C++ 2D subscript operator

Question

I know that for a simple 2d matrix like:

class Matrix{
  vector<vector<int>> data;
};

In order to support operation like Matrix[][], you just need to overload operator[] which returns the corresponding row vector like:

vector<int>& operator[](int row){return data[row]};

My question is how to implement the subscript operator if I need to perform some transformation on the col. Say for i th row, the size of that row is 10.

I want to return the actual data when j is less than 5 but some other value say i+j when j is greater than 5.

Is there a way to achieve this? Thanks!

You might return a proxy.

Jarod42
– Jarod42

2018-04-22 08:08:02 +00:00
Commented Apr 22, 2018 at 8:08 — Jarod42
– Jarod42, Commented Apr 22, 2018 at 8:08
@Jarod42 Can you elaborate?

CCC
– CCC

2018-04-22 08:16:10 +00:00
Commented Apr 22, 2018 at 8:16 — CCC
– CCC, Commented Apr 22, 2018 at 8:16

Jarod42 · Accepted Answer · 2018-04-22 08:32:34Z

3

You might return proxy, something like:

class Proxy {
public:
    Proxy(std::vector<std::vector<int>>* data, std::size_t i) : data(data), i(i) {}

    int& operator[](std::size_t j) {
         if (j < 5) { return (*data)[i][j]; }
         else { return i + j; }
    }
private:
    std::vector<std::vector<int>>* data;
    std::size_t i;
};

class Matrix{
public:
    Proxy operator[](std::size_t i) { return {&data, i}; }
private:
    std::vector<std::vector<int>> data;
};

answered Apr 22, 2018 at 8:32

Jarod42

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