iterate through std::vector ?

[SuperKoko]

I've heard that operator < only works with random-access iterators, whereas != works with other iterator types. But in this case, vector is not random access but why when we use operator <, it works.

std::vector is a random access container.
And std::vector::iterator is a random access iterator.

The example will not work for containers which provide bidrectional iterators only, as +=(int) is not defined for those, and using != instead of < max actually result in a program crash if the number of elements is odd.

It is less likely to crash with <, but it may also crash.
Because adding two to an iterator equal to end()-1, may crash or yield an invalid iterator (it is like incrementing end()).
With trivial implementations of vector, it is unlikely (except if the vector has a max address equal to 2^32-1).
But, with std:eque it is very likely to happen.