Use const T(&x)[N][M] to constructor class Matrix - c++

Question

I don't know this question is clear or not.
I want make a simple matrix like that
template<typename T>
class Matrix {
private:
T* m_buffer;
int m_row;
int m_col;
public:
template<int N, int M>
Matrix(const T (&x)[N][M]);
};
template<typename T>
template<int N, int M>
inline
Matrix<T>::Matrix(const T (&x)[N][M]) : m_row(N), m_col(M) {
m_buffer = new T[N*M];
std::copy(x, x + N*M, m_buffer);
}
int main() {
int a[2][3] = { {1, 2, 3},
{4, 5, 6} };
Matrix<int> x(a); // This not work
return 0;
}
It seem be not working and get some error when compile. I want to ask how to fix this error.
/usr/include/c++/5/bits/stl_algobase.h: In instantiation of ‘static _OI std::__copy_move<false, false, std::random_access_iterator_tag>::__copy_m(_II, _II, _OI) [with _II = const int (*)[3]; _OI = int*]’:
/usr/include/c++/5/bits/stl_algobase.h:400:44: required from ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = const int (*)[3]; _OI = int*]’
/usr/include/c++/5/bits/stl_algobase.h:436:45: required from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = const int (*)[3]; _OI = int*]’
/usr/include/c++/5/bits/stl_algobase.h:469:8: required from ‘_OI std::copy(_II, _II, _OI) [with _II = const int (*)[3]; _OI = int*]’
../main.cpp:34:13: required from ‘Matrix<T>::Matrix(const T (&)[N][M]) [with int N = 2; int M = 3; T = int]’
subdir.mk:18: recipe for target 'main.o' failed
../main.cpp:40:21: required from here
/usr/include/c++/5/bits/stl_algobase.h:340:18: error: invalid conversion from ‘const int*’ to ‘int’ [-fpermissive]
*__result = *__first;
Thank you for your support.

Answer 1

The issue seems to be the call to std::copy. You are passing a reference as the first parameter to std::copy, which is not correct. You should be passing a pointer.
To pass a pointer, the following could be done:
std::copy(&x[0][0], &x[0][0] + N*M, m_buffer);
In the above, you are passing the address of the first element in the two dimensional array, and since the data in a 2D array is contiguous, the address of one past the last element is merely specifying N*M.
Live Example

Answer 2

It's not quite what you're asking but using non-type template arguments, you can achieve a constructor like
Matrix<T,ROWS,COLS>(const T(& initializer)[ROWS][COLS])
allows
Matrix<int,2,2> a({{2,2},
{2,2}});
which has the added bonus of allowing you to enforce multiplication rules with the compiler

Answer 3

Try something like this:
// TemplateMatrix.hpp
#pragma once
#include <cstdlib>
#include <cstring>
template<class T, int M, int N>
class Matrix
{
private:
T *m_buffer;
public:
Matrix(const T matrix[M][N])
{
m_buffer = new T[M*N];
memcpy(m_buffer, matrix, M*N*sizeof(T));
}
};
Then, in the main function
#include <iostream>
#include <cstdlib>
#include "TemplateMatrix.hpp"
using namespace std;
int main()
{
int a[2][2] = {{1,1}, {2,2}};
Matrix<int, 2, 2> m(a);
}