Changing pointers in functions [duplicate] - c++

Question

This question already has answers here:
Can a local variable's memory be accessed outside its scope?
(20 answers)
Closed 4 years ago.
I am new to c++ coding and this question may seem childish to you all. But I am really not able to come up on a solution to my problem. Please help.
#include <iostream>
using namespace std;
int* getnew(int* x){
int temp = *x;
int* y = &temp;
cout << "from function address of y = " << y
<< " and value of y = " << *y << endl;
return y;
}
int main(){
int x = 100;
int* y = getnew(&x);
cout << "address of y = " << y;
cout << " and value of y = " << *y << endl;
return 0;
}
Output of this code :
from function address of y = 0x7ffee52aa914 and value of y = 100
address of y = 0x7ffee52aa914 and value of y = 1
What I thought should be the output :
from function address of y = 0x7ffee52aa914 and value of y = 100
address of y = 0x7ffee52aa914 and value of y = 100
Can someone explain where am I going wrong ?

Answer 1

Since you’re getting the address of temp, wich was declared inside a function, it’s scope belongs only to the function.
So, when you leave get_new() the temp var no longe exists ( get deallocated ). Then when you acess the same address ( the temp address) in main you access a space of memory that isn’t reserved, wich means that that space can be freely used by other vars and stuff of the program.
Try to use malloc to allocate a new space of memory and guarantee that the var still lives after the get_new returns (finishes). Here’s a example:
#include <iostream>
using namespace std;
int* getnew(int* x){
int temp = *x;
// return a memory address pointing to a ‘ sizeof(int )’ bytes reserved space.
int *y = malloc( sizeof(int) );
cout << "from function address of y = " << y << " and value of y = " << *y << endl;
return y;
}
int main(){
int x = 100;
int* y = getnew(&x);
cout << "address of y = " << y;
cout << " and value of y = " << *y << endl;
return 0;
}
EDIT:
Also do not forget to deallocate the memory when you're finished using it ( no longer needs it):
#include <iostream>
using namespace std;
int* getnew(int* x){
int temp = *x;
// return a memory address pointing to a ‘ sizeof(int )’ bytes reserved space.
int *y = malloc( sizeof(int) );
cout << "from function address of y = " << y << " and value of y = " << *y << endl;
return y;
}
int main(){
int x = 100;
int* y = getnew(&x);
cout << "address of y = " << y;
cout << " and value of y = " << *y << endl;
/*********** same code until here********/
//use free for each memory allocated by malloc()
free(y); //deallocates the memory pointed by *y*
free(other_y_if_it_exists);
return 0;
}