CODEWITHSUNDEEP
c++c++11
Brandon
Brandon

Reputation: 23498

How to overload non-const index operator for valarray class

In the class below, when I try to return std::valarray& for operator [] it says: invalid initialization of std::valarray& from R-Value reference std::valarray.

I'm using it like so:

int main()
{
    Matrix<int> MM(4, 4);
    for (int I = 0, K = 0; I < 4; ++I)
    {
        for (int J = 0; J < 4; ++J)
        {
            MM[I][J] = K++;  //Assigning to it does nothing atm :S
        }
    }

    for (int I = 0; I < 4; ++I)
    {
        for (int J = 0; J < 4; ++J)
        {
            std::cout<<MM[I][J]<<"  "; //Prints random values :l
        }
        std::cout<<"\n";
    }
}

and the class is as follows:

template<typename T>
class Matrix
{
    private:
        int Width, Height;
        std::valarray<T> Elements;
        static_assert(std::is_arithmetic<T>::value, "Argument T must be of arithmetic type.");

    public:
        Matrix(int Width, int Height);
        Matrix(T* Data, int Width, int Height);
        Matrix(T** Data, int Width, int Height);


        std::valarray<T>& operator [](int Index);
        const std::valarray<T>& operator [](int Index) const;
};

template<typename T>
Matrix<T>::Matrix(int Width, int Height) : Width(Width), Height(Height), Elements(Width * Height, 0) {}

template<typename T>
Matrix<T>::Matrix(T* Data, int Width, int Height) : Width(Width), Height(Height), Elements(Width * Height)
{
    std::copy(Data, Data + (Width * Height), &Elements[0]);
}

template<typename T>
Matrix<T>::Matrix(T** Data, int Width, int Height) : Width(Width), Height(Height), Elements(Width * Height)
{
    std::copy(Data[0], Data[0] + (Width * Height), &Elements[0]);
}


//ERROR below..
template<typename T>
std::valarray<T>& Matrix<T>::operator [](int Index)
{
    return Elements[std::slice(Index * Width, Width, 1)];
}

template<typename T>
const std::valarray<T>& Matrix<T>::operator [](int Index) const
{
    return Elements[std::slice(Index * Width, Width, 1)];
}

So my question is.. How can I overload the [] operator so I can get a single row or column so that I can index that and assign to it? I don't want to use the () subscript operator.

Upvotes: 1

Views: 266

Answers (1)

Praetorian
Praetorian

Reputation: 109119

In the non-const operator[] you have, this expression

return Elements[std::slice(Index * Width, Width, 1)];

This creates an std::slice_array<T> object, which is an rvalue. You cannot bind this to an std::valarray<T>&. The error can be fixed by changing the function to return a valarray by value instead of by reference.

template<typename T>
std::valarray<T> Matrix<T>::operator [](int Index) // <-- return by value
{
    return Elements[std::slice(Index * Width, Width, 1)];
}

Upvotes: 2

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