c++ - C++11 and C++03 differs in support for small string optimization for std::string? -

in compatibility appendix of c++11 standard, 1 of change c++03 c++11 described below:

c.2.11 clause 21: strings library
21.4.1
change: loosen basic_string invalidation rules
rationale: allow small-string optimization.
effect on original feature: valid c++ 2003 code may execute differently in international standard. const member functions, such data , c_str, no longer invalidate iterators.

the iterator invalidation rule of std::string have changed c++03 c++11 in data() , c_str() not allowed invalidate iterators anymore, don't know how leads rationale of "allow small-string optimization"? wasn't sso allowed prior c++11?

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two of differences of std::string between c++11 , c++03 knew of before came across sso notes in compatibility appendix are:

• elements guaranteed stored contiguously in c++11
• cow implementation not option in c++11 (certain operations such operator[] not allowed invalidate iterators)

i took c++11's non-invalidating guarantee of c_str() , data() direct result of change towards contiguous storage. seems there connection sso implementation, , know detail of behind-scene logic. thanks.

i believe misinterpreting "change": not referring invalidation rules of data , c_str, rather of swap.

this question gave me idea, , think correct when read quote , linked question/answers.

since basic_string's swap can invalidate iterators, 1 can implement sso. in c++03, basic_string reversible container, means had comply requirement swapping containers:

the expression a.swap(b), containers , b of standard container type other array, shall exchange values of , b without invoking move, copy, or swap operations on individual container elements.

i may immensely wrong though, ianall (i not language lawyer).