c++11 specefic overload of << operator

For an assignment, i have to code a matrix class following some clear instructions. One of the instructions is overloading the << operator , so that we can read the values this exact way for a matrix m :

m << 1,2,3,4,5,6;

i tried looking into functions with variable parameters , but then i saw that i can't overload the operator with a variable number of parameters.

i tried looking in std::initializer_list , using some reference code from cpp reference

std::vector<float> mat;


Mat<M,N>& operator<<(std::initializer_list<float> l)
{
    this->mat.insert(this->mat.begin(),l.begin(),l.end());

    return *this;
}

so my question is, what class/type of parameters can i use to achieve this, the options that i thought of did not work , or maybe i did not use them the right way.

thank you very much.