Makefile to put object files from source files different directories into a single, separate directory?

Question

I'm using UnitTest++ to allow me to create unit tests for some C++ code (that should build on Linux or Mac OS X). I have a directory structure like this:

src
- Foo.cpp
- Bar.cpp
test
- FooTest.cpp
- BarTest.cpp
- Main.cpp
- Makefile
UnitTest++
- libUnitTest++.a

And this Makefile (adapted from the UnitTest++ Makefile) works nicely (with GNU make):

test = TestFooAndBar

src = ../src/Foo.cpp \
    ../src/Bar.cpp

test_src = Main.cpp \
    FooTest.cpp \
    BarTest.cpp

lib = ../UnitTest++/libUnitTest++.a

objects = $(patsubst %.cpp,%.o,$(src))
test_objects = $(patsubst %.cpp,%.o,$(test_src))


.PHONY: all
all: $(test)
    @echo Running unit tests...
    @./$(test)

$(test): $(lib) $(test_objects) $(objects)
    @echo Linking $(test)...
    @$(CXX) $(LDFLAGS) -o $(test) $(test_objects) $(objects) $(lib)

.PHONY: clean
clean:
    -@$(RM) -f $(objects) $(test_objects) $(test) 2> /dev/null

%.o : %.cpp
    @echo $<
    @$(CXX) $(CXXFLAGS) -c $< -o $(patsubst %.cpp,%.o,$<)

But I want to put all the .o files in an "obj" subdirectory of the "test" directory. How do I modify this Makefile to do so?

I've tried adding "obj/" to the objects and test_objects variables, but I can't figure out how to modify the %.o rule so it knows where the .o files are and refers to the correct .cpp files. Do I need to create two separate rules, one for each set of .cpp files?

Would it be simpler if instead of defining the src and test_src variables, I just have the Makefile build a .o (into obj/) for all .cpp files (both in the same directory as the Makefile and in ../src/)?

Answer 1

There's more than one way to do it, but this is a pretty good one (I really should have that hotkeyed).

vpath %.cpp ../src

src = Foo.cpp Bar.cpp 
test_src = Main.cpp FooTest.cpp BarTest.cpp 

objects = $(patsubst %.cpp,obj/%.o,$(src)) 
test_objects = $(patsubst %.cpp,obj/%.o,$(test_src)) 

$(objects): | obj

obj:
  @mkdir -p $@

obj/%.o : %.cpp
  @echo $< 
  @$(CXX) $(CXXFLAGS) -c $< -o $@