Why create an include/ directory in C and C++ projects?

Question

When I work on my personal C and C++ projects I usually put file.h and file.cpp in the same directory and then file.cpp can reference file.h with a #include "file.h" directive.

However, it is common to find out libraries and other kinds of projects (like the linux kernel and freeRTOS) where all .h files are placed inside an include/ directory, while .cpp files remain in another directory. In those projects, .h files are also included with #include "file.h" instead of #include "include/file.h" as I was hoping.

I have some questions about all of this:

1. What are the advantages of this file structure organization?
2. Why are .h files inside include/ included with #include "file.h" instead of #include "include/file.h"? I know the real trick is inside some Makefile, but is it really better to do that way instead of making clear (in code) that the file we want to include is actually in the include/ directory?

question from:https://stackoverflow.com/questions/48554758/why-create-an-include-directory-in-c-and-c-projects

Answer 1

The main reason to do this is that compiled libraries need headers in order to be consumed by the eventual user. By convention, the contents of the include directory are the headers exposed for public consumption. The source directory may have headers for internal use, but those are not meant to be distributed with the compiled library.

So when using the library, you link to the binary and add the library's include directory to your build system's header paths. Similarly, if you install your compiled library to a centralized location, you can tell which files need to be copied to the central location (the compiled binaries and the include directory) and which files don't (the source directory and so forth).