Automatically break long function signatures, calls, etc

Question

void parse(std::vector<float>& io_vertices, std::vector<unsigned>& io_indices) {}

Oftentimes when we code we need to break lines repeatedly as new expressions and statements enter the picture and our rows veer ever-closer to the column limit. We break function signatures, calls, etc., and line up arguments nicely:

void parse(std::vector<float>& io_vertices,
           std::vector<unsigned>& io_indices) {}

When it comes to plain text or comments, fill-paragraph is a lifesaver. Auto-formatting code, on the other hand, is notoriously difficult but not impossible. Surely someone must've made the effort in Emacs!

I've been (fruitlessly) trying to find a set of commands or a package that can do this for me, in particular for C and C++ like languages. Either attempts are rare or options so obvious that they elude me. This is the simplest solution to doing what I want:

(defun lazy-break-args ()
  (interactive)
  (let ((tokens (split-string (thing-at-point 'line t) ", ")))
    (unless (= (length tokens) 1)
      (let* ((breaks (mapcar (lambda (p) (format "%s,\n" p)) (butlast tokens)))
             (lines (append breaks (last tokens))))
        (move-beginning-of-line nil)
        (kill-line 1)
        (mapc (lambda (l) (insert l) (indent-according-to-mode)) lines)))))

Clearly this task depends on whatever language it is you're using, but my question is purposefully open to both language-specific packages (up to full-fledged formatters) and home-brew hacks like mine ("break and indent at commas, what the heck!").

Are there any functions in Emacs already, or any packages, that can do this? I don't expect perfection.

Answer 1

I've had good luck with multi-line

which, without configuration, allows you to cycle through different white space policies with a single repeated key press.

(use-package "multi-line" :bind ("C-;"))

Answer 2

CC mode provides for automatically filling text in comments and strings out-of-the-box. See the description in the manual: Filling and Breaking

By default, filling in CC mode ignores code. If you want, you can enable it by editing the value of c-ignore-auto-fill, as explained in the manual.