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
You have the following files:
foo.hpp: the public header of the
foolibrary. 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_p.hpp: private header of the
foolibrary. This one may include third-party headers (say
zeromq.h),without having the
foo.hppheader depending on
zeromq.h,which is nice for the users of your library. If you link statically with
foowon’t even need to know about
zeromq(well, this is true if
foois 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?
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
- The private code is put in a
srcsub-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:
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.
You will note that:
- The only time we call
include_directories()is when we are staging the foo library.
foo.hppheader 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
fooheader must use
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
- We have used
KEEP_RELATIVE_PATHSargument. You could also have used it with a
SUBFOLDERargument, like this:
qi_install_header(foo/foo.hpp SUBFOLDER foo)
- Let’s assume you have two libraries,
bar, and a
foobarexecutable that needs code from
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
libbar/bar redundancy and do this
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 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.
For large libraries, also consider using submodules. The documentation can be found here