重载的流插入运算符错误，将无法编译

Question

I'm trying to overload operator<< for my Graph class but I keep getting various errors:

error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
error C2143: syntax error : missing ',' before '<'

I have the prototype for operator<< placed right above the Graph class definition. operator<< 's definition is at the very bottom of the file. Do the errors have something to do with header guards?

Here's Graph.h :

#ifndef GRAPH
#define GRAPH

#include <iostream>
#include <vector>
#include <map>
#include <sstream>
#include "GraphException.h"
#include "Edge.h"

using namespace std;

template <class VertexType>
ostream& operator<<( ostream& out, const Graph<VertexType>& graph );

/** An adjacency list representation of an undirected,
 * weighted graph. */

template <class VertexType>
class Graph
{
    friend ostream& operator<<( ostream& out, const Graph& graph );
   // stuff

}  


template <class VertexType>
ostream& operator<<( ostream& out, const Graph<VertexType>& graph )
{
    return out;
}

#endif GRAPH

and here's main :

#include <iostream>
#include "Graph.h"

using namespace std;

const unsigned MAX_NUM_VERTICES = 9;

int main()
{
    // create int graph:

Graph<int> iGraph( MAX_NUM_VERTICES );

    // add vertices and edges

    cout << iGraph;


    return 0;
}

Answer 1

The declaration of operator<< is missing a declaration for Graph . One solution is to declare the class before the operator<< declaration:

template <class VertexType>
class Graph;

Or you can omit the declaration of operator<< outside the class entirely, as a friend declaration constitutes a nonmember declaration of operator<< as well.