检查迭代器是否属于列表 [英] Check whether iterator belongs to a list

问题描述

有什么方法可以检查给定的迭代器是否属于C ++中的给定列表。

Is there any way to check whether a given iterator belongs to a given list in C++?

推荐答案

方法

您不能简单地遍历列表，将每个迭代器值与您的候选值进行比较。

The obvious but invalid approach

You can't simply iterate through the list, comparing each iterator value to your "candidate".

C ++ 03标准对于应用于不同容器的迭代器的 == 的有效性（Mankarse对Nawaz的回答链接 http://www.open-std.org/jtc1/sc22/wg21/ docs / papers / 2009 / n2948.html＃446 ），一些编译器（例如VC ++ 2005调试模式）警告，如果这样做，但尽管它可能实际工作可靠取决于您的编译器/库

The C++03 Standard is vague about the validity of == applied to iterators from different containers (Mankarse's comment on Nawaz's answer links http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2948.html#446), some compilers (eg. VC++2005 debug mode) warn if you do so, but despite all that it may actually work reliably depending on your compiler/libraries - check its documentation if you don't care about portability.

C ++ 11标准非常明确，不能将迭代器与不同容器进行比较：

The C++11 Standard is very explicit, you can't compare iterators to different containers:

§24.2.5正向迭代器的域==是同一底层序列上的迭代器。 / p>

§ 24.2.5 The domain of == for forward iterators is that of iterators over the same underlying sequence.

因此，依赖于 operator == 的问题的答案是有问题的

So, the answers to this question that rely on operator== are questionable now, and invalid in future.

你可以

• Mankarse的评论警告说，这可能不适用于提供自己的运算符& 的对象。您可以使用 std :: addressof 或C ++ 03 boost的版本

• Mankarse's comment cautions that this might not work as intended for objects providing their own operator&. You could work around this using std::addressof, or for C++03 boost's version

Martin的注释提到，您必须假定候选迭代器，您正在测试列表成员资格是安全地解引用 - 到它来自的容器的 end（）迭代器。

Martin's comment mentions that you have to assume the candidate iterator that you're testing list membership for is safely dereferenceable - i.e. not equal to an end() iterator on the container from which it came. As Steve points out - that's a pretty reasonable precondition and shouldn't surprise anyone.

（这对所有人来说都是可以的。）这是一个非常合理的前提条件，作为存储元素的标准容器从不具有相同的地址，但更一般地，用户定义的容器可以允许不相等的迭代器寻址相同的值对象（例如支持循环或飞重模式样式优化），在这种情况下，但是，如果你写这样的容器，你可能会为安全的迭代器比较而设计。）

(This is fine for all Standard containers as stored elements never have the same address, but more generally user-defined containers could allow non-equal iterators to address the same value object (e.g. supporting cycles or a "flyweight pattern" style optimisation), in which case this approach would fail. Still, if you write such a container you're probably in a position to design for safe iterator comparison.)

实现：

template <class IteratorA, class IteratorB, class IteratorC>
inline bool range_contains(IteratorA from, const IteratorB& end,
                           const IteratorC& candidate)
{
    while (from != end)
        if (&*from++ == &*candidate)
            return true;
    return false;
}

注意：

• 这采用接受一系列迭代器位置进行搜索的标准库方法。


• 每个迭代器的类型允许变化，因为有可移植性问题，例如container begin（）返回迭代器，但 end（）返回 const_iterator 。


• 以外的迭代器取自 const 引用，因为迭代器有时可能是非平凡的对象（即太大，不适合寄存器，复制相对昂贵）。 $> 是需要的值，因为它会在范围内递增。



• This adopts the Standard library approach of accepting a range of iterator positions to search.
• The types of each iterator are allowed to vary, as there are portability issues, e.g. containers where begin() returns an iterator but end() returns a const_iterator.
• Iterators other than from are taken by const reference, as iterators can sometimes be non-trivial objects (i.e. too large to fit in a register, relatively expensive to copy). from is needed by value as it will be incremented through the range.

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