CMake: how best to build multiple (optional) subprojects?

Question

Imagine an overall project with several components:

• basic
• io
• web
• app-a
• app-b
• app-c

Now, let\'s say web

Answer 1

Have multiple CMakeLists.txt.

Many open-source projects take this appraoch (LibOpenJPEG, LibPNG, poppler &etc). Take a look at their CMakeLists.txt to find out how they've done this.

Basically allowing you to just toggle features as required.

Answer 2

I ended up doing what I outlined in my question, which is to check in an empty directory (containing a .gitignore file which ignores everything) and tell CMake to GLOB any directories (which are put in there by the user). Then I can just say cmake myrootdir and it does find all the various components. This works more or less OK. It does have some side drawbacks though, such as that some third-party tools like BuildBot expect a more traditional project structure which makes integrating other tools with this sort of arrangement a little more work.

Answer 3

You can use ADD_SUBDIRECTORY for this!

https://cmake.org/cmake/help/v3.11/command/add_subdirectory.html

Answer 4

My first thought was to use the CMake import/export target feature.

Have a CMakeLists.txt for basic, io and web and one CMakeLists.txt that references those. You could then use the CMake export feature to export those targets and the application projects could then import the CMake targets.

When you build the library project first the application projects should be able to find the compiled libraries automatically (without the libraries having to be installed to /usr/local/lib) otherwise one can always set up the proper CMake variable to indicate the correct directory.

When doing it this way a make in the application project won't do a make in the library project, you would have to take care of this yourself.

Answer 5

I see two additional approaches. One is to simply have basic, io, and web be submodules of each app. Yes, there is duplication of code and wasted disk space, but it is very simple to implement and guarantees that different compiler settings for each app will not interfere with each other across the shared libraries. I suppose this makes the libraries not be shared anymore, but maybe that doesn't need to be a big deal in 2011. RAM and disk have gotten cheaper, but engineering time has not, and sharing of source is arguably more portable than sharing of binaries.

Another approach is to have the layout specified in the question, and have CMakeLists.txt files in each subdirectory. The CMakeLists.txt files in basic, io, and web generate standalone shared libraries. The CMakeLists.txt files in each app directory pull in each shared library with the add_subdirectory() command. You could then pull down all the library directories and whichever app(s) you wanted and initiate the build from within each app directory.

Answer 6

The CMake BASIS tool provides utilities where you can create independent modules of a project and selectively enable and disable them using the ccmake command.

Full disclosure: I'm a developer for the project.