C++ - fail to modify pointer in class member function - c++

Question

I defined a function getArray() in class A, like this:
void getArray(int * arr){
arr = new int[10];
arr[0] = 1;
}
I want to modify pointer arr in getArray(). And I invoked function getArray() in main(), like this:
int main(){
A obj;
int * arr = new int[2];
obj.getArray(arr);
cout<<arr[0]<<endl;
delete[] arr;
}
But when I executed main(), the output of arr[0] is 0, not 1. Why?
Actually, I want to allocate new memory to pointer arr and set the contents of the array in function getArray(). Then I want to get the contents of the array and deallocate pointer arr in main(). But I don't know what to do.

Answer 1

The array was not modified, because you reassign the variable in getArray. This code will work:
void getArray(int * arr){
arr[0] = 1;
}
You should decide where the memory for array is allocated. If the answer is in main, then you should deallocate it at the end of this function:
int main(){
A obj;
int * arr = new int[2];
obj.getArray(arr);
cout<<arr[0]<<endl;
delete[] arr;
}
If the array should be created in getArray, then you don't need the allocation in main, and you should return the new pointer from getArray, and don't forget to deallocate it:
int * getArray(){
int * arr = new int[10];
arr[0] = 1;
return arr;
}
int main(){
A obj;
int * arr = obj.getArray();
cout<<arr[0]<<endl;
delete[] arr;
}

Answer 2

Because you are calling getArray with a copy of your pointer.
You can call functions by-value or by-reference. Using a pointer may look like a by-reference call, yet it is a by-value call as a pointer only stores a reference.
You could use pass the pointer by-reference to achieve what you are trying to do: void getArray(int *& arr).
BTW: I hope that this code isn't somewhere in a productive environment as it has 2 memory leaks.
UPDATE:
Examples using std::vector:
Example 1:
#include<iostream>
#include<vector>
struct A
{
void getData(std::vector<int>& v){
v = std::vector<int>(10, 0); //vector with 10 zeros
v[0] = 1;
}
};
void printData(const std::vector<int>& v)
{
std::cout << "v.size() = " << v.size() << std::endl;
for(int value : v)
{
std::cout << value << std::endl;
}
}
int main(){
A obj;
std::vector<int> v;
obj.getData(v);
printData(v);
return 0;
}
Example 2:
#include<iostream>
#include<vector>
struct A
{
std::vector<int> getData(){
std::vector<int> v(10, 0); //vector with 10 zeros
v[0] = 1;
return v;
}
};
void printData(const std::vector<int>& v)
{
std::cout << "v.size() = " << v.size() << std::endl;
for(int value : v)
{
std::cout << value << std::endl;
}
}
int main(){
A obj;
std::vector<int> v = obj.getData();
printData(v);
return 0;
}