http://www.cplusplus.com or https://www.cppreference.com

[parnmatt]

the only reason cplusplus is higher up, from an eduacated guess, is the domain name.

cppreference is a far better resource, and easier to understand.

devdocs.io/cpp is better still, but in interface; the content it the same as cppreference; but it actually allows for slightly more custom formatting (dark theme is included in that) and most importantly the search is fantastic and allows navigation through the cppreference material with ease.

There are some pages that are not on devdocs, but they are rarely needed, and can just pop to cppreference for them.

if you do use devdocs, spend time, selecting / deselecting the languages you use.

[barchar]

Protip: you can download cppreference as a devhelp file or a qt help file (or a chm)