set_difference for a custom structure - c++

Question

I am trying to find the difference between the following two sets:
A = {(0,0), (0,1), (1,0), (1,1), (2,2)}
B = {(0,0), (0,1), (1,0), (1,1)}
The answer I am expecting is
A - B = {(2,2)}
I tried the following code. But I am stuck unable to compile. Can anyone point out the mistake I am making?
#include <vector>
#include <algorithm>
#include <iostream>
#include <utility>
using namespace std;
class compare
{
public:
bool operator()(const pair <int, int> elem1, const pair <int, int> elem2)
{
return ((elem1.first == elem2.first) &&
(elem1.second == elem2.second));
}
};
int main()
{
vector < pair<int, int> > v, va, vb;
va.push_back(make_pair(0,0));
va.push_back(make_pair(0,1));
va.push_back(make_pair(1,0));
va.push_back(make_pair(1,1));
va.push_back(make_pair(2,2));
vb.push_back(make_pair(0,0));
vb.push_back(make_pair(0,1));
vb.push_back(make_pair(1,0));
vb.push_back(make_pair(1,1));
vector < pair<int, int> >::iterator it, result;
result = set_difference (va.begin(), va.end(),
vb.begin(), vb.end(),
inserter(v, v.end()),
compare());
// for (it = v.begin( ) ; it != result; it++)
// cout << "(" << it->first << it->second << ")" << ", ";
// cout << endl;
return 0;
}
EDIT:
The compile error message is the following:
set_difference.cc: In function `int main()':
set_difference.cc:36: error: no match for 'operator=' in 'result = std::set_difference [with _InputIterator1 = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >, _InputIterator2 = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >, _OutputIterator = std::insert_iterator<std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >, _Compare = compare]((&va)->std::vector<_Tp, _Alloc>::begin [with _Tp = std::pair<int, int>, _Alloc = std::allocator<std::pair<int, int> >](), (&va)->std::vector<_Tp, _Alloc>::end [with _Tp = std::pair<int, int>, _Alloc = std::allocator<std::pair<int, int> >](), (&vb)->std::vector<_Tp, _Alloc>::begin [with _Tp = std::pair<int, int>, _Alloc = std::allocator<std::pair<int, int> >](), (&vb)->std::vector<_Tp, _Alloc>::end [with _Tp = std::pair<int, int>, _Alloc = std::allocator<std::pair<int, int> >](), std::inserter [with _Container = std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > >, _Iterator = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >](((std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > >&)(&v)), (&v)->std::vector<_Tp, _Alloc>::end [with _Tp = std::pair<int, int>, _Alloc = std::allocator<std::pair<int, int> >]()), (compare(), compare()))'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../include/c++/3.4.6/bits/stl_iterator.h:587: note: candidates are: __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >& __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >::operator=(const __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > >&)

Answer 1

(This is addressing the correctness of the algorithms, not the compilation problems.)
Read the documentation: The two input ranges already have to be sorted.
You also have to provide an output iterator for the result.
So do this:
std::sort(va.begin(), va.end(), compare());
std::sort(vb.begin(), vb.end(), compare());
set_difference(va.begin(), va.end(), vb.begin(), vb.end(),
std::back_inserter(v), compare());
Your compare function should also define a strict, weak ordering, not an equality comparison.
By the way, std::pair<S, T> and std::tuple<T...> already come with built-in lexicographic comparison, so you shouldn't usually need to define your own comparison, unless you want something exotic: std::sort(va.begin(), va.end()); etc.

Answer 2

OK, now with the error message posted it's clear what is the problem:
set_difference returns the output iterator, which in this case is an insert_iterator (constructed by your call to inserter). You try to assign it to result which is a vector iterator. This is a type mismatch.
The simplest solution would be to just omit that assignment and the iterator result because you don't need that iterator anyway; the results have been written to v.

Answer 3

Your expectation is wrong: A - B is the set of elements in A not also found in B, which in your case is the empty set because every element in A is also found in B. B - A would give the result you expect.