Calling on an element of a matrix class

[luckiejacky]

The line:

sum += (gauss[x + 2][y + 2] = exp(-(x * x + y * y) / s) / s / M_PI);

cannot be compiled.

which says:

1>e:\projects\c++\edge_detect\edge_detect\Matrix.h(14): error C2059: syntax error : '['
1>          e:\projects\c++\edge_detect\edge_detect\Matrix.h(17) : see reference to class template instantiation 'matrix<T,D0,D1>' being compiled

How can I access the following class by the [] or [][] operator and access the matrix by row or element?

template<typename T, int D0, int D1>
class matrix
{
public:
	matrix();
	matrix(const D3DXVECTOR2& m0, const D3DXVECTOR2& m1);
	matrix(const D3DXVECTOR3& m0, const D3DXVECTOR3& m1, const D3DXVECTOR3& m2);
	
	float[4]& operator[](int row) const;

	float m[4][4];
};

matrix<float, 5, 5> gaussian_kernel(float sigma) 
{
    matrix<float, 5, 5> gauss;
    float sum = 0, s = 2 * sigma * sigma;

    for (int x = -2; x <= 2; x++)
        for (int y = -2; y <= 2; y++)
            sum += (gauss[x + 2][y + 2] = exp(-(x * x + y * y) / s) / s / M_PI);

    for (auto& row : gauss)
        for (auto& x : row)
            x /= sum;

    return gauss;
}

[wolle]

The line:

sum += (gauss[x + 2][y + 2] = exp(-(x * x + y * y) / s) / s / M_PI);

cannot be compiled.

Since the matrix class represents no abstraction why not access the m member, like

sum += (gauss.m[x + 2][y + 2] = exp(-(x * x + y * y) / s) / s / M_PI);

[luckiejacky]

The original definition of the matrix is:

template<class T, size_t Rows, size_t Cols> using matrix = std::array<std::array<T, Cols>, Rows>;

I was unable to compie this with some low-end compilers. Should I use my method to adapt to this class, or should I seek another workaround to make this matrix class work again?
thanks
Jack

[wolle]

The original definition of the matrix is:

template<class T, size_t Rows, size_t Cols> using matrix = std::array<std::array<T, Cols>, Rows>;

I was unable to compie this with some low-end compilers. Should I use my method to adapt to this class, or should I seek another workaround to make this matrix class work again?

That's a nice solution and it allows for [][] accesses. It compiles with the latest VS 2017 compiler, can't you upgrade?

If not, your current matrix class approach will work with an older compiler, like

template<class T, size_t R, size_t C> // you possibly need to use int rather than size_t
class matrix {
public:
	T data[R][C];
};
//
matrix<int, 4, 4> gauss;
gauss.data[0][0] = 1; // works

[luckiejacky]

Thanks

[Marc G]

A way to make your operator[] work is to implement it as follows:

template<typename T, int D0, int D1>
class matrix
{
public:
	matrix() {};

	T* operator[](int row) { return m[row]; }

	T m[4][4];
};