Question

I am having trouble with an assignment. I have a class called Grid and Record. Record is Grid that stores how many times each position in a Grid has been accessed. Grid must contain a data member of type Record that can store whenever a change is made to Grid. I’ve managed to avoid a circular dependancy, but I seem to have a problem with a “circular construction” leading to a stack overflow. Calling the Grid constructor makes a Record, which creates a Grid, which tries to make another Record and so on. My first thought was to make Grid’s Record data member a pointer which I initialize to NULL, then actually create the Record object outside of the constructor, however the assignment specifically says Grid’s Record data member must be of type Record, not a pointer to a Record. I am also not allowed to give Record a default constructor, so I’m guessing my implementation is wrong. Any suggestions for how I can solve this problem?

Record::Record(int rows, int cols) : grid(new Grid(rows, cols)) {};

Grid::Grid(int rows, int cols) : record(new Record(rows, cols)) {};

Answer 1

I think there is simply a misunderstanding in the assignment, or that it was written poorly.

It seems that what you want:

Grid
+---+---+-
| a | b |
+---+---+-
| c | d |
+---+---+-

Record
+---+---+-
| 0 | 1 |
+---+---+-
| 2 | 2 |
+---+---+-

Which indicates that a has not been accessed and d has been accessed twice.

Now, if a Grid should contain a Record to maintain its statistics, then a Record cannot be a Grid. As you noticed it is circular. However, Record is a grid. So I suspect there is a typo/reading issue here.

My suggestion:

class Record {
public:
  Record(size_t rows, size_t cols): rows(rows), cols(cols), grid(rows*cols, 0) {}

private:
  size_t rows;
  size_t cols;
  std::vector<size_t> grid;
};


template <typename E>
class Grid {
public:
  Grid(size_t rows, size_t cols): grid(rows*cols, 0), record(rows, cols) {}

private:
  std::vector<E> grid;
  Record record;
};