How do I declare a function that can perform an assignment operation? (C++)

Question

I am trying to create a function similar to the at() function in std::vector. I know how to overload the operator for = but this is not what I am after. I have a matrix object and I am looking to perform an operation along the lines of overwriting a column vector of the matrix, i.e.

int rowNumber = 3; int columnNumber = 3;
Matrix myMatrix(rowNumber, columnNumber);
Vector myColumnVector(rowNumber);
myMatrix.col(2) = myColumnVector;

where col() is the assignment function. How do I declare this function?

Answer 1

You might use some proxy:

struct Matrix;

struct ColWrapper
{
    Matrix* mMatrix;
    int mIndex;

    ColWrapper& operator =(const std::vector<double>& d);
};

struct RowWrapper
{
    Matrix* mMatrix;
    int mIndex;

    RowWrapper& operator =(const std::vector<double>& d);
};

struct Matrix
{
    std::vector<double> mData;
    int mRow;

    Matrix(int row, int column) : mData(row * colunmn), mRow(row) {}


    ColWrapper col(int index) { return {this, index}; }
    RowWrapper row(int index) { return {this, index}; }
};

ColWrapper& ColWrapper::operator =(const std::vector<double>& ds)
{
    auto index = mIndex * mMatrix->mRow;

    for (auto d : ds) {
        mMatrix->mData[index] = d;
        index += 1;
    }
    return *this;
}

RowWrapper& RowWrapper::operator =(const std::vector<double>& ds)
{
    auto index = mIndex;

    for (auto d : ds) {
        mMatrix->mData[index] = d;
        index += mMatrix->mRow;
    }
    return *this;
}