Dynamic Memory Allocation for Two Dimensional Arrays in C++

In C++, a two-dimensional array can be declared simply like this

int a[10][20];

However, this declaration requires contanst array size and it is inconvenient when the array size vary. To avoid this issue, you can use dynamic two dimensional array. There are several ways to allocate a dynamic two-dimensional array in C++. In this post, I will show you three most frequent methods.

First one is flattening the two dimensional array into one dimensional array and using it as one dimensional array  

//Allocate
int *a = new int[m*n];

//Use a[m][n]
for( int i = 0 ; i < m ; i++)
        for ( int j = 0 ; j < n ; j++)
                 a[i*n + j] = 1;

//Deallocate
delete[] a;  

Second method is using an array of dynamic one-dimensional arrays. It makes use of dynamic 2D array be easier. However, elements of array may not be continuous in memory

//Allocate
int **a = new int*[m];
for ( int i = 0 ; i < m ; i++)
a[i] = new int[n];

 

 

//Use a[m][n]
for ( int i = 0 ; i < m ; i++)
         for ( int j = 0 ; j < n ; j++)
a[i][j] = 1;
 

//Deallocate
for ( int i = 0 ; i < m ; i++) delete[] a[i];
delete[] a; 

 

The third one allows all elements of a 2D int array to be contiguous in memory and I like this way most.

 //Allocate
int **a = new int*[m];
int a[0] = new int[m*n];
for ( int i = 1 ; i < m ; i++)  a[i] = a[i-1] + n; 

 //Use a[m][n]
for ( int i = 0 ; i < m ; i++)
        for ( int j = 0 ; j < n ; j++)
a[i][j] = 1;

  

//Deallocate
delete[] a[0];
delete[] a;

 

If you prefer to performance, I recommend you to use the first method. This may be weird but I have tested on several codes.