Question

我正在尝试重载运算符＆lt;＆lt;但我一直有这个错误。我尝试做研究但没有结果。我有一个Point2D.h和一个带有友元函数的Point2D.cpp来重载。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

以下是我的错误消息，不确定为什么该方法不匹配

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

T super Something

或者至少

#include <iostream>

另外2条建议：

包含警卫（#include <ostream>，ifndef和define）应位于标头文件的最开头和最后（{ {1}}绝不能在源文件中，而是在头文件中）
在标头中添加endif是不好的做法。至少在标题中使用endif前缀。如果你在源代码中使用using namespace std;，那么这是你的选择。我不会，但这是我个人的选择。

Answer 2

您需要包含另一个标题

#include <iostream>

只有#include <ostream>才足够。

Answer 3

更正代码（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 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

并替换

#include <iomanip>

的

#include <iostream>

或至少添加标题

#include <iostream>

到定义运算符的模块文件。

此外，运营商应声明为

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

否则，每次使用运算符时，都会创建Point2D类型的临时对象。

c ++ friend重载运算符＆lt;＆lt;

4 个答案: