How to sort a std::set with const getters - c++

Question

I have a std::set container whose elements are objects of the following class:
class LaneConnector {
public:
const Lane* getLaneFrom() const {
return From;
}
const Lane* getLaneTo() const {
return To;
}
private:
Lane* From;
Lane* To;
}
and my comparator function is as follows:
struct MyLaneConectorSorter {
bool operator() (LaneConnector * c, LaneConnector * d)
{
Lane* a = const_cast<Lane*>(c->getLaneFrom());
Lane* b = const_cast<Lane*>(d->getLaneFrom());
return (a->getLaneID() < b->getLaneID());
}
} myLaneConnectorSorter;
Now when I try to sort the elements in the set with:
//dont panic, the container just came through a const_iterator of a std::map :)
const std::set<LaneConnector*> & tempLC = (*it_cnn).second;
std::sort(tempLC.begin(), tempLC.end(), myLaneConnectorSorter);
I get a frenzy of errors starting with the following lines, Appreciate if you help me solve this problem.
Thanks:
/usr/include/c++/4.6/bits/stl_algo.h: In function ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = std::_Rb_tree_const_iterator<LaneConnector*>, _Compare = {anonymous}::MyLaneConectorSorter]’:
/home/.../dev/Basic/shared/conf/simpleconf.cpp:1104:65: instantiated from here
/usr/include/c++/4.6/bits/stl_algo.h:5368:4: error: no match for ‘operator-’ in ‘__last - __first’
/usr/include/c++/4.6/bits/stl_algo.h:5368:4: note: candidates are:
/usr/include/c++/4.6/bits/stl_iterator.h:321:5: note: template<class _Iterator> typename std::reverse_iterator::difference_type std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.6/bits/stl_iterator.h:378:5: note: template<class _IteratorL, class _IteratorR> typename std::reverse_iterator<_IteratorL>::difference_type std::operator-(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.6/bits/stl_bvector.h:181:3: note: std::ptrdiff_t std::operator-(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)
/usr/include/c++/4.6/bits/stl_bvector.h:181:3: note: no known conversion for argument 1 from ‘std::_Rb_tree_const_iterator<LaneConnector*>’ to ‘const std::_Bit_iterator_base&’

Answer 1

First, you cannot sort an std::set. It is a sorted structure, sorting happens upon construction or insertion.
Second, you can construct an std::set with your own sorting functor, and you can avoid unnecessary const_casts by making it take const pointers:
struct MyLaneConectorSorter {
bool operator() (const LaneConnector* lhs, const LaneConnector* rhs) const
{
// you may want to put some null pointer checks in here
const Lane* a = lhs->getLaneFrom();
const Lane* b = rhs->getLaneFrom();
return a->getLaneID() < b->getLaneID();
}
};
and instantiate the set like this:
std::set<LaneConnector*, MyLaneConectorSorter> s(MyLaneConectorSorter());
or, if you want to construct it from a different set, with a different ordering,
std::set<LaneConnector*> orig = ..... ;
....
std::set<LaneConnector*, MyLaneConectorSorter> s(orig.begin(), orig.end(), MyLaneConectorSorter());