GNU Make: always build pre-targets but don't rebuild target - gnu-make

Question

Imagine my Makefile has something like:
CXXFLAGS = -O3 ${INCLUDES} --std=c++17 -g ${AUTO_ARGUMENT}
COMPILE.cc = $(CXX) $(DEPFLAGS) $(CXXFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c
.PHONY: all directories
all: directories programs
directories: obj
obj:
mkdir obj
programs: Foo
Foo: obj/Foo.o
${CXX} obj/Foo.o ${LDFLAGS} -o Foo
obj/%.o : %.cpp
$(COMPILE.cc) $(OUTPUT_OPTION) $<
clean:
rm -rf Foo obj
I can execute make and it will create the obj subdirectory then do a nice compile and link. Works great. But if I do make clean Foo, it's going to fail. The clean removed the subdir and because I bypassed all to just make a single target, it doesn't recreate obj.
So I can do this:
Foo: directories obj/Foo.o
${CXX} obj/Foo.o ${LDFLAGS} -o Foo
But then it ALWAYS does the link:
$ make
g++ obj/Foo.o -o Foo
$ make
g++ obj/Foo.o -o Foo
But if I remove the directories part from Foo:
$ make
make: Nothing to be done for 'all'.
This is even worse:
obj/%.o : directories %.cpp
$(COMPILE.cc) $(OUTPUT_OPTION) $<
So, my question... Is there some way that I can tell an individual target to do some of the pre-setup without that target then always being rebuilt? I could probably make fake targets like this:
makeFoo: directories Foo
But that's annoying. I could also have all be:
all: setup programs
setup: directories
And then do make setup Foo. That's only moderately annoying. What I'd really like is the rule for the objects to ensure the directory exists without adding any spam or unnecessary rebuilds. I suppose I could add something to that particular rule to ensure the directory exists:
obj/Foo.o: Foo.cpp
if [ -d obj ]; then \
mkdir obj \
fi
$(COMPILE.cc) $(OUTPUT_OPTION) $<
Is there a cleaner way?

Answer 1

Thanks to Andreas, I made some adjustments. Here's my entire sample Makefile:
CXXFLAGS = -O3 ${INCLUDES} --std=c++17 -g ${AUTO_ARGUMENT}
COMPILE.cc = $(CXX) $(DEPFLAGS) $(CXXFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c
.PHONY: all directories
all: directories programs
directories: | obj
obj:
mkdir obj
programs: Foo
Foo: obj/Foo.o
${CXX} obj/Foo.o ${LDFLAGS} -o Foo
obj/%.o : %.cpp | obj
$(COMPILE.cc) $(OUTPUT_OPTION) $<
clean:
rm -rf obj Foo
And here are my runs:
$ make clean Foo
rm -rf obj Foo
mkdir obj
g++ -O3 --std=c++17 -g -c -o obj/Foo.o Foo.cpp
g++ obj/Foo.o -o Foo
$ make Foo
make: 'Foo' is up to date.
The trick was order-only dependencies -- the pipe thing. See the rule for obj/%.o and directories. Note that it works the same if I do it this way or if my obj/%.o rule used directories instead.