c++ - Using make to move .o files to a separate directory

Question

I've tried numerous attempts to move my .o files to my obj folder, but no matter what I do, it simply just doesn't work.

Judging from the makefile provided, what is the best method to move .o files to a specified folder?

BIN = bin/
OBJ = obj/
TARGET = opengl_03
DEPS = main.o  displayinit.o initializer.o algorithms.o matrix3f.o window.o vertex3.o
CC = g++
CFLAGS = -g 
LIBS = -lglut -lGLEW -lGL 
INCLUDEPATH = -L/usr/include/ -L/usr/lib/ -L/usr/lib/x86_64-linux-gnu/

$(TARGET) : $(DEPS)
    $(CC) $(CFLAGS) -o $(BIN)$(TARGET) $(DEPS) $(LIBS) $(INCLUDEPATH) 

displayinit.o : displayinit.cpp displayinit.h
    $(CC) $(LIBS) $(INCLUDEPATH) -c displayinit.cpp && mv displayinit.o $(OBJ)displayinit.o
initializer.o : initializer.cpp initializer.h
    $(CC) $(LIBS) $(INCLUDEPATH) -c initializer.cpp $(OBJ)
algorithms.o : algorithms.cpp algorithms.h
    $(CC) -c algorithms.cpp $(OBJ)
matrix3f.o : matrix3f.cpp matrix3f.h
    $(CC) $(LIBS) $(INCLUDEPATH)  -c matrix3f.cpp $(OBJ)
vertex3.o : vertex3.cpp vertex3.h
    $(CC) $(LIBS) $(INCLUDEPATH)  -c vertex3.cpp $(OBJ)
window.o : window.cpp window.h
    $(CC) $(LIBS) $(INCLUDEPATH) -c window.cpp $(OBJ)
main.o : main.cpp
    $(CC) $(LIBS) $(INCLUDEPATH) -c main.cpp $(OBJ)

Answer 1

To specify where the object is created use -o

window.o : window.cpp window.h
    $(CC) $(LIBS) $(INCLUDEPATH) -c window.cpp -o $(OBJ)/$@

---

Here is what you could do:

1. specify the directory where you want the object files to go

   OBJDIR    =   objdir
   

2. Create a list of object files that need to be compiled, from the list of all .cpp files by replacing .cpp with .o and add the prefix $(OBJDIR)/ to it:

   OBJ = $(addprefix $(OBJDIR)/, $(patsubst %.cpp, %.o, $(wildcard *.cpp)))
   

   So your $(OBJ) will look like: objdir/window.o objdir/main.o and so on

3. Add a target to create the directory if it does not exist:

   $(OBJDIR):
       mkdir $(OBJDIR)
   

4. Make the directory target before you make your main target:

   all: $(OBJDIR) myapp
   

5. Rule to compile all the .o object files in $(OBJDIR) from .cpp files in the current directory:

   $(OBJDIR)/%.o: %.cpp
       $(GCC) $(CPPFLAGS) -c $< -o $@
   

   This will result in something like:

   g++ -c main.cpp -o objdir/main.o
   

6. Your main target is unchanged:

   $(TARGET): $(OBJ)
       $(GCC) $(LDFLAGS) -o $@ $^ 
   

   This will look like:

   g++  -o myapp objdir/window.o objdir/main.o 
   

7. For completeness add clean target to cleanup objects:

   clean:
       @rm -f $(TARGET) $(wildcard *.o)
       @rm -rf $(OBJDIR) 
   

8. And define .PHONY targets, e.g. these will be made even if directories or files with the same name exist:

   .PHONY: all clean
   

So it should look like:

OBJDIR    =   objdir
OBJ       =   $(addprefix $(OBJDIR)/, $(patsubst %.cpp, %.o, $(wildcard *.cpp)))
TARGET    =   my app

.PHONY: all clean

all: $(OBJDIR) $(TARGET)

$(OBJDIR):
    mkdir $(OBJDIR)

$(OBJDIR)/%.o: %.cpp
    $(GCC) $(CPPFLAGS) -c $< -o $@

$(TARGET): $(OBJ)
    $(GCC) $(LDFLAGS) -o $@ $^ 

clean:
    @rm -f $(TARGET) $(wildcard *.o)
    @rm -rf $(OBJDIR) 

And if you have files such as main.cpp and a.cpp this is what make would do:

> ls
Makefile a.cpp    main.cpp

> make
mkdir objdir
g++ -I. -c a.cpp -o objdir/a.o
g++ -I. -c main.cpp -o objdir/main.o
g++ -o myapp objdir/a.o objdir/main.o 

> ls
Makefile a.cpp    main.cpp objdir   myapp

> make clean
> ls
Makefile a.cpp    main.cpp

And if you want to read more details about any of the above have a look at GNU make doc page