附加std :: vector到自身，未定义的行为？ [英] Appending std::vector to itself, undefined behavior?

问题描述

这个问题使我不确定向量本身是否附加。所以问题是：下面的代码行是否符合我的期望，但它是否符合标准？

This question made me uncertain about appending a vector to itself. So the question is: Following lines of code do what I expect, but is it standard conform?

vec.reserve(vec.size() * 2):
vec.insert(vec.end(), vec.begin(), vec.end());

以下（不含 reserve（））工作，是否甚至标准符合？

Following (without reserve()) still works, is it even standard conform?

vec.insert(vec.end(), vec.begin(), vec.end());

还是取决于实现？

推荐答案

根据C ++ 03 ISO规范（&;; 23.1.1，表67）（和@AndyProwl提到的，在23.2.3中，C ++ 11的表11 ISO规范），作为序列要求的一部分，序列容器中的操作 a.insert（p，i，j）具有此前提条件：

According to the C++03 ISO spec (§23.1.1, Table 67) (and as @AndyProwl has mentioned, in §23.2.3, table 11 of the C++11 ISO spec), as part of sequence requirements, the operation a.insert(p, i, j) in a sequence container has this precondition:

i ， j c $ c> a 。

i, j are not iterators into a.

换句话说，序列容器可以安全地假设，如果你做范围插入操作，那个范围不会从原始容器上的迭代器中定义。

In other words, sequence containers are allowed to safely assume that if you do a range insertion operation, that range will not be defined from iterators over that original container.

因此，如果您尝试将容器的元素插入到它自己，正在调用标准库函数和打破前提条件。这导致未定义的行为，意味着如果库实现者是好的人，它可能在某些平台上工作，但是它可能是非常和灾难性地失败，没有正当理由。

As a result, if you try to insert a container's elements into itself, you are calling a standard library function and breaking a precondition. This results in undefined behavior, meaning that it might work on some platforms if the library implementers are nice people, but it could terribly and catastrophically fail with no justification.

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