c++ - 模板类和 << 运算符重载

Question

我有名为“Point”的模板类，我使用 << 运算符的重载来写入ostream. 我尝试做这样的事情：

Point<int> point = Point(4, 3);
stream << point << endl;

当我编译程序时，我遇到链接器错误，提示未找到带有参数（ostream &、const Point &）的运算符 << 的重载。

//point.h -- headers definitions for Point<>
#pragma once
#include <iostream>
#include "serializable.h"

//Class that represents coordinates of point on the plane
template <class T>
class Point
{
public:
    T x;
    T y;
    Point();
    Point(T _x, T _y);
    bool operator==(Point & p)
    {
        return p.x == x && p.y == y;
    }
    Point<T> & operator=(const Point<T> &p)
    {
        x = p.x;
        y = p.y;
        return *this;
    }

    friend std::ostream & operator<<(std::ostream & stream, const Point<T> & point);
    friend std::istream & operator>>(std::istream & stream, Point<T> & point);
};

template <class T>
Point<T>::Point()
{
}

template <class T>
Point<T>::Point(T _x, T _y)
{
    x = _x;
    y = _y;
}

template <class T>
std::ostream & operator<<(std::ostream & stream, const Point<T> & point)
{
    stream << point.x << " " << point.y << std::endl;
    return stream;
}

template <class T>
std::istream & operator>>(std::istream & stream, Point<T> & point)
{
    stream >> point.x >> point.y;
    return stream;
}

那是错误文本：

Error 209   error LNK2019: reference to unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Point<int> const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Point@H@@@Z) in function "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Monster const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABVMonster@@@Z)   monster.obj The Pacman

标签： c++templates