bsl::optional::transform

If this object contains a value, invoke the specified func with the contained value as an argument and return optional<U> that contains the result of that invocation, where U is the cv‐unqualified func return type, which can be different from t_TYPE; otherwise, return an empty optional<U>. Note that func must return an object type other than array, bsl::in_place_t or bsl::nullopt. Also note that there is no requirement that U is movable. Finally note that the returned optional uses the allocator of the object returned by func, or the default allocator if func is not invoked.