Question

How to create new std::list from existing std::list taking first n-5 elements ?
n is size of existing list (list)

Answer 1

std::list<T> newlist(oldlist.begin(), std::prev(oldlist.end(), 5));

where T is the value type of the old list.

std::prev is new in C++11, but if you don’t have it you can use std::advance instead:

std::list<T>::const_iterator end = oldlist.end();
std::advance(end, -5);
std::list<T> newlist(oldlist.begin(), end);

Either way it’s your responsibility to ensure that oldlist.size() >= 5. Neither std::prev nor std::advance does that for you.