May std::vector make use of small buffer optimization?

23.2.1 / p10 / b6:

Unless otherwise specified ...

• no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. ...

Nowhere does it "specify otherwise" for vector. So this outlaws the SBO for vector.

string is not bound by this rule because it does "specify otherwise" in 21.4.1/p6:

References, pointers, and iterators referring to the elements of a basic_string sequence may be invalidated by the following uses of that basic_string object:

• as an argument to any standard library function taking a reference to non-const basic_string as an argument.^234

234) For example, as an argument to non-member functions swap() (21.4.8.8), operator>>() (21.4.8.9), and getline() (21.4.8.9), or as an argument to basic_string::swap()