Movement through argument list

Question

I'm a C++ programmer using a setup consisting of cc-mode and CEDET and of course our beloved emacs (v24.2).

A feature I'm missing is a function that moves point fast through a list of arguments. Consider this example:

void takesManyArgs( int a1, int a2, int a3, std::pair<int,int> a4, int a5 ){  
    // does something nifty!  
}  
// [...]
takesManyArgs( b1, b2, b3, make_pair( b4, b5 ), b6 );

Where point is just before the first int. Now I'd like to an easy way to quickly move through the list of arguments, i.e. a forward-argument (and backward-argument aswell) function that moves right before the first non-whitespace character past the comma (the argument separator).
I have written a small function that does it but it's not quite the way I'd like it work:

(defun arg-forward ()  
  "Move point forward in the file until we hit an argument separator, i.e. comma, colon or semicolon."  
  (interactive)  
  (progn  
    (re-search-forward "[,]")))  

In principle this function just jumps to the next comma. That's not the behaviour I want.

I'd like a function that:

• Edit: Jumps to the next argument separator (i.e. comma for C++) and places point just after that argument separator
• Checks whether there's a whitespace after the comma; in case there's one, point is moved to the first non-whitespace character (i.e. instead of ,| int it will look like , |int where | marks point)
• Stops at the end of the argument list and prints a message (e.g. "At end of argument list")
• Stays within its "scope", i.e. it'll go from |int b3 to |make_pair( int b4, int b5 ) to |int a6 where | marks point

Any help from you elisp "hackers" is appreciated!

Edit: Added some clarification.
Edit2: Fixed that example "Code"

Answer 1

This function seems to do what you requested:

(defun arg-forward ()
  "Move point forward until we hit an argument separator, the comma, colon, semicolon"
  (interactive)
  (condition-case nil
      (let ((separator-regexp "[,:;]"))
        (forward-sexp)
        (while (not (or (looking-at (format "[ \t]*%s" separator-regexp))
                        (save-excursion (forward-char -1)
                                        (looking-at (format "[]\"')}]*[ \t]*%s " separator-regexp)))))
          (forward-sexp))
        (if (looking-at separator-regexp)
            (forward-char 1))
        ;; going forward one sexp and back puts point at front of sexp
        ;; essentially skipping the whitespace, but also ensuring we don't
        ;; jump out of a larger closing set of parentheses 
        (forward-sexp)
        (forward-sexp -1))
    (error
     (beep)
     (message "At end of argument list"))))

Answer 2

You could write a function that uses forward-sexp and checks to see if you're at one of "argument separator" or "end of argument list".

I don't seem to have an emacs handy right now, so I can't trivially check an example implementation, but with those at hand, you should be well on your way.

Untested code below:

(defun forward-argument ()
  (interactive)
  (let ((argument-terminator "[),]"))              ;; Arguments stop at commas or close
     (while (not (looking-at argument-terminator)) ;; until we're at end-of-argument...
        (forward-sexp 1))))                        ;; move forward

This could probably benefit from pulling the argument-terminator out into a defvar, so it's easy to make it mode-specific. It could probably benefit from having a count prefix (wrap it in a (while (> count 0) ... (setq count (1- count)) and default count to 1 if no prefix is given) and a few other things., Not to mention actually checking the code against a live emacs instead of trying to remember what arguments functions take and what they're called.

Answer 3

Since you are using CEDET with your C++ code, you can take advantage of the TAG navigation commands. I tried this with the version of CEDET from the CEDET bzr repository.

Check in the Development menu, near the bottom is Navigate Tags. You will find keybindings for forward/backward tag on:

C-c , n - next tag

C-c , p - previous tag

These will move you between functions, between arguments, between all sorts of things.

These functions don't do all the things you were asking about, but are a good start.