Pointer address assignment in c++ [duplicate] - c++

Question

This question already has answers here:
What are the differences between a pointer variable and a reference variable?
(44 answers)
Closed 1 year ago.
i'm code beginner
#include <bits/stdc++.h>
using namespace std;
void init(int* arr){
arr = new int[10];
cout << arr << endl;
}
int main(int argc, char* argv[]){
int *arr;
init(arr);
cout << arr << endl;
}
this is a simple code.
My question is that why arr's address in init function and arr's address in main function is different?
My think is that i gave arr's address to init function and in init function, arr is assigned a new address through 'new int[10]'
So, arr's address in init function and in main will be same.
but this code doesn't work as i think.....
Can you tell me why?

Answer 1

Step back a little and think about how arguments are passed into functions.
void foo(int x) {
}
int main() {
int x;
foo(x);
}
Here the x in main() is actually passed by value to foo() which means a copy of x is created when passing to foo().
With the same logic, if you think of int * as another variable type
using intPtr = int*; //a sort of typedef
void foo(intPtr x) {
}
int main() {
intPtr x;
foo(x);
}
a copy of x is again created. This is what is happening in your program. For what you expect, you need to pass in the variable by reference
using intPtr = int*;
void foo(intPtr& x) {
}
int main() {
intPtr x;
foo(x);
}
Adapting the same to your program:
#include <iostream>
void init(int*& arr){
arr = new int[10];
std::cout << arr << '\n';
}
int main(int argc, char* argv[]){
int *arr;
init(arr);
std::cout << arr << '\n';
delete(arr);
}
There's another way with pointers that is using a pointer to a pointer (**)
#include <iostream>
void init(int** parr){
*parr = new int[10];
std::cout << *parr << '\n';
}
int main(int argc, char* argv[]){
int *arr;
init(&arr);
std::cout << arr << '\n';
delete(arr);
}
where the address of arr (&arr) is passed into the function. Inside the function, the contents of parr is modified (which is arr in this case).