declaring a template method for a template class - c++

Question

I'm trying to declare a template method on a template class and it's not working for me.
It's better to explain by giving the code so here it is:
I have this class:
matrix.h
template <class T,int a,int b>
class Matrix {
private:
int x;
int y;
public:
class IllegalOperation();
template<T,int c,int d>
Matrix<T,a,b> operator+(const Matrix<T,c,d> m);
//...
}
matrix.cpp
template<class T,int a,int b>
template<T,int c,int d>
Matrix<T,a,b> Matrix<T,a,b>::operator+(const Matrix<T,c,d> m){
if(a!=c || b!=d) throw IllegalOperation();
// add matrices and return the result
}
I'd like this code to work for any 2 types of Matrix and Matrix where a,b,c and d can be different.
for example, I want this code to compile and return an error (in run time):
const Matrix<int, 3, 2> m1;
const Matrix<int, 7, 3> m2;
// init m1 and m2
m1+m2;
While this code should compile and run successfully:
const Matrix<int, 3, 2> m1;
const Matrix<int, 3, 2> m2;
// init m1 and m2
m1+m2;
However, when I try to compile the code above I get this error:
no match for âoperator+ in m1+m2

Answer 1

Change your code to this (not considering the things that I think might be wrong here, only changed it to make it compile)
#include <type_traits>
template <typename T,int a,int b>
class Matrix {
public:
template<typename T2, int c, int d>
Matrix<T,a,b> operator+(const Matrix<T2, c, d>& m) const;
private:
int x;
int y;
};
template <typename T,int a,int b>
template <typename T2, int c, int d>
Matrix<T, a, b> Matrix<T, a, b>::operator+(const Matrix<T2, c, d>&) const {
if(a != c || b != d) {
throw IllegalOperation{};
}
/*constexpr*/ if (!std::is_same<T, T2>::value) {
throw Error{};
}
return *this;
}
int main() {
const Matrix<int, 3, 2> m1{};
const Matrix<int, 7, 3> m2{};
m1 + m2;
return 0;
}
I've made a few changes here
The operator+ is const, you were trying to call a non const member function on a const object, would not work
The matrix parameter in the addition operator is now taken by reference
The operator+ cannot be defined in the .cpp file as mentioned in the comments, it must go in the header file (if you want to split up the interface and implementation, the best you can do is In the C++ Boost libraries, why is there a ".ipp" extension on some header files)
I usually like having the public section first since it gives the reader a better idea about the interface of the class.