Why am I having trouble compiling a templated class?

0
votes

I've been stuck on this code for a while and can't get it to compile, what exactly am I doing wrong? If there are bugs present when it compiles, please ignore them as I can fix that on my own. As of right now I'm just trying to get it to run. Thank you in advance.

#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;
}

The errors from the IDE I use are as follows:

main.cpp:8:51: error: no template named 'Matrix' std::ostream & operator<<(std::ostream &os, const Matrix &matrix);

main.cpp:15:24: error: no function template matches function template specialization 'operator<<' friend std::ostream &operator<< <>(std::ostream&os,const Matrix &matrix);

main.cpp:35:32: note: in instantiation of template class 'Matrix' requested here Matrix mat;

3
c++templates
FWIW, there is no such thing as a templated class or temaplted function. There are class templates and function templates though. – NathanOliver
Your code includes something labeled "possible way of fixing" -- why only possible? What's wrong with your own answer? (Keep following the error messages. You're close.) – JaMiT

3 Answers

3
votes

If you uncomment out line 4, and change it as follows, the code you have compiles:

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

It seems your problem is that the forward-declared Matrix template parameters don't match the Matrix definition that comes later.

Also, although the code will compile after this fix, there is still a warning which you probably also want to fix:

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]
1
votes
#include <cstddef>
#include <iostream>

template<typename, std::size_t, std::size_t> class Matrix;

template<typename T, std::size_t NROWS, std::size_t NCOLS>
std::ostream& operator<<(std::ostream &os, Matrix<T, NROWS, NCOLS> const &matrix)
{
    for (std::size_t row{}; row < NROWS; ++row, os.put('\n'))
        for (std::size_t col{}; col < NCOLS; ++col)
            os << matrix.container[row][col] << ' ';
    return os.put('\n');
}

template<typename T, std::size_t NROWS = 1, std::size_t NCOLS = 1>
class Matrix {
    T container[NROWS][NCOLS] = {};
    friend std::ostream& operator<< <>(std::ostream&, Matrix<T, NROWS, NCOLS> const&);
};

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

Please get rid of the C header <string.h>.

1
votes

You need to define Matrix before it's usage:

template<class T, size_t NROWS = 1, size_t NCOLS = 1>
class Matrix;

and add a return statement to operator<<, that returns os. You also do not need duplication of operator<< declaration, you can declare it only in a class body.