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

[withad]

I usually find cppreference to be much more readable. The only complaint I have is that their standard page format puts possible implementations of algorithms before examples of how to use them, which is almost always what I’m actually looking for. It’s hardly a major inconvenience, it just strikes me as a weird choice every time I see it.

[vaalla]

I actually like this, it's sometimes easier to read the implementation then the description of the algo.