How can I store my .o files in a sepparate folder? - makefile

Question

INCDIR=include
SRCDIR=src
SRC = $(wildcard $(SRCDIR)/*.cpp)
DEPS = $(wildcard $(INCDIR)/*.h)
OBJ = $(SRC:.cpp=.o)
CFLAGS = -I$(INCDIR) -Wall -Weffc++ -Wextra -Wsign-conversion
CC=g++
preprocessor: $(OBJ)
$(CC) $(OBJ) -o $# $(CFLAGS)
$(SRCDIR)/%.o: $(SRCDIR)/%.cpp
$(CC) -c $(CFLAGS) $^ -o $#
clean:
rm $(SRCDIR)/*.o preprocessor
This is my current makefile, if I wanted to store my .o files in a sepparate directory, src/obj for example, how would I have to modify it?

Answer 1

Many ways to do it, but according to your own code, you could do:
INCDIR=include
SRCDIR=src
OBJDIR=obj
SRC= $(wildcard $(SRCDIR)/*.cpp)
DEPS= $(wildcard $(INCDIR)/*.h)
OBJ=$(patsubst %.cpp, $(OBJDIR)/%.o, $(notdir $(SRC)))
CFLAGS= -I$(INCDIR) -Wall -Weffc++ -Wextra -Wsign-conversion
CC=g++
$(OBJDIR):
#if ! [ -d $(#) ]; then\
echo "==> creating dir: $(#)";\
mkdir $(#);\
fi
preprocessor: $(OBJDIR) $(OBJ)
$(CC) $(OBJ) -o $# $(CFLAGS)
$(OBJDIR)/%.o: $(SRCDIR)/%.cpp
$(CC) -c $(CFLAGS) $^ -o $#
clean:
rm $(OBJDIR)/*.o preprocessor
printobj: $(OBJDIR)
#echo "$(OBJ)"
will output:
$ gmake preprocessor
g++ -c -Iinclude -Wall -Weffc++ -Wextra -Wsign-conversion src/a.cpp -o obj/a.o
g++ -c -Iinclude -Wall -Weffc++ -Wextra -Wsign-conversion src/b.cpp -o obj/b.o
g++ obj/a.o obj/b.o -o preprocessor -Iinclude -Wall -Weffc++ -Wextra -Wsign-conversion
NOTE: the target printobj is just here to output what you could expect, the target $(OBJDIR) ensure your directory exist before creating object files.

Answer 2

The are just a few things to change:
INCDIR := include
SRCDIR := src
OBJDIR := $(SRCDIR)/obj
SRC := $(wildcard $(SRCDIR)/*.cpp)
DEPS := $(wildcard $(INCDIR)/*.h)
OBJ := $(patsubst $(SRCDIR)/%.cpp,$(OBJDIR)/%.o,$(SRC))
CFLAGS := -I$(INCDIR) -Wall -Weffc++ -Wextra -Wsign-conversion
CC := g++
preprocessor: $(OBJ)
$(CC) $^ -o $# $(CFLAGS)
$(OBJDIR)/%.o: $(SRCDIR)/%.cpp | $(OBJDIR)
$(CC) -c $(CFLAGS) $< -o $#
$(OBJDIR):
mkdir -p "$#"
clean:
rm -f $(OBJ) preprocessor
Note some other minor modifications:
the | $(OBJDIR) order-only prerequisite and the $(OBJDIR): rule to ensure the objects directory exists before compiling,
:= instead of = for all make variable assignments because you don't need recursively expanded variables here,
$^ instead of $(OBJ) in the link recipe because using automatic variables in recipes makes them more generic,
$< instead of $^ in the compile recipe because you compile only the first prerequisite, not all of them,
rm -f $(OBJ) preprocessor instead of rm $(SRCDIR)/*.o preprocessor to remove only the object files of the project and avoid errors if none exists.