How to restrict an iterator to being a forward iterator?

Question

I have a function that needs to enumerate an iterator multiple times, but according to MSDN, \"Once you increment any copy of an input iterator, none of the other copies can

Answer 1

To expand on rodrigo's answer -- I found this solution, and thought it's worth mentioning:

struct True { unsigned char _[2]; operator bool() const { return true; } };
char is_forward_iterator(std::input_iterator_tag const *) { return 0; }
True is_forward_iterator(std::forward_iterator_tag const *) { return True(); }

Now, if you want to check it inside some function, you can say:

if (is_forward_iterator(static_cast::iterator_category*>(0)))
{
    ...
}

and if you want to check it within templates, you can check for:

sizeof(
  is_forward_iterator(static_cast::iterator_category*>(0))
) > 1

with the primary advantage of this method being that it avoids declaring templates (e.g. for better compilation speed).