In hide-ifdef-mode, code within #ifdef constructs that the C preprocessor would eliminate may be hidden from view.
Activate it using
The hidden code is marked by ellipses (…). Be cautious when editing near ellipses, since the hidden text is still in the buffer, and you can move the point into it and modify text unawares. You can make your buffer read-only while hide-ifdef-hiding by setting
‘hide-ifdef-read-only’ to a non-nil value.