减少begin（）迭代器，然后再次增加它 [英] Decreasing begin() iterator and then increasing it again

问题描述

这些语句是否通过标准有效？

Are such statements valid by the standard?

std::string str{"123"};
auto it = str.begin();
--it;
++it; // Does *it point to character '1' now?

我在g ++ 4.7.2和clang ++ 3.5 - it 返回'1'。
这是c ++ 11中的标准行为吗？

I've tried this on g++ 4.7.2 and clang++ 3.5 - *it returns '1'. Is this the standard behavior in c++11?

推荐答案

不，它无效。

行为，因为24.2.6 [bidirectional.iterators]声明 - it 的后置条件是结果必须是可解引用的。因为它在 begin（）之前，这个条件不满足，因此代码是非法的。

It is undefined behaviour, as 24.2.6 [bidirectional.iterators] states that a postcondition of --it is that the result must be dereferenceable. As it points before begin() in your example, this condition is not met and hence the code is illegal.

因为没有诊断需要，它可能似乎工作，但你不能（也不应该）依靠它。

As there is no diagnostic required it might seem to work, but you can not (and should not) rely on it.

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