gcc/g++ option to place all object files into separate directory

This is the chopped down makefile for one of my projects, which compiles the sources in 'src' and places the .o files in the directory "obj". The key bit is the the use of the patsubst() function - see the GNU make manual (which is actually a pretty good read) for details:

OUT = lib/alib.a
CC = g++
ODIR = obj
SDIR = src
INC = -Iinc

_OBJS = a_chsrc.o a_csv.o a_enc.o a_env.o a_except.o \
        a_date.o a_range.o a_opsys.o
OBJS = $(patsubst %,$(ODIR)/%,$(_OBJS))


$(ODIR)/%.o: $(SDIR)/%.cpp 
    $(CC) -c $(INC) -o $@ $< $(CFLAGS) 

$(OUT): $(OBJS) 
    ar rvs $(OUT) $^

.PHONY: clean

clean:
    rm -f $(ODIR)/*.o $(OUT)

How about changing to the directory and running the compile from there:

cd builddir/objdir
gcc ../../srcdir/file1.c ../../srcdir/file2.c ../../srcdir/file3.c

That's it. gcc will interpret includes of the form #include "path/to/header.h" as starting in the directory the file exists so you don't need to modify anything.

A trivial but effective workaround is to add the following right after the gcc call in your Makefile:

mv *.o ../builddir/objdir

or even a soft-clean (possibly recursive) after the compilation is done, like

rm -f *.o

or

find . -name \*.o -exec rm {} \;