为什么我在编译模板类时遇到困难？

Question

#include <iostream>
#include <string.h>

//template <class t> class Matrix; //possible way of fixing the friend function.

using namespace std;
  template<class T, size_t NROWS, size_t NCOLS>
  std::ostream & operator<<(std::ostream &os, const Matrix<T,NROWS, NCOLS> &matrix);


template<class T, size_t NROWS = 1, size_t NCOLS = 1>
class Matrix{
public:
  Matrix(){}
  friend std::ostream &operator<< <>(std::ostream&os,const Matrix<T, NROWS, NCOLS> &matrix);

private:
  T container[NROWS][NCOLS];
};


template<class T,size_t NROWS, size_t NCOLS>
  std::ostream &operator<<(std::ostream &os,const Matrix<T,NROWS,NCOLS>&matrix){
  for(size_t i=0;i<NROWS;++i){
    for(size_t j=0;j<NCOLS;++j){
      os  <<matrix.container[i][j]<<" ";
    }
    os <<std::endl;
  }
  os <<std::endl;
}


int main(){
  Matrix<float, 10, 5> mat;
  cout << mat;
  return 0;
}

Answer 1

如果取消对第4行的注释，并将其更改如下，则所编译的代码为：

template <class t, size_t, size_t> class Matrix; //possible way of fixing the friend function.

您的问题似乎是前向声明的矩阵模板参数与后面的矩阵定义不匹配。

此外，尽管代码将在此修复后编译，但仍然存在一个您可能也想修复的警告：

In function 'std::ostream& operator<<(std::ostream&, const Matrix<T, NROWS, NCOLS>&)':
31:1: warning: no return statement in function returning non-void [-Wreturn-type]