How to 'modify-syntax-entry' for a major mode?

Question

Following this answer, when I type (modify-syntax-entry ?_ "w") and do M-x eval-region, it has the effect I'm looking for, albeit only for that buffer. I've put (modify-syntax-entry ?_ "w") in my .emacs file, however and after restarting, it's not having effect in my C/C++ code.

I'm using Emacs GNU Emacs 26.3 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 3.24.13) of 2019-12-11. My OS is: Fedora release 31 (Thirty One).

_{There are questions with nearly the exact same title, but their accepted answers do not work for C/C++ mode, so this is a different question.}

Answer 1

Each major mode has its own syntax and syntax table. If you just put (modify-syntax-entry ?_ "w") in your init file, it gets evaluated in the buffer that is current when your init file is loaded -- not in a C/C++ buffer.

To evaluate that sexp when in a C/C++ buffer you can put it in a function, which you add to the mode hook. For example (untested):

(add-hook 'c-mode-hook (lambda () (modify-syntax-entry ?_ "w")))

But it's generally better to use a named, not an anonymous, hook function. For example:

(defun foo () (modify-syntax-entry ?_ "w"))

(add-hook 'c-mode-hook 'foo)

Answer 2

Rather than modifying the syntax tables, you can instead use the built-in superword-mode:

Superword mode is a buffer-local minor mode. Enabling it changes the definition of words such that symbols characters are treated as parts of words: e.g., in ‘superword-mode’, "this_is_a_symbol" counts as one word.

You can enable it per mode using a a hook: (add-hook 'c++-mode-hook 'superword-mode) or globally with (global-superword-mode 1) in your .emacs