Overriding << operator for templated struct - c++

Question

numcpp.h file
#include "iostream"
namespace numcpp{
template<typename T>
struct Vector
{
std::vector<T> v;
};
template<typename T>
struct Matrix
{
std::vector<T> m;
//template<typename T>
friend std::ostream& operator << (std::ostream& out, const mf& mat);
};
typedef Vector<float> vf;
typedef Matrix<vf> mf;
}
I am trying to overload << operator for cout to be able to print mf. First I tried making the overloading function a friend that directly takes mf as argument. I did this because if I took Matrix as argument, I would need to deal with template and I don't know how to do that.
numcpp.cpp file
#include "numcpp.h"
namespace numcpp{
std::ostream& operator << (std::ostream& out, const mf& mat)
{
//overloaded out here
return out;
}
}
main.cpp
#include "iostream"
#include "numcpp.h"
int main()
{
numcpp::mf inputs;
// inputs is filled with random numbers here
std::cout << inputs;
}
But this gives an error identified mf is undefined in .h file in line friend std::ostream& operator << (std::ostream& out, const mf& mat);
So I ditched this approach and tried removing the friend function declaration from Matrix without changing the .cpp file. But now I get a different error saying no operator << matches these operands.
I think this is because the overload is done in numcpp namespace so it is not visible from main which is outside the namesapce.

Answer 1

What you normally want is to define a template in your header file. The definition needs to be available at the call-site to allow the compiler to make an instatiation of the template.
If possible, the easiest way is to put the definition in the class definition.
template<typename T>
struct Matrix
{
std::vector<T> m;
// using Matrix here is allowed and refers to the current instatiation, equivalent to writing Matrix<T>
friend std::ostream& operator << (std::ostream& out, const Matrix& mat) {
...
return out;
}
};

Answer 2

Try change your numcpp.h to
namespace numcpp{
template<typename T>
struct Vector
{
std::vector<T> v;
};
typedef Vector<float> vf;
template<typename T>
struct Matrix
{
std::vector<T> m;
//template<typename T>
inline friend std::ostream& operator << (std::ostream& out, const Matrix<vf>& mat);
};
inline std::ostream& operator << (std::ostream& out, const Matrix<vf>& mat)
{
//overloaded out here
return out;
}
typedef Matrix<vf> mf;
}
You are getting this error because mf was not declared when you trying to use it.
Also, you can add the body of the function directly in the definition to get rid of the inline.

Answer 3

The typedefs are not available inside the definition of your class Matrix. You can either declare the typedef before the class definition (#Alloces' answer) of just template the overloaded operator:
#include <vector>
#include <sstream>
#include <iostream>
namespace numcpp {
template<typename T>
struct Vector
{
std::vector<T> v;
};
template<typename T>
struct Matrix
{
std::vector<T> m;
template<typename K> // template parameter must not be named T
inline friend std::ostream& operator << (std::ostream& out, const Matrix<K>& mat);
};
template<typename K>
inline std::ostream& operator << (std::ostream& out, const Matrix<K>& mat)
{
return out << "Just a test";
}
typedef Vector<float> vf;
typedef Matrix<vf> mf;
}
int main() {
numcpp::mf inputs;
std::cout << inputs << "\n"; // Just a test
}