C++ 友元重载运算符<<

c++ friend overloading operator <<

本文关键字：lt 运算符 友元 重载 C++      更新时间：2023-10-16

我正在尝试重载运算符<lt；但我一直有这个错误。我试着做研究，但没有结果。我有一个Point2D.h和一个带有朋友函数的Point2D.cpp要重载。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

下面是我的错误消息，不确定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)

您需要

#include <iostream>

或者至少

#include <ostream>

2其他建议：

  • 包含保护(ifndef、define和endif(应位于头文件的最开始和最末尾(endif不得位于源文件中，而应位于头文件中(
  • 在标头中添加using namespace std;是不好的做法。至少在标头中使用std::前缀。如果要在源中使用using namespace std;，这是您的选择。我不会，但这是我个人的选择

您需要包含另一个标头

#include <iostream>

不过，只有#include <ostream>就足够了。

正确的代码(请参阅代码中的注释(：

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

首先移动指令

#endif

从文件Point.cpp到文件Point2D.h的末尾这些文件看起来像

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>

对于

#include <iostream>

或者至少添加报头

#include <iostream>

到定义运算符的模块文件。

操作员也应该像一样声明

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

否则，每次使用运算符时，都会创建类型为Point2D的临时对象。

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