是const_iterator和迭代器的比较明确定义？

问题描述

请考虑以下代码：

#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 << "\n";
}

这里我介绍了一个拼写错误：在比较中我调用 vec.end（）而不是 vec.cend（）。这看起来像gcc 5.2一样工作。但是它实际上是根据标准很好地定义的吗？可以安全比较

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没有说你可以将迭代器与 const_iterator 。这会导致和。现在在C ++ 14中，这是由§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

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

其中 i 和 j 表示容器的迭代器类型的对象，或者
都可以由对象容器的 const_iterator
类型引用的语义无变化的相同元素。

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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