C++11 中 COW std::string 实现的合法性
据我所知，写时复制不是在 C++11 中实现符合标准的 std::string 的可行方法，但是当它最近在讨论中出现时，我发现我自己无法直接支持这种说法.
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 found myself unable to directly support that statement.
C++11 不承认基于 COW 的 std::string 实现，我是否正确?
Am I correct that C++11 does not admit COW based implementations of std::string?
如果是，这个限制是否在新标准(where)的某处明确说明?
If so, is this restriction explicitly stated somewhere in the new standard (where)?
或者这个限制是隐含的，从某种意义上说，这是对 std::string 的新要求的综合影响，排除了基于 COW 的 std::string.在这种情况下，我会对C++11 有效禁止基于 COW 的 std::string 实现"的章节和诗句风格派生感兴趣.
Or is this restriction implied, in the sense that it is the combined effect of the new requirements on std::string that precludes a COW based implementation of std::string. In this case, I'd be interested in a chapter and verse style derivation of 'C++11 effectively prohibits COW based std::string implementations'.
推荐答案
这是不允许的，因为按照标准 21.4.1 p6，迭代器/引用失效只允许
It's not allowed, because as per the standard 21.4.1 p6, invalidation of iterators/references is only allowed for
――作为任何标准库函数的参数引用将非常量 basic_string 作为参数.
― as an argument to any standard library function taking a reference to non-const basic_string as an argument.
――调用非常量成员函数，除了 operator[]、at、front、back、begin、rbegin、结束，然后撕裂.
― Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.
对于 COW 字符串，调用非常量 operator[] 将需要进行复制(并使引用无效)，这是上一段所不允许的.因此，在 C++11 中使用 COW 字符串不再合法.
For a COW string, calling non-const operator[] would require making a copy (and invalidating references), which is disallowed by the paragraph above. Hence, it's no longer legal to have a COW string in C++11.
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