为什么std :: list没有运算符[]？ [英] Why isn't there an operator[] for a std::list?

问题描述

任何人都可以解释为什么不是为std :: list实现的operator []？我搜索了一下，但没有找到答案。

Can anyone explain why isn't the operator[] implemented for a std::list? I've searched around a bit but haven't found an answer. It wouldn't be too hard to implement or am I missing something?

推荐答案

通过索引检索元素是一个O（n ）操作的链接列表，这是 std :: list 是。因此，决定提供 operator [] 将是欺骗性的，因为人们会被诱使主动使用它，然后你会看到如下代码：

Retrieving an element by index is an O(n) operation for linked list, which is what std::list is. So it was decided that providing operator[] would be deceptive, since people would be tempted to actively use it, and then you'd see code like:

 std::list<int> xs;
 for (int i = 0; i < xs.size(); ++i) {
     int x = xs[i];
     ...
 }

这是O（n ^ 2）非常讨厌。因此ISO C ++标准特别提到支持 operator [] 的所有STL序列应该在摊销常量时间（23.1.1 [lib.sequence.reqmts] / 12）这是向量和 deque ，但不是列表 。

which is O(n^2) - very nasty. So ISO C++ standard specifically mentions that all STL sequences that support operator[] should do it in amortized constant time (23.1.1[lib.sequence.reqmts]/12), which is achievable for vector and deque, but not list.

对于你真正需要这种东西的情况，你可以使用 std :: advance 算法： / p>

For cases where you actually need that sort of thing, you can use std::advance algorithm:

int iter = xs.begin();
std::advance(iter, i);
int x = *iter;

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