static dynamic 2 dimensional array in class C++

Question

I want to make something like this:

Class myclass
{
private:
static int **board;
public:
myclass();
};

and then in the cpp file

board = new int*[x];
for (int i = 0; i < x; i++) {
    board[i] = new int[y];
}

my goal is to have only one board, no matter how many objects i make.

Wrap the `board` in a [singleton](https://stackoverflow.com/questions/1008019/). And consider using `std::vector` instead of `new[]` directly. You might even consider flattening the `board` into a 1-dimensional array instead of 2-dimensional (you can still use 2-dimensional indexes with a little math). — Remy Lebeau, Sep 19 '17 at 23:26
Have you specified the 2 dimensions? square board? run time specified? — 2785528, Sep 20 '17 at 01:26

score 1 · Accepted Answer · answered Sep 19 '17 at 23:27

1

You are looking for Singleton, linked here is a wikipedia page on this

https://en.wikipedia.org/wiki/Singleton_pattern

A very simple implementation on Singleton would be:

static int ** getBoard() {
    if (!board) {
        board = new int*[x];
        for (int i = 0; i < x; i++) {
            board[i] = new int[y];
        }
    }
    else {
        return board;
    }
}

And you can use myclass::getBoard() to get the board.

You might want some variant based on your requirement.

answered Sep 19 '17 at 23:27

James Maa

1,764
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20

2

I would wrap the `board` in a class whose constructor takes the x/y values as input, and then use [Meyer's initialization](https://stackoverflow.com/questions/17712001/) to construct the `board`, eg: `static int** getBoard() { static BoardWrapper board(x, y); return board.data(); }` – Remy Lebeau Sep 19 '17 at 23:30
@RemyLebeau Definitely, If I am writing the program I would likely do the same, but once again I don't know what the author exactly wants, we can only introduce the `singleton` idea to the author and let him decide how he want his code looks like. – James Maa Sep 19 '17 at 23:33
Eeeek that's a bad example for how to implement a _Singleton_. – user0042 Sep 19 '17 at 23:55

2785528 · Answer 2 · 2017-09-20T15:11:33.097

my goal is to have only one board, no matter how many objects i make.

You are remarkably close to what you need. The key idea still missing: how determine if this is the 1st ctor, so that you only init the static board a single time. It turns out that a vector can solve this issue, too.

#include <chrono>
// 'compressed' chrono access --------------vvvvvvv
typedef std::chrono::high_resolution_clock  HRClk_t; // std-chrono-hi-res-clk
typedef HRClk_t::time_point                 Time_t;  // std-chrono-hi-res-clk-time-point
typedef std::chrono::milliseconds           MS_t;    // std-chrono-milliseconds
typedef std::chrono::microseconds           US_t;    // std-chrono-microseconds
typedef std::chrono::nanoseconds            NS_t;    // std-chrono-nanoseconds
using   namespace std::chrono_literals;          // support suffixes like 100ms, 2s, 30us

#include <iostream>
#include <vector>
#include <cassert>


class myclass
{
private:
   static std::vector<int> board;
   static int              maxCol;

   // add non-static parameters (unique to each instance)
   // here

public:

   // ctor
   myclass(int maxRows=0, int maxCols=0)
      {
         // key idea: how determine 1st ctor, so only init board 1 time
         if(0 == board.size())
         {
            std::cout
               << "  because board.size() is 0, \n"
               "  we know this is the first opportunity "
               "to reserve and initialize \n  "
               << maxRows << "*" << maxCols << " = " << (maxRows * maxCols)
               << " elements." << std::endl;

            // reserve space
            board.reserve(maxRows*maxCols); // now have all the space needed

            // with space available, initialize the elements
            for (int r=0; r<maxRows; ++r) {
               for (int c=0; c<maxCols; ++c) {
                  board.push_back(0); // init to valid number
               }
            }

            maxCol = maxCols;
         } // additional invocation do not affect board, parameters ignored
         else
            std::cout << "\n\n  parameters ignored because size is ("
                      << board.size() << ")" << std::endl;

         // add myclass ctor unique actions here
      }

   ~myclass() = default;


   // 2d access game to board
   // to 1d           from-2d
   size_t    gbIndx(int r, int c) { return ((r * maxCol) + c); }

   // void   gbIndx(size_t indx1d, int& r, int& c) {
      // simple arithmetic to determine both r and c from indx1d
   //}

   int exec()
      {
         std::cout << "\n  exec(): sizeof(board) = " << sizeof(board)
                   << "  board.size() = " << board.size() << std::endl;

         // do what now?

         return(0);
      }

   // add more methods here

}; // class myclass

std::vector<int> myclass::board; // static vector
int              myclass::maxCol = 0;


int main(int, char**)
{
   myclass mc(10,10);

   Time_t start_us = HRClk_t::now();

   int retVal = mc.exec();

   // subsequent invocation work, but do not affect board.size()
   myclass mc2;
   mc2.exec();

   myclass mc3;
   mc3.exec();


   auto  duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);

   std::cout << "\n\n  t534.exec() duration  " << duration_us.count() << " us" << std::endl;
   return(retVal);
}

Output looks like:

because board.size() is 0, we know this is the first opportunity to reserve and initialize 10*10 = 100 elements.

exec(): sizeof(board) = 24 board.size() = 100

parameters ignored because size is (100)

exec(): sizeof(board) = 24 board.size() = 100

parameters ignored because size is (100)

exec(): sizeof(board) = 24 board.size() = 100

static dynamic 2 dimensional array in class C++

2 Answers2