What is the difference between:
int* a = new int[w]; int** b= new int*[w]; and what exactly does this mean: int*[w] and int[w]?
2 Answers
In the first case, you're dynamically creating an array of int with w elements. In the second, you create an array of int * with w elements.
The notations int*[w] and int[w] are the types mentioned above: an array of int * of size w, and an array of int of size w respectively.
3 Comments
Alex
int** b= new int*[w]; Are we not creating a 2d matrix in this case with int pointers w?
Jonathan Leffler
@Alex: No; you are not creating a 2D matrix. You are creating an array of pointers to
int. If you initialize each of those pointers to point to an array of int, then you can treat it more or less as a 2D array — but it still isn't a 2D array (matrix).
dbush
@Alex Technically, no. A matrix has a fixed number of rows and columns. The pointers are not initialized, so you need to allocate an array for each one. These sub arrays can be of different sizes. Also, the array of pointers and each array of ints are separate from each other, while a true 2D array is contiguous.
int* a = new int[w] This first example dynamically allocates w integers.
int** b = new int*[w] The second example dynamically allocates w pointers to integers.
1 Comment
EsmaeelE
And second one simply can be used to implement n-D array, as each element of array is a pointer to an integer.
whas to be defined as a const and not as a variable.int w = 6; int *a = new int[w]; int **b = new int*[w];), even withg++ -O3 -g -std=c++11 -Wall -Wextra -Werror -pedantic …. That's not the same as claiming "the C++ standard allows it", but the-pedanticshould trigger a warning (error because of-Werror) if it is in contravention of the standard.int w = 6; int vla_a[w]; int *vla_b[w];then you'd be playing with VLAs, and that is not allowed by the C++ standard but G++ does allow it unless you specify-pedantic. As it stands, with thenew, you're allocating a dynamic array with a dynamic size — the analogue ofint *a = malloc(w * sizeof(int));in C. Variable sizes are allowed inint *a = new[w];, I'm practically certain.