IDisposable Thoughts

Ok, I had talked a lot about Conan in the past and how it integrates nicely with build systems like CMake. Conan has a really huge list of packages and is really simple to use but sadly, there are no straigh instructions about using Meson with Conan… Imagine that, your build in Meson but you need some dependencies that are not Meson subprojects, you just find the required package, download it using Conan and voila! life is nice again.

Let’s take a simple example, fmt is a simple formatting library, it uses CMake as build system and there is no wrapdb package for it.

#include <fmt/core.h>

int main() {
    fmt::print("Hello world!\n");
}

Of course it won’t work, we don’t have fmt installed as a dependency, let’s solve that with a conanfile.txt:

[requires]
fmt/

[generators]
cmake_find_package

Notice the cmake_find_package generator, it is important, you will get why soon. Now it is time for our first attempt for the build file:

project('meson_conan', 'cpp', default_options: ['cpp_std=c++17'])

fmt = dependency('fmt')
executable('demo', 'main.cpp', dependencies: [fmt])

Nothing new there, we just tell it to find the fmt dependency and compile our executable, but of course if we try to build this it will tell us it cannot find the dependency fmt, no worries, that is expected, the dependency is not installed yet and needs to be manually set first. We can do that automatically using the run_command in our Meson build file:

project('meson_conan', 'cpp', default_options: ['cpp_std=c++17'])

run_command('conan', 'install', '--install-folder', 
    meson.build_root(), meson.source_root(), 
    '--build=missing')

fmt = dependency('fmt')
executable('demo', 'main.cpp', dependencies: [fmt])

With this, during setup, Meson will run Conan, it will install fmt before checking the dependency (we could probably create a “dependency” between the dependency and the command, but I have no idea how to do that yet!) and guess what, it still does not work!

The reason is how the Meson build system checks for dependencies, you can see the details here, because we are generating CMake’s find package files this should work, but the problem is that Meson does not know where to find such files, we need to give it a “hint”:

project('meson_conan', 'cpp', default_options: ['cpp_std=c++17'])

run_command('conan', 'install', '--install-folder', 
    meson.build_root(), meson.source_root(), 
    '--build=missing')

fmt = dependency('fmt', cmake_module_path: meson.build_root())
executable('demo', 'main.cpp', dependencies: [fmt])

Of course you need CMake installed for this to work! and now if you try, voila! it finds the dependency and compiles nicely! :D