How to define rules in the Makefile to compile only that *.cpp files which was modified (and their dependencies), not all *.cpp files

Question

Lets say I have files:

Libs:

• one.cpp, one.h
• two.cpp, two.h
• three.cpp, three.h

Program:

• program.cpp

Answer 1

First we make the object files prerequisites of the executable. Once this is done, Make will rebuild program whenever one of the SRCS changes, so we don't need OBJS as an explicit target:

all: program

program: $(OBJS)
  g++ -Wall $(OBJS) program.cpp -o program

Then we make the header files prerequisites of the objects, so that if we change three.h, Make will rebuild three.o:

$(OBJS): %.o : %.h

And finally since one.cpp uses code from two.cpp by means of two.h (I hope), we make two.h a prerequisite of one.o:

one.o: two.h

And to make things cleaner and easier to maintain we use automatic variables:

program: $(OBJS)
  g++ -Wall $^ program.cpp -o $@

Put it all together and we get:

SRCS = one.cpp two.cpp three.cpp
OBJS = $(SRCS:.cpp=.o)

all: program

$(OBJS): %.o : %.h

one.o: two.h

.cpp.o:
  g++ -Wall -c $<

program: $(OBJS)
  g++ -Wall $^ program.cpp -o $@

clean:
  rm -f $(OBJS) program

There are a few more things we could do (like adding program.o to OBJS), but this is enough for today.