title: Legality of COW std::string implementation in C++11
body: <p>It had been my understanding that copy-on-write is not a viable way to implement a conforming <code>std::string</code> in C++11, but when it came up in discussion recently I found myself unable to directly support that statement.</p>
  
  <p>Am I correct that C++11 does not admit COW based implementations of <code>std::string</code>?</p>
  
  <p>If so, is this restriction explicitly stated somewhere in the new standard (where)?</p>
  
  <p>Or is this restriction implied, in the sense that it is the combined effect of the new requirements on <code>std::string</code> that precludes a COW based implementation of <code>std::string</code>. In this case, I'd be interested in a chapter and verse style derivation of 'C++11 effectively prohibits COW based <code>std::string</code> implementations'.</p>