Why when I changed the value of the variable using pointer, the value was still the same?-CodePudding

can someone explain why the output of firstvalue is 10 although it is manipulated at the end when writing *p=20?

Thanks

Input

#include <iostream>

using namespace std;

int main(){
int firstvalue = 5, secondvalue = 15;
int * p1, * p2;
p1 = &firstvalue; 
p2 = &secondvalue; 
*p1 = 10; 
*p2 = *p1; 
p1 = p2; 
*p1 = 20; 
 
cout << "firstvalue is " << firstvalue << endl;
cout << "secondvalue is " << secondvalue << endl;
cout << "p1 is " << p1 << endl;
cout << "p2 is " << p2 << endl;
cout << "*p1 is " << *p1 << endl;
cout << "*p2 is " << *p2 << endl;

return 0;

}

Output

firstvalue is 10
secondvalue is 20
p1 is 0x7ffc0589c4f4
p2 is 0x7ffc0589c4f4
*p1 is 20
*p2 is 20

CodePudding user response：

why the output of firstvalue is 10 although it is manipulated at the end when writing *p=20?

There is no *p=20 in the program.

There is *p1 = 20 which doesn't manipulate firstvalue because p1 doesn't point to firstvalue. It points to secondvalue because of this assignment: p1 = p2.

CodePudding user response：

With these istructions:

p1 = &firstvalue; 
p2 = &secondvalue;

p1 points to firstvalue, and p2 to secondvalue.

The next two instructions:

*p1 = 10; 
*p2 = *p1;

set the value pointed by p1 to 10, i.e., firstvalue is set to 10, and the value pointed by p2 (secondvalue) to the value pointed by p1. So now firstvalue = secondvalue = 10.

This istruction:

p1 = p2;

set the pointer p1 to p2: now p1 points to secondvalue. After that, this istruction

*p1 = 20

set the value of secondvalue to 20, while firstvalue is untouched.