Creating Min Heap from STL Priority Queue

Question

I am creating a min heap from stl priority queue. Here is my class which I am using.

class Plane
{
  private :
    int id ;
    int fuel ;
 public:
    Plane():id(0), fuel(0){}
    Plane(const int _id, const int _fuel):id(_id), fuel(_fuel) {}

    bool operator > (const Plane &obj)
    {
        return ( this->fuel > obj.fuel ? true : false ) ;
    }

} ;

In main I instantiate an object thus.

 priority_queue<Plane*, vector<Plane*>, Plane> pq1 ;
 pq1.push(new Plane(0, 0)) ;

I am getting an error from xutility which I am unable to understand.

d:\microsoft visual studio 10.0\vc\include\xutility(674): error C2064: term does not evaluate to a function taking 2 arguments

Any help to its solution would be appreciated.

Answer 1

The third template parameter must be a binary functor taking teo Plane*. Your Plane class does not provide that.

You need something of the form

struct CompPlanePtrs
{
  bool operator()(const Plane* lhs, const Plane* rhs) const {
    return lhs->fuel > rhs->fuel ;
  }
};

Answer 2

If you drop the use of pointers (which are overkill for your simple structures), then you can use std::greater from the header functional:

std::priority_queue<Plane, std::vector<Plane>, std::greater<Plane> > pq1;
pq1.push(Plane(0, 0));

Currently, you are feeding Plane as the comparison type. That won't work since the comparison type must be a type of function object, i.e. it must have an operator() that does the comparison. Plane doesn't have such a member (and adding it just for this purpose would be a bad idea).

std::greater has the appropriate method, implemented in terms of your operator>. However, it doesn't work with pointers, because then it uses a pointer comparison (based on memory addresses).

Btw., note that your comparison function can be expressed more succinctly as

bool operator>(const Plane &other)
{
    return fuel > other.fuel;
}