How do I declare a 2d array in C++ using new?

Question

How do i declare a 2d array using new?

Like, for a "normal" array I would:

int* ary = new int[Size]

but

int** ary = new int[sizeY][sizeX]

a) doesn't work/compile and b) doesn't accomplish what:

int ary[sizeY][sizeX] 

does.

Answer 1

I used this not elegant but FAST,EASY and WORKING system. I do not see why can not work because the only way for the system to allow create a big size array and access parts is without cutting it in parts:

#define DIM 3
#define WORMS 50000 //gusanos

void halla_centros_V000(double CENW[][DIM])
{
    CENW[i][j]=...
    ...
}


int main()
{
    double *CENW_MEM=new double[WORMS*DIM];
    double (*CENW)[DIM];
    CENW=(double (*)[3]) &CENW_MEM[0];
    halla_centros_V000(CENW);
    delete[] CENW_MEM;
}

Answer 2

There are two general techniques that I would recommend for this in C++11 and above, one for compile time dimensions and one for run time. Both answers assume you want uniform, two-dimensional arrays (not jagged ones).

Compile time dimensions

Use a std::array of std::array and then use new to put it on the heap:

// the alias helps cut down on the noise:
using grid = std::array<std::array<int, sizeX>, sizeY>;
grid * ary = new grid;

Again, this only works if the sizes of the dimensions are known at compile time.

Run time dimensions

The best way to accomplish a 2 dimensional array with sizes only known at runtime is to wrap it into a class. The class will allocate a 1d array and then overload operator [] to provide indexing for the first dimension. This works because in C++ a 2D array is row-major:

^{(Taken from http://eli.thegreenplace.net/2015/memory-layout-of-multi-dimensional-arrays/)}

A contiguous sequence of memory is good for performance reasons and is also easy to clean up. Here's an example class that omits a lot of useful methods but shows the basic idea:

#include <memory>

class Grid {
  size_t _rows;
  size_t _columns;
  std::unique_ptr<int[]> data;

public:
  Grid(size_t rows, size_t columns)
      : _rows{rows},
        _columns{columns},
        data{std::make_unique<int[]>(rows * columns)} {}

  size_t rows() const { return _rows; }

  size_t columns() const { return _columns; }

  int *operator[](size_t row) { return row * _columns + data.get(); }

  int &operator()(size_t row, size_t column) {
    return data[row * _columns + column];
  }
}

So we create an array with std::make_unique<int[]>(rows * columns) entries. We overload operator [] which will index the row for us. It returns an int * which points to the beginning of the row, which can then be dereferenced as normal for the column. Note that make_unique first ships in C++14 but you can polyfill it in C++11 if necessary.

It's also common for these types of structures to overload operator() as well:

  int &operator()(size_t row, size_t column) {
    return data[row * _columns + column];
  }

^{Technically I haven't used new here, but it's trivial to move from std::unique_ptr<int[]> to int * and use new/delete.}

Answer 3

In C++11 it is possible:

auto array = new double[M][N]; 

This way, the memory is not initialized. To initialize it do this instead:

auto array = new double[M][N]();

Sample program (compile with "g++ -std=c++11"):

#include <iostream>
#include <utility>
#include <type_traits>
#include <typeinfo>
#include <cxxabi.h>
using namespace std;

int main()
{
    const auto M = 2;
    const auto N = 2;

    // allocate (no initializatoin)
    auto array = new double[M][N];

    // pollute the memory
    array[0][0] = 2;
    array[1][0] = 3;
    array[0][1] = 4;
    array[1][1] = 5;

    // re-allocate, probably will fetch the same memory block (not portable)
    delete[] array;
    array = new double[M][N];

    // show that memory is not initialized
    for(int r = 0; r < M; r++)
    {
        for(int c = 0; c < N; c++)
            cout << array[r][c] << " ";
        cout << endl;
    }
    cout << endl;

    delete[] array;

    // the proper way to zero-initialize the array
    array = new double[M][N]();

    // show the memory is initialized
    for(int r = 0; r < M; r++)
    {
        for(int c = 0; c < N; c++)
            cout << array[r][c] << " ";
        cout << endl;
    }

    int info;
    cout << abi::__cxa_demangle(typeid(array).name(),0,0,&info) << endl;

    return 0;
}

Output:

2 4 
3 5 

0 0 
0 0 
double (*) [2]

Answer 4

I have left you with a solution which works the best for me, in certain cases. Especially if one knows [the size of?] one dimension of the array. Very useful for an array of chars, for instance if we need an array of varying size of arrays of char[20].

int  size = 1492;
char (*array)[20];

array = new char[size][20];
...
strcpy(array[5], "hola!");
...
delete [] array;

The key is the parentheses in the array declaration.

Answer 5

I'm using this when creating dynamic array. If you have a class or a struct. And this works. Example:

struct Sprite {
    int x;
};

int main () {
   int num = 50;
   Sprite **spritearray;//a pointer to a pointer to an object from the Sprite class
   spritearray = new Sprite *[num];
   for (int n = 0; n < num; n++) {
       spritearray[n] = new Sprite;
       spritearray->x = n * 3;
  }

   //delete from random position
    for (int n = 0; n < num; n++) {
        if (spritearray[n]->x < 0) {
      delete spritearray[n];
      spritearray[n] = NULL;
        }
    }

   //delete the array
    for (int n = 0; n < num; n++) {
      if (spritearray[n] != NULL){
         delete spritearray[n];
         spritearray[n] = NULL;
      }
    }
    delete []spritearray;
    spritearray = NULL;

   return 0;
  } 

Answer 6

Try doing this:

int **ary = new int[sizeY];
for (int i = 0; i < sizeY; i++)
    ary[i] = new int[sizeX];