C++朋友重载运算符<<

Question

我想超载运算符< <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Point2D.cpp与一个朋友函数重载。下面是我的代码：

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该方法

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

您需要

#include <iostream> 

或者至少

#include <ostream> 

---

其他2建议：

• 的包括防护装置（ifndef，define和endif）应该是在开始的时候，在的尽头头文件文件（绝不能在源文件中，但在头文件中）
• 在标题中添加using namespace std;是不好的做法。至少在标题中使用std::前缀。如果您在源文件中使用using namespace std;，则是您的选择。我不会，但这是我个人的选择。

Answer 2

您需要包括另一头

#include <iostream> 

只有#include <ostream>会虽然足够了。

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 

的从文件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 

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 - removed 

并替换

#include <iomanip> 

为

#include <iostream> 

或至少添加报头

#include <iostream> 

到操作者所定义的模块文件。

而且操作员应当操作者将被用于将被创建类型Point2D的临时对象每次声明如下

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

否则。