Question

I’m having trouble with what is apparently a linker error (‘undefined reference’) in Eclipse / C++. All the classes shown below compile fine, except for one, PlayGame.cpp, which is giving the ‘undefined reference’ error (also shown below).

Below are the relevant classes and pieces of code.

PlayerFactory.h

PlayerFactory.cpp

Game.h

Game.cpp

// constructor for game:  Game::Game (const PlayerFactory& factory)  {         cout << ' constructor' << endl;  } 

PlayGame.cpp

// start of code for game where error occurs  #include 'Game.h'  #include 'PlayerFactory.h'  int main() {    try     {        PlayerFactory factory;        Game game (factory);  <== undefined reference error       ... 

The above line gives the error ‘undefined reference to `Game(PlayerFactory const&)”

What’s causing this error, and how can it be corrected?

Answer 1

The default visibility for class declarations is private. So all the member functions of both Player and PlayerFactory classes are private — not accessible by clients. You need to make them public.

Player.h

#ifndef PLAYER_H  #define PLAYER_H class Player   {   public:    virtual ~Player() {          cout << 'Player destructor called' << endl;       }      virtual void Player::PlayerMakeMove(){          cout << 'make move' << endl;      }     };   #endif // PLAYER_H 

PlayerFactory.h

#ifndef PLAYERFACTORY_H #define PLAYERFACTORY_H class PlayerFactory   {   public:    virtual ~PlayerFactory() {          cout << 'PlayerFactory destructor called' << endl;      }       virtual std::auto_ptr<Player> PlayerFactory::MakePlayerX() const{           return PlayerFactory::MakePlayer('x');      }       virtual std::auto_ptr<Player> PlayerFactory::MakePlayerO() const{          return PlayerFactory::MakePlayer('o');       }       std::auto_ptr<Player> PlayerFactory::MakePlayer (char letter) const{          auto_ptr<Player> pt( new Player() );          return pt;      }   }; #endif // PLAYERFACTORY_H 

Also, the Game::Play() lacks a return statement.

Outcome Game::Play() {     cout << ' play' << endl;     return PlayerXWon; } 

Do add the required headers, forward declarations and using statements as required (I skip them here).