# qiBuild documentation

# How to write a library

This is merely a convention, but you are advised to follow it, especially if you are working in a large project.

Let’s say you have a `foo` library.

You have the following files:

- `foo.hpp`: the public header of the `foo` library. It contains the public API, and include the least possible number of other headers. (Use of forward declarations, and PIMPL implementations are recommended)
- `foo.cpp` : implementation of the `foo.hpp` functions
- `foo_p.hpp`: private header of the `foo` library. This one may include third-party headers (say `zeromq.h),` without having the `foo.hpp` header depending on `zeromq.h,` which is nice for the users of your library. If you link statically with `zeromq,` users of `foo` won’t even need to know about `zeromq` (well, this is true if `foo` is a dynamic library, but that’s an other topic)
- `foo_p.cpp` : private implementation.
- `test_foo.cpp` : You would not dare writing a library without unit tests, would you?

## Proposed layout

This is what your layout should look like:

    fooproject
    |__ libfoo
        | CMakeLists.txt
        |__ foo
        |   |__ foo.hpp
        |__ src
        |   |__ foo.cpp
        |   |__ foo_p.hpp
        |   |__ foo_p.cpp
        |__ test
            |__ CMakeLists.txt
            |__ foo_test.cpp

- The full path to the public header is `libfoo/foo/foo.hpp`. Note that the name of the root directory is `fooproject`
- The private code is put in a `src` sub-directory. Private and public directories are separated, it’s easy to search only in public headers.

Note: you can download an archive containing the foo project here: `fooproject.zip`

## CMake

Here’s what the `CMakeLists.txt` should look like

    include_directories(".")

    qi_create_lib(foo
      SRC foo/foo.hpp
          src/foo.cpp
          src/foo_p.hpp
          src/foo_p.cpp
      SHARED
    )

    # Add dependencies to the foo library here:
    # qi_use_lib(foo ... )

    qi_install_header(KEEP_RELATIVE_PATHS foo/foo.hpp)

    qi_stage_lib(foo)

    add_subdirectory(test)

Please note that the location of the CMake list file matters.

## Rationale

You will note that:

- The only time we call `include_directories()` is when we are staging the foo library.
- The `foo.hpp` header is in a directory named `foo`, and will be installed to `foo/foo.hpp`. It’s advised you use the same name for the target and the subdirectory.
- Everything that need a `foo` header must use

    #include <foo/...>

This way, we are sure that the code we use can be re-distributed when the headers are installed, and that the path to find the headers while in the source tree does not differ from the paths to find the installed headers. This works because:

- We have put `foo.hpp` in a `foo` subdirectory.
- We have used `qi_install_header` with the `KEEP_RELATIVE_PATHS` argument. You could also have used it with a `SUBFOLDER` argument, like this:

    qi_install_header(foo/foo.hpp SUBFOLDER foo)

- Let’s assume you have two libraries, `foo` and `bar`, and a `foobar` executable that needs code from `foo` and `bar`.

With the proposed layout, you have something like:

    foooproject
    |__ libfoo
    |    |__ foo
    |         |__ foo.hpp
    |__ libbar
    |    |__ bar
    |         |__ bar.hpp
    |__ foobar
         |__ foobar.cpp

You may want to get rid of the `libfoo/foo`, `libbar/bar` redundancy and do this instead:

    fooproject
    |__ foo
    |   |__ foo.hpp
    |__ bar
    |   |__ bar.hpp
    |__ foobar
        |__ foobar.cpp

But, let’s assume you make a mistake, and write

    qi_use_lib(foobar foo)

instead of

    qi_use_lib(foobar foo bar)

In the first layout, you will have an error during compile time, looking like:

    bar/bar.hpp : no such file or directory

because the include directory that has been staged for foo is different from the include directory that has been staged for bar. Using the second layout, you will have an error during link time, looking like:

    undefined reference to `bar_func'

because the include directory that was staged was always the same: fooproject. The additional nesting level helps you catch this king of errors early.

Note

For large libraries, also consider using submodules. The documentation can be found here

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