Question

std::list<CPoint>::iterator iter= vertices.end();
CPoint point = *(iter+1); 

In such cases I’ve tried to assign to variables the value of (iter-1) or (iter+1). Why doesn’t it work? whereas iter++ or iter-- works.

Answer 1

Simply, these operations are not part of the iterator definition. You can use the std::advance() function for that.

Obviously, the operator+(int) could be overriden to do that, just as operator++() is, but probably it is not, because this operation is not guaranteed to be of constant complexity, and a syntax like (iter + n) could suggest otherwise.

From advance:

Complexity: Linear. However, if InputIt additionally meets the requirements of RandomAccessIterator, complexity is constant.