c++ - 如何检查迭代器是否已初始化？

Question

如果我为迭代器使用默认构造函数，如何检查它是否稍后被分配？

对于指针，我可以这样做:

int *p = NULL;
/// some code
if ( NULL == p ) {
  // do stuff
}

如何为迭代器执行上述操作？ 有可能吗？

#include <iostream>
#include <list>

int main ()
{
    std::list<int>::iterator it;

  if ( NULL == it ) // this fails
  {
      std::cout<<"do stuff" << std::endl;
  }
}

Answer 1

我设法在当前标准 (c++03) 中找到了这一点。 24.1 p 5 告诉:

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 non- singular.

(强调我的)

所以答案是:不，不可能。