const_iterator 与迭代器的比较是否定义明确?

Is comparison of const_iterator with iterator well-defined?(const_iterator 与迭代器的比较是否定义明确?)

问题描述

考虑以下代码:

#include <vector>
#include <iostream>

int main()
{
    std::vector<int> vec{1,2,3,5};
    for(auto it=vec.cbegin();it!=vec.cend();++it)
    {
        std::cout << *it;
        // A typo: end instead of cend
        if(next(it)!=vec.end()) std::cout << ",";
    }
    std::cout << "
";
}

这里我引入了一个错字:在比较中我调用了 vec.end() 而不是 vec.cend().这似乎与 gcc 5.2 一样工作.但它实际上是根据标准明确定义的吗?iterator 和 const_iterator 可以安全地比较吗?

Here I've introduced a typo: in the comparison I called vec.end() instead of vec.cend(). This appears to work as intended with gcc 5.2. But is it actually well-defined according to the Standard? Can iterator and const_iterator be safely compared?

推荐答案

令人惊讶的是，C++98 和 C++11 并没有说可以将 iterator 与 进行比较const_iterator.这导致 LWG 问题 179 和 LWG 问题 2263.现在在 C++14 中，第 23.2.1[container.requirements.general]p7

Surprisingly, C++98 and C++11 didn't say that you can compare a iterator with a const_iterator. This leads to LWG issue 179 and LWG issue 2263. Now in C++14, this is explicitly permitted by § 23.2.1[container.requirements.general]p7

在表达式中

i == j
i != j
i < j
i <= j
i >= j
i > j
i - j

其中 i 和 j 表示容器的 iterator 类型的对象，或者两者都可以被容器的 const_iterator 的对象替换type 引用相同的元素，语义没有变化.

where i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

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