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 <typename T>
class matrix
{
    friend ostream& operator<<(ostream &out, const matrix<T> &m);
    friend istream& operator>>(sstream &in, matrix<T> &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 <typename T>
ostream& operator<<(ostream &out, const matrix<T> &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 <typename T>
istream& operator>>(istream &