Question

I’m trying to build a home-made min-heap with template class so I can work on Dijkstra or Prim. However the find_min() function does not work with std::min_element(). Any clue will be greatly appreciated. Thanks!

the error message from VC2010express, in short, says

error C2780: ‘_FwdIt std::min_element(_FwdIt,_FwdIt)’ : expects 2 arguments – 3 provided

and the code below:

#ifndef MIN_HEAP_H
#define MIN_HEAP_H//use (unsorted) vector and min() algorithm
#include <vector>
#include <algorithm>
#include <functional>
template <typename T>
class MinHeap{
    std::vector<T> c;//container
    typedef typename std::vector<T>::iterator iterator;
    bool compare_node(const T& lhs,const T& rhs) const {return lhs<rhs;}//define compare function for nodes
public:
    MinHeap():c(){}//default constructor

    inline void insert(T node){c.push_back(node);}

    iterator find_min(){
        iterator min_node=std::min_element(c.begin(),c.end(),compare_node);//doesn't build
        return min_node;
    }
//  deleteMin();
//  deleteNode(node);
//  decreaseKey(node);
};
#endif

Answer 1

std::min_element‘s 3rd argument is either a functor object, or a function pointer to a comparator. The comparator function must be a free function; you’re trying to give it a non-static member function.

As there is no reason for compare_node to be a member function, you may as well make it a free function instead.

Having said that, your compare_node is equivalent to the implementation of the default comparator for std::min_element, so you may as well not use it at all.