[SOLVED] CMake automatically populates variables, logic is unclear

Issue

I'm relatively new to CMake, right now I experiment with it on a Windows machine with WSL (I run CMake in WSL), and I have confusion over the way CMake handles variables.

My issue with the variables is that sometimes I feel I meet variables, that, according to how (I think) CMakeLists.txt is parsed, contain some values, but I never set any value to them yet they contain values, and I have no idea what other variables with set values exist.

I failed to turn up any information that would make it clear once and for all. Here is a specific example, from my own CMakeLists.txt. I decided I want to have Boost as a dependency:

find_package(Boost 1.74.0 COMPONENTS filesystem system REQUIRED)
message(${Boost_LIBRARIES})
target_link_libraries(app_release PUBLIC
  ${Boost_LIBRARIES}
)

And the message() (which I added as a debugging output) output is Boost::filesystemBoost::system. This CMakeLists.txt works correctly without errors, I can make-compile a hello world program with it.

I never set the variable Boost_LIBRARIES. The only reason I have it in my CMakeLists.txt is because I copied this piece of CMakeLists from another place, otherwise I would have never known that such a variable exists. I also failed to turn up anything that would hint at existence of such a variable in the documentation (CMake documentation seems to contain a lot of gaps?)

Source of "minimal complete" CMakeLists.txt example file I pulled it from StackOverflow, I use it as "inspiration" for writing my CMakeLists.txt.

Questions:

1. How am I supposed to know that the variable Boost_LIBRARIES exists? There has to be some logic to how they're created, I would think. My random guess is that _LIBRARIES is appended to package name I have just find_package()'d, but I couldn't find anything in the docs about it.
2. Can I get a list of all hidden variables and their values then?

Solution

How am I supposed to know that the variable Boost_LIBRARIES exists?

From documentation of a specific package. In this case, https://cmake.org/cmake/help/latest/module/FindBoost.html .

There has to be some logic to how they're created, I would think. My random guess is that _LIBRARIES is appended to package name I have just find_package()'d, but I couldn't find anything in the docs about it

In the beginning, there was chaos. Basically find_package all it does it executes a CMake script, that can do anything. Every package set different variables. Nowadays, it's a bit more standardized https://cmake.org/cmake/help/book/mastering-cmake/chapter/Finding%20Packages.html#built-in-find-modules , but it still depends on specific script on what it actually does.

Can I get a list of all hidden variables and their values then?

There are no "hidden" variables in CMake. All variables are normal. There are cached and non-cached variables.

After generating CMake build, you can look at all cache variables <builddir>/CMakeCache.txt.

To print all variables, see CMake: Print out all accessible variables in a script .

Answered By - KamilCuk

Answer Checked By - Candace Johnson (WPSolving Volunteer)