Question

I’m writing a matrix class and I want to be able to cast a fixed sized matrix to an fixed sized double array. Although, I have troubles implementing the appropriate cast operator. What I’ve implemented so far does not work:

template<unsigned int M, unsigned int N>
class Matrix
{
    typedef double (&ArrayType)[M][N];
public:
     operator ArrayType();
}


Matrix<3,3> mat1;
double matArr[3][3];
matArr =  mat1;

error: incompatible types in assignment of ‘sfz::Matrix<3u, 3u>’ to
‘double [3][3]’

Casting the matrix explicitly causes another error:

error: ISO C++ forbids casting to an array type ‘double [3][3]’

Is there no way to implement the syntax i want to achieve?

Answer 1

You can’t assign arrays, ever. Live with it.

To make your function work, you could make a reference:

double (&ar)[3][3] = mat1;

Alternatively, you could wrap your naked array in something like std::array<std::array<double, M>, N> and return that by value. That’s why wrappers like std::array exist – they allow you to treat arrays like values. The same trick has worked in C since day one (putting an array inside a struct), but it’s actually nice and readable in C++:

typedef typename std::array<std::array<T, M>, N> type;
operator type() const { return internal_array; }