Legality of COW std::string implementation in C++11

Question

It had been my understanding that copy-on-write is not a viable way to implement a conforming std::string in C++11, but when it came up in discussion recently I

Answer 1

From 21.4.2 basic_string constructors and assignment operators [string.cons]

basic_string(const basic_string& str);

[...]

2 Effects: Constructs an object of class basic_string as indicated in Table 64. [...]

Table 64 helpfully documents that after construction of an object via this (copy) constructor, this->data() has as value:

points at the first element of an allocated copy of the array whose first element is pointed at by str.data()

There are similar requirements for other similar constructors.