Check if iterator to std::map points to second last element - c++

Question

I'm (forward) iterating over a std::map and would like to find if the iterator points to the second last element. I can't seem to find how to do that anywhere.
I've got:
bool
isSecondLastFile(const TDateFileInfoMap::const_iterator &tsFile)
{
TDateFileInfoMap::reverse_iterator secondLastIt = mFileInfoMap.rbegin() + 1;
return (tsFile == secondLastIt);
}
Where TDateFileInfoMap is std::map
I'm getting:
error: no match for ‘operator==’ in ‘tsFile == secondLastIt’
/usr/lib/gcc/i686-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_tree.h:287: note: candidates are: bool std::_Rb_tree_const_iterator<_Tp>::operator==(const std::_Rb_tree_const_iterator<_Tp>&) const [with _Tp = std::pair<const long int, TFileInfo>]
Does that mean I can't compare the forward and reverse iterator?
How do I figure out if the forward iterator is pointing at the second last element?

Answer 1

std::map's iterator type is BidirectionalIterator. Just decrement the end iterator twice--first to get the last element since m.end() returns an iterator at the after the end position, and then again to get the second-last element:
auto penultimate = std::prev(m.end(), 2);
Then you can simply check for equality with the resultant iterator:
auto it = m.begin();
it == penultimate;
see it live on Coliru
Naturally, you should check that the map has two elements first if it's not guaranteed by other logic in your program.

Answer 2

Does that mean I can't compare the forward and reverse iterator?
Yes you can't compare them directly.
You can use base() to get the underlying base iterator.
Returns the underlying base iterator. That is
std::reverse_iterator(it).base() == it.
The base iterator refers to the element that is next (from the
std::reverse_iterator::iterator_type perspective) to the element the
reverse_iterator is currently pointing to. That is &*(rit.base() - 1) == &*rit.
e.g.
return (tsFile == (++secondLastIt).base());
BTW: mFileInfoMap.rbegin() + 1 won't compile since the iterator of std::map is not RandomAccessIterator. You might write:
TDateFileInfoMap::reverse_iterator secondLastIt = mFileInfoMap.rbegin();
++secondLastIt;
Note that we're not checking whether the map is empty or has only one element.

Answer 3

A simple solution for forward iterators:
template <typename ForwardIterator>
inline bool isNthLast(std::size_t n, ForwardIterator pos, ForwardIterator last) {
for( ;; --n, ++pos) {
if(n == 0)
return (pos == last);
if(pos == last)
return false;
}
}
bool isSecondLastFile(TDateFileInfoMap::const_iterator sFile) {
return isNthLast(2, sFile, mFileInfoMap.end());
}

Answer 4

Let's say you have a set with name s.
s= {s1,s2,...,sN-1, sN}
Now to iterate from s1.. to sN-1 (which is second last element) we will use STL functions, s.begin() and s.end().
end = s.end(); //end points to end
end--// end points to sN
Now in the for loop when itr (starts from the beginning of set) becomes equal to sN the loop will break, and you will get s1,s2,..sN-1 inside the loop.
map<int,int> s;
// to iterate till fixed range in map
auto end =s.end();
end--; // end to second last;
for(auto itr = s.begin(); itr!=end;itr++){
// do your operation
}