Modular C++ Project Build with CMake - c++

Question

I'm trying to find a way to build a big modular C++ project with CMake.
The structure of the project is the following:
--project_root
--src
--folder_1
--source_1.h
--source_1.cc
--test_source_1.cc // file containing a main with unit tests
--folder_2
--source_2.h
--source_2.cc
--test_source_2.cc // file containing a main with unit tests
--folder_3
...
And so on.
Each folder represent a project module and each module might depend on other modules, so for example source_1.h may include source_2.h.
Every module folder may also contains a test file so the whole project will have multiple executables.
How can I build the whole project with CMake? How should I write my CMakeLists.txt file?
Thank you a lot.

Answer 1

There are many, many examples out there of how to structure CMake projects for C++, many of which are referenced by the tutorial #user2485710 suggested in his comment, so I'm not going to go super in-depth here, but I'll at least give you a good starting point based on the way you want to lay out you folder structure.
The nice thing about CMake is that it can essentially do a tree-decent using the add_subdirectory command. This lets us easily divide up our CMake code to only do what is required at any specific directory level. In otherwords, each CMakeLists.txt file should only do the minimal amount of work needed to properly set up things at the current depth in the directory tree. In you example, your CMake tree might look like this:
--project_root
--src
--CMakeLists.txt
--folder_1
--CMakeLists.txt
--source_1.h
--source_1.cc
--test_source_1.cc // file containing a main with unit tests
--folder_2
--CMakeLists.txt
--source_2.h
--source_2.cc
--test_source_2.cc // file containing a main with unit tests
...
In src/CMakeLists.txt you do all of your project-level initialization, I.E. find_package, setting up your include-path, etc. Then you simply add the following at the end:
add_subdirectory(folder_1)
add_subdirectory(folder_2)
...
This tells CMake that it should look in those folders for additional stuff to do. Now in src/folder_1/CMakeLists.txt, we do the actual work of whatever combination of add_executable and add_library you need to properly build source_1.cc and test_source_1.cc, and likewise in src/folder_2/CMakeLists.txt for source_2.cc, etc.
The other nice thing is that any CMake variables you set higher up the tree are propagated down through add_subdirectory. So, for example, in src/CMakeLists.txt you can check for some sort of 'build unit-test' flag and set the CMake variable there, and then all you have to do in the other CMakeLists.txt files is check for that variable. This can also be super useful to do if you have a project where CMake is dynamically generating header files for you based on checking environment variables for path-names and the like.

Answer 2

If the structure of the project is well-regulated, you could write custom macros or function of cmake to define the modules and their dependencies.
The cmake scripts in OpenCV project is a good reference:
/libs/opencv-2.4.8/sources/
|+cmake/
|+doc/
|~modules/
| |+core/
| | |+doc/
| | |+include/
| | |+perf/
| | |+src/
| | |+test/
| | `-CMakeLists.txt
| |~imgproc/
| | |+doc/
| | |+include/
| | |+perf/
| | |+src/
| | |+test/
| | `-CMakeLists.txt
| |+ml/
| |+...
| |-CMakeLists.txt
|-CMakeLists.txt
root/modules/imgproc/CMakeLists.txt
set(the_description "Image Processing")
ocv_define_module(imgproc opencv_core)

Answer 3

You will need a CMakeLists.txt in each folder where building will occur.
project() is used to set the name of your overall project.
add_subdirectory() is used to command the configuration to process the CMakeLists.txt in that directory.
add_executable() is used to create an executable from included sources.
add_library() is used to create a static or dynamic library that can be added to executables or libraries as a dependency.
Using a build script (.bat, .cmd, or .sh) will allow you to automate some of the cmake process, such as setting up an out-of-source configuration or build.
You should look up the documentation for these commands on the cmake website, https://cmake.org/cmake/help/latest/