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c++c++11emacsindentation
Hugues
Hugues

Reputation: 3176

Emacs indenting of C++11 lambda functions (cc-mode)

The default Emacs C++ mode (cc-mode) still does not recognize many C++11 features. One annoying issue is that it applies too much indentation to lambda functions used as function parameters:

std::vector<int> ar(4);
std::generate_n(std::begin(ar), 4, [] {
        static int g_i;
        return g_i++;
    });
std::for_each(std::begin(ar), std::end(ar), [](int i) {
        std::cout << " " << i;
    });
bool b = std::is_sorted(std::begin(ar), std::end(ar), [&](int l, int r) {
        return l<r;
    });
std::cout << "   " << b << "\n";

Ideally, one would prefer:

std::vector<int> ar(4);
std::generate_n(std::begin(ar), 4, [] {
    static int g_i;
    return g_i++;
});
std::for_each(std::begin(ar), std::end(ar), [](int i) {
    std::cout << " " << i;
});
bool b = std::is_sorted(std::begin(ar), std::end(ar), [&](int l, int r) {
    return l<r;
});
std::cout << "   " << b << "\n";

Are there good solutions for this?

Upvotes: 18

Views: 4177

Answers (3)

frinkr
frinkr

Reputation: 664

In Emacs26, the accepted answer doesn't work for me anymore. I just set the 'inlambda' to 0.

  (c-offsets-alist . ((case-label . 0)
                      (inline-open . 0)
                      (substatement-open . 0)
                      (inlambda . 0) ; no extra indent for lambda
                      (block-open . 0) ; no space before {
                      (knr-argdecl-intro . -)))

Upvotes: 19

Victor Rybynok
Victor Rybynok

Reputation: 119

A slight rework of the Hugues' solution to also recognize lambda function as an argument nested in another lambda function, and lambda function as an in-class initialization value:

(defun vr-c++-looking-at-lambda_as_param ()
  "Return t if text after point matches '[...](' or '[...]{'"
  (looking-at ".*[,(][ \t]*\\[[^]]*\\][ \t]*[({][^}]*?[ \t]*[({][^}]*?$"))

(defun vr-c++-looking-at-lambda_in_uniform_init ()
  "Return t if text after point matches '{[...](' or '{[...]{'"
  (looking-at ".*{[ \t]*\\[[^]]*\\][ \t]*[({][^}]*?[ \t]*[({][^}]*?$"))

(defun vr-c++-indentation-examine (langelem looking-at-p)
  (and (equal major-mode 'c++-mode)
       (ignore-errors
         (save-excursion
           (goto-char (c-langelem-pos langelem))
           (funcall looking-at-p)))))

(defun vr-c++-indentation-setup ()
  (require 'google-c-style)
  (google-set-c-style)

  (c-set-offset
   'block-close
   (lambda (langelem)
     (if (vr-c++-indentation-examine
          langelem
          #'vr-c++-looking-at-lambda_in_uniform_init)
         '-
       0)))

  (c-set-offset
   'statement-block-intro
   (lambda (langelem)
     (if (vr-c++-indentation-examine
          langelem
          #'vr-c++-looking-at-lambda_in_uniform_init)
         0
       '+)))

  (defadvice c-lineup-arglist (around my activate)
    "Improve indentation of continued C++11 lambda function opened as argument."
    (setq ad-return-value
          (if (vr-c++-indentation-examine
               langelem
               #'vr-c++-looking-at-lambda_as_param)
              0
            ad-do-it))))

Upvotes: 4

Hugues
Hugues

Reputation: 3176

The discussion in Emacs cc-mode indentation problem with C++0x enum class fixes the enum class formatting issue.

This inspired the following advice function. It detects an open C++ lambda function in an open argument list, and cancels one level of indentation to produce the "ideal" result in the question:

(defadvice c-lineup-arglist (around my activate)
  "Improve indentation of continued C++11 lambda function opened as argument."
  (setq ad-return-value
        (if (and (equal major-mode 'c++-mode)
                 (ignore-errors
                   (save-excursion
                     (goto-char (c-langelem-pos langelem))
                     ;; Detect "[...](" or "[...]{". preceded by "," or "(",
                     ;;   and with unclosed brace.
                     (looking-at ".*[(,][ \t]*\\[[^]]*\\][ \t]*[({][^}]*$"))))
            0                           ; no additional indent
          ad-do-it)))                   ; default behavior

Upvotes: 16

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