Project paths in launch.vs.json with CMake presets

Today I was struggling with a relatively simple task in Visual Studio 2022: pass a file path in my source code folder to my running application. I am, as usual, using VS’s CMake mode, but also using conan 2.x and hence CMake presets. That last part is relevant, because apparently, it changes the way that .vs/launch.vs.json gets its data for macro support.

To make things a little more concrete, take a look at this, non-working, .vs/launch.vs.json:

{
  "version": "0.2.1",
  "defaults": {},
  "configurations": [
    {
      "type": "default",
      "project": "CMakeLists.txt",
      "projectTarget": "application.exe (src\\app\\application.exe)",
      "name": "application.exe (src\\app\\application.exe)",
      "env": {
        "CONFIG_FILE": "MY_SOURCE_FOLDER/the_file.conf"
      }
    }
  ]
}

Now I want MY_SOURCE_FOLDER in the env section there to reference my actual source folder. Ideally, you’d use something like ${sourceDir}, but VS 2022 was quick to tell me that it failed evaluation for that variable.

I did, however, find an indirect way to get access to that variable. The sparse documentation really only hints at that, but you can actually access ${sourceDir} in the CMake presets, e.g. CMakeUsersPresets.json or CMakePresets.json. You can then put it in an environment variable that you can access in .vs/launch.vs.json. Like this in your preset:

{
  ...
  "configurePresets": [
    {
      ...
      "environment": {
        "PROJECT_ROOT": "${sourceDir}"
      }
    }
  ],
  ...
}

and then use it as ${env.PROJECT_ROOT} in your launch config:

{
  "version": "0.2.1",
  "defaults": {},
  "configurations": [
    {
      "type": "default",
      "project": "CMakeLists.txt",
      "projectTarget": "application.exe (src\\app\\application.exe)",
      "name": "application.exe (src\\app\\application.exe)",
      "env": {
        "CONFIG_FILE": "${env.PROJECT_ROOT}/the_file.conf"
      }
    }
  ]
}

Hope this spares someone the trouble of figuring this out yourself!