Question

I have some code that compiles, but won’t link. I’m not sure why this is – I know that this issue is usually solved by fixing what’s linked together, but I don’t know why this is breaking. TransGame is a class that extends Game, and SearchGame is a class that extends TransGame.

Here’s the error I’m getting:

mrdmnd@dr-wily:~/ConnectFour$ make all                                                                                                                                                                             
g++    -c -o SearchGame.o SearchGame.cc
g++    -c -o TransGame.o TransGame.cc
g++    -c -o Game.o Game.cc
g++ SearchGame.o TransGame.o Game.o -o SearchGame
TransGame.o: In function `TransGame::hash()':
TransGame.cc:(.text+0x5f): undefined reference to `Game::positioncode()'
collect2: ld returned 1 exit status
make: *** [SearchGame] Error 1

Here’s my makefile:

CFLAGS = -c -Wall
SOURCES = SearchGame.cc TransGame.cc Game.cc
OBJECTS = $(SOURCES:.cc=.o)
EXECUTABLE = SearchGame
.PHONY : all clean

all : $(SOURCES) $(EXECUTABLE)

$(EXECUTABLE): $(OBJECTS)
    $(CXX) $(OBJECTS) -o $@

*.o :
    $(CXX) $(CFLAGS) $< -o $@

clean:
    rm -rf *.o

Here are the relevant bits of code:

TransGame.cc

#include "Game.h"
#include "TransGame.h"
// snip
void TransGame::hash() {
  long htemp = positioncode();
  //snip
}

TransGame.h

#ifndef TRANSGAME_H
#define TRANSGAME_H
//snip
class TransGame : public Game {
  public:
    //snip
    void hash();
    //snip
};

#endif

Game.cc

#include "Game.h"
// snip
inline long Game::positioncode() {
  return color[nplies & 1] + color[0] + color[1] + BOTTOM;
}

Game.h

#ifndef GAME_H
#define GAME_H
//snip
class Game {
  public:
    //snip
    long positioncode();
    //snip
};

#endif

Any help would be greatly appreciated – I’m sure I’m missing something obvious in my makefile.

Answer 1

Your function Game::positioncode() is an inlined function and needs to be in the header, since the compiler needs to know its definition to replace the call to positioncode with the code.

There are two ways: You can either add the function below the definition of class Game or you can add it directly to the declaration:

class Game {
  public:
    long positioncode()
    {
      return color[nplies & 1] + color[0] + color[1] + BOTTOM;
    }
};