title: Is sso used in any other standard library containers other than std::string?
body: <p>SSO (small/short string optimization) can be used for <code>std::string</code>. But is it allowed to be used in other standard library containers (e.g. <code>std::vector</code>)? Does the answer depend on whether or not the template parameters of the containers are builtin types or user-defined types?</p>