C++ programming candidate function not viable

Question

I am trying to make a priority queue which top most element contains the smallest integer. I made a function object for comparison. Everything is going well but, whenever I try to print out the top most element std::cout<<pq.top<<std::endl I get an error that says:

candidate function not viable: no known conversion from 'const value_type' (aka 'const Foo') to
  'const void *' for 1st argument; take the address of the argument with &
basic_ostream& operator<<(const void* __p);

I am really new to programming so, I really don't know what to do.

#include<iostream>
#include<queue> 
#include <vector>


class Foo
{
public:
  int data;
  Foo(int data): data(data) {}
};

class Compare
{
public:
    int operator() (Foo dat1, Foo dat2)
    {
      if( dat1.data < dat2.data )
        return dat1.data;
      else return dat2.data;
    }
};

int main()
{
  std::priority_queue<Foo, std::vector<Foo>, Compare> pq;

  pq.push(5);
  pq.push(7);
  pq.push(1);
  pq.push(2);
  pq.push(3);

  std::cout << pq.top() << std::endl; 
    return 0;
}

Answer 1

You never defined a way to output a Foo. You can use

std::cout << pq.top().data << std::endl; 

Or you can overload the operator<< for Foo to output it like

class Foo
{
public:
    int data;
    Foo(int data) : data(data) {}
    friend std::ostream & operator<<(std::ostream& os, const Foo & f)
    {
        return os << f.data;
    }
};

You also have an issue with your comparision function. The comparison function should return true if dat1.data > dat2.data in order to get the smallest element to the top. With that you should change it to:

bool operator() (const Foo& dat1, const Foo& dat2)
{
  return dat1.data > dat2.data;
}