与值初始化的迭代器进行比较是否定义明确? [英] Is it well-defined to compare with a value-initialized iterator?

问题描述

以下程序是否会调用未定义的行为?

Does the following program invoke undefined behavior?

#include <iostream>
#include <iterator>

int main(int argc, char* argv[])
{
    for (auto it = std::istream_iterator<std::string>(std::cin);
         it != std::istream_iterator<std::string>();
         ++it)
    {
        std::cout << *it << " ";
    }

    return 0;
}

此 4岁的问题表示无法将它们进行比较:

This 4 year old question says that they can't be compared:

迭代器也可以具有不与之关联的奇异值任何容器.[示例:声明未初始化后指针x(与int * x;一样)，必须始终假定x具有一个指针的奇异值.]大多数表达式的结果是未定义为奇异值；唯一的例外是作业非奇异值返回给持有奇异值的迭代器.

Iterators can also have singular values that are not associated with any container. [Example: After the declaration of an uninitialized pointer x (as with int* x;), x must always be assumed to have a singular value of a pointer. ] Results of most expressions are undefined for singular values; the only excep- tion is an assignment of a non-singular value to an iterator that holds a singular value.

但是对于C ++ 14标准，还有另一个答案:

But another answer for says for the C++14 standard:

但是，可以将值初始化的迭代器进行比较，并且应该进行比较等于其他相同类型的值初始化的迭代器.

However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type.

推荐答案

您正在混淆两个不同的问题.

You are conflating two different issues.

istream_iterator 是输入迭代器，而不是正向迭代器，因此您引用的C ++ 14更改根本不适用于它.允许您以这种方式比较 istream_iterator ，因为已明确指定它们允许进行这种比较.该标准指出(§24.6.1[istream.iterator])

istream_iterator is an input iterator, not a forward iterator, so the C++14 change you cited doesn't apply to it at all. You are allowed to compare istream_iterators in that manner because they are explicitly specified to allow such comparisons. The standard says that (§24.6.1 [istream.iterator])

不带参数的构造函数 istream_iterator()始终构造一个流结束输入迭代器对象，这是唯一的用于结束条件的合法迭代器.[...]

The constructor with no arguments istream_iterator() always constructs an end-of-stream input iterator object, which is the only legitimate iterator to be used for the end condition. [...]

两个流结束迭代器始终相等.流尾迭代器不等于非流结束迭代器.二当非流结束迭代器由以下项构造时，它们是相等的相同的流.

Two end-of-stream iterators are always equal. An end-of-stream iterator is not equal to a non-end-of-stream iterator. Two non-end-of-stream iterators are equal when they are constructed from the same stream.

通常，对于前向迭代器(还包括双向访问和随机访问的迭代器)，在

For forward iterators (which also includes bidirectional and random access ones) in general, value-initialized iterators are made comparable to each other in C++14. If your standard library implements it, then you can compare two value-initialized iterators. This allows you to create an empty range without an underlying container. However, you are still not allowed to compare a non-singular iterator to a value-initialized iterator. The following code has undefined behavior even in C++14:

std::list<int> l;

if(l.begin() == std::list<int>::iterator())
    foo();
else 
    bar();

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