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

问题描述

这个问题让我不确定向自身附加一个向量.所以问题是:以下代码行符合我的预期，但是否符合标准?

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(没有reserve())仍然有效，是否符合标准?

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)(以及@AndyProwl 在 §23.2.3 中提到的, C++11 ISO 规范的表 11)，作为序列要求的一部分，序列容器中的操作 a.insert(p, i, j) 具有以下前提条件:

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 不是a 的迭代器.

换句话说，允许序列容器安全地假设，如果您执行范围插入操作，则该范围将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自身中，您就是在调用标准库函数并破坏了前提条件.这会导致未定义的行为，这意味着如果库实现者是好人，它可能在某些平台上工作，但它可能会在没有任何理由的情况下发生可怕的灾难性失败.

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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