Question

Is there any way to do std::set_intersection on two different types of sets?

I have two sets:

std::set<X1> l_set1;
std::set<X2> l_set2;

I’m able to define some comparator for them that checks if X1 and X2 are equal.

struct sample_comparer
{
    bool operator()(const &X1 p_left, const &X2 p_right)
    {
        return p_left == p_right;
    }
};

Now, I try to do a set intersection on those two sets:

std::set<X1> l_intersect;
std::set_intersection(l_set1.begin(), l_set1.end(), l_set2.begin(), l_set2.end(),
                      std::inserter(l_intersect, l_intersect.begin()), sample_comparer());

Unfortunately, I can’t force this code to work. I’m not even sure if this is possible, but from the description of set_intersection I know that I can use two different iterators.

I tried to search for some code samples that do what I want, but didn’t found any? Could someone present me a working code sample for my problem?

Update:
the error is:

error: stl_function.h:227: no match for ‘operator<‘ in ‘__x < __y’

Thanks in advance!

Answer 1

The comment by PlasmaHH is likely the problem.

The way functions like set_intersection work is they first do: a < b and then b < a

As a result ample_comparer needs to be able to compare both ways:

struct sample_comparer
{
    bool operator()(const &X1 p_left, const &X2 p_right)
    {
        return p_left == p_right;
    }
    bool operator()(const &X2 p_left, const &X1 p_right)
    {
        return p_left == p_right;
    }
};

The following doesn’t actually do anything sensible – but it does compile cleanly:

struct A
{
  struct Compare { bool operator () (A const &, A const &) { return false;}  };
};

struct B
{
  struct Compare { bool operator () (B const &, B const &) { return false; } };
};

typedef std::set<A, A::Compare> S1;
typedef std::set<B, B::Compare> S2;

class IntersectionCompare
{
public:
  bool operator ()(S1::value_type, S2::value_type) { return false; } 
  bool operator ()(S2::value_type, S1::value_type) { return false; } 
};

void bar (S1 & s1, S2 & s2)
{
  S1 result;
  std::set_intersection (s1.begin ()
      , s1.end ()
      , s2.begin ()
      , s2.end ()
      , std :: insert_iterator< S1 > (result, result.end ())
      , IntersectionCompare ());
}