Creating a robust Makefile for C++ program - c++

Question

Consider the following trivial Makefile. Notice that there are two classes with corresponding .h and .cpp files for each.
output: main.o class1.o class2.o
g++ main.o class1.o class2.o -o output
main.o: main.cpp
g++ -c main.cpp
class1.o: class1.cpp class1.h
g++ -c class1.cpp
class2.o: class2.cpp class2.h
g++ -c class2.cpp
clean:
rm *.o output
Is this Makefile robust? How can I make it more robust? Can someone provide a more robust version of this Makefile? It will be running on a 64 bit Red Hat Linux or CentOS machine.
If it matters, I am using:
-gcc --version 4.8.4 (From the GNU Compiler Collection)
-GDB version: GNU gdb (Ubuntu 7.7.1 -0ubuntu5~14.04.2) 7.7.1

Answer 1

I don't think your original makefile is far off the mark. I've just made a few additions:
Added a default all target - it's good to be explicit
Use the CXXFLAGS environment variable in all the calls to g++
Make main.o depend upon class1.h and class2.h as discussed
Specify that clean is a PHONY target just to be on the safe side if you add build artifact called clean
ALL := output
all : $(ALL)
output: main.o class1.o class2.o
g++ $(CXXFLAGS) main.o class1.o class2.o -o output
main.o: main.cpp class1.h class2.h
g++ $(CXXFLAGS) -c main.cpp
class1.o: class1.cpp class1.h
g++ $(CXXFLAGS) -c class1.cpp
class2.o: class2.cpp class2.h
g++ $(CXXFLAGS) -c class2.cpp
.PHONY : clean
clean:
rm *.o output

Answer 2

I would write something like
CXX = g++
# mandatory build flags
AM_CXXFLAGS = -Wall -W -std=gnu++11
# optional build flags
CXXFLAGS = -O2 -Werror
# mandatory link flags
AM_LDFLAGS = -Wl,-as-neeeded
output: main.o class1.o class2.o
${CXX} ${AM_LDFLAGS} ${LDFLAGS} $(filter %.o,$^) -o $#
%: %.cpp
${CXX} ${AM_CXXFLAGS} ${CXXFLAGS} $< -c -o $#
class1.o: class1.cpp class1.h
class2.o: class2.cpp class2.h
the *FLAGS are following automake notation: AM_* mean mandatory flags, the normal flags contain local settings (e.g. debugging or optimiziation).
Usually, AM_CPPFLAGS and CPPFLAGS with preprocessor flags (-I ...) should be used too but I omitted them here
the output target might need ${LIBS} too which have been omitted here
dependency tracking is more tricky and not implemented; you will have to play with -dM compiler option here...
When your program consists only of the listed 5 files, you can write
output_SOURCES = main.c class1.c class2.c class1.h class2.h
output: ${output_SOURCES}
${CXX} ${AM_CXXFLAGS} ${CXXFLAGS} ${AM_LDFLAGS} ${LDFLAGS} $(filter %.cpp,$^) -o $# ${LIBS}
directly.
EDIT:
For automatic dependency tracking, you can write
DEPGENFLAGS = \
-MD -MF ${#D}/.${#F}.d -MT '$#'
%: %.cpp
${CXX} ${DEPGENFLAGS} ${AM_CXXFLAGS} ${CXXFLAGS} $< -c -o $#
-include .deps.main.o.d
-include .deps.class1.o.d
-include .deps.class2.o.d

Answer 3

How can I make it more robust?
Use implicit rules, keep it simple.
LDLIBS+=-lstdc++
output: main.o class1.o class2.o
clean:
rm *.o output