Check if iterator to std::map points to second last element

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.