Question

I have a class called ImageMatrix, which implements the C++ map in a recursive fashion; the end result is that I have a 3 dimensional array.

typedef uint32_t VUInt32; typedef int32_t VInt32;  class ImageMatrix { public:     ImageMatrixRow operator[](VInt32 rowIndex) private:     ImageMatrixRowMap rows; };  typedef std::map <VUInt32, VInt32> ImageMatrixChannelMap;  class ImageMatrixColumn { public:     VInt32 &operator[](VUInt32 channelIndex); private:     ImageMatrixChannelMap channels; };  typedef std::map<VUInt32, ImageMatrixColumn> ImageMatrixColumnMap;  class ImageMatrixRow { public:     ImageMatrixColumn operator[](VUInt32 columnIndex); private:     ImageMatrixColumnMap columns; };  typedef std::map<VUInt32, ImageMatrixRow> ImageMatrixRowMap; 

Each operator simply returns a map-wrapper class within, like so:

ImageMatrixRow ImageMatrix::operator[](VInt32 rowIndex) {     return rows[rowIndex]; }  ImageMatrixColumn ImageMatrixRow::operator[](VUInt32 columnIndex) {     return columns[columnIndex]; }  VInt32 &ImageMatrixColumn::operator[](VUInt32 channelIndex) {     return channels[channelIndex]; } 

Basically, when I set the value as say 100, and test the value to cout, it shows as 0, and not the number to which I had set it.

for (VUInt32 a = 0; a < GetRowCount(); a++) {     for (VUInt32 b = 0; b < GetColumnCount(); b++)     {         for (VUInt32 c = 0; c < GetChannelCount(); c++)         {             VInt32 value = 100;             matrix[a][b][c] = value;              VInt32 test = matrix[a][b][c];                              // pixel = 100, test = 0 - why?             cout << pixel << '/' << test << endl;         }     } } 

Note: I’ve altered the original code for this example so that it takes up less space, so some syntax errors may occur (please don’t point them out).

Answer 1

The following operators return by value, no writes modify the actual data.

ImageMatrixRow ImageMatrix::operator[](VInt32 rowIndex);  ImageMatrixColumn ImageMatrixRow::operator[](VUInt32 columnIndex); 

Use:

ImageMatrixRow& ImageMatrix::operator[](VInt32 rowIndex)   ImageMatrixColumn& ImageMatrixRow::operator[](VUInt32 columnIndex)