How to substract 1 from bidirectional iterator

Question

I have iterator I dont want to change, but I would like to assign another iterator to the value of that iterator -1, so
it2 = --it1; is out of the question.
Problem:
it2 = it1-1; doesnt work.

error: no match for 'operator-' in 'bigger - 1' c++/4.1.1/bits/stl_bvector.h:182: note: candidates are: ptrdiff_t std::operator-(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)

ofc I could do

it2 = --it1;
it1++;

but that is horrible.

Answer 1

In C++11, use std::prev from the header <iterator>:

it2 = std::prev(it1);

Prior to that, you're stuck making a copy and advancing the copy by -1 yourself:

it2 = it1;
std::advance(it2, -1);

or:

it2 = it1;
it2--;