c++ - C++ 模板/ostream 运算符问题

Question

试图让操作员工作，但给我带来了一堆错误：

我的头文件

template <unsigned short n>
class Vector {
public:
    std::vector<float> coords;

    Vector();
    Vector(std::vector<float> crds);
    friend std::ostream& operator <<(std::ostream& out, const Vector& v);
};

template <unsigned short n>
Vector<n>::Vector() {
coords.assign(n, 0.0);
}

template <unsigned short n>
std::ostream& operator<<(std::ostream& out, const Vector<n>& v) {
out << "(" << v.coords[1] << " - " << v.coords[2] << ")";
return out;
}

测试文件

#include <iostream>
#include "vector.h"
using namespace std;

int main() {
Vector<3> toomas;
cout << toomas;

}

错误：

C:\CodeBlocks\kool\praks3\vector.h|14|警告：友元声明 'std::ostream& operator<<(std::ostream&, const Vector&)' 声明一个非模板函数|

C:\CodeBlocks\kool\praks3\vector.h|14|注意：（如果这不是您想要的，请确保函数模板已经声明并在此处的函数名称后添加<>）|

obj\Debug\test.o||在函数'main'中：|

C:\CodeBlocks\kool\praks3\test.cpp|8|未定义引用 `operator<<(std::ostream&, Vector<(unsigned short)3> const&)'|

Answer 1

请看错误，它说，

友元声明 'std::ostream& operator<<(std::ostream&, const Vector&)' 声明一个非模板 函数|

这意味着您需要制作operator<<模板功能。

因此，在课堂上，您必须将其声明为：

template<unsigned short m> //<----note this: i.e make it template!
friend std::ostream& operator <<(std::ostream& out, const Vector<m>& v);

然后将其定义为，

template <unsigned short m>
std::ostream& operator<<(std::ostream& out, const Vector<m>& v) {
   out << "(" << v.coords[1] << " - " << v.coords[2] << ")";
   return out;
}

Answer 2

只需在类中定义朋友函数。

template <unsigned short n>
class Vector
{
public:
    std::vector<float> coords;

    Vector();
    Vector(std::vector<float> crds);
    friend std::ostream& operator <<(std::ostream& out, const Vector& v)
    {
        out << "(" << v.coords[1] << " - " << v.coords[2] << ")";
        return out;
    }
};

template <unsigned short n>
Vector<n>::Vector()
{
    coords.assign(n, 0.0);
}


int main()
{
    Vector<3> toomas;
    cout << toomas;
}

测试：http: //ideone.com/LDAR4

或者，使用前向原型预先声明模板函数：

template <unsigned short n>
class Vector;

template <unsigned short n>
std::ostream& operator <<(std::ostream& out, const Vector<n>& v);

template <unsigned short n>
class Vector
{
public:
    std::vector<float> coords;

    Vector();
    Vector(std::vector<float> crds);
    friend std::ostream& operator << <>(std::ostream& out, const Vector& v);
};

template <unsigned short n>
Vector<n>::Vector()
{
    coords.assign(n, 0.0);
}

template <unsigned short n>
std::ostream& operator <<(std::ostream& out, const Vector<n>& v)
{
    out << "(" << v.coords[1] << " - " << v.coords[2] << ")";
    return out;
}

测试：http: //ideone.com/8eTeq