> instead of dereferencing lhs on the third line: emplace_back returns a reference to the inserted item, so it's a &unique_ptr...and that reference is invalidated by the rhs emplace_back reallocating the vector.

Correct but only under the assumption that growing the backing storage of std::vector is implemented through a completely new instance of malloc, which indeed seems to be the case with libstdc++ implementation AFAICS.

Otherwise, if realloc had been used, such condition wouldn't be guaranteed since realloc can grow from existing memory and thus not invalidating all other references to it.

That said, I wonder if this behavior is mandated by the standard. https://en.cppreference.com/w/cpp/container/vector/emplace_b... implies that it is but I didn't check

    If after the operation the new size() is greater than old capacity() a reallocation takes place, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise only the end() iterator is invalidated.