Pointers as member data fields in C++ structs - c++

Question

#include <iostream>
using std::cout;
using std::cin;
void printArray(int A[], int size) {
for(int i = 0; i < size; i++) {
cout << A[i] << "\t";
}
cout << "\n";
}
struct Array {
int *A;
int size;
int length;
};
void display(Array *arr) {
for(int i = 0; i < arr->length; i++) {
cout << arr->A[i] << "\t";
}
cout << "\n";
}
void fillArray(Array *arr) {
for(int i = 0; i < arr->length; i++) {
cin >> arr->A[i];
}
}
void add(Array *arr, int x) {
if(arr->length < arr->size) {
arr->A[++arr->length] = x;
}
}
int main() {
Array arr;
cout << "arr.size before initializing: " << arr.size << "\n"; // gives a garbage value
cout << "Enter the size of the array: ";
cin >> arr.size;
cout << "Output of arr variable &arr: " << &arr << "\n";
arr.A = new int[arr.size];
cout << "arr.length before initializing: " << arr.length << "\n"; // gives garbage value
cout << "How many elements do you want to enter: ";
cin >> arr.length;
fillArray(&arr); //This is not pass by value but pass by reference because
display(&arr); // this function displays the values of arr
add(&arr, 15);
cout << "The length of the array after adding: " << arr.length << "\n";
display(&arr);
printArray(arr.A, arr.length);
}
The output of this program is like this:
$ ./array_adt
arr.size before initializing: -140717888
Enter the size of the array: 10
Output of arr variable &arr: 0x7ffe4e0ec040
arr.length before initializing: 21932
How many elements do you want to enter: 5
4 6 7 3 2
4 6 7 3 2
The length of the array after adding: 6
4 6 7 3 2 0
4 6 7 3 2 0
I am highly confused why the element 15 isn't added to the array of the struct (which is actually a pointer which is initialised to an array in the heap). If someone can throw light on this it would be great for a many people as well as for understanding c++ concepts in depth.
Thanks a lot.

Answer 1

arr->A[++arr->length] = x;
should be
arr->A[arr->length++] = x;
Array indexes start at zero in C++, so the first element past the end of an array arr of size N is arr[N]. In other words 15 did get added to the array, just not in the right place.
Also your code leaks memory because the array allocated in main is never deleted. Correctly allocating dynamic memory is a big (and very important) topic. Consult your favourite C++ book, in particular you should research the rule of three