c++ - STL 迭代器上下文中的奇异值和非奇异值是什么？

Question

C++ 标准 (2003) 的第 24.1/5 节内容如下:

Just as a regular pointer to an array guarantees that there is a pointer value pointing past the last element of the array, so for any iterator type there is an iterator value that points past the last element of a corresponding container. These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. The library never assumes that past-the-end values are dereferenceable. 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 exception is an assignment of a non-singular value to an iterator that holds a singular value. In this case the singular value is overwritten the same way as any other value. Dereferenceable values are always nonsingular.

我无法真正理解以粗体显示的文字？

• 什么是奇异值和非奇异值？它们是如何定义的？在哪里？
• 如何以及为什么可解引用的值总是是非奇异的？

Answer 1

如果我理解正确，迭代器的奇异值本质上等同于未分配的指针。它是一个尚未初始化为指向任何地方的迭代器，因此没有明确定义的元素正在迭代。例如，声明一个未设置为指向某个范围元素的新迭代器时，会将该迭代器创建为单个迭代器。

正如规范的部分暗示的那样，单数迭代器是不安全的，并且不能对它们使用任何标准迭代器操作，例如递增、赋值等。您所能做的就是为它们分配一个新值，希望将它们指向有效数据。

我认为有这个定义的原因是这样的陈述

set<int>::iterator itr;

可以被规范允许，同时具有标准化的含义。这里的“奇异性”一词可能指的是奇异性的数学定义，在不太正式的情况下也称为“不连续性”。