C++ 链接器错误，未解析的外部

Question

为什么我在尝试编译这段代码时遇到链接器错误，这基本上是一个复杂的模板类矩阵的代码，矩阵是一个方阵，所以如果输入大小“3”，它意味着 [3] 的矩阵][3] 但不知何故它给了我错误，有什么帮助吗？

#include <iostream>
#include <iomanip>
using namespace std;

template <class T>
class matrix
{
private:
  T** real;
  T** imag;
int size;
public:
  matrix(int = 0);
  friend ostream& operator<<(ostream& out, matrix<T>);
};

// constructor

template <class T>
matrix<T>::matrix(int length)
{
 size = length;

real = new T*[size];
for (int i = 0; i < size; i++)
    real[i] = new T[size];

imag = new T*[size];
for (int i = 0; i < size; i++)
    imag[i] = new T[size];

cout << "Enter real elements of matrix: ";
for (int i = 0; i < size; i++)
    for (int j = 0; j < size; j++)
        cin >> real[i][j];

cout << "Enter imag elements of matrix: ";
for (int i = 0; i < size; i++)
    for (int j = 0; j < size; j++)
        cin >> imag[i][j];
}

// functions defined here

template <class T>
ostream& operator<<(ostream& out, matrix<T> arg)
{
 out << showpos;
 for (int i = 0; i < arg.size; i++)
    for (int j = 0; j < arg.size; j++)
        out << arg.real[i][j] << arg.imag[i][j] << " ";
out << endl;
return out;
}

int main()
{
  matrix <int> obj1(3);
  cout << obj1;
}

Answer 1

因为编译器需要非模板函数。

friend ostream& operator<<(ostream& out, matrix<T>);

但是你定义为

template <class T>
ostream& operator<<(ostream& out, matrix<T> arg)
{
    //some code
}

修改类定义为

template<class N> friend ostream& operator<<(ostream& out, matrix<T>);

Here是指向 template friend operators 的链接，它提供了有关使用模板 friends 的很好的解释。

编辑 1:

根据 vsoftco 的建议，您还可以使用替代方法来定义类内:

class matrix{
//some code

//we don't need template here
friend ostream& operator<<(ostream& out, matrix<T>)
{
    out << showpos;
    for (int i = 0; i < arg.size; i++)
        for (int j = 0; j < arg.size; j++)
            out << arg.real[i][j] << arg.imag[i][j] << " ";
        out << endl;
    return out;
}
};