Undefined references in makefile

Question

Ok, ive read about 10 tutorials, but i keep getting errors all the time, i have 5 files, main.cpp class.cpp, class.h and functions.cpp and functions.h. All of those use functions from different objects meaning that functions in functions.cpp uses objects from classes.cpp.

My makefile looks as follows

CC = g++ -O2 -I./sdl/include -L.
LIBS = -lm -lSDL -lpthread -ldl
SRC = main.cpp
SDLF = SDLfunctions.cpp
CLASS = classes.cpp
CLASSH = classes.h
SDLFH = SDLfunctions.h

all: main

main: SDLfunctions.o Classes.o $(SRC)
    $(CC) -o $@ $(SRC) $(LIBS)

SDLfunctions.o: $(SDLFH) $(SDLF) $(CLASS) $(CLASSH)
    $(CC) -o $@ $(SDLF) $(LIBS)

Classes.o: $(CLASS) $(CLASSH) $(SDLF) $(SDLFH)
    $(CC) -o $@ $(CLASS) $(LIBS) 

I keeps telling me that it has undefined references. What am i missing?

What makefile outputs

/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5.2/../../../crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
/tmp/ccJG6yQA.o: In function `DrawEnemies(SDL_Surface*)':
SDLfunctions.cpp:(.text+0x3a7): undefined reference to `Enemy::sprite'
/tmp/ccJG6yQA.o: In function `rysujpociski(int, SDL_Surface*, SDL_Surface*, 
std::vector<AllyBullet, std::allocator<AllyBullet> >&, double)':
SDLfunctions.cpp:(.text+0x141f): undefined reference to `AllyBullet::sprite'
/tmp/ccJG6yQA.o: In function `global constructors keyed to width':
SDLfunctions.cpp:(.text+0x14a7): undefined reference to `Enemy::Enemy()'
collect2: ld returned 1 exit status
make: *** [SDLfunctions.o] Error 1

The files compile great when i had them in Visual C++, so it has to be my makefile.

Answer 1

You are indeed doing something strange. What you should is to compile (-c) the object files and then link them together. This would look like this:

CC = g++ -O2 -I./sdl/include -L.
LIBS = -lm -lSDL -lpthread -ldl
SRC = main.cpp
SDLF = SDLfunctions.cpp
CLASS = classes.cpp
CLASSH = classes.h
SDLFH = SDLfunctions.h

all: main

main: SDLfunctions.o Classes.o $(SRC)
    $(CC) -o $@ $(SRC) SDLfunctions.o Classes.o $(LIBS)  # you forgot to link
                                                         # the object files

SDLfunctions.o: $(SDLFH) $(SDLF) $(CLASS) $(CLASSH)
    $(CC) -o $@ -c $(SDLF)        # -c added to compile, not link

Classes.o: $(CLASS) $(CLASSH) $(SDLF) $(SDLFH)
    $(CC) -o $@ -c $(CLASS)       # -c added to compile, not link

While you are doing this, it is even better if you compiled main.o separately also. Therefore:

CC = g++ -O2 -I./sdl/include -L.
LIBS = -lm -lSDL -lpthread -ldl
MAIN = main.cpp
SDLF = SDLfunctions.cpp
CLASS = classes.cpp
CLASSH = classes.h
SDLFH = SDLfunctions.h

all: main

main: SDLfunctions.o Classes.o main.o
    $(CC) -o $@ SDLfunctions.o Classes.o main.o $(LIBS)

main.o: $(SDLFH) $(MAIN) $(CLASSH)
    $(CC) -o $@ -c $(MAIN)

SDLfunctions.o: $(SDLFH) $(SDLF) $(CLASS) $(CLASSH)
    $(CC) -o $@ -c $(SDLF)

Classes.o: $(CLASS) $(CLASSH) $(SDLF) $(SDLFH)
    $(CC) -o $@ -c $(CLASS)

Also note that I removed the $(LIBS) when using -c because linking doesn't happen then.

Answer 2

You are trying to link your .o files into executables. Add a -c to the compile flags so it compiles only for you object files.

Make it the first option like this

SDLfunctions.o: $(SDLFH) $(SDLF) $(CLASS) $(CLASSH)
    $(CC) -c -o $@ $(SDLF) $(LIBS)