c++ friend overloading operator <<

问题

I am trying overload the operator << but i keep having this error. I try doing research but with no result. I have a Point2D.h and a Point2D.cpp with a friend functions to overload. Below are my codes:

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

Below are my error message, not sure why there is no match for that method

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)

回答1:

You need

#include <iostream>

Or at least

#include <ostream>

---

2 other advises:

• the include guards (ifndef, define and endif) should be at the very beginning and at the very end of the header file (the endif MUST NOT be in the source file, but in the header file)
• Adding using namespace std; in headers is bad practice. Use std:: prefix at least in the header. It's your choice if you'll use using namespace std; in the source. I wouldn't, but it's my personal choice.

回答2:

You need to include another header

#include <iostream>

Only #include <ostream> will suffice though.

回答3:

Correct code (see comments in code):

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

回答4:

First of all move directive

#endif

from the file Point.cpp to the end of the file Point2D.h The files will look like

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

And substitute

#include <iomanip>

for

#include <iostream>

or at least add header

#include <iostream>

to the module file where the operator is defined.

Also the operator should be declared like

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

Otherwise each time when the operator will be used a temporary object of type Point2D will be created.

来源：https://stackoverflow.com/questions/30392277/c-friend-overloading-operator