struct D {int x; int y;}; 
class XEqualTo : public std::unary_function<D, bool>
{
public:
  XEqualTo(int x) : x_(x) {}
  bool operator()(const D& d) const
  {
    return d.x == x_;
  }
private:
  int x_;
};
vector<D> v;

int main()
{
  for(int i=0;i<4;i++)
  {
    D temp;
    temp.x=i;
    temp.y=i*i;
  }
  
  find(v.begin(),v.end(),XEqualTo(2));
   return 0;
}

test3.cpp:33:   instantiated from here
/usr/include/c++/3.3.3/bits/stl_algo.h:208: error: no match for 'operator==' in
   '(&__first)->__gnu_cxx::__normal_iterator<_Iterator, _Container>::operator*
   [with _Iterator = D*, _Container = std::vector<D, std::allocator<D> >]() ==
   __val'