Subtracting offsets from iterators

Hi all,

Got a quick question regarding iterators and subtracting offsets and I was wondering if you can help me out here. I had some code that was accessing a vector like such:

Code:

	for (int i = 0; i < m_linkArray.size() - 1; i++) {
		m_linkArray[i]->update();
	}

.. but wanted to change it so that it used iterators. The following compiles and runs okay - just wondering however if the subtraction of one to offset the iterator is correct:

Code:

	std::vector <CLink *>::iterator itPos = m_linkArray.begin();
	
	for (; itPos < m_linkArray.end() - 1; itPos++)
	{
		(*itPos)->update();
	}

Thanks!

No, it is incorrect. Since .end() gives you an iterator just beyond the end, iterating to end() is the correct logic, or you will miss the last element.

Additionally, if you are not going to use the iterator outside of the loop, then limit the iterator to the scope of the loop by writing it like this:

Code:

for (std::vector<CLink*>::iterator itPos = m_linkArray.begin(); itPos != m_linkArray.end(); ++itPos)
{
    (*itPos)->update();
}

Note that I used operator!= instead of operator< since you do not actually need operator< (and it helps if you need to change the container to one without random access iterators). It is also more idiomatic to use pre-increment rather than post-increment (unless post-increment is actually useful in the context) since it can be more efficient and is almost certainly no less efficient.

You could also consider using a standard generic algorithm instead of an explicit loop, e.g.,

Code:

std::for_each(m_linkArray.begin(), m_linkArray.end(), std::mem_fun(&CLink::update));

Ah yeah - thanks - I was totally reading that initial code wrong when I tried to re-implement it with iterators. Thanks guys!