C++ is not allowed to do the small buffer optimization for std::vector<T>. You'll find a number of small_vector<T> classes from third parties, though. The reas

C++ is not allowed to do the small buffer optimization for std::vector<T>. You'll find a number of small_vector<T> classes from third parties, though. The reason is that std::swap(v1, v2) must never invalidate existing iterators and/or references to elements into either vector, which the optimization would clearly break. std::string is the type you may be thinking of, where the standard does make a special exception to enable the SSO.