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 thefoo
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 thefoo.hpp
functionsfoo_p.hpp
: private header of thefoo
library. This one may include third-party headers (sayzeromq.h),
without having thefoo.hpp
header depending onzeromq.h,
which is nice for the users of your library. If you link statically withzeromq,
users offoo
won’t even need to know aboutzeromq
(well, this is true iffoo
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 isfooproject
- 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 namedfoo
, and will be installed tofoo/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 afoo
subdirectory. - We have used
qi_install_header
with theKEEP_RELATIVE_PATHS
argument. You could also have used it with aSUBFOLDER
argument, like this:
qi_install_header(foo/foo.hpp SUBFOLDER foo)
- Let’s assume you have two libraries,
foo
andbar
, and afoobar
executable that needs code fromfoo
andbar
.
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