mmm-mode: Implementing C++ mode as a submode - emacs

Question

We use an internal scripting language (let's call it pkc), which allows for embedding C++ code. The C++ code segments are delimited by {{{ and }}} markers.
I create an emacs mode for this language, using Generic Mode and mmm-mode.
Here is what I got (stripped down non-essential parts for posting here):
(require 'generic-x)
(setq pkc-imenu-generic-expression
'(("macros" "^[ \t]*macro[ \n\t]+\\([a-zA-Z0-9_]+\\)" 1)
("functions" "function[ \n\t]+\\([a-zA-Z0-9_]+\\)" 1)
))
(require 'cc-mode) ;; for c++-mode
(require 'mmm-auto)
(setq mmm-global-mode 'maybe)
(define-generic-mode
'pkc-mode ;; name of the mode to create
'("//" ("/*" . "*/")) ;; comments are same C++ comments
'( ... ) ;; some keywords
'("\\.pkc$") ;; files for which to activate this mode
;; other functions to call
'((lambda ()
(mmm-mode 1)
(setq mmm-submode-decoration-level 2)
(setq imenu-generic-expression pkc-imenu-generic-expression)
(which-function-mode 1)
(c-initialize-cc-mode t)
(c-init-language-vars-for 'c++-mode)
(c-common-init 'c++-mode)
(c-update-modeline)
(message "pkc-mode[mmm] is on")))
"A mode for pkc source files" ;; doc string for this mode
)
(mmm-add-classes
'((embedded-c++
:submode c++-mode
:face mmm-default-submode-face
:front "{{{"
:front-offset -1
:back "}}}"
:back-offset 1)))
(mmm-add-mode-ext-class 'pkc-mode nil 'embedded-c++)
When I load the source file with embedded C++ code segments, they are highlighted somewhat less than what would have been if C++ mode was the major mode (that's not my problem, however). When the cursor in C++ code, the modeline changes to pkc/l[C++/l] (as expected).
The problem is, whenever I press TAB to indent a line, I get the error Wrong type argument: stringp, nil. When I turned on debugger, this is the stack trace I see:
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
c-syntactic-skip-backward(nil nil t)
c-looking-at-decl-block(nil t)
c-guess-basic-syntax()
c-indent-line()
#[nil \302>\203)\212\303 \210\304\305x\210\303 \210\306 )i\306 X\203\"\307 !\202'\212\307 !))\20 \207" [indent-line-function column (indent-relative indent-relative-maybe) beginning-of-line "\n " nil current-indentation indent-line-to] 2 1908700 nil]()
c-indent-command(nil)
c-indent-line-or-region(nil nil)
call-interactively(c-indent-line-or-region nil nil)
Looking at the definition of c-looking-at-decl-block and particularly the call to `c-syntactic-skip-backward, I find:
(c-syntactic-skip-backward c-block-prefix-charset limit t)
Examining the first argument c-block-prefix-charset reveals that its value is nil. Doing the same from a plain C++ buffer shows a non-nil value. So, I suspect that C++-mode requires some initialization that's not being properly done.
So, my questions are:
What am I missing in the C++-mode initialization section in my implementation?
Am I using the right approach (combining generic-x and mmm-mode)? Is there a better approach?
I also see another error (File mode specification error) when I load the file, but I suspect the problem is the same or something similar.
Thanks for your time.

Answer 1

(Rewritten after the discussion in the comments, for any later visitors).
The setup in the question is basically fine, but to work with current c++-mode you need to use an updated version of mmm-mode. It's available at the GitHub project page and also at Melpa.
The relevant patches (one, two, three) added new entries to the value of mmm-save-local-variables, which mmm-mode uses to decide which local variables to save or restore when leaving or entering a submode region.
From what I understand, a future update to cc-mode can add more such vars, so the list may need to be updated from time to time.
Extra tip: to have better indentation in the subregions, you may want to wrap the submode's indent-line-function with some code that will narrow the buffer before calling it (example here). Depending on the indentation function (and whether it calls widen), it may or may not help.