Templated constructors within template class

Question

I am trying to implement a Matrix class by defining rows and columns as template parameters. Also, I consider the Vector as a 'degraded' matrix class that share most of its code.

I would like to initialize my classes using initializers lists, as it was used in old c arrays way.

The problem comes when trying to give one and only one constructor to the class depending of being a Matrix or a Vector, so it is forbides in compiling time to assign a matritial initializer list to a vector and a vectorial initializer list to a matrix. I have tried with templates and with std::enable_if, but I am not able to fully understand the weird error messages of the compiler.

I post the reduced code stating the problem, clean of all of trials to solve it. In the comments I include further explanation of what I am trying to do. Anyone can knows how to do what I need?

#include <initializer_list>     

template<int R, int C, typename T=int>
struct Matrix
{
    T data[R][C];

    Matrix(std::initializer_list<T> initializers) { }//Do some stuff inside
    Matrix(std::initializer_list<std::initializer_list<T>> initializers) { }//Do some stuff inside
};

template<int R, typename T=int>
using Vector = Matrix<R, 1, T>;

int main()
{
    Matrix<3, 3> m = { {1, 2, 3},
                       {4, 5, 6},
                       {7, 8, 9} };

    Vector<3> v = {10, 20, 30};

    Matrix<3, 3> m1 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };//This must be avoided using templates in constructors or SFINAE 
    Vector<3> v1 = { {10, 20, 30} };//This must be avoided using templates in constructors or SFINAE

    return 0;   
}

Also, I post a coliru link with this same code: http://coliru.stacked-crooked.com/a/17b5e8c504c262d1

Thank you very much in advance.

Answer 1

With C++20, you might use requires to discards overloads:

template <int R, int C, typename T=int>
struct Matrix
{
    T data[R][C];

    Matrix(std::initializer_list<T> initializers) requires (C == 1)
    { /*..*/ }

    Matrix(std::initializer_list<std::initializer_list<T>> initializers) requires (C != 1)
    { /*..*/ }
};

Demo

Prior of that, specialization:

template <int R, int C, typename T=int>
struct Matrix
{
    T data[R][C];

    Matrix(std::initializer_list<std::initializer_list<T>> initializers)
    { /*..*/ }
};

template <int R, typename T>
struct Matrix<R, 1, T>
{
    T data[R][1];

    Matrix(std::initializer_list<T> initializers)
    { /*..*/ }
};

or SFINAE can be used:

template <int R, int C, typename T=int>
struct Matrix
{
    T data[R][C];

    template <typename D = C, std::enable_if_t<D == 1, int> = 0>
    Matrix(std::initializer_list<T> initializers)
    { /*..*/ }

    template <typename D = C, std::enable_if_t<D != 1, int> = 0>
    Matrix(std::initializer_list<std::initializer_list<T>> initializers)
    { /*..*/ }
};