I'm implementing kruskal's algorithm and ran into some troubles compiling. I need to sort a vector of objects by value. Here is my code and the error I'm getting.

Code:

//These are the two functions in graph.cpp (there are more but are unrelated)
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;
#include "graph.h"
#include "edge.h"
using std::vector;
void graph::sort_edgesArray()
{
sort (edgesArray.begin(), edgesArray.end(), graph::cmp_val); <== This is line 39 in the error
}
bool graph::cmp_val (edge& a, edge& B)
{
return (a.val < b.val);
}

Code:

//This is the header file graph.h
#ifndef GRAPH_H
#define GRAPH_H
#include <vector>
#include "edge.h"
using std::vector;
class graph
{
public:
int nEdges;
int nVerts;
vector<edge> edgesArray; //array that holds edges
//might need a vertices vector
graph() :
nEdges(0),
nVerts(0)
{}
void addVert(int vert);
void addEdge(int start, int end, int value);
int get_edges();
int get_verts();
void sort_edgesArray();
bool cmp_val(edge& a, edge& b);
};
#endif

Code:

//This is the edge.h header file where graph.cpp is calling from to construct the vector.
#ifndef EDGE_H
#define EDGE_H
//Edges class in the graph
class edge
{
public:
int node1;
int node2;
int val;
//constructor
edge(int start, int end, int value) :
node1(start),
node2(end),
val(value)
{}
}; //end class edge
#endif

Code:

//This is the error I'm getting.
graph.cpp: In member function ‘void graph::sort_edgesArray()’:
graph.cpp:39:33: error: no matching function for call to ‘sort(std::vector<edge>::iterator&, std::vector<edge>::iterator&, <unresolved overloaded function type>)’
/usr/include/c++/4.5/bits/stl_algo.h:5236:18: note: candidate is: void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<edge*, std::vector<edge> >, _Compare = bool (graph::*)(edge&, edge&)]