distinguish .h files

[Andrea Crotti <andrea.crotti.0@gmail.com>]

It's a bit annoying that the header files .h BUT for c++ are not
automatically recognized (but of course understandable).

I tried the following approach but it doesn't work, maybe the function
is executed too early?
Or what else can be the problem?

--8<---------------cut here---------------start------------->8---
  (defun is-cpp-header-file ()
    "enable c++-mode when finding classes"
    (when (re-search-forward "class ")
      (c++-mode)))
  
  ;;(add-hook 'c-mode-common-hook 'is-cpp-header-file)
--8<---------------cut here---------------end--------------->8---