sdl_img | A simple image viewer based on SDL2 and stb_image

You aren't passing the -I to c++ in your makefile, when you compile the .cc [to a .o].

You only pass the flags on the c++ line used to link the .o files.

One of the .o targets needs sdl2-config as well for its c++ command.

That's your immediate problem. However ...

When you do: sdl2-config --cflags, it [partially] produces (e.g.) -I/usr/local/include/SDL2, so a source .cc file would work if it did: #include .

But, your .cc file does: #include .

That may not work because it's at a different directory level [unless you add -I/usr/local/include, which is sub-optimal because you're hardwiring the path].

If it's working, it's because the compiler assumes /usr/local/include is a default part of the path for .h files

UPDATE:

You'll have to excuse me I'm not very good at compiling yet. I tried adding -I/usr/local/include/SDL2/SDL.h in some different spots and it didn't change anything. Could you spell it out a bit more for me?

You do not want: -I/usr/local/include/SDL2/SDL.h That points to a file.

You want: -I/usr/local/include/SDL2 That points to the directory.

But, you should do two things: (1) Fix your makefile and (2) Change the #include directives in level.cc to be compatible

Here's my adjustment for your makefile:

UPDATE Your last edit shows the most important bit of the output (it's incidentally also the first part of the output):