CMake build git submodules and its dependencies

I am a novice in CMake,

I would like to use a C++ library A in my CMake project. This library A is included as a git submodule and I include it in my CMakeFile using

add_subdirectory("extern/A")

which works so far.

However, my library A has two other dependencies B and C. They are included in the CMakeFile of library A using find_package().

Now, I would like CMake to build these dependencies B and C, so that library A can use them. I want to include B and C as git submodules as well.

Simply concatenating

add_subdirectory("extern/B")
add_subdirectory("extern/C")
add_subdirectory("extern/A")

obviously doesn't work. I thought using add_dependencies could help here, but I couldn't find a way to make it work yet...

Is it even possible to do what I want to do here? If yes, could someone give me a hint which commands I might have to use?

Solution

Your mistake adding B as a dependency to RootProject where A needs it.

Solution

In the following directory structure:

RootProject
|__ .git
|__ CMakeLists.txt
|__ src
|   |__ main.c
|   |__ ...
|
|__ vendor
    |__ A
        |__ .git
        |__ CMakeLists.txt
        |__ src
            |__ ...

RootProject depends on A.
A is a library that has no dependencies.

If you want to add B as a dependency to A, you would treat A as completely separate project from RootProject and add B directly to A.

The directory structure of A becomes:

A
|__ .git
|__ CMakeLists.txt
|__ src
|    |__ ...
|
|__ vendor
    |__ B
        |__ .git
        |__ CMakeLists.txt
        |__ src
            |__ ...

Using git submodules to "manage" the dependencies:

# Adding `B` as a dependency to `A`
cd vendor/A
mkdir -p vendor
cd vendor
git submodule add https://github.com/username/B.git

This will add B as a dependency to A, and not to RootProject.

Now simply add the following to vendor/A/CMakeLists.txt:

add_subdirectory("vendor/B")
# ......
target_link_libraries(A PUBLIC B)

Also note that each of A, B and RootProject must have a top level CMakeLists.txt in order for add_subdirectory to work.