IntelliSense is a general term for various code editing features including: code completion, parameter info, quick info, and member lists.
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The VS Code indexer

VS Code provides a very elaborate code indexer, called IntelliSense.

It works by parsing the source files and creating an internal representation of the code.

With C/C++, for this process to be accurate, it needs to know exactly how the compiler is invoked for each source file, more specifically it needs to know:

  • the include folders
  • the preprocessor definitions
  • the compile options

Managed build systems, like the Eclipse CDT, having a good control over the entire project settings, can pass accurate configuration to the internal indexer.

When using external builders, which store the project settings in custom text files not parseable by editors/IDEs (make files being the common example), the users must provide the indexer details again, which is not only redundant, but also difficult to keep consistent.

To avoid this, more advanced external system build generators (like CMake/meson) can automatically provide the indexer configuration in a file called compile_commands.json.

compile_commands.json

The compile_commands.json files are created by the system build generator for each build, and list the full command lines required to compile all project files.

For the simple Hello World project, with a single source file, it may look like:

[
{
  "directory": "/Users/ilg/tmp/hello/build/debug",
  "command": "/Library/Developer/CommandLineTools/usr/bin/c++ -DDEBUG -I/Users/ilg/tmp/hello/include -g -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -std=gnu++17 -o CMakeFiles/application.dir/Users/ilg/tmp/hello/src/hello-world.cpp.o -c /Users/ilg/tmp/hello/src/hello-world.cpp",
  "file": "/Users/ilg/tmp/hello/src/hello-world.cpp"
}
]

The VS Code C/C++ extension knows how to parse these files to extract the list of include folders, preprocessor definitions and everything else needed to configure IntelliSense properly.

There must be one such file in each build folder, for each build configuration.

These files are created by the system build generator during the setup/configure/prepare step, when the project configuration is processed, and usually remain unchanged during subsequent builds.

If the project structure changes, for example when files are added/removed/moved around, this step must be repeated, not only to update the builder specific files, but also to update the indexer configuration.

c_cpp_properties.json

These files are used by the VS Code C/C++ extension to store the IntelliSense configuration. There must be one such file in each workspace folder.

The exact location is ${workspaceFolder}/.vscode/c_cpp_properties.json.

For simple projects, such a file looks like:

{
  "configurations": [
    {
      "name": "Debug",
      "configurationProvider": "ms-vscode.cmake-tools",
      "compileCommands": "${workspaceFolder}/build/debug/compile_commands.json"
    },
    {
      "name": "Release",
      "configurationProvider": "ms-vscode.cmake-tools",
      "compileCommands": "${workspaceFolder}/build/release/compile_commands.json"
    }
  ],
  "version": 4
}

The xPack extension generates/updates these files to match the existing build configurations defined in all projects.

Use separate workspace folders for projects

Since VS Code distinguishes only between workspace folders, and does not distinguish between multiple project folders, it is recommended to avoid opening parent folders with multiple projects.

Instead, for multi-project cases, create a workspace and add all projects to it (via menu File → Add Folder to Workspace…), then save the entire set of folders as a <project>-code.workspace file (via menu File → Save Workspace As…); VS Code can later open all projects at once.

The workspace file is a JSON with the paths to workspace folders. If all those folders are children of the workspace, all paths are relative, and the file is portable, which means it can be stored in repositories.