuals here
}

      

        
        
        
      

    

BasicArena.h

#ifndef __rpg__BasicArena__
#define __rpg__BasicArena__

#include <stdio.h>
#include "ArenaInterface.h"
#include "Cleric.h"

using namespace std;

class BasicArena : public ArenaInterface
{
public:
    BasicArena() {}
    ~BasicArena() {}
    virtual bool addFighter(string info);
    virtual bool removeFighter(string name);
    virtual FighterInterface* getFighter(string name);
    virtual int getSize();

private:
    vector<FighterInterface*> fighters;
};

#endif /* defined(__rpg__BasicArena__) */

      

        
        
        
      

    

BasicArena.cpp

#include "BasicArena.h"
#include <sstream>

using namespace std;
// member function definitions here

      

        
        
        
      

    

Cleric.h:

#ifndef __rpg__Cleric__
#define __rpg__Cleric__

#include <stdio.h>
#include <iostream>
#include "FighterInterface.h"

using namespace std;

class Cleric : public FighterInterface
{
public:
    Cleric(string nam, int HP, int str, int spd, int mag, int dmg) : name(nam), maxHP(HP), strength(str), speed(spd), magic(mag), damage(dmg){}
    ~Cleric() {}
    // lots of functions and data
}

      

        
        
        
      

    

main.cpp:

#include <iostream>
#include "BasicArena.h"

using namespace std;

int main() {
    BasicArena testarena;
    cout << "It works.\n";
    return 0;
}

      

        
        
        
      

    

UPDATE: Commenting out BasicArena declarations / definitions allows the program to work, but leaves me with the question of whether it is acceptable / desirable for the final program.