Question

I am using the standard (shipped) Emacs C++ mode but I have a slight itch that I am looking to get scratched. How would I go about properly highlighting the types inside of a sizeof and the casts in C++?

For example:

1. A Cast

   Type * pointer = reinterpret_cast <Type *> (original);

2. Sizeof

   std::cout << sizeof (Type) << "\n";

Answer 1

Add these expressions to your .emacs, or evaluate them with M-:.

Sizeof (that’s the easier of the two cases)

The regex highlights any combination (indicated by the bracket expression [...]; regarding syntax, please see the note below) of alphanumeric, whitespace and asterisk within parentheses and preceded by sizeof.

(font-lock-add-keywords 'c++-mode
  '(("\\<sizeof[[:space:]]*(\\([[:alnum:][:space:]*]+\\))"
  1 font-lock-type-face t)))

The number 1 tells emacs to only highlight the first subexpression (marked by \\(...\\)) using the face font-lock-type-face; t means overriding any previous highlighting.

You can see and change the available faces with M-x customize group [RET] font-lock-faces [RET].

C++ style casts

I’m not sure which typename you want to be highlighted – the “original” or the one to cast into. This highlighter marks both:

(font-lock-add-keywords 'c++-mode
  '(("\\<[[:alnum:]]+_cast[[:space:]]*<\\([[:alnum:][:space:]*]+\\)>[[:space:]]*(\\([[:alnum:][:space:]*]+\\))"
  (1 font-lock-type-face t)
  (2 font-lock-type-face t))))

Again, ‘1and2` select the corresponding subexpressions.

Please note: The regexes for the typenames do not perfectly conform to C++ syntax. For example, emacs will happily highlight nonsense like sizeof(int * 32).

Also, my solution doesn’t take into account the problem Pavel mentioned in the comment on your question; that you could also use sizeof on variables, which would need different highlighting. I don’t think this possible, short of implementing a complete C parser in the font lock code.