using pointers as private in class to access an array in c++ - c++

Question

I am using a int pointer as private to access an array. When i write separate functions for store and get values to an array, the program crashes. But if i write the get value and store value code in constructor, the program works fine. I am not able to find where the problem is.
Program 1: (Which is not working)
#include<iostream>
using namespace std;
class NewArray{
private:
int Size = 0;
int *arrAddr = NULL;
public:
NewArray(int);
void SetValue(int);
void GetValueOf(int);
};
//Array is created
NewArray::NewArray(int arSz){
int arr[arSz];
Size = arSz;
arrAddr = arr;
cout << "An array of Size " << Size << " is created" << endl;
}
// Store Value function
void NewArray::SetValue(int index)
{
cin >> *(arrAddr+(index));
}
//Get value function
void NewArray::GetValueOf(int idx)
{
if ((idx >= Size) || (idx < 0))
{
cout << "index value is out of bound" << endl;
}
else
{
cout << *(arrAddr+idx) << endl;
}
}
int main()
{
int arrSize, arrIdx;
cout << "enter the size of array" << endl;
cin >> arrSize;
if (arrSize > 0)
{
NewArray ar(arrSize);
cout << "enter " << arrSize << " values. Enter the values one after the other." << endl;
for (int i = 0; i < arrSize; i++)
{
ar.SetValue(i);
ar.GetValueOf(i);
}
cout << "enter the index to fetch the value" << endl;
cin >> arrIdx;
ar.GetValueOf(arrIdx);
}
else{
cout << "invalid input" << endl;
}
return 0;
}
Program 2: (Code which is working)
#include<iostream>
using namespace std;
// size is passed
class NewArray{
private:
int Size;
int *arrAddr;
public:
NewArray(int);
void GetValueOf(int);
};
NewArray::NewArray(int arSz){
int arr[arSz];
int idx;
Size = arSz;
arrAddr = arr;
cout << "An array of Size " << Size << " is created" << endl;
// Storing values in array
cout << "enter " << Size << " values. Enter the values one after the other." << endl;
for (int i = 0; i < Size; i++)
{
cin >> *(arrAddr+i);
}
// To get the value from the index
cout << "enter the index to fetch the value" << endl;
cin >> idx;
if ((idx >= Size) || (idx < 0))
{
cout << "index value is out of bound" << endl;
}
else
{
cout << "The value is " << *(arrAddr+idx) << endl;
}
}
int main()
{
int arrSize, arrIdx;
cout << "enter the size of array" << endl;
cin >> arrSize;
if (arrSize > 0)
{
NewArray ar(arrSize);
}
else{
cout << "invalid input" << endl;
}
return 0;
}
I have tried for this particular example, program 1 crashes when the array size is 10 and when i am trying to write to 7th index.
Can anyone please help me find out why?

Answer 1

In the constructor NewArray::NewArray() you create an array, which is stored on the stack. After leaving the constructor its lifetime is over, it is removed from the stack, so accessing it through your pointer arrAddr is Undefined Behavior.
To simply fix the problem you need to allocate the array on the heap using new and delete or store it as a class member.
These are just two ways of implementing. I do not recommend anything, they are just possibilities.

Answer 2

int arr[arSz];
is allocated on stack and hence it's lifetime is limited to the function in which it is defined - the allocated stack memory is available for others once the stack is unwound. You need to allocate memory on heap using the new operator, for the memory to remain persistent after the function call.
arrAddr = new int[arSz];
The above allocates memory on heap and is available until explicitly deleted by a call to delete [] arrAddr, which in most cases should be done only in the destructor.

Answer 3

forgot about int *. use vector instead. look here
#include <iostream>
#include <conio.h>
#include <vector>
using namespace std;
class Class1 {
private:
vector<int> intArr;
public:
void Set(int iValue) {
intArr.push_back(iValue);
}
int Get(int iIndex) {
return intArr[iIndex];
}
};
int main() {
Class1 class1;
for (size_t i = 0; i < 10; i++)
{
class1.Set(i);
}
for (size_t i = 0; i < 10; i++)
{
cout << class1.Get(i) << endl;
}
_getch();
}