IDisposable Thoughts

I had already written about using Conan and CMake to handle your C++ dependencies and that is a setup I had used for a while now. Recently, while working in a few things with the Boost library, I found the cracks in this setup, for example, this setup does not support handling components, something really useful when using Boost.

While asking around, I realised while this is the way most documentation is written, it is not the preferred way to use dependencies with Conan and CMake, it is not that we have to change much but the generator, instead of using the cmake generator, we should use the cmake_find_package generator. This generator will create files named FindXXX.cmake where XXX is the dependency we are included.

Let’s use the example from the previous blog post, but instead changing the generator to use:

[requires]
nlohmann_json/3.9.1

[generators]
cmake_find_package

Now, we need to remove from CMake the previous setup and use, instead, the native find_package facility, now created thanks to Conan:

cmake_minimum_required (VERSION 3.15)
project (sample)

find_package(nlohmann_json REQUIRED)

add_executable (main main.cpp)
target_link_libraries (main nlohmann_json::nlohmann_json)

Notice the usage of nlohmann_json::nlohmann_json in the dependencies, this is the name and component of your dependency.

Let’s try to install and compile this:

mkdir build && cd build
conan install ..
cmake ..

This will faill with a message about Findnlohmann_json.cmake not found in CMAKE_MODULE_PATH. That is weird, isn’t it? I mean, if we check that file is in the build directory, what is happening? Well, it is exactly that, the file is in the build directory and not where your CMake expects the file to live, we can fix this easily appending the file to our module path:

cmake_minimum_required (VERSION 3.15)
project (sample)

list(APPEND CMAKE_MODULE_PATH ${CMAKE_BINARY_DIR})
find_package(nlohmann_json REQUIRED)

add_executable (main main.cpp)
target_link_libraries (main nlohmann_json::nlohmann_json)

Try running the same commands again, voila! it works! now it is time to compile with the usual command:

cmake --build . --clean-first

Nice, now we have not only a simpler setup with Conan but we can use things like components as well. Another good advantage is that almost every documentation out there about using CMake with dependencies use find_package so basically we can reuse it, win-win!