How do I specify an optional dependency in a CMake package configuration file?

Question

I'm trying to specify correctly the dependencies in the MyLibConfig.cmake file of my project. In CMakeLists.txt I have something like this:

find_package(aLib REQUIRED)
find_package(bLib)

So in MyLibConfig.cmake I wrote something like:

include(CMakeFindDependencyMacro)
find_dependency(aLib REQUIRED)
find_dependency(bLib)

Now, when I write another project that needs myLib, I do:

find_package(MyLib REQUIRED)

This fails because the MyLib configuration file doesn't find bLib, but I would like it to be treated as optional. What is the best Modern CMake practice to handle this?

Answer 1

find_dependency is used only for (initially) REQUIRED packages.

find_package for non-REQUIRED sub-package remains the same when "copied" into XXXConfig.cmake script.

For CMakeLists.txt contained

find_package(aLib REQUIRED)
find_package(bLib)

corresponded content of MyLibConfig.cmake would be:

include(CMakeFindDependencyMacro)
# for REQUIRED package 'find_package' is replaced with 'find_dependency',
# but 'REQUIRED' keyword is omited.
find_dependency(aLib)
# for non-REQUIRED package 'find_package' remains.
find_package(bLib)

---

It is important that REQUIRED keyword is NOT passed to the find_dependency macro. So, would one call

find_package(MyLib) # Without 'REQUIRED'!

and aLib package would be missed, then with (failed) call

find_dependency(aLib)

CMake would treat MyLib to be missed too (and would print an appropriate message), but won't stop the configuration because of that.