Question

I’m writing a generic matrix class in c++. I want to be able to initialize a matrix from a two dimensional double array. Because I’m using templates and therefor matrix sizes need to be known at compile time, I want to make sure that just arrays of a given size can be passed to the constructor. Expressed in code this is:

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

Now, here comes the odd part: Like expected the following code works like a charm:

    double arr [3][3] = { {1,2,3},
                          {1,2,3},
                          {1,2,3} };
    Matrix<3,3>* mat3x3p = new Matrix<3,3>(arr);

But when passing the array anonymously the code is broken:

Matrix<3,3>* mat3x3p = new Matrix<3,3>({ {1,2,3},
                                         {1,2,3},
                                         {1,2,3} });

The same applies also to construction through assignment.
The exact error message is:

no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘double (&)[3][3]’

Answer 1

That would be binding a temporary to an (lvalue) reference to non-const, and that’s not allowed. If you change to a reference to const then that would work — although in the case of an array it’s a bit weird as cv-qualifiers apply to the element type, not the array itself. Nonetheless, if you have

Matrix(const double (&values)[M][N]);

then

Matrix<3, 3>({{ 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 }})

is valid in C++11, thanks to list initialization. I don’t know of a way to make it work for C++03.