C++ 运算符错误<&书信电报；重载-找不到运算符

我有一个带构造函数的类图，重载的

操作符
std:：cout
是
std:：ostream
而不是
std:：ofstream
类型的全局对象
std:：ofstream
是std:：ostream

的派生版本。看

因此，将您的friend函数（运算符）修改为

friend-ostream&operator
class Graph
{
    private:
        vector<int> setOfVertices;
    public:
        Graph(ifstream &);      //konstruktor ze souboru
        friend ofstream & operator<<(ofstream&, const Graph &);
};

ofstream & operator<<(ofstream& outputStream, const Graph & graphToPrint)
{
    //not important for minimal example
    return outputStream;
}

#include <iostream>
#include <fstream>
#include "graph.h"

using namespace std;

int main()
{
    ifstream myFile ("example.txt");
    Graph * G = new Graph(myFile);
    cout << *G;
    return 0;
}

error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'Graph' (or there is no acceptable conversion)

friend ostream & operator<<(ostream&, const Graph &);