为什么双向迭代器没有运算符+/-？ [英] Why is there no operator +/- for a bidirectional iterator?

问题描述

我自己的推理是没有随机访问，也没有办法知道边界。但是为什么我们有 std :: advance ？ 并想一想，为什么没有 std :: deadvance （因为没有更好的词）？

My own reasoning for it is that there is no random access and there is no way to know the bounds. But then why do we have std::advance? And come to think of it, why is there no std::deadvance (for lack of a better word)?

推荐答案

你是对的; 运算符+/- 的要求是它是一个 O（1）操作，双向迭代器无法满足。 std :: advance 没有这样的速度要求（但会在可用时使用它，例如，用于随机访问迭代器）。

You are right; the requirements for operator+/- is that it be an O(1) operation, which cannot be met by bidirectional iterators. std::advance has no such speed requirement (but will use it when available, e.g., for random access iterators).

请注意，boost具有 boost :: next 和 boost :: previous 迭代器的实现;我不确定他们在标准化方面的地位如何，但是如果它在增强中则很稳定。

Note that boost has boost::next and boost::prior implementations for iterators; I'm not sure what their status is on standardization but if it's in boost it's solid.

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