How to visualize a column order matrix with natvis like a 2d array

Question

I have a struct

struct Matrix2d
{
    // Column first ordered elements
    vector<int> m_elements;
    int m_numRows;
    int m_numCols;
};

m_elements stores {0, 1, 2, 3, 4, 5, 6, 7, 8} to represent the 2d matrix

0, 3, 6
1, 4, 7
2, 5, 8

I want to display this like below:

Using ArrayItems feature in Natvis, I am able to come down with:

Using natvis code:

  <Type Name="Matrix2d">
    <Expand>
      <ArrayItems>
        <Direction>Backward</Direction>
        <Rank>2</Rank>
        <Size>$i==0?m_numRows:m_numCols</Size>
        <ValuePointer>&amp;m_elements[0]</ValuePointer>
      </ArrayItems>
    </Expand>
  </Type>

But this is really ugly and I'd rather have each row be a single item than each element being an item, like how array2d is visualized.

How would you write the code in Natvis such that Matrix2d can be visualized in such way?

Answer 1

For such structures, I prefer to use CustomListItems. In your case it might look like this:

  <Type Name="Matrix2d">
    <Expand>
      <CustomListItems>
        <Variable Name="i" InitialValue="0"/>
        <Loop Condition="i  &lt; m_numRows">
          <Item Name="{i}">&amp;m_elements[i * m_numCols],[m_numCols]na</Item>
          <Exec>++i</Exec>
        </Loop>
      </CustomListItems>
    </Expand>
  </Type>

Example:

struct Matrix2d
{
    Matrix2d(int r, int c) : m_numRows(r), m_numCols(c), m_elements(r * c, 0) {}
    // Column first ordered elements
    std::vector<int> m_elements;
    int m_numRows;
    int m_numCols;
};
Matrix2d g(4, 3);