C++ 友元重载运算符 <<

Question

我正在尝试重载运算符 << 但我一直遇到此错误。我尝试做研究但没有结果。我有一个 Point2D.h 和一个 Point2D.cpp 有一个要重载的 friend 函数。以下是我的代码:

点2D.h

#include <string>
#include <iomanip>

using namespace std;

#ifndef Point2D_H
#define Point2D_H

class Point2D
{
    friend ostream& operator<< (ostream&, Point2D);

    public:

        Point2D();

        Point2D(int, int);

    protected:

        int x;
        int y;
};

点.cpp

#include <string>
#include <cmath>
#include <iomanip>
#include "Point2D.h"

Point2D::Point2D() {
    this->x=0;
    this->y=0;
}

Point2D::Point2D(int x, int y) {
    this->x=x;
    this->y=y;
}
ostream& operator<< (ostream &out, Point2D pt)
{
    out << "Point = " <<pt.x;
    return out;
}
#endif

下面是我的错误信息，不知道为什么没有匹配那个方法

Point2D.h: In function ‘std::ostream& operator<<(std::ostream&, Point2D)’:
Point2D.h:37:9: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘int’)
     out << pt.x;
         ^
Point2D.h:37:9: note: candidates are:
Point2D.h:35:10: note: std::ostream& operator<<(std::ostream&, Point2D)
 ostream& operator<< (ostream &out, Point2D pt)
          ^
Point2D.h:35:10: note:   no known conversion for argument 2 from ‘int’ to ‘Point2D’
In file included from Point2D.h:2:0,
                 from Point3D.h:2,
                 from Point3D.cpp:2:
/usr/include/c++/4.8/iomanip:235:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, std::_Setw)
 operator<<(basic_ostream<_CharT, _Traits>& __os, _Setw __f)

Answer 1

你需要

#include <iostream>

或者至少

#include <ostream>

---

另外 2 个建议:

• 包含守卫(ifndef、define 和 endif)应该在 的开头和结尾header 文件(endif 不得在源文件中，而应在头文件中)
• 在 header 中添加 using namespace std; 是不好的做法。至少在 header 中使用 std:: 前缀。您可以选择是否在源代码中使用 using namespace std;。我不会，但这是我个人的选择。