C++模板类中声明友元函数重载输入和输出运算符时，提示无法解析的外部符号解决方案

在练习模板类的重载输入输出运算符时，编译器提示“无法解析的外部符号”，代码如下：

template 
class matrix
{
    friend ostream& operator<<(ostream &out, const matrix &m);
    friend istream& operator>>(sstream &in, matrix &m);
public:
    ......
private:
    int theRows;  //矩阵行数
        theColumns;  //矩阵列数
    T *element;  //矩阵元素用一维数组存储
};
......
template 
ostream& operator<<(ostream &out, const matrix &m)
{
    for (int i = 0; i < m.theRows; i++)
    {
        for (int j = 0; j < m.theColumns; ++j)
            out << m.element[i * m.theColumns + j] << " ";
        out << endl;
    }
    return out;
}
template 
istream& operator>>(istream &in, matrix &m)
{
    cout << "Enter " << m.theRows * m.theColumns << " element: ";
    for (int i = 0; i < m.theRows * m.theColumns; i++)
        in >> m.element[i];
    if (!in)
        throw illegalInputData("filed to input");
    return in;
}

这是因为代码中用到模板类template 而在类内声明友元函数的时候也用到了,所以此时友元函数是依赖于类的实现而实现的，编译器才会报错。

对此我们要两种解决方法。
1、依赖于两个成员函数output()和input()，而可以省去用友元声明，代码如下：

......
template 
class matrix
{
public:
    ......
    void output(ostream &out) const;
    void input(istream &in);
    ......
private:
    int theRows;  //矩阵行数
        theColumns;  //矩阵列数
    T *element;  //矩阵元素用一维数组存储
};
......
template 
void matrix::output(ostream &out)
{
    for (int i = 0; i < theRows; i++)
    {
        for (int j = 0; j < theColumns; ++j)
            out << element[i * theColumns + j] << " ";
        out << endl;
    }
}
template 
ostream& operator<<(ostream &out, const matrix &m)
{
    m.output(out);
    return out;
}

template 
void matrix::input(istream &in)
{
    cout << "Enter " << theRows * theColumns << " element: ";
    for (int i = 0; i < theRows * theColumns; i++)
        in >> element[i];
    if (!in)
        throw illegalInputData("filed to input");
}
template 
istream& operator>>(istream &in, matrix &m)
{
    m.input(in);
    return in;
}

2、改变友元与类模板的对应关系为多对多（即若T为int，U为vector依然为友元关系)，代码如下：

template <typename T>
class matrix
{
    template <typename U>
    friend ostream& operator<<(ostream &out, const matrix &m);
    template <typename U>
    friend istream& operator>>(sstream &in, matrix &m);
public:
    ......
private:
    ......
};
......
template <typename U>
ostream& operator<<(ostream &out, const matrix &m)
{
    ......
}
template <typename U>
istream& operator>>(istream &in, matrix &m)
{
    ......
}

3、改变友元与类模板的对应关系为一对一（友好关系被限定在相同类型的实例化)，代码如下：

//前置声明
template <typename T>  
ostream& operator<<(ostream &out, const matrix &m);
template <typename T>
    friend istream& operator>>(sstream &in, matrix &m);

template <typename T>
class matrix
{
    friend ostream& operator<<(ostream &out, const matrix &m);
    friend istream& operator>>(sstream &in, matrix &m);
public:
    ......
private:
    ......
};
......
template <typename T>
ostream& operator<<(ostream &out, const matrix &m)
{
    ......
}
template <typename T>
istream& operator>>(istream &in, matrix &m)
{
    ......
}

关于更具体的模板类中的友元声明，参见《C++ primer》第十六章：模板与泛型编程