I happened across this gem the other day and I thought I'd share (hint).
This is an almost complete reference for C/C++ produced by the good people at cppreference.com. I say almost as it is an ongoing project.
Follow this link to their Archives for offline viewing page. You will find a bunch of files. At the top is the Html book which is the 'official' version currently dated 11 March 2018. Below that you will see the 'Unofficial Release' dated 23 September 2018. Download one or both (29.8MB official and 34.1MB unofficial for the zips) and unpack in your favourite directory. In the \reference\en directory you will find Main_Page.html. Double click on this (or create a shortcut on your desktop) and enjoy. It's one of the better references I've found, it's up to date and appears as though an honest effort is being made to keep it so, and it's free (although I suspect the lads wouldn't say no to donations).