c++ - Is this a singular iterator and, if so, can I compare it to another one?

Question

I always thought that a "singular" iterator was one that has been default-initialised, and these could serve as comparable sentinel values of sorts:

typedef std::vector<Elem>::iterator I;
I start = I();

std::vector<Elem> container = foo();

for (I it = container.begin(), end = container.end(); it != end; ++it) {
   if ((start == I()) && bar(it)) {
      // Does something only the first time bar(it) is satisfied

      // ...

      start = it;
   }
}

But this answer suggests not only that my definition of "singular" is wrong, but also that my comparison above is totally illegal.

Is it?

Answer 1

Obviously this will work for some iterators - T* being a clear example - but it's definitely not guaranteed correct behavior for all iterators. C++11 24.2.1 [iterator.requirements.general] p5:

Singular values are not associated with any sequence ... Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value, the assignment of a non-singular value to an iterator that holds a singular value, and, for iterators that satisfy the DefaultConstructible requirements, using a value-initialized iterator as the source of a copy or move operation.

You can replicate your desired behavior with a simple bool flag:

std::vector<Elem> container = foo();
bool did_it_already = false;

for (I it = container.begin(), end = container.end(); it != end; ++it) {
   if (!did_it_already && bar(it)) {
      // Does something only the first time bar(it) is satisfied

      // ...

      did_it_already = true;
   }
}