hide-ifdef-header-regexp

hide-ifdef-header-regexp is a variable defined in `hideif.el'.
Its value is

"\\.h\\(h\\|xx\\|pp\\)?\\'"

Documentation:
C/C++ header file name patterns to determine if current buffer is a header.
Effective only if `hide-ifdef-expand-reinclusion-protection' is t.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 25.1 of Emacs.