I have question on 'Declaring pointers'

[Flampard]

Write your question here.

Hi I am Joon who is studying C++ with this website.

I have question about one of part. At the part 'Declaring pointers' the example

// more pointers
#include <iostream>
using namespace std;

int main ()
{
int firstvalue = 5, secondvalue = 15;
int * p1, * p2;

p1 = &firstvalue; // p1 = address of firstvalue
p2 = &secondvalue; // p2 = address of secondvalue
*p1 = 10; // value pointed to by p1 = 10
*p2 = *p1; // value pointed to by p2 = value pointed by p1
p1 = p2; // p1 = p2 (value of pointer is copied)
*p1=20; // value pointed by p1 = 20

cout << "firstvalue is " << firstvalue << '\n';
cout << "secondvalue is " << secondvalue << '\n';
return 0;
}


I don't really understand why with all '*p1=20' the firstvalue has to be 10 not 20. When it comes to secondvalue, it makes sense since *p2=*p1.

could you explain why is it like that?

I really aprreciate on this website to lecture me informative C++.
Thank you.

[TarikNeaj]

welcome to the website.

Please use code tags for all of your code - http://www.cplusplus.com/articles/jEywvCM9/

Read below :)

[germanchocolate]

If you're wondering why firstvalue stayed at the value of 10, it's because the pointer p1 was changed to point to the address of secondvalue. Then when you did *p1 = 20, it actually set the secondvalue to 20, but not firstvalue since p1 was no longer pointing toward it.

Edit:

p1 = p2; // p1 = p2 (value of pointer is copied)

The code above is where the p1 was changed to point toward the secondvalue instead of firstvalue

[Gamer2015]

I think the output is that both are 20, right?

But what do you want to know?

[germanchocolate]

Gamer2015 wrote:

I think the output is that both are 20, right?

But what do you want to know?

Actually, firstvalue would output 10, since the value of firstvalue was only changed once.

[Flampard]

└ TarikNeaj Thank you for reply. I haven' read codes tags yet but next time I will follow that steps

└ germanchocolate Thank you very much now I get the concept more clearly. really appreciate it :)