Subject: Re: bug#14008: Better autoindent for C++11 code?

From: Alan Mackenzie <[email-masked]>
Date: Sun, 13 Oct 2019 18:09:49 +0000

Hello, Stefan and Andrew.

On Sun, Oct 13, 2019 at 10:03:00 +0200, Stefan Kangas wrote:
> Andrew Pennebaker <[email-masked]> writes:

> > C++11 introduced lambda syntax []() { ... } and other strange things that Emacs indents strangely.

> > for_each(range.begin(), range.end(), [=](int i) {
> >     cout << strings[i] << endl;
> >   });

> > I would like the final line `});` to have the same indentation level
> > as the first line `for_each...`.

> I'm seeing the same thing here.

I think this is the minimum indentation imposed on all lines within "code
blocks" in "gnu" style.  (See the CC Mode manual for a description of CC
Mode's style system.)  If you indent the for_each line by, say, 4 columns
(put it inside braces if needed) the line with }); then gets indented
under the for_each.  As for_each is a function (pretty much equivalent to
Lisp's mapc), it's unlikely to be put at column 0 in real source code.

> With "emacs -Q" on current master, this indents like:

> for_each(range.begin(), range.end(), [=](int i) {
>   cout << strings[i] << endl;
>  });

> However, note that I have only one space before the final "}"
> character, whereas the reporter had two.  I'm not sure if this
> behaviour is intentional or not, or if it could be configured.

I'm surprised about the two spaces too, I don't understand how it could
have happened, unless there's been some disruption in the copying from
Emacs screen to email.

> > Here's another example:

> > for_each(range.begin(), range.end(), [&](int i) {
> >              std::async(
> >                         launch::async,
> >                         [&]() { strings[i] = fizzy(i); }
> >                         );
> >   });

> > The arguments to std::async and its closing parenthesis are indented
> > much too far; I would like them indented only one level further than
> > where std::async is itself indented. Any tips for achieving this?

Configure CC Mode, either by activating a different style (with C-c ., or
M-x c-set-style if some minor mode is using that key binding), or by
directly setting syntactic symbols' "offsets".

For the first possibility, I'd recommend trying out, say styles "bsd" or
"linux".

For the second, there are several ways to set symbols' offsets (see page
"Config Basics" in the CC Mode manual).  A good way is by using a hook
function.

To find out what syntactic symbols you need to change, put point on a
pertinent line and type C-c C-s.  For example on the "launch::async" line
you'd see something like "((arglist-intro 394 404))", the two numbers
being the "anchor points" from which the indentation is done.  To see
(and change temporarily) the "offset" for a symbol, type C-c C-o.  The
"gnu" style setting of the "offset" for arglist-intro is the function
c-lineup-arglist-intro-after-paren, which does pretty much what its name
says.

I think you want a simple extra level of indentation from point 394, so
you could construct your hook function something like this:

(defun my-c-indent ()
  (c-set-offset 'arglist-intro '+)
  ;; Add any further syntactic symbols here.
  )
(add-hook 'c-mode-common-hook #'my-c-indent) ; c++-mode-hook could also
                                             ; be used

Again, I strongly recommend reading the relevant pages in the CC Mode
manual.

> I'm seeing something similar here:

> for_each(range.begin(), range.end(), [&](int i) {
>   std::async(
>          launch::async,
>          [&]() { strings[i] = fizzy(i); }
>          );
>  });

> Perhaps Alan could clarify if this is a bug or if this is just a case
> of missing configuration?

A bit of a mixture of both, I think.

> Best regards,
> Stefan Kangas

-- 
Alan Mackenzie (Nuremberg, Germany).