問題描述
據我所知，寫時復制不是在 C++11 中實現符合標準的 std::string 的可行方法，但是當它最近在討論中出現時，我發現我自己無法直接支持這種說法.
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 不承認基于 COW 的 std::string 實現，我是否正確?
Am I correct that C++11 does not admit COW based implementations of std::string?
如果是，這個限制是否在新標準(where)的某處明確說明?
If so, is this restriction explicitly stated somewhere in the new standard (where)?
或者這個限制是隱含的，從某種意義上說，這是對 std::string 的新要求的綜合影響，排除了基于 COW 的 std::string.在這種情況下，我會對C++11 有效禁止基于 COW 的 std::string 實現"的章節和詩句風格派生感興趣.
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'.
推薦答案
這是不允許的，因為按照標準 21.4.1 p6，迭代器/引用失效只允許
It's not allowed, because as per the standard 21.4.1 p6, invalidation of iterators/references is only allowed for
——作為任何標準庫函數的參數引用將非常量 basic_string 作為參數.
— as an argument to any standard library function taking a reference to non-const basic_string as an argument.
——調用非常量成員函數，除了 operator[]、at、front、back、begin、rbegin、結束，然后撕裂.
— Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.
對于 COW 字符串，調用非常量 operator[] 將需要進行復制(并使引用無效)，這是上一段所不允許的.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.
這篇關于C++11 中 COW std::string 實現的合法性的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！