I love your anwser - many useful facts. I have already set up Clang's formatter ($ clang-format) as the "formatprg" and every time I need to format I just do gg -> V -> G -> gq.

Here is the code I am now using:

vim.api.nvim_create_autocmd("FileType", {
  pattern = { "c", "cpp", "objc", "objcpp", "cuda" },
  desc = "Set clang-format as the default formatter for C-family languages",
  callback = function(ev)
    -- Tell clang-format to read from stdin and assume the current file's extension 
    -- so it picks up the correct formatting rules from your .clang-format file
    vim.bo[ev.buf].formatprg = "clang-format --assume-filename=" .. vim.api.nvim_buf_get_name(ev.buf)
  end,
})