Question

I have a list like below

typedef std::list<std::string> SegmentValue;

then in a iteration I need check if this is last iteration.

     for(Field::SegmentValue::const_iterator it = m_segmentValue.begin();It != 
           m_segmentValue.end();It++){
              if((segIt + 1) == m_segmentValue.end())//last iteration
              ...
        }

but I get error in compile that:

 error C2678: binary '+' : no operator found which takes a left-hand operand of type 'std::list<_Ty>::_Const_iterator<_Secure_validation>'

how I can check if this is last itration?

Answer 1

You can’t use binary + and - operators with std::list iterators. std::list iterators are bidirectional iterators, but they are not random access iterators, meaning that you can’t shift them by an arbitrary constant value.

Use unary ++ and -- instead

Field::SegmentValue::const_iterator it_last = m_segmentValue.end();
--it_last;

Now it_last is the last element iterator. Just make sure it remains valid. If you are not making any iterator-invalidating modifications to your container, you can pre-compute it_last and use it in the cycle. Otherwise, you’ll have to re-compute it as necessary.

In fact, in generic algorithms it is always a good idea to prefer using -- and ++ with iterators whenever possible (instead of binary + 1 and - 1), since it reduces your algorithm’s requirements: binary + and - require random access iterators, while ++ and -- work with bidirectional ones.