g++: fatal error: cannot specify -o with -c, -S or -E with multiple files

Question

I am trying to compile a library file using other library files. I am using the following line in my makefile to create gameobject.o:

lib/gameobject.o: src/gameobject.cpp src/vector.hpp lib/objectevent.o lib/sprite.o
g++ $^ -c -o $@ $(SFML_FLAGS)

All the dependencies comile correctly, but I get the following error when it tries to compile gameobject.o:

g++: fatal error: cannot specify -o with -c, -S or -E with multiple files

I'm still a bit new to using make/separating compilation, so I'm not quite sure what I should do. Do I just have to compile it without setting an output? Do I have to compile gameobject.o without using any of my other .o files? If that's true, wouldn't compile times get pretty big for large objects if you can't compile libraries with other libraries? Or am I just reading this error completely wrong?

Answer 1

You're not building a library file here. A .o file is an object file. Typically there is one object file per source file. When you use the compiler's -c option, it takes a single source file and compiles it into a single object file. You cannot add other object files into an existing object file, so adding both .o and .cpp files into the same compiler line with -c is not going to work.

If you want to create a library, that would be something like libfoo.a (the "a" here stands for "archive"). If you want to create an executable you can do that as well.

You need to be more clear about exactly what result you want before we can describe how to get it.