How to increment an iterator by 2?

std::advance( iter, 2 );

This method will work for iterators that are not random-access iterators but it can still be specialized by the implementation to be no less efficient than iter += 2 when used with random-access iterators.

http://www.cplusplus.com/reference/std/iterator/advance/

std::advance(it,n);

where n is 2 in your case.

The beauty of this function is, that If "it" is an random access iterator, the fast

it += n

operation is used (i.e. vector<,,>::iterator). Otherwise its rendered to

for(int i = 0; i < n; i++)
    ++it;

(i.e. list<..>::iterator)

You could use the 'assignment by addition' operator

iter += 2;

If you don't know wether you have enough next elements in your container or not, you need to check against the end of your container between each increment. Neither ++ nor std::advance will do it for you.

if( ++iter == collection.end())
  ... // stop

if( ++iter == collection.end())
  ... // stop

You may even roll your own bound-secure advance function.

If you are sure that you will not go past the end, then std::advance( iter, 2 ) is the best solution.

The very simple answer:

++++iter

The long answer:

You really should get used to writing ++iter instead of iter++. The latter must return (a copy of) the old value, which is different from the new value; this takes time and space.

Note that prefix increment (++iter) takes an lvalue and returns an lvalue, whereas postfix increment (iter++) takes an lvalue and returns an rvalue.