Set array dimension at runtime - c++

Question

I have a struct, which, depending on user inputs at runtime, will either require a 1D array or a 3D array. It will never need both. Right now, I have it set up like in the sample code below, with separate variables that can point to either a 1D array, or a 3D array. I would like to have just one variable in the struct that can point to either a 1D array or a 3D array, where the dimension is set at runtime. I have intermediate knowledge of C, and am a beginner with C++. I'd be willing to accept an answer based on C++ concepts but only if there is no slowdown (or negligible slowdown) compared to using C when iterating over the values. If it's a 3D array, then the for loops that access and change the array's values are the biggest bottleneck in my code. Once the array is set up, I won't need to change the dimension or size of the array.
Is there a way to do this, or should I just settle for always having an extraneous variable in my struct?
#include <iostream>
using namespace std;
typedef struct {
int dim;
int *one_d_arr;
int ***three_d_arr;
} Struct;
int main() {
int count = 0;
int *arr1 = (int*) malloc(2 * sizeof(int));
arr1[0] = 0;
arr1[1] = 1;
int ***arr3 = (int***) malloc(2 * sizeof(int**));
for (int i=0; i<2; i++) {
arr3[i] = (int**) malloc(2 * sizeof(int*));
for (int j=0; j<2; j++) {
arr3[i][j] = (int*) malloc(2 * sizeof(int));
for (int k=0; k<2; k++) {
arr3[i][j][k] = count++;
}
}
}
Struct s;
s.one_d_arr = NULL;
s.three_d_arr = NULL;
cout << "Enter number of dimensions: ";
cin >> s.dim;
if (s.dim==1) {
s.one_d_arr = arr1;
cout << s.one_d_arr[0] << ", " << s.one_d_arr[1] << endl;
}
else if (s.dim==3) {
s.three_d_arr = arr3;
cout << s.three_d_arr[0][0][0] << ", " << s.three_d_arr[0][0][1] << endl;
cout << s.three_d_arr[0][1][0] << ", " << s.three_d_arr[0][1][1] << endl;
cout << s.three_d_arr[1][0][0] << ", " << s.three_d_arr[1][0][1] << endl;
cout << s.three_d_arr[1][1][0] << ", " << s.three_d_arr[1][1][1] << endl;
}
else {
cout << "Must enter 1 or 3" << endl;
}
}

Answer 1

My recommendation is to use two different types here, instead of a single struct. Using an abstract base class, you can make both subclasses conform to a single interface, but they would have different underlying behavior. A very basic example:
class ArrayBase {
int dim;
public:
// This function is pure virtual, which means it's impossible to
// instantiate an instance of ArrayBase. Any class that inherits from
// ArrayBase must implement printArray().
virtual void printArray() = 0;
}
class Array1D : public ArrayBase {
int* array;
void printArray() {
// some code to print this one-dimensional array
}
}
class Array3D : public ArrayBase {
int*** array;
void printArray() {
// some code to print this three-dimensional array
}
}
Later, when you need to use the array, you can dynamically allocate the type you need, like this:
ArrayBase* inputArray;
// if the user wants a 1D array
inputArray = new Array1D();
// if the user wants a 3D array
inputArray = new Array3D();
// this will call the appropriate function to print the array
inputArray->printArray();
If you really want to have a single type, using boost::any is one way to condense your two array pointers into one. I would not recommend this approach, but it would work.

Answer 2

One of the juicy things about the C/C++ pointers is the existence of void pointers. A void pointer can point to anything you want, from int to int ***.
So you can simply use the following code:
#define CAST1(arr) ((int *)arr)
#define CAST3(arr) ((int ***)arr)
#define CAST(arr,i) CAST##i(arr)
typedef struct {
int dim;
void *arr;
} Struct;
int main()
{
Struct s;
cin >> s.dim;
int count = 0;
if (s.dim == 1){
s.arr = malloc(2 * sizeof(int));
CAST(s.arr, 1)[0] = 0;
CAST(s.arr, 1)[1] = 1;
}
else if (s.dim == 3){
s.arr = malloc(2 * sizeof(int ***));
for (int i = 0; i < 2; i++){
CAST(s.arr, 3)[i] = (int **) malloc(2 * sizeof(int **));
for (int j = 0; j < 2; j++){
CAST(s.arr, 3)[i][j] = (int *)malloc(2 * sizeof(int *));
for (int k = 0; k < 2; k++){
CAST(s.arr, 3)[i][j][k] = count++;
}
}
}
}
if (s.dim == 1) {
cout << CAST(s.arr, 1)[0] << ", " << CAST(s.arr, 1)[1] << endl;
}
else if (s.dim == 3) {
cout << CAST(s.arr, 3)[0][0][0] << ", " << CAST(s.arr, 3)[0][0][1] << endl;
cout << CAST(s.arr, 3)[0][1][0] << ", " << CAST(s.arr, 3)[0][1][1] << endl;
cout << CAST(s.arr, 3)[1][0][0] << ", " << CAST(s.arr, 3)[1][0][1] << endl;
cout << CAST(s.arr, 3)[1][1][0] << ", " << CAST(s.arr, 3)[1][1][1] << endl;
}
else {
cout << "Must enter 1 or 3" << endl;
}
system("pause");
return 0;
}