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 有一个要重载的朋友函数。以下是我的代码：

Point2D.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;
};

Point.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的方法

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>

或至少

#include <ostream>

2 其他建议：

包含守卫（ifndef、define 和 endif）应该在 header 的最开始和最后 文件（endif 不得在源文件中，而应在 header 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。

Answer 2

您需要添加另一个 header

#include <iostream>

不过 #include <ostream> 就够了。

Answer 3

正确的代码（见代码中的注释）：

Point2D.h:

#include <string>
// #include <iomanip> // this includes io manipulators you do not need here
#include <iosfwd> // minimalist forward declarations for io streams

// using namespace std; // don't do this :(

#ifndef Point2D_H // should be above the includes
#define Point2D_H // should be above the includes

class Point2D
{
    // friend ostream& operator<< (ostream&, Point2D);
    friend std::ostream& operator<< (std::ostream&, const Point2D &);
    // observe pass by const reference

Point2D.cpp

#include <string>
#include <cmath>
// #include <iomanip> // not needed
#include <iostream> // std::ostream class definition
#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)
std::ostream& operator<< (ostream &out, const Point2D& pt)
{
    out << "Point = " << pt.x;
    return out;
}
// #endif // this shouldn't be here

Answer 4

首先移动指令

#endif

从文件Point.cpp到文件末尾Point2D.h 这些文件看起来像

Point2D.h

#include <string>
#include <iomanip>

using namespace std;

#ifndef Point2D_H
#define Point2D_H

//...

#endif

Point.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 - removed

并替代

#include <iomanip>

为了

#include <iostream>

或至少添加 header

#include <iostream>

到定义运算符的模块文件。

运算符也应该像这样声明

ostream& operator<< (ostream &out, const Point2D &pt);

否则，每次使用运算符时，都会创建 Point2D 类型的临时 object。

C++ 友元重载运算符 <<

c++ friend overloading operator <<

c++

overloading

friend