I don't think you ever stop learning C++... :)

But I trust cppreference more for the details, which are often necessary for C++ because, well, the devil's in the details. (And to be totally honest, the styling is just nicer.)

Compare the docs for std::vector for example, cplusplus[0] and cppreference[1]. The former spends three paragraphs explaining the internals, whereas the latter only spends one. The latter also includes information about what's been introduced on which language version (11, 17, 20) and assorted facts that the former doesn't discuss, e.g. the various container contracts that vector fulfills.

[0] http://www.cplusplus.com/reference/vector/vector/

[1] https://en.cppreference.com/w/cpp/container/vector