a couple of Makefile issues - c++

Question

I've got this Makefile:
CFLAGS = -c -Wall
CC = g++
EXEC = main
SOURCES = main.cpp listpath.cpp Parser.cpp
OBJECTS = $(SOURCES: .cpp=.o)
EXECUTABLE = tp
DIR_SRC = /src/
DIR_OBJ = /obj/
all: $(SOURCES) $(OBJECTS)
$(EXECUTABLE): $(OBJECTS)
$(CC) $(CFLAGS) $(OBJECTS) -o $#
.cpp.o:
$(CC) $(CFLAGS) $< -o $#
clean:
rm $(OBJECTS) $(EXECUTABLE)
Note this:
I'm in the directory "." which contains the makefile
The folder "./src" EXISTS, and has all the .h and .cpp files
The folder "./obj" doesn't exist, I want makefile to create it and put all the .o there
The error I get is:
No rules to build "main.cpp", necessary for "all". Stopping.
Help!

Answer 1

All right, from the top:
CFLAGS = -c -Wall
CC = g++
# EXEC = main never used, not needed
SOURCES = main.cpp listpath.cpp Parser.cpp
So far, so good. Note that this SOURCES doesn't mention DIR_SRC, so we'll have to make that connection later (and $(DIR_SRC)$(SOURCES) won't work, because the path must be appended to each member of the list). But OBJECTS really needs paths (e.g. /obj/main.o):
OBJECTS = $(patsubst %.cpp, $(DIR_OBJ)%.o, $(SOURCES))
EXECUTABLE = tp
DIR_SRC = /src/
DIR_OBJ = /obj/
(Personally I don't like putting the trailing slash in the variable, but it's a matter of taste.) The first target is the default target, so it should build what you actually want built:
all: $(EXECUTABLE)
Don't worry about listing the sources as prerequisites; they will sort themselves out later.
$(EXECUTABLE): $(OBJECTS)
$(CC) $(CFLAGS) $^ -o $# # <-- note the automatic variable $^
The .cpp.o convention doesn't really work here; we'll have to spell it out. And we must tell Make to search $(DIR_SRC) for .cpp files:
$(OBJECTS): $(DIR_OBJ)%.o: %.cpp $(DIR_OBJ)
$(CC) $(CFLAGS) $< -o $#
$(DIR_OBJ):
mkdir $#
vpath %.cpp $(DIR_SRC)
And tell Make that clean is not a real target, just to be safe:
.PHONY: clean
clean:
rm $(OBJECTS) $(EXECUTABLE)
EDIT:
I shouldn't have attempted so much in one step. Let's try something simpler:
$(DIR_OBJ)%.o: $(DIR_SRC)%.cpp $(DIR_OBJ)
$(CC) $(CFLAGS) $< -o $#

Answer 2

Edit the SOURCES to include the source directory (e.g. src/main.cpp etc.).
For the object files, consider something like this:
OBJECTS = $(subst src/,obj/,$(SOURCES:%.cpp=%.o))
# ...
all: $(SOURCES) build
.PHONY: build
build: pre_build $(EXECUTABLE)
.PHONY: pre_build
pre_build: obj
obj:
-mkdir obj
$(EXECUTABLE): $(OBJECTS)
$(CC) $(CFLAGS) $^ -o $#