copying static array to dynamic array c++ - c++

Question

So I was playing around with some basics in c++ when I noticed this:
int * ptr = new int[3], arr[3] = { 11,13,15 };
for (int i = 0; i < 3; i++)
ptr[i] = arr[i];
cout << ptr[0] << ' ' << ptr[1] << ' ' << ptr[2] << endl;
this code will perfectly insert and print out the values: 11 13 15.
Whereas when I write it this way:
int size=0, * ptr = new int[size], arr[3] = { 11,13,15 };
for (int i = 0; i < 3; i++)
{
ptr = new int[++size];
ptr[i] = arr[i];
}
cout << ptr[0] << ' ' << ptr[1] << ' ' << ptr[2] << endl;
When I increment the size one value at a time, it prints out:
-842150451 -842150451 15
Why is it that when I define the total size at the beginning, the values are properly inserted, however when I increment one step at a time it does not? Also, is there any way to write the second code but in a way where it works please?

Answer 1

As #Ch3esteR points out in the comment. The problem is that you are reallocating your
array on each iteration, and thereby zeroing (or in your case, catching some random
values on the heap).
I would say that the bigger solution might be to use standard containers for handling memory. Here is a solution for fixing your original code original() and a proposed alternative for how to handle dynamic arrays with std::vector.
#include <vector>
#include <iostream>
using namespace std;
void original() {
const int arr[3] = { 11,13,15 };
const int size=3;
auto ptr = new int[size]; // this is what you want to do
std::copy(arr, arr + size, ptr); // Standard function for copying
cout << ptr[0] << ' ' << ptr[1] << ' ' << ptr[2] << endl;
}
int main() {
original();
const int arr[3] = { 11,13,15 }; // You might want to make this a std::vector too
std::vector<int> result{arr, arr + 3};
// std::vector<int> result; // Or if the question was how to do it dynamically
// for (auto i: arr)
// {
// result.push_back(i); // Dynamic add
// }
cout << result[0] << ' ' << result[1] << ' ' << result[2] << endl;
}
On request: How to do it with pointers (don't do this unless you have some external constraint that forces you to do this, like a teacher)
#include <iostream>
using namespace std;
// Reallocate memory and delete the old pointer
int *resize(int *arr, size_t oldSize, size_t newSize) {
auto newArr = new int[newSize];
auto copySize = std::min(oldSize, newSize); // Handle both growing and shrinking resize
std::copy(arr, arr + copySize, newArr); // Copy your old data
// Or do it like this
// for (size_t i = 0; i < copySize, ++i) {
// newArray[i] = arr[i]
// }
delete [] arr;
return newArr;
}
int main() {
auto arr = new int[2] { 11,13 }; // Observe 'new' here
size_t size = 3;
// "resize" the array arr (in practice you are just creating a new one)
arr = resize(arr, size, size + 1);
// Assign your new value
arr[2] = 14;
cout << arr[0] << ' ' << arr[1] << ' ' << arr[2] << endl;
delete [] arr; // Remember, try to not manual memory management,
// only use it in class when you are learning the basics, otherwise use
// std::unique_ptr or std::shared_ptr
}